Two-dimensional gel electrophoresis/matrix-assisted laser desorption/ionisation mass spectrometry of a milk powder

Post translational modifications of milk proteins in geographically diverse goat breeds

Goat milk is a source of nutrition in difficult areas and has lesser allerginicity than cow milk. It is leading in the area for nutraceutical formulation and drug development using goat mammary gland as a bioreactor. Post translational modifications of a protein regulate protein function, biological activity, stabilization and interactions. The protein variants of goat milk from 10 breeds were studied for the post translational modifications by combining highly sensitive 2DE and Q-Exactive LC-MS/MS. Here we observed high levels of post translational modifications in 201 peptides of 120 goat milk proteins. The phosphosites observed for CSN2, CSN1S1, CSN1S2, CSN3 were 11P, 13P, 17P and 6P, respectively in 105 casein phosphopeptides. Whey proteins BLG and LALBA showed 19 and 4 phosphosites respectively. Post translational modification was observed in 45 low abundant non-casein milk proteins mainly associated with signal transduction, immune system, developmental biology and metabolism pathways. P_asp is reported for the first time in 47 sites. The rare conserved peptide sequence of (SSSEE) was observed in αS1 and αS2 casein. The functional roles of identified phosphopeptides included anti-microbial, DPP-IV inhibitory, anti-inflammatory and ACE inhibitory. This is first report from tropics, investigating post translational modifications in casein and non-casein goat milk proteins and studies their interactions.

PEPPI-MS: Polyacrylamide-Gel-Based Prefractionation for Analysis of Intact Proteoforms and Protein Complexes by Mass Spectrometry

Prefractionation of complex mixtures of proteins derived from biological samples is indispensable for proteome analysis via top-down mass spectrometry (MS). Polyacrylamide gel electrophoresis (PAGE), which enables high-resolution protein separation based on molecular size, is a widely used technique in biochemical experiments and has the potential to be useful in sample fractionation for top-down MS analysis. However, the lack of a means to efficiently recover the separated proteins in-gel has always been a barrier to its use in sample prefractionation. In this study, we present a novel experimental workflow, called Passively Eluting Proteins from Polyacrylamide gels as Intact species for MS ("PEPPI-MS"), which allows top-down MS of PAGE-separated proteins. The optimization of Coomassie brilliant blue staining followed by the passive extraction step in the PEPPI-MS workflow enabled the efficient recovery of proteins, separated on commercial precast gels, from a wide range of molecular weight regions in under 10 min. Two-dimensional separation combining offline PEPPI-MS with online reversed-phase liquid chromatographic separation resulted in identification of over 1000 proteoforms recovered from the target region of the gel (≤50 kDa). Given the widespread availability and relatively low cost of traditional sodium dodecyl sulfate (SDS)-PAGE equipment, the PEPPI-MS workflow will be a powerful prefractionation strategy for top-down proteomics.

Dairy products and the Maillard reaction: A promising future for extensive food characterization by integrated proteomics studies

Heating of milk and dairy products is done using various technological processes with the aim of preserving microbiological safety and extending shelf-life. These treatments result in chemical modifications in milk proteins, mainly generated as a result of the Maillard reaction. Recently, different bottom-up proteomic methods have been applied to characterize the nature of these structural changes and the modified amino acids in model protein systems and/or isolated components from thermally-treated milk samples. On the other hand, different gel-based and shotgun proteomic methods have been utilized to assign glycation, oxidation and glycoxidation protein targets in diverse heated milks. These data are essential to rationalize eventual, different nutritional, antimicrobial, cell stimulative and antigenic properties of milk products, because humans ingest large quantities of corresponding thermally modified proteins on a daily basis and these molecules also occur in pharmaceuticals and cosmetics. This review provides an updated picture of the procedures developed for the proteomic characterization of variably-heated milk products, highlighting their limits as result of concomitant factors, such as the multiplicity and the different concentration of the compounds to be detected.

Proteomic comparison of equine and bovine milks on renneting

Rennet-induced coagulation of bovine milk is a complex mechanism in which chymosin specifically hydrolyzes κ-casein, the protein responsible for the stability of the casein micelle. In equine milk, this mechanism is still unclear, and the protein targets of chymosin are unknown. To reveal the proteins involved, the rennetability of equine milk by calf chymosin was examined using gel-free and gel-based proteomic analysis and compared to bovine milk. RP-HPLC analysis of bovine and equine milks showed the release of several peptides following chymosin incubation. The hydrolyses of equine and bovine casein by chymosin were different, and the major peptides produced from equine milk were identified by mass spectrometry as fragments of β-casein. Using two-dimensional electrophoresis, equine β-casein was confirmed as the main target of calf chymosin over 24 h at 30 °C and pH 6.5. The gel-based analysis of equine milk discriminated between the different individual proteins and provided information on the range of isoforms of each protein as a result of post-translational modifications, as well as positively identified for the first time several isoforms of κ-casein. In comparison to bovine milk, κ-casein isoforms in equine milk were not involved in chymosin-induced coagulation. The intensity of equine β-casein spots decreased following chymosin addition, but at a slower rate than bovine κ-casein.

Proteomic approach based on MALDI-TOF MS to detect powdered milk in fresh cow's milk

Milk and cheese are expensive foodstuffs, and their consumption is spread among the population because of their high nutritional value; for this reason they are often subjected to adulterations. Among the common illegal practices, the addition of powdered derivatives seems very difficult to detect because the adulterant materials have almost the same chemical composition of liquid milk. However, the high temperatures (180-200 °C) used for milk powder production could imply the occurrence of some protein modifications (e.g., glycation, lactosylation, oxidation, deamidation, dehydration). The modified proteins or peptides could then be used as markers for the presence of powdered milk. In this work, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was employed to analyze tryptic digests relevant to samples of raw liquid (without heat treatment), commercial liquid, and powdered cow's milk. Samples were subjected to two-dimensional gel electrophoresis (2-DE); differences among liquid and powder milk were detected at this stage and eventually confirmed by MALDI analysis of the in gel digested proteins. Some diagnostic peptides of powdered milk, attributed to modified whey proteins and/or caseins, were identified. Then, a faster procedure was optimized, consisting of the separation of caseins from milk whey and the subsequent in-solution digestion of the two fractions, with the advantage of obtaining almost the same information in a limited amount of time. Finally, analyses were carried out with the fast procedure on liquid milk samples adulterated with powdered milk at different percentages, and diagnostic peptides were detected down to 1% of adulteration level.

MALDI-TOF MS characterization of glycation products of whey proteins in a glucose/galactose model system and lactose-free milk

The major modifications induced by thermal treatment of whey proteins α-lactalbumin (α-La) and β-lactoglobulin (β-Lg) in a model system mimicking lactose-free milk (L(-) sugar mix) were investigated by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). The analysis of the intact α-La revealed species with up to 7 and 14 adducts from lactose and sugar mix, respectively, whereas for β-Lg 3 and up to 5 sugar moieties were observed in the case of lactose and sugar mix experiments, respectively. A partial enzymatic hydrolysis with endoproteinase AspN prior to mass spectrometric analysis allowed the detection of further modifications and their localization in the amino acid sequence. Using α-cyano-4-chlorocinnamic acid as MALDI matrix, it could be shown that heating α-La and β-Lg with glucose or galactose led to the modification of lysine residues that are not glycated by lactose. The higher glycation degree of whey proteins in a lactose-free milk system relative to normal milk with lactose reflects the higher reactivity of monosaccharides compared to the parent disaccharide. Finally, the analysis of the whey extract of a commercial lactose-free milk sample revealed that the two whey proteins were present as three main forms (native, single, and double hexose adducts).

Proteomic analysis of temperature-dependent changes in stored UHT milk

Molecular changes in milk proteins during storage of UHT-treated milk have been investigated using two-dimensional electrophoresis (2-DE) coupled to MALDI-TOF mass spectrometry. UHT-treated samples were stored at three different temperatures, 4 °C, 28 °C, and 40 °C, for two months. Three main changes could be observed on 2-DE gels following storage. They were (1) the appearance of diffuse staining regions above the position of the monomeric caseins caused by nondisulfide cross-linking of α and β-caseins; (2) the appearance of additional acidic forms of proteins, predominantly of α(S1)-casein, caused by deamidation; and (3) the appearance of "stacked spots" caused by lactosylation of whey proteins. The extent of the changes increased with increased storage temperature. Mass spectrometric analysis of in-gel tryptic digests showed that the cross-linked proteins were dominated by α(S1)-casein, but a heterogeneous population of cross-linked forms with α(S2)-casein and β-casein was also observed. Tandem MS analysis was used to confirm deamidation of N(129) in α(S1)-casein. MS analysis of the stacked spots revealed lactosylation of 9/15 lysines in β-lactoglobulin and 8/12 lysines in α-lactalbumin. More extensive analysis will be required to confirm the nature of the cross-links and additional deamidation sites in α(S1)-casein as the highly phosphorylated nature of the caseins makes them challenging prospects for MS analysis.

Liquid chromatography and mass spectrometry in food allergen detection

Food allergy is an important issue in the field of food safety because of the hazards for affected persons and the hygiene requirements and legal regulations imposed on the food industry. Consumer protection and law enforcement require suitable analytical techniques for the detection of allergens in foods. Immunological methods are currently preferred; however, confirmatory alternatives are needed. The determination of allergenic proteins by liquid chromatography and mass spectrometry has greatly advanced in recent years, and gel-free allergenomics is becoming a routinely used approach for the identification and quantitation of food allergens. The present review provides a brief overview of the principles of proteomic procedures, various chromatographic set ups, and mass spectrometry instrumentation used in allergenomics. A compendium of published liquid chromatography methods, proteomic analyses, typical marker peptides, and quantitative assays for 14 main allergy-causing foods is also included.

Proteomic quantification of disulfide-linked polymers in raw and heated bovine milk

Disulfide bond formation between milk protein molecules was quantified in raw and heated bovine milk using reducing and nonreducing two-dimensional electrophoresis. Analysis of protein profiles in raw milk indicated that 18% of alpha(S2)-casein, 25% of beta-lactoglobulin, and 46% of kappa-casein molecules were involved in disulfide-linked complexes (calculated through differences in spot volumes on two-dimensional electrophoretograms under reducing and nonreducing conditions), whereas levels of alpha(S1)- and beta-caseins were similar under both conditions. Following heat treatment at 90 degrees C for 30 min, spot volumes of serum albumin, beta-lactoglobulin, and kappa-casein decreased by 85%, 75%, and 75%, respectively, with the formation of several spots on nonreducing gels corresponding to polymers. Homopolymers and heteropolymers of kappa-casein and alpha(S2)-casein were identified by mass spectrometry in raw milk samples; polymers involving only alpha(S2)-casein or only kappa-casein accounted for 43% and 12% of the total polymers present, respectively. In addition, 45% of polymers in raw milk involved alpha(S2)-casein in association with other proteins as heteropolymers, indicating the key role of this protein in intermolecular disulfide bridging between proteins in raw milk. The intensity of monomeric kappa-casein spots decreased progressively with heating time at 90 degrees C, with greatest changes in spots with acidic isoelectric points. Interactions and association of milk proteins via disulfide bridges are discussed in relation to the proteins involved and their potential protective function against formation of fibril aggregates.

Use of reducing/nonreducing two-dimensional electrophoresis for the study of disulfide-mediated interactions between proteins in raw and heated bovine milk

The composition and interactions of proteins in bovine milk, and modifications resulting from milk storage and processing, are complex and incompletely understood. Analysis of the milk proteome can elucidate milk protein expression, structure, interaction, and modifications. Raw milk was analyzed by two-dimensional electrophoresis (isolelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) under reducing and nonreducing, or combined, conditions, followed by mass spectrometry of separated protein spots; a small number of high-abundance proteins, that is, caseins (alpha(S1)-, alpha(S2)-, beta-, kappa-, and gamma-), beta-lactoglobulin, alpha-lactalbumin, and serum albumin, represented the vast majority of protein spots on the two-dimensional electrophoretograms of raw milk samples, but some cross-linked protein complexes (mainly homopolymers of kappa-casein and alpha(S2)-casein but also some heteropolymeric complexes) were resolved under native/unheated conditions. When skim milk was heated to 90 degrees C for up to 10 min, the level of native whey proteins decreased in parallel with an increase in disulfide-linked complexes, including very complex heteropolymers, for example, casein/whey protein polymers containing multiple species. The analysis strategy used in this study reveals numerous disulfide-mediated interactions and can be proposed to analyze reduction/oxidation of milk and dairy product proteins following processing treatments applied for processing and storage.

Identification and site-specific relative quantification of beta-lactoglobulin modifications in heated milk and dairy products

During milk processing, proteins can be severely modified by oxidation, condensation, and Maillard reaction, leading to changes in their nutritional and technological properties. In this study, major modifications of beta-lactoglobulin, formed during the heating and processing of milk, were screened by mass spectrometry. For this purpose, beta-lactoglobulin was isolated from the milk samples by gel electrophoresis and analyzed by matrix-assisted laser desorption/ionization mass spectrometry after in-gel digestion with endoproteinase AspN. In heated milk, lactulosyllysine was detected at lysine 47 and 138 or 141 as well as methionine sulfoxide at methionine 7, 24, and 145. All these modifications increased gradually when raw milk was heated for 20, 40, and 60 min at 120 degrees C. The major modifications were also relatively quantified in dairy products, such as raw, high-temperature, ultra-high-temperature, sterilized, and condensed milk as well as infant formulas. The highest contents of lactulosyllysine at Lys47 were detected in powdered infant formulas, whereas lactulosyllysine at Lys138/141 was predominant in condensed milk samples. Methionine sulfoxide at Met7 and Met24 showed a trend toward higher modification rates in more severely processed products.

Determination of proteins in infant formula by high-performance liquid chromatography–electrospray tandem mass spectrometry

To determine the protein content of formula, gel electrophoresis was performed on the infant formula samples and the entire protein patterns were analyzed by nano-high performance liquid chromatography-electrospray tandem mass spectrometry (nano-HPLC/ESI/MS/MS). From the commercial infant formula profiled in this study, a total of 154 peptides, corresponding to 31 unique proteins were identified by nano-HPLC/ESI/MS/MS. Each of the identified peptides was reconfirmed by a strict integrated approach using tandem mass spectra. This protein profiling method using gel electrophoresis coupled with nano-HPLC/ESI/MS/MS and manual evaluation is a sensitive and accurate method for protein identification as well as a powerful tool for monitoring various types of food products.

Carbonylation of milk powder proteins as a consequence of processing conditions

During industrial treatments, milk proteins could be oxidatively modified, thus leading to the formation of modified/oxidised amino acid residues. The apparition of such modified residues may contribute to the formation of new immunologically reactive structures. Some of these adducts could, in an advanced stage, lead to cross-linked protein species whose proteolytic susceptibility would be drastically decreased. Such protein species, that are resistant to digestion, could also constitute major food allergens. Therefore, these oxidative protein modifications tend to increase the natural allergenicity of milk proteins. For these reasons, monitoring milk protein oxidative modifications could be very useful regarding both product quality and allergenicity issues. In the present paper, we highlight, using different analytical approaches, the preferential carbonylation of beta-lactoglobulin (beta-Lg) during industrial treatments of milk. This result is particularly interesting since native beta-Lg represents one of the major milk allergens.

Cow's milk allergens identification by two-dimensional immunoblotting and mass spectrometry

Cow's milk allergy (CMA) has become a common disease in early childhood, its prevalence ranging from 1.6% to 2.8% among children younger than 2 years of age. The role of different cow's milk protein (CMP) in the pathogenesis of CMA is still controversial. Even if the proteins most frequently and most intensively recognized by immunoglobulin E (IgE) seem to be the most abundant in milk (caseins and beta-lactoglobulin), with an although great variability all milk proteins appear to be potential allergens, even those that are present in trace amounts (i.e., lactoferrin, IgG, and BSA). In this work proteomics techniques have been applied for CMP allergens analysis. Allergens have been identified by immunoblotting following resolution of CMP components by two-dimensional electrophoresis. Sera from 20 milk-allergic subjects, as proven by oral provocation test, CAP-RAST and skin prick test, have been used for cow's milk major allergen identification. Cow's milk proteins and their isoforms were identified by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF)-mass spectrometry. In our group of patients, the prevalence of CMP allergens, i.e., the total number of subjects sensitized to CMP divided by the total number of the subjects enrolled in the study, was: 55% alpha(s1)-casein, 90% alpha(s2)-casein, 15% beta-casein, 50% kappa-casein, 45% beta-lactoglobulin, 45% BSA, 95% IgG-heavy chain, 50% lactoferrin, and 0% alpha-lactalbumin.

Solid-state glycation of beta-lactoglobulin monitored by electrospray ionisation mass spectrometry and gel electrophoresis techniques

Glycation of beta-lactoglobulin (beta-Lg) with either lactose or galactose in a solid-state medium was monitored using gel electrophoresis techniques and liquid chromatography coupled to electrospray ionisation mass spectrometry (LC/ESI-MS). The kinetics of glycation monitored by SDS polyacrylamide gel electrophoresis showed a molecular weight increase over time of the beta-Lg bands for both sugars, but no significant amounts of aggregated proteins were observed. The isoelectric point of the protein, observed by isoelectric focusing gel electrophoresis, was dramatically affected by galactosylation. LC/MS measurements of beta-Lg variants A and B, over the whole glycation reaction time, showed a larger extent of glycation with galactose (from 4 up to 22 adducts) as compared with lactose (from 0 up to 14 adducts), and confirmed that early Maillard reaction products were the main species observed. Based on the relative abundances obtained from the deconvoluted mass spectra after a 8 h 15 min incubation time at 60 degrees C, the mean values of lactose and galactose molecules bound to the protein species were calculated to be 10.4 and 17.9, and 10.5 and 18.6, for variants A and B, respectively. Furthermore, the charge state distribution data obtained by ESI-MS was studied using different methanol percentages, and indicated that adduct formation with lactose, but more significantly galactose, tends to improve the stability properties of the native protein towards denaturation.

Characterization of lactosylated proteins of infant formula powders using two-dimensional gel electrophoresis and nanoelectrospray mass spectrometry

Infant formula powders were analyzed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) to assess the whey proteins quality, which may be altered by the heat treatment used during the processing conditions. Lactosylation was found to be the major chemical modification occurring in whey proteins. In parallel, a two-dimensional (2-D) gel electrophoresis was performed on the milk sample and the entire protein patterns were analyzed by nano-ESI-MS after cutting the different gel spots and in-gel trypsin digestion. A highly selective and specific tandem MS technique has been developed to characterize and localize up to ten lactosylation sites in beta-lactoglobulin (beta-Lg) and alpha(S2)-casein. alpha-Lactalbumin (alpha-La), with five lactosylated peptides, was found to be an interesting protein marker in the milk powder sample to detect chemical modification induced by the processing/storage conditions.

Identification of caseins in goat milk

The importance of goat milk in infant diet is growing, because it is reported that goat's milk in some cases is less allergenic than cow's milk. This is due probably to the lower presence of caseins associated with a specific type of alpha(s1)-casein. In caprine breeds, four types of alpha(s1)-casein alleles are identified and associated with various amounts of this protein in milk. The contribution of strong alleles to the goat milk is approximately 3.6 g/L of alpha(s1)-casein, while for middle alleles is only 1.6 g/L, weak alleles 0.6 g/L. The contribution of null allele is very low (or non-existent). The quantity of total caseins in caprine milk is positively correlated with the amount of alpha(s1)-casein. Milk from animals possessing strong alleles contain significantly more total caseins than milk from animals without those alleles. This is important because animals with mild alleles can be employed to produce milk for allergic subjects while the other animals can be used to produce milk for the dairy industry. This work shows casein profiles of two types of classified goat milk (B, strong alpha(s1) allele, 0, null alpha(s1) allele) with two-dimensional electrophoresis coupled with matrix-assisted laser desorption/ionization-time of flight mass spectrometry, and it confirms the different polymorphisms at locus alpha(s1) casein.

Two-dimensional gel electrophoresis/matrix-assisted laser desorption/ionisation mass spectrometry of commercial bovine milk

Proteins in commercial bovine milk have been separated by two-dimensional gel electrophoresis and examined by matrix-assisted laser desorption/ionisation mass spectrometry. Gel separation was conducted in two different pH gradients, 3-10 and 6-11; the latter range resulted in a higher spot resolution and favoured the basic proteins. We have limited the time-of-flight mass spectrometry analysis to the linear mode to examine the capability of reliable relative molecular masses of the intact proteins in their characterisation. The present study draws attention to the difficulty of identifying basic proteins with low molecular masses (below 12000 Da) that are commonly encountered in milk samples.