Carnitine-Metabolism and Functions

Transcription

PHYSIOLOGICAL REVIEWS
Vol. 63, No. 4, October 1983
Prided i?i U.S.A.
Carnitine-Metabolism
and Functions
JON BREMER
Institute
of Medical
Biochemistry,
University
of Oslo, Oslo, Norway
1420
0031-9333/83
$1.50
Copyright
0 1983 the
American
Physiological
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I. Introduction
..........................................................
..........................................
II. Occurrence
and Distribution
III. Biosynthesis
..........................................................
...............
A. S-adenosylmethionine-6-N-L-lysine
methyltransferase
.................................
B. Protein
(lysine)
methyltransferase
....................
C. 6-N-trimethyllysine,
2-oxoglutarate
dioxygenase
............................
D. 3-Hydroxy-6-N-trimethyllysine
aldolase
...........................
E. y-Butyrobetaine
aldehyde
dehydrogenase
........................
F. y-Butyrobetaine,2-oxoglutarate
dioxygenase
.....................................
IV. Transport
of Carnitine
Precursors
V. Transportofcarnitine
................................................
A. Liver
.............................................................
.........................................
B. Skeletal
muscle
and heart
C. Kidney
............................................................
D. Epididymis
........................................................
E. Brain
.............................................................
........................................................
F. Other
cells
............................................
VI. Excretion
and Degradation
A. Excretion
.........................................................
....................................
B. Degradation
in microorganisms
.............................................
C. Degradationininsects
..........................................
D. Degradation
in mammals
.....................................
VII. Regulation
of Carnitine
Turnover
...........
VIII.
Function
of Carnitine
in Mitochondrial
Fatty
Acid Oxidation
..........................
IX. Function
of Carnitine
in Acetate
Metabolism
A. Fatty
acid synthesis
...............................................
..................................
B. Acetate
and acetyl-CoA
oxidation
.............
X. Function
of Carnitine
in Peroxisomal
Fatty
Acid Oxidation
................................
XI. How Many
Carnitine
Acyltransferases?
...........................................
XII. Carnitine
Acetyltransferase
...........................................
A. Isolation
and properties
............................................
B. Kinetics
and specificity
................................
C. Inhibitors
and “suicidal”
substrates
.........................................
XIII.
Carnitine
Octanoyltransferase
........................................
XIV.
Carnitine
Palmitoyltransferase
.........................................
A. Purification
and properties
............................................
B. Kinetics
and specificity
C. Effectofdetergents
...............................................
.............................
D. Properties
in mitochondrial
membrane
E. Inhibitors
.........................................................
.................................................
XV. CarnitineTranslocase
October
1983
XVI.
XVII.
XVIII.
XXIII.
I.
AND
FUNCTIONS
A. Propertiesand
kinetics
............................................
B. Inhibitors
.........................................................
Acylcarnitine
Hydrolase
..............................................
Regulation
of Fatty
Acid Oxidation
in Liver
...........................
A. Effect
of metabolites
and cofactors
.................................
B. Changes
in mitochondria
..........................................
C. Allover
regulation
.................................................
Fatty
Acid Oxidation
in Extrahepatic
Tissues
..........................
A. Heart
and skeletal
muscle
.........................................
B. Brown
adipose
tissue
..............................................
Carnitine
Acyltransferases
in Newborns
...............................
Carnitine
and Fertility
................................................
Carnitine
and Branched-Chain
Amino
Acid Metabolism
................
Carnitine
in Pathogenic
Mechanisms
..................................
A. Loss and lack of carnitine
.........................................
B. Inborn
errors
of carnitine
metabolism
..............................
Summary
............................................................
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INTRODUCTION
Carnitine has a long history in biochemistry.
It was discovered to be a
quantitatively
important
compound in muscle tissue in 1905 (El), and its
chemical structure was determined
in 1927 (404). Although
the chemical
similarity between carnitine and choline inspired extensive physiological and
pharmacological
studies in the 193Os, such studies did not reveal any definite
evidence either for the physiological role of carnitine or for its biosynthesis
or degradation.
In 1952 Carter et al. (89) created new interest in carnitine when they
established that it is a growth factor for the meal worm Tenebrio moZitcw
[hence carnitine’s other name, vitamin BT (T for Tenebrio)]. Subsequent studies
showed that carnitine-deficient
larvae died “fat” when they were starved;
that is, they were unable to utilize their fat stores in order to survive (151).
The recent steadily growing interest in the function of carnitine has its
origin in two papers published in 1955. At that time Friedman and Fraenkel
(153) discovered that carnitine can be reversibly acetylated by acetyl coenzyme A (acetyl-CoA), and Fritz (155) showed that carnitine stimulates fatty
acid oxidation in liver homogenates. These studies led to the discovery that
carnitine carries activated fatty acids across the mitochondrial
membrane.
The present review deals mainly with carnitine research since 1955. The
early literature
has been extensively covered in reviews by Fraenkel and
Friedman (151, 154) and by Fritz (156).
Two symposia on carnitine have been published in separate books (152,
430), and reviews of carnitine palmitoyltransferase
(EC 2.3.1.21; 196) and of
carnitine metabolism in humans (279-281) have also appeared.
Analytical and experimental procedures are not discussed in this review.
XIX.
XX.
XXI.
XXII.
CARNITINE-METABOLISM
1422
II.
OCCURRENCE
JON
AND
BREMER
Vdume
63
DISTRIBUTION
III.
BIOSYNTHESIS
The ability to synthesize carnitine seems to be nearly as ubiquitous as
carnitine itself. Only a few insect larvae of the tenebrionidae
family require
carnitine (i.e., vitamin BT) in their diets (89, 154). The first convincing evidence for carnitine biosynthesis in animals was obtained from chick embryos,
which contained significant amounts of carnitine, whereas none was found
in eggs. When grown on a carnitine-free
synthetic medium, the microorganism Neurospora crassa also contained carnitine (150).
The first clues to the mechanism of carnitine biosynthesis were obtained
in 1961, when it was shown that the methyl groups of carnitine come from
methionine (but not from choline; 59,431) and that y-butyrobetaine
(but not
y-aminobutyric
acid or y-dimethylaminobutyrate)
is converted to carnitine
(60, 242, 243). These results, however, left the origin of the carnitine fourcarbon chain unknown. The origin of this chain was established 10 years
later when several investigators
showed that labeled lysine is converted to
carnitine in N. crassa (201,202) and in the rat (397), with 6-N-trimethyllysine
as an intermediate
(127, 128, 397). Figure 1 shows the conversion of lysine
to carnitine.
Carnitine most likely is present in all animal species, in many microorganisms, and in many plants (150, 279, 326). A similar, nearly ubiquitous
distribution
of carnitine palmitoyltransferase
has been found (129,304). The
flight muscle of the blowfly (Phormia regina) is an exception: it has a high
concentration
of carnitine but no detectable carnitine palmitoyltransferase,
although a high activity of carnitine acetyltransferase
(EC 2.3.1.7) is found
(109). The general occurrence of carnitine and of its accompanying
carnitine
acyltransferases shows that it must have been developed at a phylogenetically
early stage, probably during a time closely associated with the development
of mitochondria.
The concentration of carnitine in different species and in different tissues
varies over a wide range. The highest concentrations
reported have been
found in horseshoe crab muscle (152) and in rat epididymal fluid, in which
carnitine can reach a concentration
of 60 mM (80). In mammalian
tissues
the concentration
usually varies between 0.1 and a few millimoles per liter
(with the highest concentrations
in heart and skeletal muscle), and there
are relatively great interspecies variations.
The carnitine concentration
is
-1 mM in rat skeletal muscle (81), is -3 mM in human muscle (97), and
may be up to 15 mM in ruminant
muscle (376, 377).
When carnitine is found in plants, the concentration
is only a few micromoles per liter (326).
October
1983
(Protein)
I ysine
AND
IkH 33)
I
CH2
I
CH2
I
II
CH
kH 331
I
CH2
I
CH2
I
III
CH2 -
CH2
I
CHNH,
I
COOH
CHOH
I
CHNH2
FH2NH2
COOH
COOH
I2
(Protein)
6-N-trimethyllysine
-
I
I
i(CH3)3
I
CH2
I
CH2
I
CH
I2
CHO
+
3-Hydroxy-
If-Butyrobetaine
6-N-trimethyk
lysine
aldehyde
+ glycine
1423
FUNCTIONS
IV
-
i(CH331
I
CH2
I
CH
I 2
CH2
I
COOH
8-Butyrobetaine
i(CH3)3
I
CH2
I
CHOH
I
CH2
I
COOH
I-ICarnitine
FIG. 1. Enzyme
steps in the biosynthesis
of carnitine.
I: S-adenosyl-6-N-L-lysine
methyltransferase
(yeast) or protein
(lysine)
methyltransferase
(animal
tissues).
II: 6JWtrimethyllysine,
2-oxoglutarate
dioxygenase
in animal
tissues
trimethyllysine
must first be liberated
from methylated
proteins
by peptidases.
III: 3-hydroxy-6-N-trimethyllysine
aldolase.
IV: butyrobetaine
aldehyde
dehydrogenase.
V: y-butyrobetaine,%oxoglutarate
dioxygenase.
In N. crassa free lysine is methylated
with S-adenosylmethionine
as the
methyl donor (55, 340). In animals, however, 6-N-trimethyllysine
is formed
by the methylation
of lysine residues in proteins such as myosin, actin, and
histones (321). Carnitine
is formed from this 6-N-trimethyllysine
after its
liberation
in protein breakdown, presumably in the lysosomes (139, 235).
In the next step the trimethyllysine
is hydroxylated
to 3-hydroxy-6-Ntrimethyllysine
(197,203,223,308,350).
The hydroxytrimethyllysine
is cleaved
to butyrobetaine
aldehyde and glycine (193,203). The butyrobetaine
aldehyde
is then oxidized to butyrobetaine
(204), which is finally hydroxylated
to carnitine (243, 244). Most animal tissues, including brain and skeletal muscle,
contain the enzymes necessary to convert trimethyllysine
to butyrobetaine
(128, 178, 342, 398). However, the last enzyme [butyrobetaine
hydroxylase;
also called y=butyrobetaine,2=oxoglutarate
dioxygenase
(EC 1.14.11.1)] is
present only in few tissues, and it also shows species variations
in tissue
distribution.
In all species it is found in the liver. In the rat it is also found
to a small extent in the testis, but it is absent from all other tissues (38,95,
128,178). It is present in the kidneys in hamster, rabbit, rhesus monkey, and
cat but not in the kidneys of dog, guinea pig, mouse, or rat (145). In humans
the enzyme is present in liver, kidney, and brain (144, 342).
In liver the conversion of trimethyllysine
to butyrobetaine
seems to take
place in both parenchymal
and nonparenchymal
cells, whereas the hydroxylation of butyrobetaine
to carnitine occurs only in parenchymal
cells (139).
NH2
I
CH2
I
CH2
I
I
CH2 I
CH2
I
CHNH,
I
COOH
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JON
BREMER
A. S-Adenosylmethionine-6-N-h-Lysine
Volume
63
Methyltransferase
In N. crassa S-adenosylmethionine-6-N-L-lysine
methyltransferase
(EC
2.1.1.-) is a soluble enzyme with a molecular weight of -22,000. It contains
only one peptide chain. The same enzyme transfers all three methyl groups.
The second methyl group is transferred
about five times faster than the first,
and the third methyl group is transferred
about three times faster than the
second. Thus any accumulation
of partially methylated
lysine is prevented.
No cofactor or prosthetic group for this enzyme has been found (55).
(Lysine) Methyltransferase
Protein (lysine) methyltransferase
(EC 2.1.1.43) is present in all tissues
of the rat. It is localized in the nuclei. The Michaelis-Menten
constant (K,)
for S-adenosylmethionine
of the nuclear enzyme is -3 PM, a value well below
the intracellular
concentrations
of S-adenosylmethionine.
Partially purified
enzyme from calf thymus methylates
histone and a variety of proteins. It
is not known whether more than one enzyme is active in protein lysine
methylation.
Mono-, di-, and trimethylated
lysines are all found in different
proteins (321), but only the trimethyllysine
is converted to carnitine (235).
The same derivatives have been isolated from human urine (220). Thus only
part of the liberated trimethyllysine
is converted to carnitine.
C’ 6-N-Trimethyllysine,
%Oxoglutarate
Dioxygenase
This enzyme (EC 1.14.11.-) is a dioxygenase localized in the mitochondria
of kidney, liver, heart, skeletal muscle, and probably other tissues (389). The
enzyme requires a-ketoglutarate
and oxygen as cosubstrates and ferrous ions
and ascorbic acid for optimal activity (203). The enzyme presumably has a
reaction mechanism similar to that of y-butyrobetaine,Z-oxoglutarate
dioxygenase and other cy-ketoglutarate-requiring
hydroxylases (see sect. 1118’).
This is the only enzyme of this type shown to be present in mitochondria.
The hydroxylation
in isolated mitochondria
is stimulated
by Ca2+, but
this may be an effect on the substrate uptake in the mitochondria
(350). So
far the enzyme has not been purified. In accordance with the requirement
for ascorbic acid, scorbutic guinea pigs show a decreased capacity to convert
trimethyllysine
and butyrobetaine
to carnitine and have less carnitine than
normal in heart and skeletal muscle, although the carnitine content in liver
and plasma is normal (293).
D. 3-Hydroxy-6-N-Triwwthyllysine
Aldolase
This enzyme cleaves its substrate to butyrobetaine
aldehyde and glycine.
3-Hydroxy-6-N-trimethyllysine
aldolase may be identical to serine hydrox-
B. Protein
October
1983
AND
FUNCTIONS
1425
ymethyltransferase
(EC 2.1.2.1), because this enzyme catalyzes the reaction
(203). Nevertheless the existence of a separate enzyme for the cleavage of
3-hydroxy-6-N-trimethyllysine
has not been excluded. In human tissues the
highest activity is found in liver (342).
The enzyme is inhibited by l-amino-D-proline,
an agonist of vitamin Be,
leading to accumulation
of 3-hydroxy-6-N-trimethyllysine
in the cell (138).
E. T-Butyrobetaine
Aldehyde
Dehydrogenase
F. y-Butyrobetaine,Z-Oxoglutarate
Dioxygenase
This enzyme was the first enzyme of carnitine biosynthesis to be studied.
The enzyme requires ar-ketoglutarate
and oxygen as cosubstrates and ferrous
ions and a reducing agent for its activity. Ascorbic acid is most efficient as
the reducing agent. The enzyme catalyzes the reaction
butyrobetaine
+ a-ketoglutarate
+ O2 -
carnitine
+ succinate
+ CO2
The ferrous ions are assumed to activate oxygen in the reaction, and
ascorbic acid most likely protects the ferrous ions and enzyme sulfhydryl
(-SH) groups from oxidation. An activating effect of catalase (EC 1.11.1.6)
in vitro is probably explained by the removal of inactivating
hydrogen peroxide formed
by spontaneous
oxidation
of ascorbic acid and ferrous
ions (243).
With an ‘*02 atmosphere [‘*O]oxygen is incorporated
into both carnitine
and succinate. It has therefore been suggested that formation
of a peroxide
bridge between butyrobetaine
and a-ketoglutarate
initiates
the reaction
(241). The enzyme thus is a dioxygenase, incorporating
one oxygen in each
of the products. Several similar dioxygenases are known (244).
The purified enzyme of calf liver is a dimer with two 46,000-dalton subunits. The K, values for butyrobetaine
and a-ketoglutarate
are 0.5 and 0.8
mM, respectively (231).
A similar inducible enzyme has been isolated from a Pseudormmas strain
able to grow on butyrobetaine
as the sole carbon source (245, 246).
Recently it has been found that y-butyrobetaine,2-oxoglutarate
dioxygenase catalyzes the decarboxylation
of a-ketoglutarate
in the presence of
This enzyme has been purified from the cytosol of bovine liver. It shows
a high specificity for butyrobetaine
aldehyde and nicotine adenine dinucleotide (NAD). The &, for butyrobetaine
is -4 PM, and the molecular weight
of the enzyme has been determined
to be 160,000, with subunits of -55,000
daltons (204). In human tissues the highest activity is found in liver and
kidney (342).
1426
JON
BREMER
Vobne
63
both (+)- and (-)carnitine
but with no conversion of the carnitine. Hence
the enzyme is “uncoupled”
by one of its reaction products (195). It is still
unknown whether this reaction has any regulatory
function in carnitine
biosynthesis.
IV.
TRANSPORT
OF CARNITINE
PRECURSORS
V.
TRANSPORT
OF CARNITINE
Several in vivo studies have appeared on the uptake and exchange of
carnitine in different tissues (81,96,441). Because most tissues have a carnitine
concentration
that is >lO-fold higher than that of blood plasma, an active
uptake of carnitine must take place; this uptake, however, must occur with
widely varying rates. Brooks and McIntosh (81) have shown that the turnover
times for carnitine in kidney, liver, heart, skeletal muscle, and brain in the
and 220 h, respectively. Tissue distribution
and uptake
rat are 0.4,1.3,21,105,
are at least partially controlled by hormones, particularly
in the liver and
epididymis. Fasting or diabetes increase the liver concentration
of carnitine
(272). This increase is particularly
great in sheep, where the liver carnitine
increases 7- to 300fold in diabetic animals (377). In the kidneys the concentration is about doubled, whereas heart and skeletal muscle show no change.
In rats the implantation
of a tumor producing great amounts of prolactin, growth hormone (somatotropin),
and adrenocorticotropic
hormone
(ACTH) increased the liver carnitine nearly lo-fold. At the same time cardiac
carnitine dropped (327). Injection
of glucagon increases liver carnitine
in
rats (272), and glucagon also stimulates carnitine uptake in isolated hepatocytes (114). In humans glucagon reduces plasma carnitine (167), presumably because liver uptake is stimulated.
Choline-deficient
rats have a lowered liver carnitine that is rapidly replenished when choline is given (88). Because choline is not a carnitine precursor (59), it must therefore influence carnitine uptake in the liver by an
unknown mechanism.
The formation of 6-N-trimethyllysine
and its conversion to butyrobetaine
take place in most tissues, but the hydroxylation
of butyrobetaine
to carnitine
occurs only in the liver (and in some species in the kidney). Therefore interorgan transport of butyrobetaine
and carnitine takes place.
Intestinal digestion of proteins will also liberate trimethyllysine,
which
can be absorbed. The trimethyllysine
is partially converted to butyrobetaine
in the intestinal wall. Circulating
trimethyllysine
is not readily taken up by
the liver, but it is taken up and converted to butyrobetaine
by the kidneys
(444). Nothing is known about the release of butyrobetaine
from the tissues.
Circulating
butyrobetaine
is taken up by the perfused liver but is poorly
absorbed by perfused kidneys (444).
October
1983
AND
FUNCTIONS
1427
n
‘p
0
v
-x 15E
A
-53 %
‘E lon
u0
2.>
G
:
-
50
2
I
5e--4
II
III
trace
-1 7%
1
B
4
-16%
trace
20
Fraction
trace
40
60
no.
FIG. 2. Metabolism
of r3H]CH3(+)
carnitine
(A) and PH]CH3(-)carnitine
(B) in rat. Different
rats (weighing
-250 g) were injected
subcutaneously
with
2 radioactive
isomers
(~5 pmol),
and urine was collected
for 24 h. After
treatment
with alkali
to hydrolyze
acylcarnitines,
the
urine was evaporated;
the residue
was extracted
with ethanol,
and the extract
chromatographed
on a column
of Dowex
50 H, 200-400
mesh, 1 X 30 cm, with 1.5 N HCl as eluent.
Fractions
of
5 ml were collected.
Percentage
of injected
radioactivity
recovered
in different
chromatographic
peaks is given. I: carnitine;
II: methylcholine
(?); and III: trimethylaminoacetone.
Sex hormones may also influence carnitine distribution.
Female rats
have been reported to have a significantly
higher liver/plasma
carnitine
concentration
ratio than male rats (52), and the high carnitine uptake and
concentration
in the epididymis depend on an androgen-induced
transport
mechanism (42).
As Smoswell and Henderson (376) have pointed out, the carnitine content of skeletal muscles in sheep appears to be inversely related to the coenzyme A (CoA) content. Such a relation is not found in liver, however, where
both carnitine (272) and CoA (374) increase in fasting animals.
The great differences in carnitine uptake in different tissues have been
confirmed in studies on isolated organs or cells. Differences in kinetics and
stereospecificity
have also been observed. (+)Carnitine
is eliminated
from
the body more quickly than (-)carnitine
(Fig. 2). Nevertheless
even after
24 h a significant fraction of (+)carnitine
is retained in different
tissues,
apparently with a distribution
that agrees with the variable stereospecificity
of carnitine uptake in the tissues (Table 1).
Carnitine is also released by tissues, particularly
by liver, gut, and kidneys, which furnish other tissues with carnitine after synthesis, absorption,
and reabsorption,
respectively.
1428
JONBREMER
TABLE 1. Relative
68
uptake of labeled (-#-)- and (-)carnitine
Tissu6
(-)Carnitine
in tissues
injection;
3.2
8.1
19
7.7
31
54
2.4
0.9
(blood
-70%
= 1).of rH]CH3(-)of (+)carnitine
(+)Carnitine
3.6
6.6
8.6
4.8
9
9.5
1.1
0.8
and rH]CH3(
+)carnitine
(-5 pmol)
and 16% of (-)carnitine
were recovered
A. Liver
Isolated liver cells have the ability to accumulate both butyrobetaine
and carnitine (116). The maximum rate of carnitine uptake (2.4 nmol. mg-’
protein emin-‘) was found to be about twice that of butyrobetaine.
The K,
for carnitine uptake (“5 mM), however, was 10 times higher than for butyrobetaine (0.5 mM). Mutual competitive studies and the effect of structural
analogues suggest that both compounds are transported
by the same carrier,
and the lower K, for butyrobetaine
shows that this compound is the preferred
“substrate.”
The liver carrier shows no or almost no stereospecificity,
(+)carnitine being taken up about as fast as (-)carnitine.
This lack of stereospecificity has been confirmed in vivo (see Table 1). With butyrobetaine
the concentration in the cells reached ~100 times that in the medium, whereas with
carnitine an accumulation
with a factor of -10 was reached. This accumulation of carnitine is about the same as that in the intact animal.
Liver cells incubated in a carnitine-free
medium release both carnitine
and acetylcarnitine.
The release of acetylcarnitine
is even faster than the
release of free carnitine. This release of both carnitine and acetylcarnitine
can also be seen in intact rats injected with labeled butyrobetaine
(116). A
significant stimulation
of carnitine uptake by glucagon in isolated liver cells
has also been observed (114). These results suggest that the variable acetylcarnitine/carnitine
ratios in blood plasma and urine (57, 162, 166) reflect
corresponding
acetylcarnitine/carnitine
ratios in liver, ratios modified by
the transport
specificities of liver and kidneys.
B. Skeletal Muscle and Heart
Carnitine
uptake in muscle has been studied in heart cells in tissue
culture (41,284,285,287,288),
in heart myocytes (9), and in isolated skeletal
muscle (339, 425).
Liver
Kidney
Heart
Skeletal
muscle
Caput
epididymidis
Cauda epididymidis
Testis
Brain
Relative
uptake
24 h after subcutaneous
in urine (Fig. 1).
Vdume
October
1983
AND
FUNCTIONS
1429
C. Kidney
Kidneys show the fastest rate of carnitine turnover in the body (81),
presumably
because of efficient reabsorption
from the glomerular
filtrate,
and kidney slices absorb carnitine from the medium. The Km for (-)carnitine
The properties of the system for active uptake of carnitine are rather
different from those of the liver system. The Km values reported for (-)carnitine vary for different muscle preparations,
but all are much lower than
the Km found with liver cells (“5 PM in tissue culture heart cells, 60 PM in
heart myocytes, and 0.06-0.5 mM in isolated muscles, vs. 5 mM in liver cells).
The maximum uptake rate may’ also be only l/l,ooo of the maximum rate in
liver. This agrees quite well with the turnover time, which was 100-200 times
longer for muscle carnitine than for liver carnitine in vivo (81). The muscle
system also is more specific for (-)carnitine,
because the Km is significantly
higher and the uptake rate significantly
lower with both (+)carnitine
and
butyrobetaine.
This stereospecificity
has been confirmed in vivo (Table 1).
In heart cell cultures the capacity for carnitine uptake increased when
cells were grown in the presence of carnitine and when cells were grown in
the presence of prednisolone.
These effects seemed to be additive (285) and
may be why patients with lipid-storage myopathy due to low muscle carnitine
levels showed improvement
on treatment
with carnitine
and prednisone (412).
Carnitine is also slowly released from muscle cells, and carnitine and
carnitine analogues stimulate this release. Apparently
release is not simply
a reversal of uptake (284). In this connection it is interesting
that (+)carnitine injections in rats provoke a carnitine deficiency in heart and skeletal
muscle but not in the liver (332) and that (+)carnitine
[but not (-)carnitine]
can cause muscle weakness (myasthenia)
in patients treated with carnitine (18).
Recently, Cantrell and Borum (85) have extracted from heart a carnitinebinding protein with properties that agree closely with the carnitine-uptake
characteristics
of the heart. The highest binding capacity was found in a
fraction rich in plasma membrane.
It is likely therefore that this protein
represents the carrier that transports
carnitine across the cell membrane,
a carrier different from the carnitine translocase in the mitochondria
(see
sect. XV). A peculiar feature of this protein is the slow but relatively specific
binding of (-)carnitine.
Maximum binding was obtained after incubation for
1 h at 25OC. The relatively slow uptake of carnitine in heart may be connected
with this slow binding. It is also striking that butyrobetaine
is a relatively
weak inhibitor
of carnitine binding to this heart protein. In liver cells butyrobetaine
is taken up with a lower Km than is carnitine (116). It therefore
seems likely that the carnitine carrier in the hepatocyte cell membrane is
different from that of muscle tissues.
1430
JON
BREMER
l-+&me
63
uptake in kidney slices was found to be -90 PM; the maximum uptake rate
fluid, which is close to the calculated
was 22 nmol min-’ ml-’ intracellular
normal rate of carnitine reabsorption
in the kidneys. The system shows only
a moderate stereospecificity,
because the Km and the maximum
rate for
(+)carnitine
were 166 PM and 14 nmol min-’ ml-l fluid, respectively (206).
No studies have been performed on the renal clearance of (+)carnitine,
and
thus it is not known whether these results agree with the more rapid elimination of (+)carnitine
from the body (Fig. 1).
Evidently
the kidneys must also possess a system for the release of
carnitine into the bloodstream.
A relatively rapid “loss” of carnitine from
kidney slices was observed, but the nature of this release has not been studied. When (-)carnitine
and (+)carnitine
were administered
separately to
rats, 47% of the radioactive
(+)carnitine
was recovered in the urine after
4 h and 71% was recovered after 24 h, whereas with (-)carnitine
~10% and
16% were recovered in the urine after 4 and 24 h, respectively (Fig. 2). These
results agree quite well with the excretion of (+)carnitine
(247). Although
it is impossible from this experiment
to decide exactly to what extent the
kidneys selectively clear (+)carnitine
from the blood, the results suggest a
stereospecific reabsorption
of (-)carnitine
in the kidneys. A significant uptake of (+)carnitine
into the tissues does take place, however, and may explain why (f)carnitine
has some toxic effects (18, 332).
l
l
l
l
Marquis and Fritz (265) were the first to observe the extremely high
concentrations
of carnitine in the cauda epididymidis
and the epididymal
fluid and to determine
that this high concentration
depends on the action
of androgen hormones. In the epididymis the androgens induce a carnitinetransport system with the ability to concentrate carnitine in the epididymal
lumen (42). In rats this concentration
can reach 60 mM, or 2,000 times the
concentration
in blood (80, 191). As in the kidney, a double transport mechanism must be assumed, because the carnitine passes through the epididymal
cells into the lumen. The main concentration
gradient is across the brush
border at the luminal side of the cell, and androgens seem to induce this
transport
from the cell into the lumen (45). This assumption
of a double
transport
system is supported by the observation that carnitine uptake in
dispersed cells is nonstereospecific,
with a K, of ~1 mM for both (+)- and
(-)carnitine
(213). The overall process is specific for (-)carnitine
(440), however, and this is confirmed by in vivo experiments
(see Table 1).
E. Brain
Although carnitine uptake in the brain is extremely slow in vivo @I),
brain slices show a relatively rapid uptake (205). The K, for (-)carnitine
D. Epididymis
October
1983
AND
FUNCTIONS
1431
F. Other Cells
Carnitine uptake has been studied in fetal lung fibroblasts and in smooth
muscle cells from the aorta (87). The Km for uptake was found to be 6-8
PM [i.e., similar to that found in heart cells in tissue culture (41)].
In Pseudowmnas ueruginosa an active uptake mechanism stereospecific
for (-)carnitine
is induced when the cells are grown in a carnitine-containing
medium (7,229). The induction of this transport system, which also catalyzes
a carnitine/carnitine
exchange, is inhibited by chloramphenicol,
suggesting
that a protein carrier is synthesized. An Na+-K+-ATPase
also seems to be
involved in the active transport. The Km for the carnitine of this system was
found to be 0.7 mM.
VI.
EXCRETION
AND
DEGRADATION
A. Excretion
In normal animals carnitine is lost mainly by excretion in the urine. In
normal rats 1-2 pmol of carnitine are excreted per day per 100 g of body
weight (96, 406). This represents l/15-?&-, of the total body pool and is in
agreement with the slow turnover in quantitatively
important
tissues, such
as skeletal muscle (81). In one turnover study, however, the loss of carnitine
in rat was estimated to be about three times the amount recovered in the
urine. No explanation
was found for this discrepancy (81).
(56). The
In rats the renal clearance of carnitine is co.05 liters/day
glomerular
filtration
rate is ~1.7 liters. day-’ 100 g-l body wt (17). Thus
>95% of the carnitine in the glomerular filtrate is reabsorbed. This is similar
to amino acid reabsorption
in the kidneys. In fasting rats both the plasma
concentration
and the renal excretion drop at first but then increase above
the initial values after fasting for 3-4 days. Both free carnitine and acylcarnitines are excreted (56).
l
was ~3 mM, whereas (+)carnitine
was taken up with a Km of ~10 mM.
Thus the affinity of the transporter
in brain is similar to that in liver, but
the brain shows a more pronounced stereospecificity
than the liver, which
takes up (-)- and (+)carnitine
at about the same rate (116). All other tissues
investigated
have lower Km values (5 PM-0.5 mM).
The brain system is strongly inhibited by y-aminobutyric
acid [inhibitor
constant (Ki) = 0.6 mM], but this compound has no inhibitory
effect in kidney
and muscle (147, 205, 339).
The discrepancy between the rates of carnitine uptake in vivo and in
vitro suggests that the blood-brain
barrier limits the access of carnitine to
the brain. The nature of the transport
across this barrier is unknown.
1432
JON
vdurne
BREMER
63
Similar observations have been made in humans. Normal serum clearance is -1 ml/min,
and the daily excretion of carnitine
is 100-300 pmol.
Excretion increases after oral administration,
and after 6 h -10% of a l-g
oral dose is recovered in the urine. Tubular reabsorption
in the kidneys was
found to be 96-99% (140, 162, 257). Carnitine is also excreted in milk. The
concentration
in cow’s milk is 0.1-0.5 mM (146), in goat’s milk is -0.1 mM,
in sheep’s milk is nearly 1 mM (378), and in human milk is ~0.05 mM (53).
No studies on possible losses of carnitine to the gastrointestinal
tract have
appeared.
in Microurganisms
Several mechanisms are known for the degradation
of carnitine (Fig.
can
metabolize
carnitine
by the following
3). Different strains of Pseudomonas
reaction (246, 362)
(-)carnitine
+ NAD
g y-trimethylamino-P-ketobutyric
acid + NADH
where NADH is the reduced form of NAD. The enzyme (EC 1.1.1.108) is
inducible and specific for (-)carnitine.
The product can decarboxylate spontaneously to trimethylaminoacetone.
In the presence of ATP and CoA, however, extracts of Pseudomonas convert
i(CH331
I
CHOH \r
I
i(CH313
+
CH2
I
COOH
COOH
I
CH2
I
CHOH
CH
I 2
\IV
CO
I
L
COOH
I
COOH
(4 orl-1carnit
ine
Trimethylamine
+ Malate
Dehydro carnitine
i(CH331
I
II ~
tilCH313
I
CH
2
I
COOH
+
CH3
I
COOH
Betaine
+acetate
Trimet
CH2
I
co
I
CH3
+
CO
2
\
V
ihCH3)
3
I
CH2
I
CHOH
I
CH3
hylaminoacetone
(+) Methyl
choline
FIG. 3. Catabolism
of carnitine.
I: Carnitine
dehydrogenase.
Inducible
and specific
for
(-)carnitine
and NAD in some microorganisms.
[In insects and mammals
both (-)- and (+)carnitine
may be oxidized
by a similar
enzyme.]
II: This decarboxylation
may be spontaneous
(nonenzymatic).
III: Cleavage
in some microorganisms
by unknown
enzyme(s).
IV: CoA- and
ATP-dependent
cleavage
in some microorganisms.
CoA ester(s)
are probable
intermediate(s).
V: Trimethylaminoacetone
is reduced
to methylcholine
in some insects and possibly
in mammals.
Enzyme
unknown.
-
B. Degradation
October
1983
AND
FUNCTIONS
1433
the y-trimethyl-P-ketobutyric
acid to glycine and acetate, presumably with
the CoA ester(s) as intermediate(s)
(246). This reaction sequence can explain
Pedicoccus
why carnitine can substitute for betaine in the betaine-requiring
sogae (358).
The microorganism
Serratia murcescens can form trimethylamine
and
malate from carnitine (411). Escherichia coli can reduce carnitine to r-butyrobetaine,
presumably by hydrogenation
after dehydration
of carnitine to
y-crotonbetaine
(365).
in Insects
The larvae of some choline -requiring i nsects [Mwca donwstica (housefly),
regina (blowfly), and Drosophila melanogaster (fruit fly)] can form
methylcholine
from carnitine and can use this product instead of choline for
phospholipid
synthesis (29, 79, 164). Both (-)carnitine
and (+)carnitine
can
be utilized (33, 164), and it is striking that both isomers are converted to
(+)methylcholine,
showing that the hydroxyl group of carnitine is epimerized
in the process (33). To explain this phenomenon,
it was suggested that both
isomers are converted to trimethylaminoacetone,
which subsequently is reduced to (+)methylcholine.
The trimethylaminoacetone
was not isolated, but
it was shown to be a methylcholine
precursor.
The formation
of methylcholine
has also been explained by the action
of the enzyme carnitine decarboxylase
(EC 4.1.1.42), which has been found
of this
both in rat and in Phormia regina (175, 227). The stereospecificity
enzyme is not known because the assays were done with labeled racemic
(_+)carnitine. The (+)methylcholine
found in the phospholipids,
however,
would have to be formed from the unphysiological
(+)isomer.
To complicate matters further,
some evidence has been presented for
the formation
of phosphatidylcarnitine
in Phormia regina, and some researchers have suggested that phosphatidylmethylcholine
is formed by decarboxylation
of phosphatidylcarnitine
(275, 277).
Phormia
D. Degradation
in Mammals
Feeding experiments in which mice and rats were given heavy oral loads
of carnitine have shown increased urinary excretion of trimethylamine
and
trimethylamineoxide,
and even of butyrobetaine
and crotonobetaine
(367,
392, 393). These metabolites
are probably formed by intestinal microorganisms (335, 367).
Small amounts of trimethylaminoacetone
are found in normal urine
(171) and may also be of intestinal,
microbiological
origin. One striking bit
of evidence, however, is that the administration
of the unphysiological
isomer
(+)carnitine
to rats and mice leads to a far higher excretion of trimethylaminoacetone than does administration
of (-)carnitine,
and parenteral
admin-
C. Degradation
1434
JON
BREMER
Vohme
63
VII.
REGULATION
OF
CARNITINE
TURNOVER
Our knowledge of possible regulatory
mechanisms in the turnover of
carnitine is scarce. In the neonatal period rat pups apparently
receive most
of their carnitine from mother’s milk (346), and in human babies carnitine
probably is an essential nutrition
factor (53, 307, 373). In the growing rat,
however, the capacity for carnitine synthesis is sufficient, even with no carnitine in the diet (52, 150). It has been shown that a deficiency of lysine or
protein leads to a moderately
reduced carnitine content in heart, skeletal
muscle, and epididymis
(54, 228, 396, 400) and that carnitine can promote
growth in rats on a low-methionine
diet, suggesting that carnitine may have
a methionine-sparing
action (226). But carnitine synthesis depends on methylated lysine in proteins (235), and only a minor fraction of the methyl groups
of methionine
is finally incorporated
into carnitine (59). It seems unlikely
istration is more efficient than administration
per OS (366). Thus by some
unknown mechanism animal tissues seem to metabolize (+)carnitine
to trimethylaminoacetone
far more efficiently than (-)carnitine.
We have recently
confirmed this preference after injection of rats with rH]CH,(+)and (-)carnitine (Fig. 2). We have also found small amounts of a compound that is
chromatographed
like methylcholine.
Previous studies on the metabolism
of labeled racemic (+)carnitine
showed conversion of carnitine to what was identified as methylcholine
(227,
276). The results of Seim et al. (366), which have been confirmed in this
laboratory
(Fig. l), suggest that these earlier studies really represent primarily the conversion of (+)carnitine
to trimethylaminoacetone.
However,
the role of carnitine decarboxylase partially purified from mitochondria
remains unexplained
(227).
Carnitine decarboxylation
reportedly is stimulated
in diabetic and fasting rats, and in accordance with this finding the diabetic rats were found
to have a decreased body pool and a decreased half-life of carnitine (276).
As mentioned above, however, these studies were performed with labeled
racemic (&)carnitine.
In a more recent study the decreased carnitine content
in the skeletal muscles of diabetic rats could not be confirmed (388).
There is also other evidence for the degradation
of (-)carnitine
in mammals. Carboxyl-labeled
butyrobetaine,
which is rapidly converted to (-)carnitine in the body, gave rise to some radioactive
CO2 in rats (60), and
[‘4C]CH3(-)carnitine
also led to the formation of radioactive COz (247). However, the slow rates of formation
of labeled COz make it difficult to exclude
participation
of intestinal
microorganisms.
Altogether
it is possible that
some degradation
of (-)carnitine
takes place in mammals,
but it seems
doubtful that this degradation
is of any quantitative
importance.
Most of
the carnitine seems to be excreted unchanged in the urine.
October
1983
AND
1435
FUNCTIONS
VIII.
FUNCTION
OF CARNITINE
IN
MITOCHONDRIAL
FATTY
ACID
OXIDATION
In his 1955 paper Fritz (155) identified carnitine as the active component
in muscle extracts that had previously been observed to stimulate fatty acid
oxidation in perfused livers (255). In subsequent papers carnitine was also
shown to stimulate
fatty acid oxidation
in skeletal and heart particulate
fractions, and the effect was shown to be specific for carnitine. A stimulating
effect of CoA in the presence of carnitine was also observed (156).
By that time it was well known that isolated mitochondria
contain the
fatty acid @-oxidation enzymes and that mitochondria
normally retain CoA
and other cofactors through isolation and washing procedures.
Because of these different observations and because of the established
acetylation
of carnitine (153), acetylcarnitine
and fatty acid esters of carnitine (61, 62) were tested and were found to be excellent oxidizable substrates for isolated mitochondria.
The reversibility
of the acetyl group transfer to the mitochondria
was
demonstrated
by the formation of acetylcarnitine
in the presence of carnitine
and pyruvate (61). It was suggested that carnitine transports activated fatty
acids through the mitochondrial
membrane
and that this carrier function
represents an important
“loophole” in the compartmentation
of the cell (Fig.
4). Subsequently a CoA-dependent
formation
of palmitoylcarnitine
(63) and
therefore that protein methylation,
which may have other functions, is influenced by the availability
of carnitine. After breakdown
of the proteins
the trimethyllysine
is either converted to carnitine (139,235) or excreted in
the urine (220). Thus the conversion of trimethyllysine
may be regulated,
but it is not known whether this conversion is influenced by carnitine. Holme
and co-workers (195) have recently reported that the y-butyrobetaine,Z-oxoglutarate
dioxygenase is “uncoupled”
by carnitine. A similar mechanism
might regulate trimethyllysine,
Z-oxoglutarate
dioxygenase (EC 1.14.11.-),
which has a similar reaction mechanism and which represents the initial
step in the conversion of trimethyllysine
to carnitine (203).
In normal animals the excretion in urine of unchanged carnitine seems
to be the main pathway of loss. This excretion is increased in thyrotoxic and
decreased in hypothyroid
patients (256). Possibly this just corresponds to
an increased turnover of protein in the hyperthyroid
state. At present it
seems unlikely that carnitine breakdown is quantitatively
important
in carnitine turnover in animals, although intestinal microorganisms
may degrade
this compound. Because plasma carnitine is elevated in kidney failure (44,
108), the kidneys must take part in regulation of the plasma carnitine level.
As, mentioned
in section VIA, carnitine per OS increases urinary carnitine
excretion, whereas fasting first decreases and then increases carnitine excretion.
1436
JON
Ceil
BREMER
membrane
i’iium
63
Mitochondrion
/3-OH-but
t
AcAc---
p--c
Peroxisome
I
-AC-CoA
FFA
h8'
-Acyl-CoA
+
c24
CoA
p-ox.
= Acyl-CoA
C20
G1yc.P
FFA
c4-c
Triacylglyc.
P-lip.
1
10
I
I
FIG. 4. Compartmentation
of fatty
acid metabolism
in liver. Activation
of free long-chain
fatty acids to acyl-CoA
takes place in endoplasmic
reticulum
or in outer mitochondrial
membrane.
Circles
across
mitochondrial
membranes
represent
carnitine-dependent
transfer
of acetyl
and
in detail in
long-ch ain acyl groups
from extramitochondrial
to intramitochondrial
CoA (shown
Fig. 5). In this scheme
it is assumed
that acetyl-CoA
and shortened
fatty
acids formed
in
peroxisomes
are transported
into mitochondria
via this carnitine-dependent
mechanism.
FFA,
free fatty acids; Glyc.P,
glycerol-3-phosphate;
P-lip., phospholipids;
Triacylglyc.,
triacylglycerol;
CoA, coenzyme
A; Ac-CoA,
acetyl-CoA;
AcAc,
acetoacetate;
P-OH-but,
p-hydroxybutyrate;
pox., B-oxidation.
a carnitine-dependent
oxidation of palmitoyl-CoA
in isolated mitochondria,
with palmitoylcarnitine
as an intermediate
(160), were shown.
Even though the carnitine acyltransferases
were assumed to be present
inside and outside the mitochondria,
the carnitine palmitoyltransferase
was
later shown to be localized exclusively in the inner mitochondrial
membrane
(305, 306). Functional studies with isolated mitochondria
showed that mitochondria contain an external (overt) and an internal (latent) carnitine
palmitoyltransferase
(3’71, 438), and digitonin extraction showed that the
external transferase is more easily solubilized. After such solubilization
of
the outer transferase the mitochondria
can no longer oxidize palmitoyl-CoA
in the presence of carnitine, whereas the oxidation of palmitoylcarnitine
is
unhampered (199).
Early studies of mitochondrial
permeability
suggested that the inner
mitochondrial
membrane was impermeable
to carnitine (65, 176). It was
therefore assumed that the inner carnitine palmitoyltransferase
is a vecextramitochondrial
(acyl-) carnitine to react with
torial enzyme permitting
intramitochondrial
CoA. The isolated heart mitochondria,
however, retain
FFA
clO‘
------co2
'-Ox-
October
1983
AND
1437
FUNCTIONS
Outer
membrane
Inner
membrane
Matrix
AMP+ PP;
RCOO-+ ATP
tine
FIG. 5. Carnitine-dependent
transport
of activated
fatty
acids across
inner mitochondrial
membrane.
I: Acyl-CoA
synthase;
II: outer carnitine
acyltransferase;
III: inner (latent)
carnitine
acyltransferases;
and IV: carnitine/acylcarnitine
translocase.
AMP, adenosine
monophosphate;
ATP, adenosine
triphosphate;
CoA, coenzyme
A; PPi, inorganic
pyrophosphate;
RCOO-,
longchain fatty
acid.
an intramitochondrial
carnitine pool that is exchanged against added extramitochondrial
carnitine or acylcarnitines (322,337). Thus the inner membrane of heart mitochondria
contains a carnitine translocase catalyzing a
one-to-one exchange of carnitine and/or acylcarnitine across the membrane.
Later the translocase was also demonstrated in liver mitochondria
(329) and
in spermatozoa (84). In isolated liver mitochondria
the carnitine pool is small,
thus preventing detection of carnitine permeation when carnitine space is
measured.
At present the carnitine-dependent
acyl group-transfer
system of mitochondria is believed to consist of an outer and an inner carnitine acyltransferase connected by a carnitine translocase (322,337). So far, however,
no separate translocase protein has been identified (Fig. 5).
It has been suggested that the uptake of acylcarnitine
in mitochondria
is an energy-dependent
process prevented by uncouplers of oxidative phosphorylation
(130, 240). However, the observed inhibition
of acylcarnitine
oxidation in uncoupled mitochondria
was probably due to osmotic dehydration of the mitochondrial
matrix under the conditions used (316).
The function of carnitine in the oxidation of fatty acids must be understood mainly in relation to the cellular localization of the acyl-CoA synthases. The long-chain acyl-CoA synthase was originally found only in the
endoplasmic reticulum (234) but was later discovered in the outer membrane
of the mitochondria
as well (1, 163, 306, 4X), that is, was found outside the
permeation barrier to CoA in the inner membrane of the mitochondria.
In
contrast the short- and medium-chain-length
acyl-CoA synthases are localized in the mitochondrial
matrix (1, 2, 360). This explains why the oxi-
1438
JON
BREMER
Vohme
63
dation of short- and medium-chain-length
fatty acids is carnitine independent (423).
A certain ability of the medium-chain-length
acyl-CoA synthase inside
the acyl-CoA barrier to activate high concentrations
of long-chain fatty acids
(371) explains the repeated observation that isolated mitochondria
can oxidize long-chain fatty acids in the absence of carnitine (170). This oxidation
of long-chain fatty acids, however, is of doubtful physiological
significance
in the intact cell. The intramitochondrial
capacity for activation
of longchain fatty acids amounts to only 1% of the extramitochondrial
capacity in
rat liver (170). The overlapping
and organ variation of specificities explain
why the oxidation of octanoate is stimulated
by carnitine skeletal muscle
mitochondria
(169).
The mitochondria
also contain a guanosine 5’-triphosphate
(GTP)-dependent acyl-CoA synthase [guanosine 5’0diphosphate (GDP)-forming]
in their
matrix (348), which theoretically
should permit carnitine-independent
oxidation of long-chain fatty acids. But careful studies with isolated liver mitochondria have made it doubtful that this enzyme of low activity is of any
normal physiological
significance in the intact cell (15).
Isolated mitochondria
can also oxidize long-chain acyl-CoA esters in the
absence of carnitine and in the presence of free fatty acids, or when relatively
high concentrations
of acyl-CoA are used (73, 439); such oxidation presumably can occur because of detergent
effects that make the mitochondrial
membrane permeable. This carnitine-independent
oxidation is probably of
no physiological
significance.
Nevertheless, even if the carnitine dependence of the oxidation of longchain fatty acids in mitochondria
is the rule in animals, it is not without
exception. Flight muscle mitochondria
from a moth (Prodenic eridania) oxidize palmitate
at a rapid rate in the absence of carnitine. These mitochondria contain no carnitine palmitoyltransferase
but have a very active longchain acyl-CoA synthase (390).
Isolated mitochondria
from animal tissues normally oxidize acylcarnitines of all chain lengths from C2 to C= (62,317), and they contain carnitine
acyltransferases
that may show even higher activities with short- and medium-chain-length
fatty acids than with long-chain fatty acids (121, 381).
Because the medium- and short-chain
acyl-CoA synthases are localized
in the mitochondria,
it has been difficult to understand the function of mediumand short-chain carnitine acyltransferase
activities. Again our understanding
derives from the overlapping
chain-length
specificities of the extra- and
intramitochondrial
acyl-CoA synthases. This overlapping assures that fatty
acids of any chain length (including unusual fatty acids with branched chains
or other side groups) can be metabolized.
The medium- and short-chain
carnitine acyltransferase
activities ensure that any medium-chain
fatty acid
CoA ester formed “by accident” outside the mitochondria
can be transferred
to the mitochondria
for oxidation. Note that octanoate oxidation has been
found to be carnitine dependent in skeletal muscle mitochondria
(169).
October
IX.
1983
FUNCTION
OF CARNITINE
IN
ACETATE
AND
FUNCTIONS
1439
METABOLISM
A. Fatty Acid Synthesis
B. Acetate and AcetyGCoA
Oxidation
There is no general agreement on whether carnitine is active in the
oxidation of extramitochondrial
acetyl-CoA. No significant oxidation of added
acetyl-CoA in the presence of carnitine was obtained in isolated mitochondria
of ox liver (141) or in the flight muscle of Phomzia regina (109) despite a high
carnitine acetyltransferase
content. Apparently
almost all the enzyme was
latent (inner transferase),
which permits reaction only with mitochondrial
acetyl-CoA. From these and similar results it has been suggested that carnitine acts mainly as a buffer or as a sink for mitochondrial
acetyl-CoA,
allowing a shift of the “acetyl pressure” from the mitochondria
to the cytosol.
This has been suggested for tissues as different as sheep liver (375,377), rat
heart (333), frog muscle (4), and blowfly flight muscle (109). Because carnitine
is usually present in a much higher concentration
than CoA, the extramitochondrial acetylcarnitine/carnitine
ratio will prevent great fluctuations
in
the mitochondrial
acetyl-CoA/CoA
ratio. A similar function for acetylcarnitine as “spare fuel” can be visualized for spermatozoa (see sect. xx).
The metabolism
of acetate and of acetyl-CoA requires specific consideration, because acetyl-CoA synthase is found both-intraand extramitochondrially,
especially in lipogenic tissues such as liver, gut, and mammary
gland (13, 360), and because acetyl groups are transferred
from the mitochondria to the cytosol during lipogenesis.
Because acetylcarnitine
is formed from carnitine and pyruvate in isolated
mitochondria
(61, 109, 208), it was assumed that carnitine might be active
in the transfer of acetyl groups from the mitochondria
to the cytosol for
fatty acid synthesis (61). Although the idea gained some support from studies
on the effect of carnitine on fatty acid synthesis in guinea pigs (77), this
seems generally not to be the case.
Even in the guinea pig the activity of the outer carnitine acetyltransferase
OF he tota2 (II). In rats and other animals
is very low- only a Pew percent
the acetyl group transfer from the mitochondria
takes place via citrate synthesis in the mitochondria
and extramitochondrial
citrate cleavage to oxaloacetate and acetyl-CoA by ATP citrate (vo-3S)-lyase
(EC 4.1.3.8) (420).
In ruminants,
which lack citrate lyase, fatty acid synthesis seems to take
place from free acetate formed in the rumen (23,182). In addition the highest
activities of carnitine
acetyltransferase
are found in nonlipogenic
tissues
such as heart, skeletal muscle, and testis, whereas the enzyme’s activity is
low in lipogenic tissues such as liver and adipose tissue (264).
1440
JON
Vohme
BREMER
In some studies with isolated mitochondria,
however,
a rapid oxidation of added acetyl-CoA; this may occur,
tochondria
from rat heart (161), from pig kidney (58),
muscle of the locust (Locusta migratoria)
(19). Carnitine
be involved in the oxidation of excess extramitochondrial
cially acetyl-CoA formed in the peroxisomes.
X.
FUNCTION
OF CARNITINE
IN
PEROXISOMAL
FATTY
ACID
63
carnitine stimulates
for example, in miand from the flight
therefore may also
acetyl-CoA, espe-
OXIDATION
The recent demonstration
of a P-oxidation system for fatty acids in the
peroxisomes (237) suggests a function for the short- and medium-chain
carnitine acyltransferase(s),
that is, the transfer of acyl-CoA esters formed
outside the mitochondria
for oxidation in the mitochondria.
Isolated peroxisomes have been found to contain carnitine acetyltransferase
and medium-chain carnitine acyltransferase
but no carnitine palmitoyltransferase
(31, 219, 261, 263).
Peroxisomal fatty acid oxidation does not require the participation
of
carnitine. Apparently
the peroxisomal membrane contains a direct carrier
or a permease for acyl-CoA (5, 319). But peroxisomal fatty acid oxidation
is only partial; that is, the fatty acids are shortened (237, 320). This shortening seems to be particularly
important
in the metabolism
of very-longchain fatty acids (e.g., C,), which are poorly oxidized in mitochondria
(74,
118, 119).
The reaction products (acetyl-CoA and a shortened acyl-CoA) must be
transferred
to the mitochondria
for complete oxidation. It is therefore reasonable to assume these products are transferred
via the carnitine
acyltransferases. Because carnitine acyltransferases
are present in peroxisomes,
a transfer of this acetyl-CoA and of the shortened acyl-CoA would not require
the presence of external transferase(s)
in the mitochondria.
A direct demonstration of such transfer is still lacking, however, although some evidence
has been obtained for very-long-chain
fatty acids. It has been shown that
the chain shortening
of erucic acid (C,) is unaffected by (+)decanoylcarnitine, an inhibitor
of carnitine-dependent
fatty acid oxidation in the mitochondria,
or by carnitine depletion of the cell, but that the complete oxidation of erucic acid to ketone bodies and CO2 is carnitine dependent (118).
In this connection it is striking that clofibrate, which induces increased
peroxisomal
oxidation
of fatty acids in the peroxisomes, also induces increased activity of the carnitine acyltransferases
(particularly
the acetyltransferase)
in both mitochondria
and peroxisomes (219,262,289,382).
AcylCoA hydrolases in the cytosol are also induced under these conditions (49).
Therefore the transfer of the peroxisomal products to the mitochondria
as
free acetate and free fatty acids is also a possibility.
October
XI.
HOW
1983
MANY
CARNITINE
AND
FUNCTIONS
1441
ACYLTRANSFERASES?
The varying ability of mitochondria
from different
tissues to oxidize
acylcarnitines
of different chain lengths suggested that more than one carnitine acyltransferase
existed (62). This assumption was first confirmed by
the separation of carnitine acetyltransferase
from carnitine palmityltransferase (159, 160, 297, 298). The separated enzymes have overlapping
chainlength specificities. Shortly afterward
the carnitine acetyltransferase
was
prepared in crystalline form (103). The palmitoyltransferase
proved to be
more difficult to purify, but homogeneous preparations
of this enzyme have
also been prepared (121, 233).
After the initial studies, a bewildering
number of different carnitine
acyltransferases
were postulated,
first in mitochondria
and later also in
peroxisomes and in other parts of the cell. The demonstration
by Yates and
Garland (438) of different functional
pools of the transferases
[one easily
soluble outer transferase
and one firmly bound inner transferase
(199)] in
the mitochondria
led to attempts to characterize these different pools. West
et al. (421) found that the outer palmityltransferase
was inhibited
by Zbromopalmitoyl-CoA,
whereas the inner transferase was not. This difference
in sensitivity still remains unexplained.
Fritz et al. (83, 233) found immunological and kinetic differences between the isolated inner and outer transferases and suggested that the outer enzyme could be converted to the inner
enzyme.
Studies of chain-length
specificity that were performed
on intact mitochondria and on soluble enzyme preparations
have shown essentially three
activity peaks depending on fatty acid chain length. One peak occurs at chain
lengths C2-3 (acetyltransferase),
one at C &l() [octanoyl
transferase
(EC
(232, 380-382). Thus these
2.3.1.-)], and one at C 14-16 (palmitoyltransferase)
studies imply a total of six carnitine acyltransferases
in mitochondria
(i.e.,
3 outer and 3 inner enzymes). In addition carnitine acetyltransferase
and
medium-chain
have been found in peroxisomes (31,
224, 262, 263, 344) and in the endoplasmic
reticulum
(261), and palmityltransferase has been reported to be present in the cell membrane of erythrocytes (428), cells that lack intracellular
organelles.
Recent studies, however, again seem to reduce the number of carnitine
acyltransferases.
In a careful study Bergstrom and Reitz (24) have shown
that outer and inner palmitoyltransferases
purified with identical chromatographic
procedures most likely are identical proteins. Clarke and Bieber
(121) also used chromatographic
procedures to show that heart mitochondria
contain only two carnitine acyltransferases,
one short-chain acyltransferase
and one long-chain
acyltransferase,
again with overlapping
chain-length
specificities. They have also shown that in heart the octanoyl transferase
activity may be explained by this overlap and by the kinetic properties of
the long-chain transferase. Recently, however, a separate octanoyl transferase
1442
JON
BREMER
Vobmne
63
XII.
CARNITINE
A. Isolation
ACETYLTRANSFERASE
and Properties
Carnitine acetyltransferase
is relatively easily extracted from the mitochondrial
membrane with salt solutions, and it has been crystallized from
pigeon breast muscle (103). The molecular weight of the enzyme from pigeon
breast muscle is -55,000, from beef heart is 62,000 (121), and from rat liver
with slightly
is 56,000 (282). In Candidu tropicalis two acetyltransferases
different properties have been isolated, one from mitochondria
and the other
from peroxisomes. Both enzymes have two subunits with molecular weights
of -60,000 and 70,000 (410). The rat liver enzyme also appears to have two
subunits with molecular weights of 34,000 and 25,000 (282). Only one of the
subunits has a catalytic site; the function of the second subunit is not known.
B. Kinetics
and SpeciJicity
The reaction
acetyl-CoA
+ carnitine
e acetylcarnitine
+ CoA
is freely reversible, with the equilibrium
constant (K,,-J equal to 0.6-0.7 (68,
159). The K,, decreases with the chain length of the acyl group. The Keg
value close to unity shows that the free energy of the O-ester bond of acetylcarnitine
is similar to that of the thioester bond of acetyl-CoA, or cit. -7.9
kcal/mol. This unusually active character of an O-ester bond is explained
by the inductive effect caused by the closeness of the molecule’s quaternary
has been purified to homogeneity
from mouse liver peroxisomes (148). This
third transferase probably is also present in liver mitochondria
(381). Edwards
and co-workers (141) have presented evidence indicating that mitochondria
contain only one acetyltransferase.
Thus the number of carnitine acyltransferases in heart mitochondria
at present seems to be back to the original
two, whereas in liver three transferases
have been isolated.
It is still uncertain whether the extramitochondrial
transferases in animal tissues represent unique enzymes. Nevertheless Japanese workers have
and peroxirecently shown that Candida tropic&& contains mitochondrial
somal carnitine acetyltransferase;
in their purified forms these enzymes have
different molecular weights and can be separated with chromatographic
and
electrophoretic
procedures (224, 410). Further work is needed to establish
how the extramitochondrial
transferases
are related to the two firmly established mitochondrial
ones.
October
1983
AND
FUNCTIONS
1443
C. Inhibitors
and
“Suicidal”
Substrates
Bromoacetylcarnitine
and bromoacetyl-CoA
inhibit the enzyme rapidly,
and studies of their effects have shed light on the reaction mechanism of
nitrogen (148a). Chase and Tubbs (101,102,104-106)
have thoroughly
studied
the kinetics and reaction mechanism of carnitine acetyltransferase.
The following K, values for the four substrates (2 in each direction)
have been found: carnitine, 120 PM; acetyl-CoA, 37 PM; acetylcarnitine,
350
PM; and CoA, 37 PM. The reaction products in one direction are competitive
inhibitors of the corresponding
substrates in the same direction. The enzyme
has a relatively broad pH optimum between 7 and 8.5.
The enzyme shows a pronounced specificity for CoA and carnitine, but
it does react with norcarnitine (P-hydroxy-y-dimethylaminobutyric
acid) (158)
or with dephospho-CoA
(lo@, although these compounds have much higher
K, values than its physiological
substrates. Thiocarnitine
(P-sulfhydryl-ytrimethylaminobutyric
acid) also reacts with the enzyme (149).
The K, values for the different straight-chain
acyl-CoA substrates are
nearly independent
of acyl length. The enzyme from pigeon breast muscle
shows the highest reaction rate with acetyl-CoA and with gradually declining
rates as the chain length of the acyl gro.up increases. With decanoyl-CoA
the rate is only 4% of that with acetyl-CoA. The rat heart and pig heart
enzymes show the highest rates with propionyl-CoA
(121). The enzymes from
Candida tropicalis react almost exclusively with acetyl-CoA. The rate with
propionyl-CoA
is only a few percent of that with acetyl-CoA (410). Thus the
Candida enzymes have a much narrower chain-length
specificity than the
animal enzymes.
The enzyme from animal tissues also reacts with branched-chain
acyl
groups, because branched-chain
acylcarnitines
are formed when mitochondria are incubated with branched-chain
cu-keto acids (383). Acetoacetylcarnitine is also a substrate for the enzyme. Because of the low concentration
of acetoacetyl-CoA
in the tissues, however, it is unlikely that acetoacetylcarnitine is formed, although it has been used as a substrate in experimental
studies of ketogenesis (68).
Long-chain acyl-CoA esters inhibit the enzyme (102). This inhibition has
a peculiar feature: palmitoyl-CoA
acts as a competitive inhibitor toward both
carnitine and acetyl-CoA. Free palmitate is also a weak competitive inhibitor
(but surprisingly
only against carnitine). These results have been interpreted
to mean that the enzyme has a hydrophobic
site that binds long-chain acyl
groups and that enhances the binding of the CoA moiety. At the same time
the long-chain acyl group prevents the binding of carnitine to its reactive
is as
site. In accordance with this interpretation
the Ki of palmitoyl-CoA
low as 1 PM, whereas for palmitate
it is several hundred times higher.
1444
JON
BREMER
vohmie
63
XXII.
CARNITINE
OCTANOYLTRANSFERASE
Carnitine
octanoyltransferase
#Zen.eity from mouse liver peroxisomes,
(EC 2.3.1.-) has been purified to homofrom which it can be extracted without
the enzyme. Bromoacetylcarnitine
can inactivate the enzyme by two mechanisms. In the presence of CoA its -SH group is alkylated on the enzyme
active site, and an S-carboxymethyl-CoA
carnitine
ester is formed. It is
firmly bound to the enzyme [dissociation
constant (Kd) = 10-lo], and the
enzyme activity is blocked. With bromoacetylcarnitine
alone (in the absence
of CoA) a histidine residue in the enzyme is irreversibly
alkylated. These
results confirm that the enzyme forms a ternary complex with its substrates
and that a histidine residue takes part in its catalytic process at the carnitine
site (105, 106). With bromoacetyl-CoA
(in the presence of carnitine)
bromoacetylcarnitine
is first formed by the enzyme. The -SH group of CoA is
then alkylated, as with added bromoacetylcarnitine.
Thus bromoacetyl-CoA
is a “suicidal” substrate for the enzyme (105).
(+)Carnitine
and (+)acetylcarnitine
are competitive
inhibitors
of the
enzyme when tested against (-)carnitine
and (-)acetylcarnitine.
[The Ki
values are 170 and 250 PM, respectively (104)].
Fluoroacetylcarnitine
is as good a substrate for the enzyme as acetylcarnitine. In isolated mitochondria
fluoroacetyl-CoA
is rapidly formed in the
matrix; intramitochondrial
fluorocitrate
is presumably formed, and the citric
acid cycle is inhibited. Fluoroacetylcarnitine
therefore can be used to inhibit
the citric acid cycle in isolated heart mitochondria,
for example, where added
fluorocitrate
has no effect because the mitochondrial
membrane is impermeable to fluorocitrate
(69).
Pent-4-enoylcarnitine
reacts rapidly with carnitine acetyltransferase
and
is converted to pent-4-enoyl-CoA
in the mitochondria,
where it is further
metabolized to pent-2,4-dienoyl-CoA
and 3-keto-pent-4-enyl-CoA.
Both these
compounds are inhibitors
of acetyl-CoA acyltransferase
(EC 2.3.1.16) and
therefore of fatty acid oxidation (194,363). Low concentrations
of pentenoate
do not inhibit fatty acid oxidation because the compound can be completely
metabolized by a separate route. This metabolism of pentenoate depends on
a reduction of the pent-2,4-dienoyl-CoA
to pent-Z-enoyl-CoA
with reduced
NAD phosphate (NADPH) by 2,4-dienoyl-CoA
reductase, which is followed
by a normal P-oxidation (50,190). Here NADPH can be supplied from NADH
via the energy-dependent
transhydrogenase
in mitochondria.
This is analogous
to the function of this enzyme in the metabolism of unsaturated
fatty acids
(318). These findings explain why clofibrate treatment,
which dramatically
increases the activity of 2,4-dienoyl-CoA
reductase (EC 1.3.1.-), protects
against toxic effects of pent-4-enoylcarnitine
and why uncoupling increases
the inhibitory
effect of pent-4-enoylcarnitine
(50).
October
1983
AND
FUNCTIONS
1445
XIV.
CARNITINE
PALMITOYLTRANSFERASE
A. Purification
and Pvwpmties
Carnitine palmitoyltransferase
is more firmly bound to the mitochondrial
membrane than is carnitine acetyltransferase,
but its properties are in many
ways similar. Norum (298) obtained a partially purified enzyme by salt extraction of lyophilized, washed calf liver mitochondria,
but highly purified
preparations
of the enzyme are obtained only by the use of detergents (24,
121, 232, 233). Kopec and Fritz (233) isolated the enzyme from calf liver in
two forms, one of which was very labile and was probably a partially denatured
enzyme. The enzyme was found to be a dimer with a molecular weight of 2
X 75,000. Bergstrom and Reitz (24) and Clarke and Bieber (121) isolated the
enzyme from rat liver and beef heart, respectively, as polymers with molecular
weights of 400,000-500,000 and with subunit weights of ~65,000. The amino
acid compositions of the acetyl- and palmitoyltransferases
were found to be
similar (121).
It is not yet known whether all subunits have catalytic sites.
B. Kinetics
and Spea$icity
Like the acetyltransferase,
the palmitoyltransferase
reacts with norcarnitine (300) and with dephospho-CoA (76) but with reduced reaction rates.
A peculiar feature of the palmitoyltransferase
is that one of its substrates (palmitoyl-CoA)
behaves as a competitive inhibitor to its cosubstrate
the use of detergents (148). Its molecular weight is ^160,000. It is stable in
pyrophosphate
buffer, but it is rapidly inactivated by freezing.
Its chain-length
specificity overlaps those of the acetyl- and palmitoyltransferases. Thus the maximum reaction rates with acetyl-CoA and palmitoyl-CoA
are 26 and 8%, respectively, of that with octanoyl-CoA.
The
relative reaction rates in the reversed direction are similar.
The & value for octanoyl-CoA is 15 PM (for acetyl-CoA and palmitoylCoA the Km values are 130 and 70 PM, respectively) and for carnitine is 155
and CoA
PM. In the reverse direction the Km values for octanoylcarnitine
are 100 and 780 PM, respectively.
The activity of octanoyltransferase
in the liver is increased by clofibrate
and other hypolipidemic
drugs, as are the activities of the other carnitine
acyltransferases
(148).
The effects of inhibitors on the purified enzyme have not been studied,
but in isolated mitochondria
malonyl-CoA
inhibits the formation of octanoylcarnitine
from octanoyl-CoA
(254, 353).
1446
JON
BREMER
Vohme
63
carnitine, thus increasing the apparent K, of carnitine (71,72). This property
of the enzyme, which is also present in intact mitochondria
(73,436), explains
a strong substrate inhibition
by palmitoyl-CoA
in the forward reaction (the
reaction of the outer carnitine palmitoyltransferase)
palmitoyl-CoA
+ carnitine
-
palmitoylcarnitine
and a strong product inhibition
of the reversed reaction
inner palmitoyltransferase)
’
+ CoA -
palmitoyl-CoA
(the reaction
of the
+ carnitine
The possible physiological
significance of this enzyme property is that
it may act to slow down the transfer of acyl groups to the mitochondrial
matrix when a rapid flux of fatty acids into the cell gives a high extramitochondrial level of acyl-CoA. Thus “flooding” of the mitochondria
with longchain acyl-CoA may be prevented.
The inhibitory
effect of palmitoyl-CoA
on carnitine binding to the enzyme is evidently similar to the effect of palmitoyl-CoA
on carnitine binding
the difference is that palmito carnitine acetyltransferase
(see sect. XII@;
toyl-CoA is also the substrate of the enzyme in the case of palmitoyltransferase. Hence palmitoyltransferase,
like acetyltransferase,
seems to contain
a hydrophobic
region binding the long-chain acyl groups of the substrates.
This binding evidently contributes to the substrate binding and explains why
the K, values for acylcarnitines
and acyl-CoA esters decrease with the length
of the fatty acid (102, 232).
With palmitoyl-CoA
the following K, values have been found for the
four substrates: palmitoyl-CoA,
lo-20 PM; carnitine, 250-450 PM (extrapolated to 0 palmitoyl-CoA);
palmitoylcarnitine,
40-140 PM; and CoA, 5-50
(71, 232).
PM (5 PM when extrapolated
to 0 palmitoylcarnitine)
The spread of the determined
values probably has to do with the low
solubility and the micellar properties of the long-chain acyl substrates and
the complicated kinetics of the enzyme.
The acyl group chain-length
specificity of carnitine palmitoyltransferase
is apparently different in the two directions of the reversible reaction (122).
In the forward reaction (acylcarnitine
formation)
the optimum chain length
of the acyl-CoA was Co, whereas in the reverse reaction palmitoylcarnitine
gave the highest rate (122, 381). Increasing substrate inhibition,
which was
due to a lowered Ki of acyl-CoA as the length of the acyl group is increased,
most likely explains this phenomenon.
The enzyme reacts with a variety of acyl derivatives,
although often
with reduced rates. Mono-CoA esters of long-chain dicarboxylic acids react
with carnitine palmitoyltransferase,
but carnitine esters of long-chain
dicarboxylic acids are poor substrates for mitochondria
(334). Recent results
indicate that long-chain dicarboxylic
acids are preoxidized (shortened)
in
palmitoylcarnitine
+ CoA
October
1983
AND
FUNCTIONS
1447
the peroxisomes. Their oxidation is therefore carnitine independent
(290).
@Keto- and @hydroxyacyl-CoA
(75, ZOO,258) are substrates for the enzyme.
Carnitine has also been shown to increase the breakdown of prostaglandins
(217) and cyclopropane
carboxylic acids (173) in isolated mitochondria.
A
bulky acyl group like pyrenebutyrylcarnitine,
however, is not a substrate for
the enzyme (432).
C Eflect of Detergents
D. Pmperties
in Mitochondrial
Membrane
The kinetic properties of carnitine palmitoyltransferase
in the intact
mitochondrial
membrane deviate in important ways from the properties of
the soluble enzyme. A physiologically
important property of the membranebound outer transferase is its inhibition
by malonyl-CoA
(270). Little or no
such inhibition is found with the soluble enzyme. In the presence of malonylCoA the substrate saturation curve with palmitoyl-CoA
changes to a sigmoid
form (67, 354). The inhibition by malonyl-CoA
is more pronounced in heart
and skeletal muscle than in liver and kidney (353). Inhibition
is prevented
by disulfides such as 3,3’-dithiobis[6nitro-lbenzoic
acid (DTNB) (354), suggesting that -SH groups in the enzyme or in the membrane are involved.
Disulfides can also inhibit the solubilized enzyme, however (301). Solubilization of the enzyme with detergents also abolishes the malonyl-CoA
inhibition (351). The activity of the outer carnitine palmitoyltransferase
is
modified by K+ and Mg+, whereas these ions have no effect on the solubilized
enzyme (351).
In liver the inhibition
by malonyl-CoA
also seems to be modified in
different nutritional
and endocrine states (see sect. XVII.@.
The activities of the outer and inner palmitoyltransferases
have been
measured indirectly in intact mitochondria
by observing the reduction rates
of the mitochondrial
flavoproteins (acyl-CoA dehydrogenases and electrontransfer protein). The outer transferase activity is measured as the flavo-
The kinetics of carnitine palmitoyltransferase
are strongly influenced
by detergents, partly because of interaction with long-chain acyl substrates.
Nevertheless direct effects on the enzyme (via its hydrophobic binding site?)
are also a possibility (72,122). A peculiar feature of detergent effects on the
enzyme is that the inhibitory
effect of palmitoyl-CoA
on carnitine binding
is more affected by detergents than is palmitoyl-CoA’s
function as substrate;
that is, detergents increase the Ki of palmitoyl-CoA
more than its Km. Thus
with low palmitoyl-CoA
concentrations,
detergents inhibit the enzyme; with
high concentrations, detergents have a strong activating effect. The strongest
activating effect was obtained with (+)palmitoylcarnitine,
the unphysiological isomer of one of its substrates (72).
1448
JON
BREMER
Vohme
63
E. Inhibitors
2-Tetradecyl
glycidate is a potent inhibitor
of the outer carnitine palmitoyltransferase
and hence of ,&oxidation. At a concentration
of 1 PM this
substance causes 80-90s inactivation
of the transferase.
The CoA ester of
2tetradecyl
glycidic acid is probably the active inhibitor
(409).
2Substituted
oxiran-2-carbonoyl-CoA
esters are potent inhibitors of the
outer carnitine palmitoyltransferase
(14). At concentrations
of 30-300 nM,
some of these CoA esters reportedly cause 50% inhibition
of palmitoyl-CoA
oxidation by isolated liver mitochondria;
such data indicate that these compounds are some of the most potent known inhibitors
of the outer transferase. The inner transferase
is not inhibited,
as judged by the absence of
inhibition
oxidation.
Palmitoyl-CoA
analogues in which the fatty acyl carbonyl group (or CoA
thioester moiety) has been replaced by a methylene group are also potent
inhibitors
(120).
2-Bromopalmitoyl-CoA
in the presence of carnitine
inhibits the outer
carnitine palmitoyltransferase
in intact mitochondria.
This inhibition
presumably occurs by a similar mechanism
because the acetyltransferase
is
inhibited by bromoacetyl-CoA.
The inner transferase is not inhibited, however,
even after partial purification
(107, 421). Note that bromopalmitate
is not a
specific inhibitor of carnitine palmitoyltransferase
in the intact cell. In isolated
hepatocytes bromopalmitate
appears to inhibit the transport of fatty acids
across the cell membrane,
possibly by competing with normal fatty acids
for binding to the fatty acid-binding
protein in the cytosol(260). This process
is believed to be essential for fatty acid uptake in the cells.
Recent evidence suggests that the outer and inner palmitoyltransferases
are identical enzymes (24). If this is so, the inner transferase
must be protected against bromopalmitoyl-CoA,
even after solubilization
and partial
purification
(421). This discrepancy remains’ unexplained.
protein-reduction
rate after addition of palmitoyl-CoA
and carnitine, and
the inner transferase activity is measured as the reduction rate after addition
(296). In such studies the activity of the inner transferase
has been found to exceed the activity of the outer transferase by a factor of
500-1,000, both in brown adipose tissue mitochondria
(296) and in liver mitochondria
(24). After extraction
and solubilization,
however, the ratio of
the same activities is only -5:l (24).
These different studies show that the activity of the transferase is profoundly influenced by its environment
in the membrane. In this connection
it should be remembered
that the P-oxidation enzymes in the mitochondrial
matrix may be organized in an enzyme complex (387). Studies of the activity
of the inner carnitine palmitoyltransferase
in situ suggest that this enzyme
may be an integrated
part of the @-oxidation complex.
October
XV.
1983
CARNITINE
A. Properties
AND
FUNCTIONS
1449
TRANSLOCASE
and Kinetics
B. Inhibitors
The 16-trimethylaminohexadecanoyl
ester of DL-Carnitine
inhibits the
action
carnitine translocase with a Ki value of -1 PM (408). The inhibitory
may be due to the inability of the carnitine/acylcarnitine
exchange carrier
to transport
the cationic acyl group through
the inner mitochondrial
membrane.
Sulfobetaines
(N-alkyl-NJV-dimethyl-3-ammonio-l-propane-sulfonates) are also potent inhibitors
of the carnitine/acylcarnitine
exchange
carrier (328). Consequently
these compounds are potent inhibitors
of mitochondrial fatty acid oxidation.
Acyl esters of (+)D-carnitine
are potent inhibitors of fatty acid oxidation
(157), in isolated hepatocytes (115), and in perfused
livers (271). The solubilized carnitine palmitoyltransferase
is not inhibited
to the same extent (IYI). With isolated mitochondria
the inhibition
is apparently due to inhibition
of the carnitine translocase (322).
Pyrenebutylcarnitine,
which has useful spectroscopic properties,
is a
Carnitine translocase catalyzes a one-to-one exchange of carnitine and
acylcarnitines
across the inner mitochondrial
membrane (323, 324, 338). A
specific carnitine translocase protein has not been isolated, but its existence
as a separate entity is assumed on the basis of functional
studies in intact
mitochondria.
The translocase apparently also catalyzes a slow unidirectional
transport of carnitine (324, 338). The Km for external carnitine is ~0.5-1.5
mM when measured close to O°C and is much lower for acylcarnitines
(209).
The Km decreases with the chain length of the fatty acid (329). The exchange
rate also depends on the intramitochondrial
concentration
of carnitine. The
exchange is 5-10 times faster with the (-)carnitine
isomer than with the
(+)isomer [in contrast to the active transport
of carnitine across the liver
cell membrane, which shows little or no stereospecificity
(116)].
is in the same range
The Km for carnitine exchange in the mitochondria
as the concentration
of carnitine in the tissues, and the lower Km for acylcarnitines
indicates that the acylcarnitines
are preferentially
transported
into the mitochondria.
At 37OC the exchange capacity probably exceeds the
capacity of the mitochondria
to oxidize fatty acids with a wide margin. At
low tissue concentrations
of acylcarnitines,
however, the translocase may
be rate limiting for fatty acid oxidation, and the intramitochondrial
acylCoA concentration
may be kept low by a significant difference in the extraand intramitochondrial
acylcarnitine/carnitine
ratios.
1450
JON
Volume
BREMER
very potent inhibitor
of the i nner carnitine palmitoyltransferase
exch .ange (432).
of the acylcarnitine/carnitine
XVI.
ACYLCARNITINE
63
and also
HYDROLASE
XVII.
REGULATION
OF FATTY
ACID
OXIDATION
IN LIVER
Numerous studies on perfused livers, isolated liver cells, and perfused
hearts have shown that the rates of fatty acid oxidation and of ketogenesis
depend on the concentration
of fatty acids offered to the tissues (114, 271,
314). Increased rates of fatty acid oxidation also correlate with high levels
of long-chain acylcarnitines
in the tissues (43, 333). The high level of acylcarnitine may be a direct consequence of the increased uptake of fatty acids.
Hence lipolysis in adipose tissue is an important
regulatory
process with
respect to rates of fatty acid oxidation in all animal tissues. The rates of
fatty acid oxidation and ketogenesis also depend, however, on the nutritional
and endocrine state of the tissue, especially in the liver. This is illustrated
by the findings that isolated perfused livers or isolated hepatocytes
from
fasted (271), diabetic (274), or thyrotoxic
animals (385) oxidize more and
esterify less of the incoming fatty acids than do livers from normal animals
fed carbohydrates.
Perfused livers from female rats esterify relatively more
and oxidize relatively less of the incoming fatty acids than do livers from
male rats (379). Feeding the antilipemic
drug clofibrate to rats also increases
oxidation and decreases esterification
of fatty acids (115).
Changes in both cellular metabolites
and cofactors and in the mitochondrial carnitine-dependent
transport
system for activated fatty acids
seem to explain these shifts in the metabolism
of fatty acids.
A. Eflect of Metabolites
and Cofactors
1. Makmyl-CoA
(270);
Malonyl-CoA
strongly inhibits the outer carnitine palmitoyltransferase
it also inhibits carnitine acetyltransferase
in liver but not in other
An enzyme hydrolyzing the fatty acid esters of (-)carnitine
but not the
esters of (+)carnitine
has been found in the microsomal fraction of liver,
kidney, pancreas, small intestine, and brain (21,259). Acylcarnitine
hydrolase
(EC 3.1.1.28) hydrolyzes carnitine esters with chain lengths of C6-C18, with
Cl0 giving the highest rate. The K, for acylcarnitines
is 3-5 mM. Thus one
would expect the enzyme to have a low activity in intact cells. Although the
function of the enzyme is not known, Mahadevan and Sauer (259) suggested
that it prevents excessive accumulation
in the
tissues.
October
1983
AND
FUNCTIONS
1451
tissues (254). In intact animals (174, 443) and in isolated hepatocytes (123,
269) there is an inverse relation between malonyl-CoA
content and fatty acid
oxidation
rates. Insulin stimulates
acetyl-CoA carboxylase and increases
malonyl-CoA
concentration,
whereas glucagon has the opposite effect (28);
therefore a decreased level of malonyl-CoA
is probably important
in the
stimulation
of fatty acid oxidation in hepatocytes isolated from fasted or
diabetic rats or in hepatocytes stimulated with glucagon (114). Nevertheless
several observations
discussed below suggest that additional
mechanisms
modify the effect of malonyl-CoA.
It has repeatedly
been suggested that glycerophosphate
may inhibit
fatty acid oxidation by trapping the acyl-CoA for triacylglycerol
synthesis,
thus preventing the formation
of acylcarnitine.
This possibility seems likely
from studies of how small amounts of glycerol affect fatty acid oxidation in
isolated hepatocytes with a previously low glycerophosphate
level (133,253).
In intact animals, however, no inverse relationship
between the glycerophosphate level and fatty acid oxidation has been found (443). Therefore any
variable competing or trapping effect of fatty acid esterification
must more
likely be explained by varying activities of the triacylglycerol-synthesizing
enzymes (see sect. XVII@.
3. &enzyme
A
In liver the total concentration
of CoA about doubles both in whole
tissue and in the mitochondria
during fasting (374). The initial (and ratelimiting) step in the biosynthesis of CoA is pantothenate
kinase (EC 2.7.1.33).
The enzyme is inhibited by acetyl-CoA (179). This seems paradoxical because
the concentration
of acetyl-CoA in the liver rises in fasting animals (407).
But it is the amount of mitochondrial
acetyl-CoA that increases in fasting
and ketotic states, whereas the cytosolic acetyl-CoA/CoA
ratio probably
decreases (369). In this way the activity of pantothenate
kinase will increase
and more phosphopantothenate,
phosphopantetheine,
and CoA will be
formed. The mitochondrial
CoA is synthesized from the extramitochondrial
phosphopantetheine
transferred
to the mitochondria,
where synthesis of
mitochondrial
CoA is completed (372). Owing to the close correlation between
the level of long-chain acyl-CoA in tissues and the rate of fatty acid oxidation
(51, 369, 407), it seems likely that the availability
of CoA is important.
4 Carnitine
Fasting or diabetes increases the level of carnitine in the liver (272).
This increase is particularly
pronounced in sheep. The carnitine concentration
2. Glycerophosphate
1452
JON
BREMER
t%hmne
63
5. Fatty acid-binding
protein
Perfused livers from female rats take up and esterify relatively more
and oxidize fewer fatty acids than do livers from male rats (379). These
differences correlate with a higher concentration
of a specific fatty acidbinding protein in the livers of female rats. The level of this protein is
increased by estradiol and lowered by testosterone (309). Clofibrate also increases the level of this protein (345). In the cytosol of rat liver the concentration of fatty acid-binding
protein amounts to 4-5% of the total protein (310).
From studies with isolated hepatocytes,
Wu-Rideout
and co-workers
(437) found that flavispidic acid, which competes with free fatty acids for
binding to the fatty acid-binding
protein, increases fatty acid oxidation while
in the livers of diabetic sheep may be more than lo-fold that of normal sheep
(377). The regulatory
importance
of such data is uncertain.
Carnitine can
stimulate fatty acid oxidation in isolated hepatocytes because these cells lose
most of their carnitine during preparation
(114). An increase in carnitine
seems “unnecessary,”
however, because isolated hepatocytes from fasted rats
require a lower concentration
of carnitine to give maximum rates of fatty
acid oxidation than do hepatocytes from rats fed carbohydrate
(117). In intact
rats the liver carnitine
has been increased about fivefold by intravenous
injection of carnitine. This increase in liver carnitine led to increased levels
of long-chain acylcarnitines,
long-chain acyl-CoA, and acetylcarnitine
in the
liver; no change in ,&hydroxybuty rate was observed, suggesting that the rate
of fatty acid ox .idation remained unchanged (57). It was suggested that an
increased transfer
of activated fatty acids into the mitochondrial
matrix
could explain the increased level of acyl-CoA. The authors concluded that
this transfer could not be rate limiting in fatty acid oxidation.
There is a possible interpretation
of these results: The extramitochondrial acylcarnitine/carnitine
ratio is in equilibrium
with the intramitochondrial acylcarnitine/carnitine
and acyl-CoA/CoA
ratios via the active
carnitine translocase and the inner carnitine palmitoyltransferase.
The lessactive outer transferase (and malonyl-CoA)
determines the extent to which
the extramitochondrial
acyl-CoA/CoA
ratio deviates from equilibrium
with
the acylcarnitine/carnitine
ratio and the acyl-CoA/CoA
ratio of the mitochondrial matrix.
In such a system the rate of fatty acid oxidation can be visualized as
being regulated both by th e activity of the outer tra .nsferase, which influences the level of oxidizable acylcarni tin es, a ‘nd by the competing subst rates,
which will influence the redox situation in the electron-transport
chain in
the mitochondria
and the availability
of cofactors (oxidized electron-transfer
protein, NAD, and CoA); thus the oxidation rate at a certain acylcarnitine
level can be determined.
October
1983
AND
FUNCTIONS
1453
it inhibits esterification.
Fatty acids bound to this protein therefore seem
to be more available for activation
and esterification
in the endoplasmic
reticulum than for activation, acylcarnitine
formation,
and oxidation in the
mitochondria.
The mechanisms of these effects are not known.
6. Competing
oxidizable
substrates
B. Changes in Mitochmzdria
Liver mitochondria
isolated from fasted rats show an increased activity
of outer carnitine
in the inner membrane
and a decreased activity of glycerophosphate
acyltransferase
(EC 2.3.1.15) in the outer
Lactate is a strong inhibitor of fatty acid oxidation in hepatocytes from
fasted rats. Still, lactate has only a marginal effect on the malonyl-CoA
level
in hepatocytes from fasted rats (273). Octanoylcarnitine
is taken up and
oxidized by isolated hepatocytes. In this oxidation only the carnitine translocase and the inner carnitine acyltransferase
are supposed to be active. Nevertheless the oxidation rate is stimulated
by fasting, by glucagon, and by
anti-insulin
serum to a similar extent, as is the oxidation of oleate (274).
In the mitochondria
the incoming acyl-CoA must compete with other
substrates for a shared pool of NAD and for a shared electron-transport
chain. As a result it is feasible that a variable availability
of such substrates
or a variable activity of competing enzymes will influence the rate of fatty
acid oxidation. Such effects have been demonstrated.
Activation
of pyruvate
dehydrogenase (lipoamide) (EC 124.1) with dichloroacetate
accelerates glucose oxidation and inhibits fatty acid oxidation in perfused hearts (267). In
isolated liver mitochondria
the NAD-linked
intermediates
of the citric acid
cycle have only a weak effect, but high concentrations
of the flavoproteinlinked succinate can suppress oxidation of palmitoylcarnitine
(75). However,
the allover competing ability of the citric acid cycle is weak. On the contrary,
a rapid fatty acid oxidation suppresses the citric acid cycle (239, 250, 251).
Long-chain
acylcarnitines
are also dominating
substrates compared with
pyruvate (64) and glutamate
(112).
The extramitochondrial
NADH is a more efficient competing substrate
than the NADH generated within the mitochondria
(252), a seeming paradox
explained by the active transfer of reducing equivalents into the mitochondria by the malate/aspartate
shuttle. The active transport of aspartate out
of the mitochondria
combined with transamination
will efficiently transfer
an elevated NADH/NAD
ratio in the cytosol to the mitochondria.
Because
lactate and ethanol generate extramitochondrial
NADH, this may be a mechanism by which lactate (270) and ethanol (312) suppress fatty acid oxidation.
This inhibition
by ethanol seems to take place without any decrease in the
acylcarnitine/carnitine
ratio in the liver (36).
1454
JON
BREMER
Vohmw
63
membrane. Refeeding of carbohydrate
reverses these effects (16,47,48). The
outer carnitine palmitoyltransferase
also has a higher activity in the livers
of male rats than in those of female rats (356). The activity of the mitochondrial glycerophosphate
acyltransferase
is also decreased in diabetes, and
its activity again increases after treatment
with insulin (16). Because the
glycerophosphate
acyltransferase
has a lower Km for acyl-CoA than carnitine
palmitoyltransferase,
a preferential
acylation of glycerol phosphate in the
presence of both glycerol phosphate and carnitine can be predicted [as demonstrated in isolated mitochondria
(47, 48)].
The inhibitory
effect of malonyl-CoA
on fatty acid oxidation is decreased
in liver homogenates
from fasted rats (124, 313). This decreased effect of
malonyl-CoA
is partly due to the increased activity of the outer carnitine
and partly due to a decreased sensitivity of the enzyme
(Fig. 6; 67, 353). The inner carnitine palmitoyltransferase
shows no or little
change in its activity after 24 h of fasting. The enzyme therefore shows a
changed distribution
(less latency) in the inner mitochondrial
membrane
after fasting. Consequently it is tempting to suggest that more enzyme that
is less sensitive to malonyl-CoA
is exposed on the membrane surface. Recently we have found similar behavior of carnitine acetyltransferase
in rat
liver (254).
In liver mitochondria
from rats fasted for a long period (48 h) or from
diabetic rats an increased total (outer + inner) carnitine palmitoyltransferase
has been observed. This increase was not prevented by inhibitors of protein
synthesis (299). A similar observation has been done in ketotic and nonketotic
alloxan-diabetic
rats. In this case increased activity of both the outer and
inner enzymes was found (185). In streptozocin-diabetic
rats, however, no
significant increase in the activity of the outer transferase
was found, nor
was its sensitivity
to malonyl-CoA
changed (352). In hepatocytes in tissue
culture the activity of palmitoyltransferase
has been reported to increase
after stimulation
by glucagon and to decrease after stimulation
by insulin (183).
Liver mitochondria
isolated from rats treated for two days with 3,5,3’triiodothyronine
(hyperthyroid
rats) show about a doubling of the activity
of the outer carnitine palmitoyltransferase.
Mitochondria
from thyroidectomized rats or from rats treated with 6-propyl-2-thiouracil
(hypothyroid
rats) show a decreased activity. Total transferase showed no or little change,
and the malonyl-CoA
sensitivity was also essentially preserved independent
of the thyroid state (357, 385, 386). In agreement with these changes, the
mitochondria
from the hyperthyroid
rats oxidize palmitoyl-CoA
in the presence of carnitine 3-4 times faster than do mitochondria
from hypothyroid
rats (Fig. 6). The difference in the rate of palmitoylcarnitine
oxidation was
far less (385,386). These changes may explain why both fatty acid oxidation
and fatty acid synthesis can show increased rates in the liver of thyrotoxic
rats (134).
A different picture is found in rats fed the drug clofibrate, which causes
hypolipidemia.
This drug induces both an increased mitochondrial
mass and
October
1983
AND
1455
FUNCTIONS
8
5
25
Malonyl-CoA (FM)
FIG. 6. Effect
of fasting,
of thyroid
state,
and of malonyl-CoA
on carnitine-dependent
oxidation
of palmitoyl-CoA
in isolated
rat liver mitochondria.
[1-14C]palmitoyl-CoA
(70 PM),
(-)carnitine
(1 mM), malate
(5 mM), phosphate
(10 mM), and adenosine
diphosphate
(2 mM)
were incubated
in presence
of 1% bovine
serum
albumin
with liver mitochondria
from fed rats
(ClosecE squares),
fasted
rats (open squares),
hypothyroid
rats treated
with
propylthiouracil
(closed c-ides),
hypothyroid
fasted
rats (open cirdes),
and hyperthyroid
rats treated
with triiodothyronine
(open triangles).
Oxidation
was measured
as perchloric
acid-soluble
radioactivity.
Bars represent
standard
deviation
of results
obtained
with 3-5 different
mitochondrial
preparations.
increased activities of carnitine acetyltransferase
and carnitine palmitoyltransferase in liver. The acetyltransferase
in particular
shows a great increase in activity (lo-400fold) (219,382), but it is mainly the inner carnitine
acyltransferases
that show increased activity (381). In addition clofibrate
induces increased activity of the peroxisomal P-oxidation and chain shortening of fatty acids and of peroxisomal carnitine acetyltransferase
and medium-chain acyltransferase
(31, 74, 237). Clofibrate does not affect the malonyl-CoA sensitivity of the palmitoyltransferase
(386). Thus the oxidation
of fatty acids in the liver of clofibrate-treated
animals is stimulated by mechanisms different from those of fasted or thyrotoxic rats.
The carnitine translocase has also been found to increase its activity in
the livers of fasted, diabetic, and clofibrate-fed
rats (325, 329) and possibly
also after treatment with glucagon (443).
Researchers have not yet determined the mechanisms by which fasting
0
1456
JON
BREMER
Vohww
and hormones change the latency and the malonyl-CoA
sensitivity
as well as the activity of the translocase.
C. Allover
63
of the
Regulation
XVIII.
FATTY
Carnitine
ACID
OXIDATION
(181, 302, 353), as are carnitine
and long-chain
IN
acylcarnitines
EXTRAHEPATIC
TISSUES
is present in all animal and human tissues
acetyltransferase,
carnitine, acetylcarnitine,
(43,264,333).
Variations in the tissue contents
These different studies show that the rate of fatty acid oxidation in the
liver is probably regulated by a series of mechanisms.
Lipolysis in adipose tissues is of primary importance because it regulates
the availability
of free fatty acids to the liver.
Inhibition
by malonyl-CoA
probably helps to shut off fatty acid oxidation under lipogenic conditions. A
variable rate of triacylglycerol
synthesis may trap fatty acids, and the fatty
acid-binding
protein may direct fatty acids to esterification.
Pyruvate and
extramitochondrial
NADH products of glycolysis (and activation of the pyruvate dehydrogenase
and of the malate/aspartate
shuttle) may compete
with acylcarnitines in the mitochondria,
particularly when acylcarnitine levels
are low.
Along with these extramitochondrial
changes, the mitochondrial
carnitine-dependent
transfer mechanism for fatty acids seems to change-it
is
activated by fasting, by thyroid hormones, and possibly by other factors
(clofibrate).
Such changes in the mitochondrial
transfer system itself are
still poorly understood.
It is striking
that the outer palmitoyltransferase
and the carnitine
translocase change their activities in the same direction in the liver. At
present it is assumed that the outer transferase,
the carnitine translocase,
and the inner transferase
all are separate (conjugated?)
units. Perhaps a
in the membrane catalyzes all the reactions presently supposed to be the functions of three protein molecules. If this were
the case, one must again consider the idea of a vectorial enzyme in the membrane (439).
It is necessary, however, to interpret
observations
on isolated mitochondria with caution. Siess et al. (370) have recently shown that treatment
of rats with glucagon may stabilize rather than activate different
mitochondrial functions during the subsequent isolation procedures. Activation
or inhibition
of phospholipases
in the membranes may be involved. Treatment of the animals with other agents may provoke similar phenomena.
Further work may show whether the observed changes in liver mitochondria
correspond to events in the intact cell.
October
1983
AND
FUNCTIONS
1457
A. Heart
and Skeletal Muscle
Owing to the large muscle mass, fatty acid oxidation in muscle tissue
is quantitatively
important.
Muscle tissue is responsible for the oxidation
of a major fraction of circulating
fatty acids (188). Oxidation of fatty acids
in skeletal muscle and heart is mainly controlled, by the supply of fatty acids
(i.e., by the concentration
of free fatty acids in the circulation)
but also by
work load (86, 314).
Training increases the capacity of muscle tissue to utilize fatty acids,
and an integrated
part of this adaptation
is induction of an increased mitochondrial
mass and an increased activity of carnitine palmitoyltransferase
(283). Conversely inactivation
of the muscle by denervation
leads to a drop
in the activity of both carnitine palmitoyltransferase
and carnitine acetyltransferase.
Carnitine is also lost from denervated muscle (225).
In the heart -85% of the tissue content of CoA is found in the mitochondria, whereas 90% of the carnitine is outside the mitochondria
(315).
These distributions
are likely to cause activated fatty acids to be funneled
toward oxidation rather than toward lipid synthesis. This reasoning is supported by observations on pathological
conditions associated with carnitinedepleted muscle tissue. In such conditions intracellular
accumulation
of triacylglycerol is usually found (see sect. XXII; 78, 142).
In muscle tissues, which lack the ketogenesis occurring in liver and
kidney, ,&oxidation
of fatty acids must be rigidly coupled to the citric acid
cycle through the common mitochondrial
pool of CoA, especially under conditions in which, for example, the heart uses fatty acids almost exclusively
for its energy needs. Feedback mechanisms must exist by which a variable
use of acetyl-CoA in the mitochondria
(due to a variable work load) can
with the nutritional
state of the animal show
that carnitine most likely is involved in regulation
of fatty acid oxidation
in all tissues (43,333). This assumption is supported by the observation that
is inhibited
by malonyl-CoA
in all tissues
tested: liver, kidney, adipose tissue, mammary gland, skeletal muscle, heart,
and brown adipose tissue (353,355). Less is known about the distribution
of
malonyl-CoA
in the tissues, but recently we have found that, besides the
liver, both heart and kidney have measurable levels of malonyl-CoA
and that
these levels decrease in fasting (B. Singh, J. Bremer, and B. Borrebak, unpublished results).
The regulation of fatty acid oxidation has been studied mainly in liver,
heart, and brown adipose tissue and to some extent in skeletal muscle. Carnitine-dependent
fatty acid oxidation has also been directly demonstrated
in a series of other tissues, however, such as adipose tissue (186), salivary
gland (ZOO), kidney (62), placenta (394), adrenals (184), sperm (207), and
probably brain (61, 62).
1458
JONBREMER
v&mw
63
B. Brown Adipose
Tissue
Brown adipose tissue
chondrial system for fatty
has a highly active, carnitine-dependent
mitoacid oxidation (192). Mitochondrial
metabolism
adjust the rate of fatty acid uptake, activation, and acylcarnitine
formation
in the extramitochondrial
compartment.
Neely and co-workers (210,314,315)
have suggested that such a regulatory system exists in a coupling of the two
compartments
through carnitine acetyltransferase.
From studies on perfused
rat hearts with variable work loads, they have shown that the acetyl-CoA/
CoA ratio in the mitochondria
is probably in equilibrium
with the extramitochondrial
acetylcarnitine/carnitine
ratio and possibly with the extramitochondrial
acetyl-CoA/CoA
ratio. Thus an increase in the mitochondrial
acetyl-CoA/CoA
ratio will adjust the extramitochondrial
pools of free CoA
and free carnitine available for fatty acid activation and acylcarnitine
formation.
The carnitine palmitoyltransferase
of muscle tissues is more sensitive
to malonyl-CoA
than, for example, the liver enzyme (353); the carnitine
acetyltransferase
is completely insensitive (254). Recently we have found
that the heart contains measurable amounts of malonyl-CoA
that decrease
in fasting. Thus malonyl-CoA
probably modifies the regulatory
mechanism
mentioned
above (e.g., when pyruvate is available for oxidation).
In this
connection, note that fatty acid oxidation in the heart is inhibited when the
(267). Acetyl-CoA
oxidation of pyruvate is stimulated
with dichloroacetate
and acetylcarnitine
increase concomitantly.
Fatty acid oxidation in muscle may also be influenced by feedback inhibitions through the P-oxidation chain. Acetyl-CoA is a product inhibitor
of the acetyl-CoA acyltransferase
(311), and ,&ketoacyl-CoA
is an inhibitor
of the acyl-CoA dehydrogenase
(131, 359).
Long-chain
acyl-CoA and long-chain acylcarnitine
accumulate
in the
anoxic heart, whereas the levels of acetyl-CoA and acetylcarnitine
decrease,
probably because accumulation
of NADH and reduced flavoproteins
in the
mitochondria
inhibits fatty acid oxidation (368, 422).
Thus the regulation
of fatty acid oxidation in muscle tissues appears
to be a complicated
interplay of the availabilities
of fatty acids, pyruvate,
and lactate; the acetyl-CoA/CoA
and acetylcarnitine/carnitine
ratios; the
level of malonyl-CoA;
and the redox situation in the mitochondria.
Palmitoylcarnitine
is a relatively
strong inhibitor
of the Na+-K+ATPase, and it has been suggested that the accumulation
of long-chain
acylcarnitine
in the ischemic heart explains the loss of K+ from heart in
ischemia (434). Palmitoylcarnitine
also inhibits phospholipid-sensitive
protein kinases (3). It is still uncertain whether these effects represent more
than unspecific detergent
effects or whether they represent physiological
regulatory
mechanisms.
October
1983
AND
FUNCTIONS
1459
XIX.
CARNITINE
ACYLTRANSFERASES
IN NEWBORNS
In the newborn a switch from carbohydrate
to fatty acid oxidation for
energy takes place. In fetal liver and heart the rate of fatty acid oxidation
is low (6, 418, 419, 427). Immediately
after birth a rapid increase in the
activity of carnitine palmitoyltransferase
and in the capacity to oxidize fatty
acids takes place in liver (6, 32) and in heart (20, 418). An increase in the
activity of the acetyltransferases
also takes place in different tissues after
birth (177, 238,417). In the chick embryo a similar increase in the activities
of the carnitine acyltransferase
takes place a few days before hatching (94,
230). In the fetal heart it is the outer carnitine palmitoyltransferase
in particular that shows a low activity, whereas the inner transferase
is more
“normal.”
Thus although fetal calf heart mitochondria
oxidize palmitoylCoA only at a slow rate in the presence of carnitine, palmitoylcarnitine
is
rapidly oxidized. Shortly after birth palmitoyl-CoA
is also rapidly oxidized
(419). The outer transferase evidently is present in a latent form. Preincubation of isolated mitochondria
on ice for 2 h (82), incubation of mitochondria
in this tissue is unusual in that the mitochondria
physiologically
can operate
reversibly in a loosely coupled state (113, 295). This is the mechanism for
nonshivering
thermogenesis.
Fatty acids made available by a catecholamine-triggered
lipolysis seem
to regulate the rate of fatty acid oxidation (336). The reversible uncoupling
is regulated by a specific translocase protein that renders the inner mitochondrial membrane permeable to hydrogen ions (294). This translocase is
inactivated by GTP, which binds to the translocase, and it is probably activated
by free fatty acids (248). Thus the free fatty acids trigger both acylcarnitine
formation
and uncoupling,
which together
make rapid heat production
possible.
In the brown adipose tissue of the hamster, which is a true hibernator,
free acetate can be the end product of acylcarnitine
oxidation. This is due
to the presence of an acetyl-CoA hydrolase in the mitochondria
(25,26). An
interesting
feature of this system is that the &oxidation
of palmitoylcarnitine to acetate is as rapid at 10°C as at 37”C, whereas the citric acid cycle
cannot function at 10°C. Hence the oxidation to free acetate may represent
an initial “ignition”
in the early heating phase of this hibernator.
Even in
the cold-adapted
rat, brown adipose tissue is very active in fatty acid synthesis (268, 405). Thus malonyl-CoA
must be formed.
The outer carnitine palmitoyltransferase
in this tissue is strongly inhibited by malonyl-CoA,
even in the cold-adapted state (355). This may seem
paradoxical in a tissue geared to heat production through fatty acid oxidation,
and an interesting
problem is how catecholamine-induced
lipolysis, uncoupling, and malonyl-CoA
inhibition
interact in the regulation
of fatty acid
oxidation in this tissue.
1460
JON
BREMER
Vokwrne
63
XX.
CARNITINE
AND
FERTILITY
In 1965 Marquis and Fritz (264) found that epididymis contains an extremely high concentration
of carnitine and that spermatozoa have a >high
activity of carnitine acetyltransferase.
They also showed that this high concentration
and high enzyme activity depend on an effect of testosterone.
These observations led to extensive studies on the function of carnitine
in spermatogenesis
and fertility.
The development
of the high activity of
in the spermatozoa coincides with the maturation
of sperm cells (416). As they pass through the epididymis, the sperm cells
take up relatively
large amounts of carnitine
from the epididymal
fluid,
which contains up to 60 mM carnitine. By the time the spermatozoa reach
the cauda epididymidis
they have lost the ability to take up carnitine, but
they now contain high concentrations
of carnitine and acetylcarnitine
and
a high carnitine acetyltransferase
activity (80,91,92). A similar development
takes place in the spermatozoe of Drosophila mlanogaster
(165).
The spermatozoa
have the ability to utilize a series of substrates for
energy. During transit through the epididymis the oxidation of fatty acids
seems to be important.
These fatty acids may be derived from the breakdown
of phospholipids
(187), and their oxidation is carnitine dependent (90). In
this connection it is interesting
that the epididymis contains an androgeninduced phospholipase
(34) and that the epididymal fluid contains high concentrations of glycerophosphocholine
(80). In mature ejaculated sperm, however, fatty acid oxidation seems to be relatively unimportant,
and the fertilizing capacity of the spermatozoa seems to depend on glycolysis of fructose
and glucose and on oxidation of lactate and pyruvate (93, 207, 278, 391).
in a medium with KC1 instead of sucrose, preincubation
of mitochondria
with palmitoyl-CoA
and carnitine (in the absence of adenosine diphosphate;
403), or pretreatment
of mitochondria
with phospholipase C (EC 3.1.4.3) (433)
all lead to exposure of latent palmitoyltransferase
and more rapid oxidation
of palmitoyl-CoA
in the presence of carnitine.
It is striking that mitochondria
isolated from adult ischemic dog hearts
show a low activity of outer carnitine palmitoyltransferase
similar to that
of fetal mitochondria
(435). In this case a decrease in the K, for carnitine
was also found, and the authors have suggested that lipid changes in the
membrane, which affect the hydrophobic
region of the carnitine palmitoylmight cause the observed phenomena.
transferase (see sect. XIV@,
The variable latency of the heart carnitine palmitoyltransferase
may
be a phenomenon similar to the variable latency of the same enzyme in liver;
more enzyme seems to be exposed on the surface of the inner mitochondrial
membrane in the livers of animals after fasting (48, 67) or after treatment
385, 386). Fasting
of the animal with 3,5,3’-triiodothyronine
(see sect. XVIIB;
has no effect, however, on the heart enzyme (385).
October
1983
AND
FUNCTIONS
1461
XXI.
CARNITINE
AND
BRANCHED-CHAIN
AMINO
ACID
METABOLISM
When isolated mitochondria
are incubated with branched-chain
cu-keto
acids and carnitine, branched-chain
acylcarnitines are formed (383). Acetyl-,
propionyl-, and butyrylcarnitine
are formed with pyruvate, a-ketobutyrate,
and a-ketopentanoate
(66, 303). Thus mitochondria
contain carnitine acyltransferase(s)
able to transfer both straight-chain
and branched-chain
acyl
groups to carnitine.
In mature spermatozoa
the relation of carnitine and carnitine acetyltransferase to pyruvate metabolism is similar to that found in other tissues.
When sperm is incubated with pyruvate or with pyruvate precursors such
as lactate, glucose, or fructose, a rapid oxidation of pyruvate takes place. At
the same time the endogenous carnitine is acetylated by the carnitine acetyltransferase
and by the acetyl-CoA formed from pyruvate. The acetylcarnitine level remains high as long as pyruvate is available (93,207,278). When
pyruvate is exhausted, the stored acetyl groups are transferred
back to CoA
and oxidized in the citric acid cycle. This is similar to what takes place in
mitochondria
from heart (61) and from blowfly flight muscle (109).
Mature spermatocytes contain over 1,000 times more carnitine than CoA.
Thus sperm can accumulate significant amounts of easily available activated
acetate [up to 2-3 pmol/lO’ cells, or 150 pmol/g dry matter (93)].
In the absence of external substrates the oxidation of endogenous acetylcarnitine
in the citric acid cycle can account for the entire oxygen uptake
in bovine spermatozoa.
Even when sperm motility is stimulated
by cyclic
adenosine monophosphate
after addition of caffeine, the acetylcarnitine
can
support-respiration
for several minutes (278). Because the cell membrane
in mature sperm is impermeable
to acetylcarnitine
(91), this substrate is not
lost by diffusion in a substrate-free
environment.
In this connection it is
noteworthy
that the motility of ejaculated sperm (one of the indexes of their
fertilizing
capacity) correlates with their acetylcarnitine
content (216).
Perhaps carnitine has specific functions in fertility
other than storing
acetyl groups. High concentrations
of external carnitine, such as those found
in the epididymis, suppress respiration
in mature bovine spermatozoa, and
this might have a function while the spermatozoa are stored in the cauda
epididymidis
(180). This effect of carnitine seems to be connected with its
binding to the cell membrane.
The effect is observed only with ejaculated
bovine sperm, however, not with spermatozoa taken from the cauda epididymidis. The effect is also absent from human sperm (215).
Tanphaichitr
(395) reported that external acetylcarnitine
stimulates
motility in human sperm. The nature of this effect is not clear. In summary,
only its function as a store for activated acetyl groups has been established
as a general function of carnitine in spermatogenesis
and fertility.
1462
JON
BREMER
Volume
63
After perfusion of rat hearts and rat hindquarters
with branched-chain
cu-keto acids or with a)-ketobutyrate,
substantial
amounts of the same acylcarnitines are formed in the muscle tissues (132,384). Even in normal intact
rats small amounts of propionylcarnitine
(37, 40) and branched-chain
acylcarnitines can be identified in the tissues (30, 110). In sheep liver there may
even be more propionylcarnitine
than acetylcarnitine
(375).
In isolated heart and muscle mitochondria
and in muscle homogenates
carnitine stimulates
the oxidation of branched-chain
cu-keto acids (70, 331,
414), and the corresponding
branched acylcarnitines
inhibit such oxidation
(168, 413). Fatty acid esters of carnitine also inhibit the oxidation of cu-keto
acids (70). On the other hand, isolated mitochondria
oxidize isobutyrylcarnitine (111).
These different observations have led to speculations that carnitine funtions specifically in the catabolism of branched-chain
amino acids by permitting
transport
into the mitochondria
of the activated branched-chain
carboxylic acids, which are formed by the branched-chain
cu-keto acid dehydrogenase. This idea was reasonable because the mitochondrial
branchedchain ar-keto acid dehydrogenase was originally reported to be located on the
outer surface of the inner mitochondrial
membrane
(218). Isolated mitochondria can decarboxylate
branched-chain
cu-keto acids in the absence of
carnitine, however, and also in the absence of external CoA and NAD, which
are cofactors in the reaction (70, 266). The cu-keto acid dehydrogenase
(as
pyruvate dehydrogenase)
is therefore
located inside the CoA permeation
barrier, presumably
in the matrix. The formation
of branched-chain
acylcarnitines
thus seems to be a “dead-end”
reaction, because the carnitine
esters must be transferred
back into the mitochondria
for further
metabolism.
Because small amounts of free branched-chain
carboxylic acids are found
in blood (8), Van Hinsbergh
and colleagues (413) suggested that carnitine
has a function in the release of these carboxylic acids from muscle tissue.
Mitochondria
contain acyl-CoA hydrolases (22, 49), however, whereas only
a high-K,
acylcarnitine
hydrolase has been found extramitochondrially
(21,
259). Hence carnitine probably does not play a major role in the release of
these carboxylic acids.
At present it is certain only that the presence of extramitochondrial
propionyl and branched-chain
acylcarnitines
in the tissues mirrors the intramitochondrial
acyl-CoA situation, because the short-chain carnitine acyltransferase(s)
has a relatively broad acyl group specificity. This formation
of branched-chain
acylcarnitines
by the mitochondria,
like the formation
of
acetylcarnitine,
may be part of a general buffering effect of carnitine on the
mitochondrial
acyl-CoA/CoA
ratios, preventing “flooding” of the mitochondrial matrix by CoA esters. The level of propionyl and branched-chain
acylcarnitines in the tissues may be a metabolic indicator of the catabolic rate
of branched-chain
amino acids.
October
XXII.
1983
CARNITINE
IN PATHOGENIC
AND
FUNCTIONS
1463
MECHANISMS
A. Loss and Lack of Car&tine
With the key role of carnitine in fatty acid oxidation in mind, it is not
surprising that disturbances
in the function of carnitine may cause pathological phenomena. When carnitine is lacking, a common feature seems to
be the accumulation
of triacylglycerol
in the cell; under these conditions an
inhibited fatty acid oxidation diverts fatty acids from oxidation to esterification (lipidosis). Wittels and Bressler (78, 426) reported the first example
of such a condition. They found that the well-known
lipidosis and heart
failure in diphtheria
may be explained by a loss of carnitine from the heart.
In guinea pigs the administration
of the toxin of Neisseria diphtheria induced
lipidosis of the heart, and homogenates
of these hearts had a decreased
ability to oxidize fatty acids. The addition of carnitine normalized the oxiof carnitine to diphtheritic
animals in vivo
dation rate (78). Administration
has a protective effect against the toxin and prolongs the survival of the
animals (98, 99).
Malstad and Brshmer (286) studied the effect of the toxic in tissue culture
and suggested that the cells lose carnitine because the toxin, which is an
inhibitor of protein synthesis, prevents the synthesis of the carnitine carrier
in the cell membrane. Prednisolone in the medium had a protective effectpresumably it slowed the breakdown of the carrier (285). This effect of prednisolone may be the basis for its beneficial effect in hereditary
carnitine
deficiency (see below; 212, 412).
Loss of carnitine may also contribute to heart failure in other conditions.
In experimental
heart failure produced by constriction
of the aorta, heart
carnitine
decreases (429), as it does in severe ischemia of the heart
(364, 368).
In severe liver cirrhosis the carnitine content in plasma and in different
tissues was found to be only one-fourth
to one-third of normal, and urinary
excretion of carnitine was even more reduced (349). Both spontaneous dietary
intake and biosynthesis
of carnitine
appeared to be far below normal in
these patients.
Carnitine deficiency also may be an iatrogenic condition. In kidney patients on hemodialysis
great amounts of carnitine are lost in the dialysis
fluid, and the carnitine content in plasma and muscle is often low (12, 39).
This is believed to contribute
to the hypertriglyceridemia,
cardiomyopathy,
and cardiac failure seen in these patients. Substitution
therapy wth carnitine
has been found to diminish hypertriglyceridemia
(172, 339).
Plasma carnitine is often elevated in untreated
renal insufficiency (44,
108), presumably
because of a lowered clearance in the kidneys.
Carnitine
deficiency may also be provoked in infants given artificial
carnitine-free
diets, presumably because of a slow endogenous rate of carnitine
1464
JON
BREMER
Vohm
63
biosynthesis in the neonatal period (373). Newborn infants normally receive
substantial
amounts of carnitine from their mothers’ milk (53).
Disturbed carnitine metabolism has also been suggested in other clinical
conditions, although its significance seems uncertain (281).
B. I-
Errors
of Carnitine
Metabolism
In 1973 the first two cases of inborn errors of carnitine metabolism were
reported. Engel and Angelini (142) found a case of lipid-storage
myopathy
associated with muscle fatigue and a low carnitine content in skeletal muscle,
and DiMauro and DiMauro (136) reported a case in which muscle cramps
and myoglobinuria
after exercise were associated with a low activity of carnitine palmitoyltransferase
in muscle, whereas the muscle carnitine level
was normal. Subsequently
a series of similar cases of varying severity has
been reported in the literature. DiMauro and co-workers (137) have reviewed
these and other disorders of lipid metabolism
in muscle.
Carnitine deficiency may vary in severity. In some cases the condition
seems to be limited to skeletal muscle-the
serum carnitine and the carnitine
contents in other tissues are normal. In some cases the carnitine deficiency
is general (systemic carnitine deficiency), with low carnitine concentrations
in plasma and in all tissues. Several fatal cases have been reported (100,125,
221,412). The age when symptoms appear may range from early childhood
for systemic cases to late adolescence for cases limited to muscle. In the less
severe cases a normal development of ketonemia in fasting has been observed
(thus showing a normal ability of the liver to utilize fatty acids), whereas
the systemic cases have a reduced or absent fasting ketonemia.
A corresponding decrease in carnitine content in the liver has been found (135).
It has been assumed that the isolated muscle deficiency is due to defective
transport of carnitine into the tissue, because the plasma carnitine is normal
(412). In accordance with this view, the addition of carnitine to muscle homogenates gave normal fatty acid oxidation (142), although in another case
no such normalization
was obtained (424).
In systemic carnitine deficiency a defect in carnitine biosynthesis has
been suggested. However, a normal conversion of &V-trimethyl+lysine
to
to carnitine was found in three patients (341, 343). In one case a defect in
the conversion of y-butyrobetaine
to carnitine was found (C. Hoppel, personal
communication).
Although a moderately decreased reabsorption
of carnitine
in the kidneys and an increased clearance of carnitine were found in one
study of systemic carnitine deficiency, this defect was also found in a control
with normal carnitine in the tissues (143). The investigators
therefore concluded that the renal abnormality
could at most only contribute
to the carnitine deficiency in the tissues. Possibly a general defect in carnitine transport
in the tissues (including
the kidneys and gut) could explain the general
deficiency. The absorption of carnitine in the gut has not been systematically
October
1983
AND
FUNCTIONS
1465
investigated, either in normal persons or in those with carnitine deficiency.
In some cases of carnitine deficiency, however, patients have responded favorably to carnitine substitution
therapy. The plasma carnitine was restored
to normal, showing that carnitine is absorbed, but muscle content was not
restored (100).
So far it is not established with certainty whether muscle and systemic
carnitine deficiencies represent different
disease entities or whether they
represent varying severities of the same disease in different stages of development.
In one case of muscle deficiency VanDyke and co-workers (412) found
moderately lowered carnitine contents in the skeletal muscles of a patient’s
parents, both of whom were clinically. normal. Consanguinity
of the parents
could be traced in another study of two affected sisters (424). These and other
cases suggest therefore that the disease is inherited as an autosomal recessive
disorder.
In systemic carnitine deficiency increased amounts of dicarboxylic acids
are found in the urine. Electron-microscopic
investigation
of the liver showed
proliferation
of the endoplasmic reticulum and an increased number of peroxisomes (221). These observations suggest that the inhibited mitochondrial
oxidation of fatty acids leads to increased w-oxidation of fatty acids and to
peroxisome proliferation
and increased peroxisomal
P-oxidation,
which is
carnitine independent
(118). Apparently
the auxiliary @oxidation system of
the peroxisomes and the o-oxidation
system of the endoplasmic reticulum
represent adaptive systems that are stimulated when the mitochondrial
carnitine-dependent
P-oxidation
system shows a (relative) insufficiency.
This
may occur in a number of situations:
in starvation
and diabetes (35), in
feeding of high-fat
diets (292, 401), or when mitochondrial
fatty acid oxidation is inhibited by poorly oxidizable fatty acids (74), by inhibitors
(46),
or by suboptimal
carnitine concentrations.
At least 15 cases of carnitine palmitoyltransferase
deficiency of different
severities have been reported (214). The patients suffer from muscle cramps
and myoglobinuria,
especially after prolonged exercise, and the condition is
aggravated by fasting and/or high-fat diets. The carnitine palmitoyltransferase activity in skeletal muscle has been found to range from almost none
to nearly 50% of normal. Those with severe deficiency experience symptoms
early in childhood, whereas those with moderately depressed enzyme activity
experience their first symptoms as teenagers after prolonged exercise. The
severe cases also have a depressed ketone body production in fasting, showing
that the liver may also be affected. This has been confirmed in one case by
direct measurement
of enzyme activity in the liver (27). In other cases leukocytes, thrombocytes,
and fibroblasts have reduced levels of the enzyme.
Thus the disease most likely is general, with symptoms primarily
in the
muscles. In two cases carnitine
acetyltransferase
was also measured and
was found to be reduced, but to a lesser extent than palmitoyltransferase
(136, 189).
1466
JON
BREMER
vohme
63
XXIII.
SUMMARY
Carnitine was detected at the beginning of this century, but it was nearly
forgotten among biochemists until its importance in fatty acid metabolism
was established 50 years later. In the last 30 years, interest in the metabolism
and functions of carnitine has steadily increased.
Carnitine is synthesized in most eucaryotic organisms, although a few
insects (and most likely some newborn animals) require it as a nutritional
factor (vitamin BT). Carnitine biosynthesis
is initiated by methylation
of
lysine. The trimethyllysine
formed is subsequently converted to butyrobetaine
in all tissues; the butyrobetaine
is finally hydroxylated
to carnitine in the
liver and, in some animals, in the kidneys (see Fig. 1). It is released from
these tissues and is then actively taken up by all other tissues. The turnover
of carnitine in the body is slow, and the regulation of its synthesis is still
incompletely
understood.
Microorganisms
(e.g., in the intestine) can metabolize carnitine to trimethylamine,
dehydrocarnitine
(@keto-y-trimethylaminobutyric
acid), betaine, and possibly to trimethylaminoacetone.
In some insects carnitine can
be converted to methylcholine,
presumably with trimethylaminoacetone
as
an intermediate(see
Fig. 3). In mammals the unphysiological
isomer (+)carnitine is converted to trimethylaminoacetone.
The natural isomer (-)car-
Most patients with carnitine palmitoyltransferase
deficiency have no
significant accumulation
of triacylglycerol
in their muscle cells, despite a
normal content of long-chain acyl-CoA synthase. They do, however, have
elevated plasma triacylglycerol
levels (10, 136, 361). Thus fatty acid uptake
in the tissues seems to be inhibited. This difference from muscle carnitine
deficiency is not understood.
In some cases both the outer and inner carnitine palmitoyltransferases
showed reduced activity (136); in two cases, however, the defect has been
assumed to be limited to the inner transferase (330, 361) and in one case to
the outer transferase (198). In the case where the deficiency was localized
to the outer transferase, isolated muscle mitochondria
oxidized palmitoylcarnitine but showed no activity with palmitoyl-CoA
plus carnitine.
Because of these differences some researchers have proposed that the
outer and inner transferases are under different genetic controls (361). Others have suggested that the outer and the inner transferases are one and
the same enzyme (24). In most cases it has been assumed that the disease
is inherited as an autosomal disorder.
A family with a combined deficiency of carnitine (40% of normal) and
(SO-65% of normal) has also been reported.
The family had six members in four generations with similar symptoms
(muscle cramps and dark urine), suggesting an autosomal dominant inheritance (211).
October
1983
AND
FUNCTIONS
1467
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Vohme

Carnitine-Metabolism and Functions

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