The interaction of cocoa polyphenols with milk proteins studied by

Food Research International 54 (2013) 406–415
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Food Research International
journal homepage: www.elsevier.com/locate/foodres
The interaction of cocoa polyphenols with milk proteins studied by
proteomic techniques
Monica Gallo a, Giovanni Vinci b, Giulia Graziani b, Carmela De Simone b, Pasquale Ferranti b,c,⁎
a
b
c
Department of Molecular Medicine and Medical Biotechnology, University of Naples Federico II, via Pansini 5, 80131 Naples, Italy
Department of Agriculture, University of Naples Federico II, via Università 100, 80055 Portici, Naples, Italy
Institute of Food Science, National Research Council, via Roma 52, 83100 Avellino, Italy
a r t i c l e
i n f o
Article history:
Received 30 March 2013
Accepted 3 July 2013
Keywords:
Cocoa
Polyphenols
Antioxidant activity
Mass spectrometry
Milk proteins
a b s t r a c t
The molecular interaction of cocoa polyphenols with milk proteins were investigated in vitro by combined
proteomic and biochemical strategies. Mass spectrometry and antioxidant activity assays allowed monitoring
the binding of casein and whey protein fractions to cocoa polyphenols. In particular, the nature of interaction
of β-lactoglobulin (β-Lg) with catechin and epicatechin was characterized and the amino acid residue at the
binding site was identified. On the other side, antioxidant activity assays also showed a significant effect of the
various milk protein fractions in decreasing the in vitro antioxidant activity of polyphenols, suggesting the
existence of other types of protein–polyphenol interactions, probably weaker non-covalent bonds. From a nutritional point of view, these data indicate that the β-Lg covalent modification by polyphenol alone do not support
the hypothesis of a decrease in the bioavailability of polyphenols themselves (Scalbert & Williamson, 2000). This
might also explain the maintenance of the antioxidant properties of cocoa polyphenols in cocoa-based beverages.
These results suggest the perspective use of the model system developed to study other complex food matrices.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Phenolic compounds are one of the most represented groups of
substances in the plant kingdom: there are currently more than
8000 known phenolic structures produced by secondary metabolism
of plants (Bravo, 1998). Among these compounds, phenolic acids and
flavonoids account for 30% and 60% of total polyphenols ingested
with diet, respectively (Manach, Scalbert, Morand, Rémésy, & Jiménez,
2004; Ramos, 2007; Xiao et al., 2011). Phenolic compounds are raising
great interest in medical and scientific research for their health benefits,
which include anti-carcinogenic, anti-atherogenic, anti-inflammatory,
anti-microbial, anti-hypertensive activities (Gryglewski, Korbut, Robak,
& Swies, 1987; Kaul, Middleton, & Ogra, 1985; Mascolo, Pinto, &
Capasso, 1988).
Until now, the polyphenols in green tea, black tea, grape and wine
(especially red) have been extensively studied and characterized. Only
recently attention has also focused on the characterization of the phenol
components of cocoa (Sánchez-Rabaneda et al., 2003; Zumbea, 1998).
Cocoa beans are a rich source of polyphenols contributing to about
10% of the dry weight of the whole seed. Cocoa polyphenols have a
short half-life in plasma and a rapid excretion rate and, therefore,
have a relatively low bioavailability (Manach, Williamson, Morand,
Scalbert, & Rémésy, 2005). Among the different classes of flavonoids,
⁎ Corresponding author at: Department of Agriculture, University of Naples, Italy.
Tel.: + 39 081 2539359; fax: + 39 081 7762580.
E-mail address: [email protected] (P. Ferranti).
0963-9969/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.foodres.2013.07.011
procyanidins appear to be from 10 to 100 times less absorbed of their
monomeric constituents (Tsang et al., 2005).
If assumed daily in the diet, beneficial effects of phenolic compounds
can be cumulative (Cooper, Donovan, Waterhouse, & Williamson, 2008),
especially at high doses (Heiss et al., 2007). However, Manach et al.
(2005) suggested differences in individual adsorption of polyphenols
probably linked to specific polymorphisms. This could explain the
high variability in reported flavonoid absorption rates in vivo
(Lamuela-Raventós, Romero-Pérez, Andrés-Lacueva, & Tornero,
2005). Absorption and metabolism of polyphenols are determined
by their chemical structure, which is correlated with the degree of
glycosylation, acylation, polymerization, conjugation with other
phenolic compounds, molecular size and solubility (Bravo, 1998).
When ingested, the first stage in the interaction between food
polyphenols and proteins occurs in the mouth, where flavanols react
first with proline-rich salivary proteins forming insoluble complexes
responsible for the perception of astringency and for the characteristic
taste of various food products (e.g. fruit, cocoa, coffee, tea, beer and
wine) (Baxter, Williamson, Lilley, & Haslam, 1996; Jobstl, O'Connell,
Fairclough, Mike, & Williamson, 2004).
In some foods, proteins and polyphenols combine to form soluble
complexes, which can reach colloidal size, causing turbidity of beverages and limiting the shelf life of these products. In the beer, wine and
juice production, for example, polyphenols may induce formation of
protein precipitates with the appearance of negative sensory characteristics for the consumer (Baxter, Lilley, Haslam, & Williamson, 1997;
Siebert, 1999; Siebert, Carrasco, & Lynn, 1996a; Siebert, Troukhanova,
M. Gallo et al. / Food Research International 54 (2013) 406–415
& Lynn, 1996b). Polyphenols, in general, interact with globular proteins
and can cause structural and conformational changes of the protein. It has
been shown that binding affinity depends on the molecular size of the
polyphenol molecule: the higher the molecular size of the polyphenol,
the greater the tendency to form complexes with proteins (De Freitas &
Mateus, 2001).
Structural changes in globular proteins have already been investigated in milk/tea mixtures. Interaction of milk proteins with tea polyphenols induces structural changes in both whey proteins and caseins
(Hasni et al., 2011; Hudson, Ecroyd, Dehle, Musgrave, & Carver, 2009;
Jobstl, Howse, Fairclough, Mike, & Williamson, 2006; Kanakis, Hasni,
Bourassa, Polissiou, & Tajmir-Riahi, 2011). These changes may explain
the effect of milk addition on the antioxidant activity of tea as well as
other food polyphenols (Stojadinovic et al., 2013). However, despite
the numerous studies, the binding mechanism of tea polyphenols with
milk proteins has neither been clarified yet, nor is it known whether
this interaction is preferential to certain amino acids (Stanner, Hughes,
Kelly, & Buttriss, 2004). With regard to the interactions between milk
proteins and polyphenols of cocoa and chocolate, it has been shown
that, following intake of chocolate, the absorption of epicatechin
(one of the major cocoa flavanols) is very low, and is still lower if
cocoa is assumed together with milk. This suggested that milk
proteins sequester cocoa polyphenols limiting their adsorption in
the gastrointestinal tract (Serafini et al., 2003). However, data in
the literature are still conflicting, as later studies have found no
reduction in the bioavailability of epicatechin when cocoa was
consumed with milk (Schroeter, Holt, Orozco, Schmitz, & Keen,
2003). In the light of these contrasting reports on the bioavailability
of cocoa polyphenols in the presence of milk, the purpose of this study
was to investigate the interaction between polyphenols and proteins in
cocoa/milk systems, by means of complementary mass spectrometry
techniques. A simplified model system was then developed, in which
the polyphenols were incubated with isolated milk protein fractions.
The time-dependent protein–polyphenol in vitro interactions were
studied by either MALDI-TOF or ESI mass spectrometry, showing a
major reactivity of β-lactoglobulin (β-Lg) towards cocoa polyphenols.
The data acquired allowed us to characterize the molecular basis of the
interaction between the main cocoa flavanols (catechin and epicatechin)
with β-Lg by MS/MS structural analysis. The effect of non-enzymatic
glycosylation, always occurring in commercial milk–cocoa products due
to thermal treatments, on β-Lg interaction with polyphenols was
studied. These data were complemented by total antioxidant capacity
assays on the protein–polyphenol mixtures in order to correlate
polyphenol activity changes to structural interactions with proteins.
Finally, the model system in vitro results were verified on a commercial
chocolate/milk beverage.
2. Materials and methods
2.1. Reagents and standards
The phenol standards catechin (CAT), epicatechin (EPI), procyanidins,
β-Lg, ferulic acid, caffeic acid, coumaric acid, protocatechuic acid,
chlorogenic acid, gallic acid as well as trypsin (proteomic grade), 6hydroxy-2,5,7,8-tetramethylchromano-2-carboxylic acid (Trolox), the
ammonium salt of 2,2′-azinobis-(3 ethylbenzothiazoline-6-sulfonic
acid) (ABTS), potassium persulfate (K2O8S2) and formic acid were
supplied from Sigma-Aldrich (Italy). Oligomeric proanthocyanidins
(OPCs) were purified as previously reported (Rigaud, Escribano-Bailon,
Prieur, Souquet, & Cheynier, 1993). α-Cyano-4-hydroxycinnamic acid
(α-cyano), 3.5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid) and
2.3 dihydroxybenzoic acid (DHB) were supplied by Fluka (Italy).
Ultrapure water and all solvents of chromatographic purity were
purchased from Romil (Italy). Hydrochloric acid and Folin reagent
were supplied by Riedel de-Haën (Seelze, Germany), ethanol and
trifluoroacetic acid were supplied from Carlo Erba (Italy).
407
2.2. Phenol extraction, quantitation and characterization by LC/MS/MS
A commercial sample (1 g) of defatted cocoa powder (Perugina, Italy)
was extracted with 10 mL of methanol/water (70/30 v/v) in ultrasonic
bath for 1 min. The sample was centrifuged at 4000 rpm for 10 min
and the supernatant defatted with 10 mL hexane. The polyphenol extract
was centrifuged at 13,000 rpm for 3 min and the supernatant filtered
with Phenex filters RC 0.22 μm.
Total phenol concentration in the different extracts were determined
spectrophotometrically by the Folin–Ciocalteu assay (Box, 1983) using
gallic acid as a standard. An aliquot of 125 μL of each extract was mixed
with 125 μL of Folin–Ciocalteu phenol reagent and reaction carried out
for 6 min. Then, 1.25 mL of saturated Na2CO3 solution (7.5%) was
added and allowed to stand for 90 min before the absorbance of the
reaction mixture was measured in triplicate at 760 nm. The total phenol
content was expressed as mg gallic acid equivalents/mL of sample.
The identification of polyphenol compounds was performed by liquid
chromatography coupled with tandem mass spectrometry (LC/MS/MS).
Chromatographic separations were obtained using a HPLC equipped
with two micropumps series 200 (Perkin Elmer, Canada) and a Prodigy
ODS3 100 Å column (250 × 4.6 mm, particle size 5 μM) (Phenomenex,
CA, USA). The eluents were: A, water 0.2% formic acid; B, CH3CN/MeOH
(60:40 v/v). The gradient used was as follows: 20–30% B (6 min), 30–
40% B (10 min), 40–50% B (8 min), 50–90% B (8 min), 90–90% B
(3 min), 90–20% B (3 min) at a constant flow of 0.8 mL/min. A flow of
0.2 mL/min was sent into the mass spectrometer. The injection volume
was 20 μL. MS and MS/MS analyses of cocoa extracts were performed
with an API 3000 triple quadrupole (Applied Biosystems, Canada) instrument equipped with a TurboIonspray source. Acquisition was performed
in negative ion mode by multiple reaction monitoring (MRM).
2.3. Extraction of milk proteins
Raw cow milk was defatted by centrifugation at 5000 rpm for 30 min
at 4 °C. Separation of the casein fractions was carried out by isoelectric
precipitation according to Aschaffenburg & Drewry (1957). Casein was
lyophilized and stored for subsequent experiments. Whey proteins
(WP) were precipitated from the milk whey with 12% (v/v) trichloroacetic acid (TCA). The pellet was recovered after centrifugation at 4500 rpm
for 30 min and washed three times with acetone at −20 °C for 30 min,
and finally centrifuging at 4500 rpm for 30 min. The dry residue was
used for analysis.
2.4. RP-HPLC analysis of whey proteins
HPLC analysis of WP was performed using a modular HP 1100
(Agilent) instrument equipped with a C4 Reverse Phase 5 μm;
250 mm × 2.1 mm (Vydac, 218TP54) column. Solvent A was 0.1%
trifluoroacetic acid (TFA) in H2O and solvent B 0.1% TFA in acetonitrile
(ACN). The chromatographic separation was carried out with a B gradient
from 35 to 55% in 65 min, at the constant flow of 1 mL/min. The eluate
from the column was monitored 220 and 280 nm. For analysis of WP,
150 μL sample dissolved at a concentration of 2 mg/mL in H2O/TFA 0.1%
were injected. The chromatographic peaks containing α-lactalbumin
(α-La) and β-Lg were dried, freeze-dried and finally stored in a freezer
at −20 °C.
Lactosylated β-Lg (Lac-β-Lg) was isolated by RP-HPLC separation in
the same conditions as a commercial sample of pasteurized whole milk.
2.5. Study of in vitro interaction of polyphenols with caseins and WP
Interactions between polyphenol components of cocoa and milk
proteins were studied using a simplified model obtained by incubating
milk individual caseins and WP with cocoa polyphenols extracts and
with polyphenol standards. Proteins were solubilized in 5 mM ammonium bicarbonate (AMBIC) at pH 6.8 (close to milk pH of the milk), at
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M. Gallo et al. / Food Research International 54 (2013) 406–415
concentrations of 5.4 mg/mL, 5.7 mg/mL, and 2.9 mg/mL for caseins,
whey proteins and β-Lg, respectively. The cocoa polyphenol extract
and individual polyphenols were dissolved in methanol and added
at a 10-fold molar excess compared to that of protein. Protein–
polyphenol mixtures were incubated at 37 °C and the interaction
monitored for 0–48 h by antioxidant activity assays and mass
spectrometry analysis.
2.6. Alkylation and tryptic hydrolysis of β-Lg
Alkylation of β-Lg (IAA-β-Lg). β-Lg (3 mg/mL) was dissolved in
5 mM ammonium bicarbonate, pH 8.4, containing iodoacetamide (IAA)
in molar ratio 10:1, to obtain alkylation of free cysteine residues. The
reaction was carried out for 45 min in the dark. IAA excess was eliminated by diafiltration with 3 kDa cut-off filters Centricon (Amicon, Italy).
Trypsin hydrolysis was carried out by adding proteomic-grade trypsin
and incubating at 37 °C for 4 h.
2.7. MALDI-TOF-MS analysis
MALDI-TOF-MS experiments were carried out on a Voyager DE-Pro®
(Applied Biosystems, Framingham, USA) instrument. Spectra were
acquired either in linear or reflector mode using the delayed extraction
technology; external calibration was performed with a mixture of
peptides standard low molecular masses. Spectra of proteins incubated
the polyphenols were acquired by loading 1 μL of the protein sample
and using sinapinic acid (10 mg/mL in 50% ACN/TFA0.1%) as matrix. For
peptide analysis, 1 μL of peptide mixture was loaded on the appropriate
target steel using α-cyano acid as matrix (10 mg/mL in 50% ACN/TFA
0.1%). MS/MS analysis of selected ions was made with Post-Source
Decay (PSD). The identification of the peptides was performed by
comparing the measured m/z values with those obtained by simulating
the theoretical tryptic digestion of β-Lg using General Protein/Mass
Analysis for Windows software (GPMAW).
2.8. ESI–MS/MS analysis
ESI–MS/MS data were obtained using a Q-STAR MS (Applied
Biosystems, Foster City, CA) equipped with a nanospray interface
(Protana, Odense, Denmark). The dried samples were resuspended in
H2O and TFA 0.1%, were purified by the use of micro ZipTip C18 columns
(Millipore, Milan, Italy), and sprayed through gold-coated borosilicate
capillaries (Protana). The capillary voltage used was 800 V. The collision
energy ranged from 20 to 40 eV, depending on the size of the peptide.
The collision induced dissociation spectra were processed using the
Analyst 5 software (Applied Biosystems). LC/ESI-MS analysis of β-Lg
tryptic peptides was carried out using a C18 reversed-phase column
(218TP52, 5 μm, 250 × 2.1 mm, Vydac, Hesperia, CA, USA) with a
flow rate of 0.2 mL/min on an Agilent MSD 1100 system. Solvent A
was 0.03% TFA in water, and solvent B was 0.02% TFA in ACN. Separation
was carried out in a linear gradient of 5–70% solvent B for 60 min. MS
acquisition was in positive selected ion monitoring mode.
2.9. Antioxidant activity assays
Antioxidant activity was determined on samples prepared as described in paragraph 2.5 using the ABTS method (Pellegrini et al.,
2003). In particular, different samples were opportunely diluted
with methanol, centrifuged for 5 min at 4000 rpm and used for the
determination of antioxidant activity. The absorbance at 734 nm of
the ABTS discolored solution was recorded on a Spectrophotometer
UV/VIS UV2100 (Shimadzu, Japan). The values were expressed in
mmol equivalent of Trolox/L.
2.10. Statistical analysis
Data are expressed as mean (SD). Significance of differences was
assessed by one way analysis of variance (ANOVA) and (when the
F value was significant) by the Tukey–Kramer test for multiple comparisons or by Student's t test for comparison between two means.
Differences were considered to be significantly different if p b 0.05.
3. Results
3.1. Analysis of phenolic components in cocoa by LC/ESI–MS/MS
As a preliminary step to study of interaction of polyphenols with
milk proteins, the polyphenol composition of the commercial cocoa
samples uses in this study was determined by LC–MS/MS (not
shown). The TIC chromatogram of cocoa polyphenol extract showed
all the expected compounds previously described in cocoa (Cooper
et al., 2007; Hammerstone, Lazarus, Mitchell, Rucker, & Schmitz,
1999), including catechin and epicatechin and oligomeric procyanidins.
Minor amounts of protocatechuic acid, chlorogenic acid and glucoside
esters of apigenin, naringenin, kampferol, luteolin and quercetin were
also detected. Because of the complexity of milk/cocoa mixtures, a simplified model study was preliminary set up mixing single polyphenol
compounds (catechin, epicatechin, and oligomeric proanthocyanidins)
as well as with those extracted from the cocoa powder with isolated
cow milk protein fractions (casein, whey protein and β-Lg). The
mixtures were incubated at 37 °C for times ranging from 0 to 24 h
and aliquots were taken at time intervals for MS analysis and antioxidant assays.
3.2. Antioxidant activity of polyphenol compounds
To set up the optimal conditions to measure antioxidant activity,
ABTS assays (Huang, Ou, & Prior, 2005) were preliminarily carried
out testing solutions of pure catechin, epicatechin, OPCs (oligomeric
proanthocyanidins) and polyphenol extracts from cocoa.
The ABTS assay results (Fig. 1a) showed that after 4 h incubation at
37 °C, pH 7.4, polyphenol samples showed no significant changes of
total antioxidant capacity. This indicated that the incubation conditions
did not introduce undesired polyphenol oxidation. OPCs showed a slight
but significant (p b 0,05) increase of antioxidant activity, possibly a
mere effect of their slow solubilization process, or a consequence of
the time/temperature conditions for incubation which induced slow
conversion to large-sized polymers with consequent increase of antioxidant capacity. Actually, it has been shown that aged wine, in which the
polymerization of polyphenols to tannins increased antioxidant capacity (for a review see Cheynier, 2005). Total polyphenol measurement
(Fig. 1b) confirmed the results of ABTS assay. No quantitative variation
of total polyphenol content was observed after 24 h. Therefore, based
on both ABTS and Folin data, it was possible to conclude that the assay
conditions did not affect the polyphenol antioxidant activity.
3.3. Total antioxidant activity of polyphenol/protein mixtures
To measure changes of total antioxidant capacity determined by
polyphenol/protein interaction, ABTS assays were performed after
incubation of casein, whey proteins or β-Lg with individual polyphenols
and total polyphenols extracted from cocoa.
Fig. 2 shows the antioxidant activity by ABTS assay as for polyphenols
incubated with the purified milk protein fractions. In the mixture casein/
polyphenol (Fig. 2a), the antioxidant activity decreased significantly
(p b 0,05) after 24 h of incubation, thus showing a significant effect of
casein on polyphenol activity. This decrease was more pronounced in
the case of casein incubated with either catechin or epicatechin.
These results were in agreement with previous studies which showed
that incubation of polyphenols affected casein structure, causing a
M. Gallo et al. / Food Research International 54 (2013) 406–415
409
Fig. 1. (a) ABTS assay for standards and polyphenol extracts from cocoa powder (values are expressed as millimoles of Trolox/L). (b) Folin–Ciocalteu assay of polyphenol standards and of
the extracts from cocoa powder (values are expressed as mg gallic acid/mL). Mean (SD) from three separate experiments run in duplicate. *p b 0.05 versus t = 0.
reduction of α-helix and β-sheet structures and an increase of unfolding
(Hasni et al., 2011; Hudson et al., 2009; Jobstl et al., 2006), thus
suggesting the existence of interaction with phenols.
Fig. 2b–c shows the results of ABTS assay after incubation of individual
phenols with whey proteins and with purified β-Lg, respectively. In both
cases, the trend of the antioxidant activity after 24 h of incubation was
similar to that observed for caseins, suggesting also in this case that
protein/polyphenol interaction reduced antioxidant activity of the
polyphenol system by subtracting free polyphenols. Interestingly, this
decrease was more pronounced in the case of β-Lg alone. These data
were in good agreement with previous reports (Stojadinovic et al.,
2013).
3.4. Characterization of protein/polyphenols interactions by MALDI-TOF-MS
The mixtures previously analyzed for antioxidant activity were
characterized by mass spectrometry. The time-dependent formation
of covalent protein–polyphenol complexes was first investigated by
MALDI-TOF-MS on the basis of the protein mass increase.
MALDI-TOF mass spectra of incubated caseins did show any stable adduct between the individual caseins neither with catechin/epicatechin
nor with extracted cocoa polyphenols.
In the system whey protein/polyphenol, mass spectra showed no
reactivity of α-La with extracts of cocoa polyphenols, catechin or
epicatechin over 24–48 h incubation time (Fig. 3). The same spectra,
instead, showed a significant reactivity for β-Lg (both variants A and
B, molecular mass 18,367 Da and 18,281 Da respectively) towards
catechin or epicatechin, either pure or present in the cocoa extract.
In fact, after incubation with the cocoa polyphenols (Fig. 4), a
288 Da mass increase (18,655 Da for variant A and 18,569 Da for
variant B) was observed in the protein molecular mass, corresponding to addition of a single epicatechin or catechin molecule.
In addition, the spectra performed at various incubation times
showed that the interaction between β-Lg and polyphenol was
detectable starting from 4 h of incubation. At that time, the relative
abundance of the β-Lg/polyphenol adducts was still much lower
than that of the native proteins. The abundance of the adducts increased with the incubation time. These findings also excluded that
the ions observed were due to cluster artifacts of MALDI analysis.
The stable nature of the β-Lg/polyphenol adducts was supported by
their persistence in MALDI TOF MS following denaturing RP-HPLC
separation.
3.5. In vitro reactivity of catechin and epicatechin with β-Lg
Once proved that β-Lg was the only major milk protein to stably
bind cocoa polyphenols, we investigated the nature of the protein binding site.
a) Native β-Lg (constituted by a mixture of the two genetic variants A
and B).
b) β-Lg alkylated to block the single free cysteine at position 121, in
order to study its possible involvement in polyphenol binding.
c) Lactosylated β-Lg obtained by RP-HPLC purification from total WP
extracted from a sample of heat-treated milk. In this latter case, we
aimed to define whether heat treatment employed to prepare commercial milk/chocolate beverages might change β-Lg binding to
polyphenols. In β-Lg, it is known that the preferential site of
lactosylation is located at lysine 100 (Fogliano et al., 1998).
Fig. 5 shows the MALDI-TOF-MS analysis of native β-Lg (control in
Fig. 5a) incubated with catechin (Fig. 5b) and an epicatechin (Fig. 5c)
for 24 h: both A and B variants added a single polyphenol molecule. On
the contrary, β-Lg treated with IAA, in which the free cysteine was
blocked, showed a molecular mass of 18,423 and 18,339 Da for variants
A and B, respectively, corresponding to the mass increase of 57 mass
units for alkylation of the single free cysteine of the protein (Fig. 5d–f).
However, upon incubation with polyphenols, no mass increase was
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M. Gallo et al. / Food Research International 54 (2013) 406–415
Fig. 2. ABTS assay of catechin, epicatechin, OPC and of the polyphenols extracted from cocoa powder after incubation with (a) casein, (b) whey proteins and (c) β-Lg of cow milk (values are
expressed as millimoles of Trolox/liter); Mean (SD) from three separate experiments run in duplicate. *p b 0.05 versus t = 0; the ratio decrease (%) with respect to the initial time (t = 0)
is indicated.
observed, indicating that the polyphenol binds specifically to the free
cysteine residue.
In order to confirm this hypothesis, and to assign the reactive site of
β-Lg with polyphenols, tryptic hydrolysis was performed on the
samples after a diafiltration step to remove polyphenol excess from
the reaction mixture. Fig. 6b shows the partial MALDI spectrum of the
tryptic digest of native β-Lg before and after incubation with catechin
(control in Fig. 6a). The identification of peptides was performed on
the basis of the measured peptide masses compared with those
expected on the basis of trypsin specificity (Table 1) using the data
processing software GPMAW (Applied Biosystems).
The mass spectra obtained after incubation of epicatechin were very
similar to that obtained for catechin incubation, demonstrating the
same extent of reactivity of the two phenols with β-Lg. These data are
in agreement with literature data that demonstrate two flavanols
exhibit the same behavior in terms of reactivity, chemical properties,
etc. (Mendoza-Wilson & Glossman-Mitnik, 2006).
Peptide mass mapping allowed to verify the protein sequence
completely. In particular, signals at m/z 2647.09 and 2675.14
corresponded to peptides 102–124 of β-Lg variants B and A respectively. These signals, in the β-Lg sample incubated with catechin
(Fig. 6b), were accompanied by signals at m/z 2935.83 and 2963.89
where the mass of the two peptides was increased by 288 units
corresponding to addition of a single catechin monomer. A similar result
was obtained for epicatechin. In the case of IAA-β-Lg (Fig. 6c), peptide
102–124 was present only in the alkylated form but no evidence of catechin binding was found. In fact, peptide 102–124 contains three cysteine
residues, two of which involved in a disulfide bridge and the third one
free. This result supported the hypothesis that the polyphenol could
have reacted with the free cysteine residue on the peptide, and alkylation
of this residue prevented interaction either flavanols. For unambiguous
structural characterization, peptides at m/z 2961.40 and m/z 2933.36
were analyzed by ESI-Q-TOF MS/MS (not shown) confirming binding of
polyphenols to cysteine 121. Moreover, analysis showed the absence of
these peptides in the case of alkylated β-Lg, confirming, also in this
case, the data provided by MALDI-TOF-MS.
The same analysis was also carried out on the lactosylated form
of β-Lg. The tryptic map of Lac-β-Lg presented the same peptides
102–124 with addition of catechin (signals m/z 2933.83 and
2961.89) (Fig. 6d). This demonstrated that lactosylation did not
prevent polyphenol binding, and that lactosylated β-Lg covalently
bound polyphenols at the same cysteine 121 residue.
M. Gallo et al. / Food Research International 54 (2013) 406–415
411
Fig. 3. MALDI-TOF mass spectra of whey proteins after 0 (a), after 24 h (b) and 48 h (c) incubation with polyphenol extracts from cocoa, excluding formation of covalent complexes of α-La
with cocoa polyphenols. *: Adduct with the matrix; Lac: Lactose.
3.6. LC ESI MS analysis of polyphenol/β-Lg adducts in commercial products
LC–ESI-MS analysis of the intact whey protein fraction isolated from a
commercial pasteurized chocolate/milk did not evidence any adduct possibly because of the low extent of modification (not shown). However,
the same analysis carried out on the tryptic digest of the same fraction
monitoring the triply charged ions of reacted peptides (Fig. 7d) showed
the presence of the same peptides 102–124 of β-Lg with addition
of catechin/epicatechin which had been observed in the samples
incubated in vitro (Fig. 7a–c) although to a very low extent. These
findings proved that covalent interactions β-Lg/polyphenol occur
also in real milk and milk/chocolate beverages and adduct formation
can be monitored using the proposed peptidomic approach, once a
robust quantification method has been developed.
4. Discussion
Several studies have highlighted the nutritional implications of
interactions of polyphenols with proteins in food, which may determine
a decreased bioavailability of polyphenols. The loss of food nutritional
quality has been ascribed to modification of essential amino acid residues and to reduction of protein digestibility due to inhibition of
proteolytic and glycolytic enzymes. According to some studies, polyphenols may interact irreversibly with dietary proteins and digestive
enzymes in the gut and be transported in vivo bound to plasma
proteins (Brunet, Blade, Salvado, & Arola, 2002). This protein binding
can affect the physiological effects of polyphenols, depending on
their intake and structure. Indeed, binding to food proteins may
have implications in terms of bioavailability (Serafini, Ghiselli, &
Ferro-Luzzi, 1996; Wollgast & Anklam, 2000), as the antioxidant
capacity of polyphenols can be modified by the presence of proteins
(Arts, Haenen, Voss, & Bast, 2001; Arts et al., 2002; Riedl &
Hagerman, 2001). Therefore, the proteins present in the food matrix,
the digestive environment and the blood have the potential to significantly affect the biological activity of polyphenols.
Despite this, the structural definition of the nature of these protein–
polyphenol complexes has never been carried out. Several kinds of
interactions have been hypothesized: strong (covalent, ionic) or weak
(hydrogen bridges, π bonds, hydrophobic) bonds. With regard to the
covalent adducts, there are few data in the literature to support
Fig. 4. MALDI-TOF mass spectra showing evidence of a complex between β-Lg and (b) polyphenols extracted from cocoa, (c) catechin, (d) epicatechin, after 24 h incubation, compared to
control (a).
412
M. Gallo et al. / Food Research International 54 (2013) 406–415
Fig. 5. MALDI-TOF-MS analysis of β-Lg: a) control; b) after 24 h incubation with epicatechin; c) after 24 h incubation with catechin. MALDI-TOF-MS analysis of IAA-β-Lg: d) control;
e) after 24 h incubation with epicatechin, f) after 24 incubation with catechin. *: adduct of the protein with the sinapinic acid matrix.
experimentally their formation and to identify binding sites of proteins.
This study demonstrates for the first time the formation of the complex protein–polyphenol involving a milk protein, β-Lg and cocoa
polyphenols, through covalent binding of free-SH group of the free
cysteine residue of the protein.
To this aim, a strategy combining proteomic with biochemical approaches has been applied to the characterization of a simplified model
system and then of a commercial products, a chocolate–milk drink.
The polyphenol extract of commercial cocoa was characterized by
LC–ESI–MS/MS and incubated with milk proteins. The interaction and
complex formation at several incubation times was studied by mass
spectrometry and antioxidant activity assays. Antioxidant activity of
polyphenols was measured by ABTS in a model assay system, which
was shown to preserve antioxidant activity of polyphenols under the
conditions of time/temperature/medium used for the interaction study.
The measurement of antioxidant activity of casein/polyphenol mixtures revealed a time-dependent decrease of activity. It has been reported
that the interaction with polyphenols modifies casein conformation
resulting in a reduction of α-helices and β-sheets (Hasni et al., 2011).
This decrease of antioxidant activity was observed also for polyphenol/
WP and polyphenol/β-Lg systems. Considering that polyphenols incubated in the absence of proteins did not change significantly their antioxidant activity it was possible to hypothesize that the reduced
antioxidant activity of polyphenol–protein system is due to a stable
protein–polyphenol interaction, resulting in a decrease of free
polyphenol, with a decrease of the antioxidant activity of the system.
Proteomic analysis, however, confirmed the formation of covalent
complexes only in the case of WP, not for caseins. More precisely,
while α-La presents no reactivity with polyphenol extracts of cocoa as
well as with phenol standards, β-Lg showed a significant reactivity to
catechin and epicatechin experimentally measurable starting from 4 h
of incubation and remained stable up to 48 h. Taken together, these
data suggest that covalent binding occurs with β-Lg, while other weaker,
likely non-covalent interactions take place in the case of casein with
cocoa polyphenols.
MALDI TOF MS and ESI-Q-TOF MS/MS analyses of tryptic peptides of
β-Lg have allowed the identification of the binding site as the free thiol
group of cysteine 121. In support of our findings, reactivity of grape
flavan-3-ols, including catechins, with cysteine and cysteine derivatives
has already been described (Tanaka, Kusano, & Kouno, 1998; Torres &
Bobet, 2001; Torres, Lozano, & Maher, 2005; Torres et al., 2002). In
any case, this is the first structural report of a food polyphenol covalent
adducts with milk proteins.
Interestingly, grape polyphenol adducts with cysteine thiol groups
showed a higher antiradical capacity than their underivatized counterparts (Torres et al., 2002), and this suggests that functional properties of
β-Lg/polyphenol adducts deserve further detailed investigation, possibly on pure compounds obtained by chemical synthesis.
At the same time, our data showed also that only a small part of
the protein interacts with the polyphenol; therefore, it can be concluded that the implications of nutritional point of view are not
such as to support the hypothesis of a quantitative decrease in the
M. Gallo et al. / Food Research International 54 (2013) 406–415
413
Fig. 6. Partial MALDI-TOF-MS analysis of the tryptic digest of β-Lg incubated for 4 h with polyphenols: (a) control, and (b) native β-Lg, (c) alkylated β-Lg, (d) lactosylated β-Lg
incubated with catechin (CAT).
Table 1
Identification of tryptic peptides of native β-Lg, Lac-β-Lg and IAA-β-Lg after 24 h of incubation with catechin (CAT) or epicatechin (EPI). CM: peptide carboxymethylated. x: peptide
detected.
Peptide
Measured MH+
Theoretical MH+
Peptide sequence
β-Lg
Lac-β-Lg
IAA-β-Lg
71–75
78–83
142–148
84–91
1–8
92–100
125–138
149–162
41–60
102–124 B
102–124 A
15–40
61–69 B + 149–162
61–69 A + 149–162
61–70 B + 149–162
61–70 A + 149–162
102–124 B + CAT
102–124 A + CAT
102–124 B + EPI
102–124 A + EPI
61–70 A + 149–162 + lactose
(102–124B) CM
(102–124A) CM
573.55
674.19
837.55
916.51
933.65
1065.22
1635.89
1658.85
2313.54
2647.11
2675.42
2707.23
2721.59
2777.97
2848.70
2906.43
2935.89
2963.89
2935.83
2963.89
3173.29
2704.44
2704.37
573.36
674.42
837.48
916.47
933.54
1065.58
1635.77
1658.78
2313.26
2647.20
2675.20
2707.38
2721.111
2779.11
28848.30
2906.30
2935.83
2963.44
2935.42
2963.40
3173.39
2704.33
2704.33
IIAEK
IPAVFK
ALPMHIR
IDALNENK
LIVTQTMK
VLVLDTDYK
TPEVDDEALEKFDK
LSFNPTQLEEQCHI
VYVEELKPTPEGDLEILLQK
YLLFCMENSAEPEQSLACQCLVR
YLLFCMENSAEPEQSLVCQCLVR
VAGTWYSLAMAASDISLLDAQSAPLR
WENGECAQK + LSFNPTQLEEQCHI
WENDECAQK + LSFNPTQLEEQCHI
WENGECAQKK + LSFNPTQLEEQCHI
WENDECAQKK + LSFNPTQLEEQCHI
YLLFCMENSAEPEQSLACQCLVR + CAT
YLLFCMENSAEPEQSLVCQCLVR + CAT
YLLFCMENSAEPEQSLACQCLVR + EPI
YLLFCMENSAEPEQSLVCQCLVR + EPI
WENGECAQKK + LSFNPTQLEEQCHI + lactose
YLLFCMENSAEPEQSLACQCLVR
YLLFCMENSAEPEQSLVCQCLVR
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
414
M. Gallo et al. / Food Research International 54 (2013) 406–415
Fig. 7. LC–ESI-MS analysis in selected ion monitoring mode of the tryptic digest of whey proteins incubated with catechin (a: total ion current); b: triply charged ion of 102–124 from β-Lg
variant B reacted with CAT (979.0 m/z); c: triply charged ion of 102–124 from β-Lg variant A reacted with CAT (988.3 m/z), compared to the tryptic digest of the whey protein fraction of a
commercial sterilized milk-chocolate beverage (d: monitoring of the modified 102–124 peptide forms from both β-Lg variants A and B).
bioavailability of polyphenols themselves. Furthermore, LC ESI MS
analysis of a commercial chocolate/milk drink demonstrated the formation of the same covalent complex also in this product.
5. Conclusions
A proteomic approach has been developed to monitor the formation of stable polyphenol/protein complex in either laboratory or
commercial milk products. The results obtained allow us to hypothesize the use of this methodology for similar products like chocolate
milk drinks, chocolate milk, and formulated protein containing
cocoa. The extension of this method to the quantitative level could
be useful to understand the type of polyphenol–protein interaction
in these products taking into account the characteristics of ingredients and the technological parameters of the production process. At
the same time, the observed protein effects on the antioxidant
activity decrease measured in vitro have allowed us to assume the
presence of other types of weak protein–polyphenol interactions
that reduce this activity. Currently, little is known about polyphenol
absorption, bioavailability, biodistribution, and metabolism, although
there is probably a common path. Consequently, further studies are
needed to identify the nature and strength of these interactions, in
order to assess the possible structural changes upon food ingestion
and digestion As a matter of fact, the consequences of the interactions
in nutritional terms are not so easily measurable and generalizable as
it should be taken into consideration a number of other factors, not
least the type of polyphenol and the nature of the protein matrix.
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