Quantitative proteomic analysis of bacterial enzymes released in cheese during ripening

Due to increasingly available bacterial genomes in databases, proteomic tools have recently been used to screen proteins expressed by micro-organisms in food in order to better understand their metabolism in situ. While the main objective is the systematic identification of proteins, the next step will be to bridge the gap between identification and quantification of these proteins. For that purpose, a new mass spectrometry-based approach was applied, using isobaric tagging reagent for quantitative proteomic analysis (iTRAQ), which are amine specific and yield labelled peptides identical in mass. Experimental Swiss-type cheeses were manufactured from microfiltered milk using Streptococcus thermophilus ITG ST20 and Lactobacillus helveticus ITG LH1 as lactic acid starters. At three ripening times (7, 20 and 69 days), cheese aqueous phases were extracted and enriched in bacterial proteins by fractionation. Each sample, standardised in protein amount prior to proteomic analyses, was: i) analysed by 2D-electrophoresis for qualitative analysis and ii) submitted to trypsinolysis, and labelled with specific iTRAQ tag, one per ripening time. The three labelled samples were mixed together and analysed by nano-LC coupled on-line with ESI-QTOF mass spectrometer. Thirty proteins, both from bacterial or bovine origin, were identified and efficiently quantified. The free bacterial proteins detected were enzymes from the central carbon metabolism as well as stress proteins. Depending on the protein considered, the quantity of these proteins in the cheese aqueous extract increased from 2.5 to 20 fold in concentration from day 7 to day 69 of ripening.

Inhibitory effect of αS1- and αS2-casein hydrolysates on angiotensin I-converting enzyme in human endothelial cells in vitro, rat aortic tissue ex vivo, and renovascular hypertensive rats in vivo

A great number of milk-derived peptides have been shown to exhibit angiotensin converting enzyme (ACE) inhibitory properties and thus potential utility in the regulation of blood pressure. The present work aimed to investigate the effects of 2 milk trypsin hydrolysates from alpha(S1)- and alpha(S2)-casein (CH1 and CH2, respectively) on ACE activity evaluated in human umbilical vein endothelial cells (HUVEC) in vitro, rat aortic tissues ex vivo, and renovascular hypertensive rat in vivo. Incubation of HUVEC and rat aortic tissues with CH1 or CH2 induced a concentration-dependent inhibition of hydrolysis of the ACE substrate hippuryl-histidyl-leucine (HHL), the hydrolysates being much less potent than perindopril (an ACE inhibitor). However, in contrast to perindopril, CH1 and CH2 failed to modify angiotensin I-induced aortic ring vasoconstriction. The HPLC profiles of rat plasma after intragastric administration were variable among individuals but none of the observed peaks corresponded to peptides comprising CH1 or CH2 or to fragments of these peptides. During 4 wk of cardiovascular monitoring, in hydrolysate-fed renovascular hypertensive rats, systolic blood pressure weakly decreased compared with the control group. However, the CH1-fed hypertensive rats exhibited a decrease of heart rate during the nocturnal period of activity. To conclude, our results show that CH1 and CH2 inhibited ACE activity in HUVEC and rat aortic tissue but failed to antagonize the aortic-constricting effects of the natural agonist angiotensin I. Moreover, we demonstrated that CH1, to a greater extent than CH2, can slightly affect cardiovascular parameters although the ingested bioactive peptides could not be detected in the blood.

Identification of phosphorylation sites of equine beta-casein isoforms

Equine beta-casein is phosphorylated at variable degrees and isoforms carrying 3 to 7 phosphate groups (3P-7P) have been found in milk, but the phosphorylated amino acid residues of each isoform are not yet identified. In the present work, the different phosphorylation variants were first isolated by ion-exchange chromatography and then hydrolysed by trypsin to generate caseinophosphopeptides (CPPs), each containing all the potential phosphorylation sites. The equine CPPs were prepared by metal oxide affinity chromatography, a method based on the affinity of phosphate groups towards titanium dioxide immobilized onto a micro-column. This method turned out to be an efficient tool to separate the CPPs Arg(1)-Lys(34) and Glu(4)-Lys(34) from non-phosphorylated peptides. Purification was achieved by reversed-phase high-performance liquid chromatography (RP-HPLC) and each CPP was hydrolyzed by endoproteinase Glu-C. Finally, the digests were analyzed by RP-HPLC/electrospray ionization mass spectrometry (RP-HPLC/ESI-MS) and identified by nano-electrospray ionization tandem mass spectrometry (nESI-MS/MS) to locate the phosphorylated sites of the beta-casein isoforms 4P-7P with accuracy. Thus, the isoform 4P was found to be phosphorylated on residues Ser(9), Ser(23), Ser(24), and Ser(25). Addition of phosphate groups on Ser(18), Thr(12), and Ser(10) led to the formation of the isoforms 5P-7P, respectively. The results indicated that the in vivo phosphorylation of the equine beta-casein follows a sequential way and is not randomly performed.

Identification in milk of a serum amyloid A peptide chemoattractant for B lymphoblasts

Background

Normal mammary gland contains an extravascular population of B lymphoblasts, precursors of the immunoglobulin plasma cells that play a key role in the passive protection of neonates by secreting immunoglobulins to colostrum and milk. We investigated the presence of chemoattractants in the milk by analysing the chemoattractant activity of various fractions of this secretion. Milk chemoattractants are potentially involved in the recruitment of lymphocytes from the maternal bloodstream in lactating mammary glands.

Results

The dilution-related lymphoid cell chemoattraction of whey was associated with a < 10 kDa ultrafiltrate. Active fractions were purified by reverse-phase high performance liquid chromatography. Two peptides of 2.7 kDa (DMREANYKNSDKYFHARGNYDAA) and 1 kDa (RPPGLPDKY) were identified as fragments of the SAA protein family, tentatively identified as SAA2. Only the 2.7 kDa synthetic peptide displayed chemotactic activity, at two different optimal concentrations. At the lower concentration (3.7 nM), it attracted B-cell lymphoblasts, whereas at the higher (3.7 μM), it attracted B lymphocytes. Then, the SAA mRNA expression was analysed and we observed more SAA transcripts during lactation than gestation.

Conclusion

These data are consistent with the SAA_23–45 fragment being involved in preplasma B-cell recruitment to the mammary gland and resultant benefit to the neonate.

Comparative secretome analyses of two Trichoderma reesei RUT-C30 and CL847 hypersecretory strains

BACKGROUND

Due to its capacity to produce large amounts of cellulases, Trichoderma reesei is increasingly been researched in various fields of white biotechnology, especially in biofuel production from lignocellulosic biomass. The commercial enzyme mixtures produced at industrial scales are not well characterized, and their proteinaceous components are poorly identified and quantified. The development of proteomic methods has made it possible to comprehensively overview the enzymes involved in lignocellulosic biomass degradation which are secreted under various environmental conditions.

RESULTS

The protein composition of the secretome produced by industrial T. reesei (strain CL847) grown on a medium promoting the production of both cellulases and hemicellulases was explored using two-dimensional electrophoresis and MALDI-TOF or LC-MS/MS protein identification. A total of 22 protein species were identified. As expected, most of them are potentially involved in biomass degradation. The 2D map obtained was then used to compare the secretomes produced by CL847 and another efficient cellulolytic T. reesei strain, Rut-C30, the reference cellulase-overproducing strain using lactose as carbon source and inducer of cellulases.

CONCLUSION

This study provides the most complete mapping of the proteins secreted by T. reesei to date. We report on the first use of proteomics to compare secretome composition between two cellulase-overproducing strains Rut-C30 and CL847 grown under similar conditions. Comparison of protein patterns in both strains highlighted many unexpected differences between cellulase cocktails. The results demonstrate that 2D electrophoresis is a promising tool for studying cellulase production profiles, whether for industrial characterization of an entire secretome or for a more fundamental study on cellulase expression at genome-wide scale.

Epitope characterization of a supramolecular protein assembly with a collection of monoclonal antibodies: the case of casein micelle

In milk, kappa-, beta-, alphas(1)- and alphas(2)-casein (CN) are associated into a supramolecular assembly, the micelle. In this work, CN micelles contained in fresh skim milk were used to produce over 100 monoclonal antibodies. The specificity of these probes was determined using libraries of synthetic peptides and peptides fractionated from tryptic hydrolysis of purified CNs. Although kappa-CN and alphas(2)-CN are minor proteins in the micelle (ratio 1:1:4:4 for kappa, alphas(2), alphas(1), beta) a proportionally high number of clones were produced towards these two proteins (32 for each), compared to 9 and 29 for alphas(1)-CN and beta-CN, respectively. Most of the beta-CN and kappa-CN epitopes were identified, while about 50% of alphas(1)-CN and alphas(2)-CN antibodies were suspected to react to conformational linear or discontinuous epitopes, since no peptide binding could be identified. Antibody binding to the phosphoserine rich regions of the three calcium sensitive CNs was weak or non-existing, suggesting them to be hidden in the micelle structure together with alphas(1)-CN. The C-terminal glycomacropeptide of kappa-CN and the C-terminal moiety of beta-CN were well exposed generating the majority of the antibodies specific for these two proteins. The two major antigenic sites of alphas(2) were alphas(2)-CN (f96-114) and (f16-35). Cross-reaction between alphas(2)-CN specific antibodies with alphas(1)-CN illustrated the tangled structure between the two proteins. Immuno-dominant epitopes identified in the present study totally differ from those known for the purified caseins suggesting they were specific for the micelle supramolecular structure.

Study of Caprine β-casein using Reversed-phase High-performance Liquid Chromatography and Mass Spectroscopy: Identification of a New Genetic Variant

The genotypes and the main phosphorylation levels of beta-casein of goat milk were studied using RP-HPLC/ESI-MS. A new variant of caprine beta-casein named E has been characterized using RP-HPLC/ESI-MS, MALDI-MS and NanoESI MS/MS methods. Its sequence differed from that of variant A in the mono amino acid substitution D47 --> Y47, which resulted in a 48 Da experimental mass difference between them. The calculated molecular mass of the new variant E 6 P was estimated as 23,869 Da. Its phosphorylation pattern was similar to that of variant A, the most abundant types being those with 5 and 6 P in similar quantities.

Determination of exposed sulfhydryl groups in heated beta-lactoglobulin A using IAEDANS and mass spectrometry

This paper takes a new approach to determining which sulfhydryl groups are exposed during the heat denaturation of bovine beta-lactoglobulin A. The sulfhydryl groups exposed after heating were blocked with 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid (IAEDANS). The results show that IAEDANS is a suitable blocking agent, and its absorbance at 336 nm enabled the quantification of exposed sulfhydryl groups in a mixture of protein species by gel permeation chromatography. Combined with the specific fragmentation of bound IAEDANS by matrix-assisted laser desorption ionization (MALDI) MS/MS in negative ionization mode, this facilitated the identification of peptides that contained blocked cysteines after enzymatic digestion of the protein. During MALDI MS/MS of the peptides, in positive ionization mode, the IAEDANS molecule remained bound to the cysteines, making it possible to identify exactly which cysteine had been exposed after heating. In beta-lactoglobulin A it was found that cysteine 66 and cysteine 160 were predominantly exposed regardless of the length of exposure to heat.

The primary structure of a low-Mr multiphosphorylated variant ofβ-casein in equine milk

Highly phosphorylated casein with a low molecular mass was isolated from Haflinger mare's milk by RP-HPLC. It accounts for 4.0% of the casein content. Its mass was determined by LC-ESI-MS before and after treatment by alkaline phosphatase. The molecular mass found for the apo-form (10,591 +/- 2 Da) is in agreement with its primary structure, which was established by ESI-MS/MS from tryptic peptides. It appeared that this short protein (94 amino acid residues) is an internally truncated form of the full-length equine beta-casein (226 residues). This low-Mr variant of equine beta-casein displays a large deletion (residues 50-181), due to a cryptic splice site usage occurring within exon 7 during the course of primary transcripts processing. The phosphorylation pattern of this equine beta-casein variant was investigated by LC-ESI-MS and 2-DE. Seven phosphorylation forms were identified with one to seven phosphate groups with pIs ranging between 4.67 and 4.01. The major isoforms carry five and six phosphate groups.

Determination of the phosphorylation level and deamidation susceptibility of equine β-casein

beta-Casein was isolated from Haflinger mare's milk by RP-HPLC, and displayed microheterogeneity by urea-electrophoresis and 2-DE probably due to a variable degree of phosphorylation. To investigate the degree of phosphorylation, the primary structure of equine beta-casein was determined by tryptic hydrolysis and MS of peptides released and by MS of the protein treated by alkaline phosphatase. The molecular mass found for the apo-form of Haflinger mare's beta-casein (25 514 +/- 3 Da) was close to the theoretical mass of the reported sequence (GenBank AAG43954) modified by insertion of a region (residues 27-34) encoded by an exon sometimes out-spliced (25 511.40 Da). Hence, the beta-casein isolated from Haflinger mare's milk corresponded to a variant of 226 amino acid residues. The latter was composed by highly multi-phosphorylated isoforms with three to seven phosphate groups, and pIs, determined by 2-DE, ranging from 4.74 to 5.30. Moreover, the equine beta-casein was able to deamidate spontaneously, at the level of Asn in the potential deamidation motif (135)Asn-Gly(136). Approximately 80% of the protein was deamidated after 96 h of incubation under physiological conditions.

Proteomic analysis of hen egg white

Hen egg white is an original biological fluid in which major proteins have been widely studied, unlike the minor components. In this study, two-dimensional electrophoresis associated with mass spectrometry enabled the separation of 69 protein spots and their matching with major proteins, which were already known, and with minor proteins. Sixteen proteins were identified, and among them, two had never been previously detected in hen egg white, i.e., Tenp, a protein with strong homology with a bacterial permeability-increasing protein family (BPI), and VMO-1, an outer layer vitelline membrane protein. Thirteen proteins present a very wide polymorphism (ovotransferrin, ovomucoid, clusterin, etc.), some of them up to nine isoforms (ovoinhibitor). Eleven functional protein families were identified (serpin, transferrin, protease inhibitors Kazal, glycosyl hydrolases, lipocalin, bactericidal permeability-increasing protein, clusterin, UPAR/CD59/Ly6/ snake neurotoxin, cysteine protease inhibitor, VMO-1, and folate receptor families). These various biological functions could be interesting for further valorizations. In addition, three spots remain unidentified, probably because these proteins are not yet indexed in the international protein databanks.

Improved absorption of caseinophosphopeptide-bound iron: role of alkaline phosphatase

Hydrolysis of proteins could lessen their inhibiting effect on the poor absorption of cow's milk iron (Fe), which is responsible for the high incidence of Fe deficiency worldwide. When bound to Fe, caseinophosphopeptides (CPP) derived from milk proteins resist luminal digestion, enhance Fe solubility and could improve its bioavailability; brush border enzyme alkaline phosphatase activity could influence iron absorption by releasing free Fe; this study assessed its role in the absorption of CPP-bound Fe. Rat duodenal loops were perfused with Fe gluconate or Fe bound to the CPP of beta casein [beta-CN (1-25)], with or without the addition of an inhibitor of alkaline phosphatase, Na2WO4. The uptake of Fe-beta-CN (1-25) was greater than Fe gluconate. Na2WO4 enhanced the uptake of Fe-beta-CN (1-25) and not of Fe gluconate. So the release of free, insoluble Fe, by alkaline phosphatase seems to be prevented by providing Fe in the Fe-beta-CN (1-25) complex form. Its good disappearance rate makes beta-CN (1-25)-bound Fe a candidate for food fortification.

Identification and characterization of ovalbumin gene Y in hen egg white

Ovalbumin gene Y has been known as a member of the ovalbumin gene family since 1982, when its encoding gene was sequenced. In the present study, ovalbumin gene Y has been demonstrated as a new minor protein of hen egg white. This protein has been isolated by isoelectrofocalization and two-dimensional polyacrylamide gel electrophoresis and has been characterized using peptide mass fingerprinting. The concentration ratio of ovalbumin gene Y:ovalbumin is about 13:100. Unlike ovalbumin, ovalbumin gene Y is not phosphorylated, but like ovalbumin, this protein is glycosylated. Ovalbumin gene Y exists as a mixture of three molecular species, which differ in their isoelectric points. The polymorphism of this protein cannot be explained by various glycosylation levels.

Spectroscopic characterization of heat-induced nonnative beta-lactoglobulin monomers

Previous studies have shown that two altered monomeric species were formed in the early steps of thermal denaturation of bovine beta-lactoglobulin (beta-lg), the well-known Cys121-exposed intermediate (Mcys121), and a new, stable monomer with exposed nonnative Cys119 (Mcys119). In this study, circular dichroism and fluorescence spectroscopies were used to characterize the structural features of these molecules. The structural characteristics of MCys121 after heating and cooling cycles are similar to those of native beta-lg. In contrast, Mcys119 monomer exhibits some characteristics of the well-known molten-globule state. Combined with other published data, these results indicate that heating induces at least two molten globule-like states of beta-lg, a highly reactive Mcys121 that returns to native state after cooling, and a less-reactive Mcys119 that is trapped and stabilized in a molten globule-like state by nonnative disulfide bond.

Stable monomeric intermediate with exposed Cys-119 is formed during heat denaturation of beta-lactoglobulin

The role of the free sulfhydryl group of beta-lactoglobulin in the formation of a stable non-native monomer during heat-treatment of beta-lactoglobulin solutions was investigated. Two concomitant events occurred at the earlier stage of heating: unfolding of native globular monomer and intramolecular sulfhydryl/disulfide exchange reaction. Thus, two denatured monomeric species were formed: a non-native monomer with exposed Cys-121 (Mcys121) which became reversible after cooling, and a stable non-native monomer with exposed Cys-119 (Mcys119) which exhibited both a larger hydrodynamic conformation than native monomer and low solubility at pH 4.7. The results also show that the formation of these monomeric species throughout heat-induced denaturation of native beta-lg monomers is faster than their subsequent aggregation. A mechanism describing the behavior of beta-lg denaturation/aggregation during heat-treatment under selected conditions (5.8 mg/ml, low ionic strength, pH 6.6, 85 degrees C) is presented.

Heterogeneity of caprine beta-casein elucidated by RP-HPLC/MS: genetic variants and phosphorylations

Casein variants occurring in milks from goats homozygous at the alpha(s1)-Cn locus were separated and identified by an RP-HPLC/ESI-MS method. Preferential haplotypes arose as well as some particularities in posttranslational modifications. A new variant of caprine beta-Cn, named C, as well as the phosphorylations pattern of the protein were characterized by the combined use of peptide mass fingerprinting and sequencing by tandem mass spectrometry. The molecular mass of the new variant in its 6P form was measured as 23854 Da and it differs in a mono amino acid substitution, A177 --> V177, from the variant A. The phosphorylation pattern of caprine beta-Cn is homologous to bovine beta-Cn concerning the 5P located on Ser15, 17, 18, 19, 35 but it presents a specific additional phosphorylation site on Thr12 that is comparable to human beta-Cn phosphorylation located on Thr3.

Heat-induced covalent complex between casein micelles and beta-lactoglobulin from goat's milk: identification of an involved disulfide bond

Goat milk is characterized by a very low heat stability that could be attributed, in part, to the covalent interaction between whey proteins and casein micelles. However, the formation of such a complex in goat milk has never been evidenced. This study was designed to assess whether heat-induced covalent interaction occurs between purified casein micelles and beta-lactoglobulin. We used a multiple approach of ultracentrifugation of heated mixture, chromatographic fractionation of resuspended pellets, sequential enzyme digestion of disulfide-linked oligomers, and identification of disulfide-linked peptides by on-line liquid chromatography-electrospray ionization mass spectrometry (LC-ESI/MS), and tandem MS. We identified three different types of disulfide links: (1) expected intermolecular bridges between beta-Lg molecules; (2) disulfide bond involving two kappa-casein molecules; and (3) a disulfide bond between two peptides, one from beta-Lg and the other from kappa-casein. The involved sites in this last bond were Cys(160) of beta-Lg and Cys(88) of kappa-casein. Although the identified heterolinkage is possibly only one of several different types, the results of this study constitute the first direct evidence of the formation of a covalent complex between casein micelles and beta-lactoglobulin derived from goat milk.

Evidence of the glycation and denaturation of LTP1 during the malting and brewing process

The influence of malting and brewing processes on the chemical and structural modifications occurring on LTP1 was investigated by mass spectrometry and circular dichroism. Proteins were first purified from malt, and samples were collected at various steps of beer processing performed on two barley cultivars. The levels of LTP1 found in malt were not significantly different from the amounts in barley seed. However, in malt, both LTP1b, a post-translational form of LTP1, and a third isoform named LTP1c were isolated. Moreover, both of these proteins were found to be heterogeneously glycated but still exhibited an alpha-helix structure. Both glycated LTP1 and LTP1b were recovered during mashing. It was also shown that glycated LTP1 was unfolded during heat treatment of wort boiling, which is in agreement with the denatured form previously isolated from beer.

Peptides identified during Emmental cheese ripening: origin and proteolytic systems involved

To determine the proteolytic changes occurring during Emmental cheese ripening, peptides released in cheese aqueous phase were analyzed by reversed-phase HPLC and identified by tandem mass spectrometry sequencing, for which different strategies were illustrated by some examples. Among the 91 peptides identified, most of them arose from alpha(s1)- (51) and beta-caseins (28), and a few arose from alpha(s2)- (9) and kappa-caseins (1). An attempt was made to correlate the released peptides with the proteolytic systems potentially involved during Emmental cheese manufacture. Besides the well-known action of plasmin on beta- and alpha(s2)-caseins, and in the absence of residual fungal coagulant from Endothia parasitica, two other proteinases seem to be involved in the hydrolysis of alpha(s1)-casein in Emmental cheese: cathepsin D originated from milk and cell-envelope proteinase from thermophilic starters. Moreover, peptidases from starters were also active throughout ripening, presumably like those from nonstarter lactic acid bacteria, in contrast to those from propionic acid bacteria.

Identification of a new form of lipid transfer protein (LTP1) in wheat seeds

Recently, this laboratory has isolated from barley and beer extract an isoform of lipid transfer protein (LTP1), which was not fully sequenced (Jégou, S.; Douliez, J. P.; Mollé, D.; Boivin, P.; Marion, D. J. Agric. Food Chem. 2000, 48, 5023--5029). It was named LTP1b and exhibited a molecular weight 294 Da higher than that of the known LTP1. This paper reports the finding of an LTP1 isoform in wheat that also exhibits an excess of 294 Da compared to the native protein. Amino acid sequencing, reduction and alkylation, and mass spectrometry showed that this new LTP1b possesses the same N-terminal sequence as the native LTP1, suggesting that the difference resides in the binding of an adduct which has a molecular weight of 294 Da. The aim of the present paper is to highlight various biophysical techniques that afford the identification of such an isoform-like LTP1 and to correlate this finding with other isoforms of LTP1 that were isolated from other plants but not fully sequenced.

Disulfide bond assignment, lipid transfer activity and secondary structure of a 7-kDa plant lipid transfer protein, LTP2

The 7-kDa lipid transfer proteins, LTP2s, share some amino-acid sequence similarities with the 9-kDa isoforms, LTP1s. Both proteins display an identical cysteine motif and, in this regard, LTP2s have been classified as lipid transfer proteins. However, in contrast with LTP1s, no data are available on their structure, cysteine pairings, lipid transfer and lipid binding properties. We reported on the isolation of two isoforms of 7-kDa lipid transfer protein, LTP2, from wheat seeds and showed for the first time that they indeed display lipid transfer activity. Trypsin and chymotrypsin digestions of the native LTP2 afforded the sequence of both isoforms and assignment of disulfide bonds. The cysteine pairings, Cys10--Cys24, Cys25--Cys60, Cys2--Cys34, Cys36--Cys67, revealed a mismatch at the Cys34-X-Cys36 motif of LTP2 compared to LTP1. Moreover, the secondary structure as determined by circular dichroism suggested an identical proportion of alpha helices, beta sheets and random coils. By analogy with the structure of the LTP1, we discussed what structural changes are required to accommodate the LTP2 disulfide pattern.

Influence of pH on the heat-induced proteolysis of casein molecules

Proteolysis of sodium caseinate solution (24.5 g/l) induced by heat treatment at 120 degrees C at different pH values was studied by measuring nitrogen content and relative fluorescence intensity in the 4% trichloroacetic acid filtrates. The low molar mass pepticles corresponding to the soluble nitrogen were identified using liquid chromatography/tandem mass spectrometry. Increase in proteolysis, deduced from the increase in soluble nitrogen content, was observed with heating time (10, 20 and 30 min) and pH (6.0, 7.0, 8.0 and 9.0). The fluorescence measurements showed that the release of peptides containing tryptophan was minimal at pH approximately 7.0. In parallel, eighteen low molar mass peptides were characterized, of which four came from kappa-casein, nine from beta-casein and five from alphas1-casein. Peptides were preferentially released under alkaline conditions.

Binding of two mono-acylated lipid monomers by the barley lipid transfer protein, LTP1, as viewed by fluorescence, isothermal titration calorimetry and molecular modelling

The binding of two mono-acylated lipid monomers by plant lipid transfer proteins (LTP1s) presents an attractive field of research that could help our understanding of the functional role of this protein family. This task has been investigated in the case of barley LTP1 because it is known to exhibit a small cavity in its free state. The titration with lipids could not be followed by fluorescence with the native protein. Indeed, this LTP1 possesses a tyrosine residue on its C-terminus, Tyr91, which is not sensitive to lipid binding but mainly contributes to the fluorescence signal intensity. However, the binding of 1-myristoylglycerophosphatidylcholine (MyrGro-PCho) could be monitored by fluorescence after removal of Tyr91 by a carboxypeptidase. These experiments returned a dissociation constant of about 1 microM and showed that the protein can indeed bind two monomers. This result was corroborated by molecular modelling where the structure of the complex between barley LTP1 and MyrGro-PCho was derived from that determined in the case of wheat [Charvolin, D., Douliez, J.P., Marion, D., Cohen-addad, C. & Pebay-Peyroula, E. (1999) Eur. J. Biochem. 264, 562-568.]. Results from isothermal titration calorimetry experiments indicated non-classic titration behaviour but also suggested that two lipids could be bound by the protein. Finally, barley LTP1 binds two omega-hydroxypalmitic acid, a compound found in the family of cutin monomers. The fact that the binding of two lipids could be related to the physiological role of this protein family is discussed.

Purification and structural characterization of LTP1 polypeptides from beer

We report on the purification of lipid transfer proteins (LTP) from barley seeds and beer with the aim of investigating the chemical modifications that occur during the brewing process. In seeds, the well-known LTP of 9 kDa (LTP1) has been found together with a second form named LTPb that displays comparable amino acid composition but was not fully sequenced. These two forms have been recovered in beer with marked chemical modifications including disulfide bond reduction and rearrangement and especially glycation by Maillard reaction. The glycation is heterogeneous with variable amounts of hexose units bound to LTPs. Circular dichroism shows that glycated LTP1 having all their disulfide bridges reduced are totally unfolded. These results provide a first basis for understanding how barley LTPs become foam-promoting agents during the malting and brewing process.

Identification of beta-oxidation and thioesterase activities in Staphylococcus carnosus 833 strain

Staphylococcus carnosus 833, inoculated into sausage meat, increased the level of methyl ketones, which contributed to the cured aroma. These ketones can arise from incomplete beta-oxidation followed by two enzymatic activities: a thioesterase and a decarboxylase. In this study we identified the beta-oxidative pathway (through the measure of 3-hydroxyacyl-CoA dehydrogenase activity) and the thioesterase activity in extracts of S. carnosus cells grown in the presence of different methyl esters. The beta-oxidative system was induced by methyl esters and highest induction was found with a 12-carbon substrate. It was specific for medium chain length fatty acyl CoA substrates. Its maximal activity was observed at the end of stationary growth phase. HPLC analyses of acyl-CoA after incubation of cell extracts with palmitoyl-CoA showed that the beta-oxidation system released preferentially long chain hydroxyacyl-CoAs, enoyl-CoAs, and acyl-CoAs. The time-course of intermediate formation indicated a precursor product relationship indicative of a model of free intermediates which could be further deacylated by a thioesterase. The thioesterase activity was enhanced when S. carnosus was grown in the presence of methyl esters with at least 12 carbons and this enzyme was specific for short chain acyl-CoAs. The maximal activity was reached at the stationary growth phase.

Application of chromatography and mass spectrometry to the characterization of food proteins and derived peptides

The following review describes the development of mass spectrometry off-line and on-line coupled with liquid chromatography to the analysis of food proteins. It includes the significant results recently obtained in the field of milk, egg and cereal proteins. This paper also outlines the research carried out in the area of food protein hydrolysates, which are important components in foodstuffs due to their functional properties. Liquid chromatography and mass spectrometry have been particularly used for the characterization of food peptides and especially in dairy products.

New genetic variants identified in donkey's milk whey proteins

Novel genetic variants for donkey milk lysozyme and beta-lactoglobulins I and II have been identified by the combined use of peptide mass mapping and sequencing by tandem mass spectrometry in association with database searching. The novel donkey lysozyme variant designated as lysozyme B (Mr 14,631 Da) differed in three amino acid exchanges, N49 --> D, Y52 --> S, and S61 --> N, from the previously published sequence. Three novel genetic variants for donkey beta-lactoglobulins were identified. One of them is a type beta-lactoglobulin I with three amino acid exchanges at E36 --> S, S97 --> T, and V150 --> I (beta-lactoglobulin I B, Mr 18,510 Da). The two others are type beta-lactoglobulins II with two amino acid exchanges at C110 --> P and M118--> T (beta-lactoglobulin II B, Mr 18,227 Da) and with three amino acid exchanges at D96 --> E, C110 --> P, and M118 -->T (beta-lactoglobulin II C, Mr 18,241 Da). All these primary structures are closely related to those of homologous proteins in horse milk (percent identity >96%).

Sensitivity of β-casein phosphopeptide-iron complex to digestive enzymes in ligated segment of rat duodenum

Binding iron to the phosphorylated beta(1-25) peptide derived from beta-casein improves iron bioavailability in the rat. The aim of the present work was to learn how injected beta(1-25) and iron-beta(1-25) complex behave in the duodenum of rats using the technique of intestinal ligation in situ and reversed-phase (RP)-high performance liquid chromatography-electrospray mass spectrometry analysis of the lumen contents. The results demonstrate that beta(1-25) is sensitive to digestive enzymes including proteases/peptidases and phosphatases during duodenal transit. The lumen contents of rats perfused with iron free beta(1-25) contained all peptidic sequences derived from beta(1-25). In contrast, the phosphorylated part of beta(1-25) [i.e., beta(15-24)] was not detected in lumen of rats perfused with iron-beta(1-25) complex.