Proteomics of major bovine milk proteins: Novel insights

Unveiling the power of proteomics in advancing tropical animal health and production

Proteomics, the large-scale study of proteins in biological systems has emerged as a pivotal tool in the field of animal and veterinary sciences, mainly for investigating local and rustic breeds. Proteomics provides valuable insights into biological processes underlying animal growth, reproduction, health, and disease. In this review, we highlight the key proteomics technologies, methodologies, and their applications in domestic animals, particularly in the tropical context. We also discuss advances in proteomics research, including integration of multi-omics data, single-cell proteomics, and proteogenomics, all of which are promising for improving animal health, adaptation, welfare, and productivity. However, proteomics research in domestic animals faces challenges, such as sample preparation variation, data quality control, privacy and ethical considerations relating to animal welfare. We also provide recommendations for overcoming these challenges, emphasizing the importance of following best practices in sample preparation, data quality control, and ethical compliance. We therefore aim for this review to harness the full potential of proteomics in advancing our understanding of animal biology and ultimately improve animal health and productivity in local breeds of diverse animal species in a tropical context.

Proteomics and Peptidomics As a Tool to Compare the Proteins and Endogenous Peptides in Human, Cow, and Donkey Milk

Cow's milk is the most widely used ingredient in infant formulas. However, its specific protein composition can cause allergic reactions. Finding alternatives to replace cow's milk and fill the nutritional gap with human milk is essential for the health of infants. Proteomic and peptidomic techniques have supported the elucidation of milk's nutritional ingredients. Recently, omics approaches have attracted increasing interest in the investigation of milk because of their high throughput, precision, sensitivity, and reproducibility. This review offers a significant overview of recent developments in proteomics and peptidomics used to study the differences in human, cow, and donkey milk. All three types of milks were identified to have critical biological functions in human health, particularly in infants. Donkey milk proteins were closer in composition to human milk, were less likely to cause allergic reactions, and may be developed as novel raw materials for formula milk powders.

Effect of pH treatment on egg white protein digestion and the peptidomics of their in vitro digests

The pH treatment significantly enhanced the functional properties of egg white protein (EWP), but little is known about the relationship between pH treatment and in vitro digestion of EWP. In this paper, we explored the effect of pH treatment (pH 2, pH 2-7, pH 12 and pH 12-7) on the digestibility of egg white protein and peptide profiling using the digestion kinetics and peptidomics methods, separately. The results implied that all pH treatment reduced the protein digestibility in gastric phase, while alkaline pH (pH 12 and pH 12-7) showed greater digestion level and more gastric peptides, and more importantly, produced a greater amount of potentially bioactive peptides than acid treated samples. Besides, the least number of potentially bioactive peptides was obtained at pH 2, but this could be improved by adjusting pH 2 back to 7. Notably, the unique bioactive peptides induced by pH were mainly relevant to DPP IV inhibitor. These differences of digestibility and peptide profiling might be attributed to the change of protein structure and the formation of molten sphere, altering cleavage sites of digestive enzymes. This work would give an enlightening insight into the digestive and nutritional characteristics of the pH-induced EWP to expand their application in the field of food and healthcare.

Digestion Profiles of Protein in Edible Pork By-Products

Edible pork by-products are widely consumed in many areas, whereas their digestion characteristics have rarely been evaluated. This work compared the digestibility of protein in boiled pork liver, heart, tripe and skin with tenderloin as a control. Cooked skin showed the highest digestibility in the simulated gastric digestion, whereas its gastric digests were less digested in the simulated intestinal stage. In contrast, cooked tripe showed the lowest gastric digestibility but relatively higher intestinal digestibility. All the edible by-products showed lower digestibility than tenderloin, especially for pork liver, in which large undigested fractions (>300 μm) could be observed. Corresponding to these results, larger amount of bigger peptides was found in the digests of pork liver and skin. In addition, peptides in tripe (average bioactive probability = 0.385) and liver digests (average bioactive probability = 0.386) showed higher average bioactive probability than other samples. Tripe digests contained the highest level of free Asp, Gln, Cys, Val, Phe, Pro, Ser, Thr, Ile and Asn, whereas heart digests contained the highest level of free Leu, Met and Arg. These results could help to reveal the nutrition value of pork by-products.

Use of MALDI-TOF MS technology to evaluate adulteration of small ruminant milk with raw bovine milk

Detection of adulteration of small ruminant milk is very important for health and commercial reasons. New analytical and cost-effective methods need to be developed to detect new adulteration practices. In this work, we aimed to explore the ability of the MALDI-TOF mass spectrometry to detect bovine milk in caprine and ovine milk using samples from 18 dairy farms. Different levels of adulteration (0.5, 1, 5, 10, 20, 40, 60, and 80%) were analyzed during the lactation period of goat and sheep (in May, from 60 to 90 d in milk, and in August, from 150 to 180 d in milk). Two different ranges of peptide-protein spectra (500-4,000 Da; 4-20 kDa) were used to establish a calibration model for predicting the concentration of adulterant using partial least squares and generalized linear model with lasso regularization. The low molecular weight part of the spectra together with the generalized linear model with lasso regularization regression model appeared to have greater potential for our aim of detection of adulteration of small ruminants' milk. The subsequent prediction model was able to predict the concentration of bovine milk in caprine milk with a root mean square error of 11.4 and 17.0% in ovine milk. The results offer compelling evidence that MALDI-TOF can detect the adulteration of small ruminants' milk. However, the method is severely limited by (1) the complexity of the milk proteome resulting from the adulteration technique, (2) the potential degradation of thermolabile proteins, and (3) the genetic variability of tested samples. Additionally, the root mean square error of prediction based only on one individual sample adulteration series can drop down to 6.34% for quantification of adulterated caprine milk and 6.28% for adulterated ovine milk for the full set of concentrations or down to 2.33 and 4.00%, respectively, if we restrict only to low concentrations of adulteration (0, 0.5, 1, 5, 10%).

Phosphorylation and glycosylation isoforms of bovine κ-casein variant E in homozygous Swedish Red cow milk detected by liquid chromatography-electrospray ionization mass spectrometry

Variations in the phosphorylation and glycosylation patterns of the common κ-casein (CN) variants A and B have been explored, whereas studies on variant E heterogeneity are scarce. This study reports for the first time the detailed phosphorylation and glycosylation pattern of the κ-CN variant E in comparison with variants A and B. Individual cow milk samples representing κ-CN genotype EE (n = 12) were obtained from Swedish Red cows, and the natural posttranslational modifications of its κ-CN were identified and quantified by liquid chromatography-electrospray mass spectrometry. In total, 12 unique isoform masses of κ-CN variant E were identified. In comparison, AA and BB milk consisted of 14 and 17 unique isoform masses, respectively. The most abundant κ-CN E isoform detected in the EE milk was the monophosphorylated, unglycosylated [1P 0G, ∼70%; where P indicates phosphorylation from single to triple phosphorylation (1-3P), and G indicates glycosylation from single to triple glycosylation (1-3G)] form, followed by diphosphorylated, unglycosylated (2P 0G, ∼12%) form, resembling known patterns from variants A and B. However, a clear distinction was the presence of the rare triphosphorylated, nonglycosylated (3P 0G, ∼0.05%) κ-CN isoform in the EE milk. All isoforms detected in variant E were phosphorylated, giving a phosphorylation degree of 100%. This is comparable with the phosphorylation degree of variants A and B, being also almost 100%, though with very small amounts of nonphosphorylated, glycosylated isoforms detected. The glycosylation degree of variant E was found to be around 17%, a bit higher than observed for variant B (around 14%), and higher than variant A (around 7%). Among glycosylation, the glycan e was the most common type identified for all 3 variants, followed by c/d (straight and branched chain trisaccharides, respectively), and b. In contrast to κ-CN variants A and B, no glycan of type a was found in variant E. Taken together, this study shows that the posttranslational modification pattern of variant E resembles that of known variants to a large extent, but with subtle differences.

High-Performance Thin-Layer Chromatography-Immunostaining as a Technique for the Characterization of Whey Protein Enrichment in Edam Cheese

Whey protein-enriched cheese can be produced by means of a high-temperature treatment of a part of the cheese milk. In this way, the nutritional quality of the resulting cheeses can be increased while resources are conserved. High-performance thin-layer chromatography-immunostaining (HPTLC-IS) using specific β-lactoglobulin (β-LG) antibodies was applied to study the implementation and stability of β-LG in two different sample sets of whey protein-enriched Edam model cheeses, including industrial-scale ones. Two methods were compared for the extraction of the proteins/peptides from the cheese samples. By applying tryptic hydrolysis directly from a suspended cheese sample instead of a supernatant of a centrifuged suspension, a better yield was obtained for the extraction of β-LG. When applying this method, it was found that selected epitopes in the tryptic β-LG peptides remain stable over the ripening period of the cheese. For four of the tryptic β-LG peptides detected by immunostaining, the amino acid sequence was identified using MALDI-TOF-MS/MS. One of the peptides identified was the semi-tryptic peptide VYVEELKPTP. A linear relationship was found between the content of this peptide in cheese and the proportion of high-heated milk in the cheese milk. β-LG enrichment factors of 1.72 (n = 3, sample set I) and 1.33 ± 0.19 (n = 1, sample set II) were determined for the cheese samples containing 30% high-heated milk compared to the non-enriched samples. The relative β-LG contents in the cheese samples with 30% high-heated milk were calculated to be 4.35% ± 0.39% (sample set I) and 9.11% ± 0.29% (sample set II) using a one-point calibration. It can be concluded that the HPTLC-IS method used is a suitable tool for the analysis of whey protein accumulation in cheese, being therefore potentially directly applicable on an industrial scale. For more accurate quantification of the whey protein content in cheese, an enhanced calibration curve needs to be applied.

The Promise and Challenges of Determining Recombinant Bovine Growth Hormone in Milk

Recombinant bovine growth hormone (rbGH) is produced in large quantities and widely used in a number of countries worldwide to stimulate milk production in dairy animals. The use of this compound in animal production is strictly regulated by food safety directives in force, in particular in the European Union (EU). Although analytical strategies for the detection of rbGH in blood have been successfully reported over the past 15 years, they do not fully answer the expectations of either competent authorities or industrials that would expect measuring its occurrence directly in the milk. As a matrix of excretion but also of consumption, milk appears indeed as the matrix of choice for detecting the use of rbGH in dairy animals. It also allows large volumes to be collected without presenting an invasive character for the animal. However, rbGH detection in milk presents several challenges, mainly related to the sensitivity required for its detection in a complex biological matrix. This review article presents the specific difficulties associated with milk and provides an overview of the analytical strategies reported in the literature and whether they concern indirect or direct approaches to the detection of rbGH administration to animals, with applications either for screening or confirmation purposes.

Identification of rare genetic variants of the αS-caseins in milk from native Norwegian dairy breeds and comparison of protein composition with milk from high-yielding Norwegian Red cows

Several factors influence the composition of milk. Among these, genetic variation within and between cattle breeds influences milk protein composition, protein heterogeneity, and their posttranslational modifications. Such variations may further influence technological properties, which are of importance for the utilization of milk into dairy products. Furthermore, these potential variations may also facilitate the production of differentiated products (e.g., related to specific breeds or specific genetic variants). The objective of this study was to investigate the genetic variation and relative protein composition of the major proteins in milk from 6 native Norwegian dairy breeds representing heterogeneity in geographical origin, using the modern Norwegian breed, Norwegian Red, as reference. In total, milk samples from 144 individual cows were collected and subjected to liquid chromatography-electrospray ionization/mass spectrometry-based proteomics for identification of genetic and posttranslational modification isoforms of the 4 caseins (α_S1-CN, α_S2-CN, β-CN, κ-CN) and the 2 most abundant whey proteins (α-lactalbumin and β-lactoglobulin). Relative quantification of these proteins and their major isoforms, including phosphorylations of α_S1-CN and glycosylation of κ-CN, were determined based on UV absorbance. The presence and frequency of genetic variants of the breeds were found to be very diverse and it was possible to identify rare variants of the CN, which, to our knowledge, have not been identified in these breeds before. Thus, α_S1-CN variant D was identified in low frequency in 3 of the 6 native Norwegian breeds. In general, α_S1-CN was found to be quite diverse between the native breeds, and the even less frequent A and C variants were furthermore detected in 1 and 5 of the native breeds, respectively. The α_S1-CN variant C was also identified in samples from the Norwegian Red cattle. The variant E of κ-CN was identified in 2 of the native Norwegian breeds. Another interesting finding was the identification of α_S2-CN variant D, which was found in relatively high frequencies in the native breeds. Diversity in more common protein genetic variants were furthermore observed in the protein profiles of the native breeds compared with milk from the high-yielding Norwegian Reds, probably reflecting the more diverse genetic background between the native breeds.

Protein Quality in Infant Formulas Marketed in Brazil: Assessments on Biodigestibility, Essential Amino Acid Content and Proteins of Biological Importance

Infant formulas, designed to provide similar nutritional composition and performance to human milk, are recommended when breastfeeding is not enough to provide for the nutritional needs of children under 12 months of age. In this context, the present study aimed to assess the protein quality and essential amino acid content of both starting (phase 1) and follow-up (phase 2) formulas from different manufacturers. The chemical amino acid score and protein digestibility corrected by the amino acid score were calculated. The determined protein contents in most formulas were above the maximum limit recommended by FAO and WHO guidelines and at odds with the protein contents declared in the label. All infant formulas contained lactoferrin (0.06 to 0.44 g·100 g⁻¹) and α-lactalbumin (0.02 to 1.34 g·100 g⁻¹) below recommended concentrations, whereas ĸ-casein (8.28 to 12.91 g·100 g⁻¹), α-casein (0.70 to 2.28 g·100 g⁻¹) and β-lactoglobulin (1.32 to 4.19 g·100 g⁻¹) were detected above recommended concentrations. Essential amino acid quantification indicated that threonine, leucine and phenylalanine were the most abundant amino acids found in the investigated infant formulas. In conclusion, infant formulas are still unconforming to nutritional breast milk quality and must be improved in order to follow current global health authority guidelines.

Identification and in silico characterization of structural and functional impacts of genetic variants in milk protein genes in the Zebu breeds Guzerat and Gyr

Whole genome sequencing of bovine breeds has allowed identification of genetic variants in milk protein genes. However, functional repercussion of such variants at a molecular level has seldom been investigated. Here, the results of a multistep Bioinformatic analysis for functional characterization of recently identified genetic variants in Brazilian Gyr and Guzerat breeds is described, including predicted effects on the following: (i) evolutionary conserved nucleotide positions/regions; (ii) protein function, stability, and interactions; (iii) splicing, branching, and miRNA binding sites; (iv) promoters and transcription factor binding sites; and (v) collocation with QTL. Seventy-one genetic variants were identified in the caseins (CSN1S1, CSN2, CSN1S2, and CSN3), LALBA, LGB, and LTF genes. Eleven potentially regulatory variants and two missense mutations were identified. LALBA Ile60Val was predicted to affect protein stability and flexibility, by reducing the number the disulfide bonds established. LTF Thr546Asn is predicted to generate steric clashes, which could mildly affect iron coordination. In addition, LALBA Ile60Val and LTF Thr546Asn affect exonic splicing enhancers and silencers. Consequently, both mutations have the potential of affecting immune response at individual level, not only in the mammary gland. Although laborious, this multistep procedure for classifying variants allowed the identification of potentially functional variants for milk protein genes.

Chemometrics for the Identification of Nitrogen and Acid Compounds in Milk-Whey as By-Products from Crescenza and Grana Padano Type Cheese-Making

Proteomics and metabolomics are analytic tools used in combination with bioinformatics to study proteins and metabolites which contribute to describing complex biological systems. The growing interest in research concerning the resolution of these systems has stimulated the development of sophisticated procedures and new applications. This paper introduces the evolution of statistical techniques for the treatment of data, suggesting the possibility to successfully characterize the milk-whey syneresis process by applying two-dimensional correlation analysis (2DCOR) to a series of CE electropherograms referring to milk-whey samples collected during cheese manufacturing. Two cheese-making processes to produce hard cheese (Grana type) and fresh cheese (Crescenza) were taken as models. The applied chemometric tools were shown to be useful for the treatment of data acquired in a systematically perturbed chemical system as a function of time.

Ruminant Milk-Derived Extracellular Vesicles: A Nutritional and Therapeutic Opportunity?

Milk has been shown to contain a specific fraction of extracellular particles that are reported to resist digestion and are purposefully packaged with lipids, proteins, and nucleic acids to exert specific biological effects. These findings suggest that these particles may have a role in the quality of infant nutrition, particularly in the early phase of life when many of the foundations of an infant's potential for health and overall wellness are established. However, much of the current research focuses on human or cow milk only, and there is a knowledge gap in how milk from other species, which may be more commonly consumed in different regions, could also have these reported biological effects. Our review provides a summary of the studies into the extracellular particle fraction of milk from a wider range of ruminants and pseudo-ruminants, focusing on how this fraction is isolated and characterised, the stability and uptake of the fraction, and the reported biological effects of these fractions in a range of model systems. As the individual composition of milk from different species is known to differ, we propose that the extracellular particle fraction of milk from non-traditional and minority species may also have important and distinct biological properties that warrant further study.

Proteomic Advances in Milk and Dairy Products

Proteomics is a new area of study that in recent decades has provided great advances in the field of medicine. However, its enormous potential for the study of proteomes makes it also applicable to other areas of science. Milk is a highly heterogeneous and complex fluid, where there are numerous genetic variants and isoforms with post-translational modifications (PTMs). Due to the vast number of proteins and peptides existing in its matrix, proteomics is presented as a powerful tool for the characterization of milk samples and their products. The technology developed to date for the separation and characterization of the milk proteome, such as two-dimensional gel electrophoresis (2DE) technology and especially mass spectrometry (MS) have allowed an exhaustive characterization of the proteins and peptides present in milk and dairy products with enormous applications in the industry for the control of fundamental parameters, such as microbiological safety, the guarantee of authenticity, or the control of the transformations carried out, aimed to increase the quality of the final product.

New applications of advanced instrumental techniques for the characterization of food allergenic proteins

Current approaches based on electrophoretic, chromatographic or immunochemical principles have allowed characterizing multiple allergens, mapping their epitopes, studying their mechanisms of action, developing detection and diagnostic methods and therapeutic strategies for the food and pharmaceutical industry. However, some of the common structural features related to the allergenic potential of food proteins remain unknown, or the pathological mechanism of food allergy is not yet fully understood. In addition, it is also necessary to evaluate new allergens from novel protein sources that may pose a new risk for consumers. Technological development has allowed the expansion of advanced technologies for which their whole potential has not been entirely exploited and could provide novel contributions to still unexplored molecular traits underlying both the structure of food allergens and the mechanisms through which they sensitize or elicit adverse responses in human subjects, as well as improving analytical techniques for their detection. This review presents cutting-edge instrumental techniques recently applied when studying structural and functional aspects of proteins, mechanism of action and interaction between biomolecules. We also exemplify their role in the food allergy research and discuss their new possible applications in several areas of the food allergy field.

The Impact of Seasonality in Pasture-Based Production Systems on Milk Composition and Functionality

Seasonal calving, pasture-based dairy systems are widely practiced in countries with a temperate climate and plentiful rainfall such as Ireland and New Zealand. This approach maximizes milk production from pasture and, consequently, is a low-cost, low-input dairy production system. On the other hand, the majority of global milk supply is derived from high input indoor total mixed ration systems where seasonal calving is not practiced due to the dependence on ensiled silages, grains and concentrated feeds, which are available year-round. Synchronous changes in the macro and micronutrients in milk are much more noticeable as lactation progresses through early, mid and late stages in seasonal systems compared to non-seasonal systems-which can have implications on the processability and functionality of milk.

Development of an All-in-One Protein Digestion Platform Using Sorbent-Attached Membrane Funnel-Based Spray Ionization Mass Spectrometry

In this work, the sorbent-attached microfunnels used in funnel-based spray ionization mass spectrometry were evaluated for the all-in-one digestion of proteins. Sorbent materials, including C₁₈ and TiO₂ powders, were used as substrates to support in-funnel digestion and subsequent solid-phase extraction and purification of the digested products. In-funnel digestion protocols with and without reductive alkylation were developed for the analysis of proteins with and without disulfide linkages. Compared with in-solution digestion of the same loadings, the sequence coverage of in-funnel digestion of ovalbumin (with one disulfide bond) and ovocystatin (with two disulfide bonds) increased from 36% to 65% and from 21% to 81%, respectively. Loading 100 fmol of ovalbumin was sufficient to generate detectable tryptic fragments on C₁₈-attached funnels. Notably, some phosphorylated digestion fragments were solely detected on C₁₈-attached funnels and some nonphosphorylated digestion fragments were detected only on TiO₂-attached funnels. Complex biological protein mixtures (i.e., bovine milk) and mouse liver protein extract could also be digested on C₁₈- and TiO₂-attached funnels. Using this platform, 30 samples were digested at the same time with enhanced digestion efficiency and were analyzed by funnel-based spray ionization mass spectrometry. This approach is potentially useful for sensitive and high-throughput bottom-up proteomic studies of complex biological samples.

Optimization of Protein Extraction Method for 2DE Proteomics of Goat's Milk

Two-dimensional electrophoretic (2DE)-based proteomics remains a powerful tool for allergenomic analysis of goat’s milk but requires effective extraction of proteins to accurately profile the overall causative allergens. However, there are several current issues with goat’s milk allergenomic analysis, and among these are the absence of established standardized extraction method for goat’s milk proteomes and the complexity of goat’s milk matrix that may hamper the efficacy of protein extraction. This study aimed to evaluate the efficacies of three different protein extraction methods, qualitatively and quantitatively, for the 2DE-proteomics, using milk from two commercial dairy goats in Malaysia, Saanen, and Jamnapari. Goat’s milk samples from both breeds were extracted by using three different methods: a milk dilution in urea/thiourea based buffer (Method A), a triphasic separation protocol in methanol/chloroform solution (Method B), and a dilution in sulfite-based buffer (Method C). The efficacies of the extraction methods were assessed further by performing the protein concentration assay and 1D and 2D SDS-PAGE profiling, as well as identifying proteins by MALDI-TOF/TOF MS/MS. The results showed that method A recovered the highest amount of proteins (72.68% for Saanen and 71.25% for Jamnapari) and produced the highest number of protein spots (199 ± 16.1 and 267 ± 10.6 total spots for Saanen and Jamnapari, respectively) with superior gel resolution and minimal streaking. Six milk protein spots from both breeds were identified based on the positive peptide mass fingerprinting matches with ruminant milk proteins from public databases, using the Mascot software. These results attest to the fitness of the optimized protein extraction protocol, method A, for 2DE proteomic and future allergenomic analysis of the goat’s milk.

Peptidomic Investigation of the Interplay between Enzymatic Tenderization and the Digestibility of Beef Semimembranosus Proteins

This work investigated the influence of enzymatic tenderization on digestibility changes of beef semimembranosus proteins using peptidomics methods. Hydrolysis by proteinase K and bromelain elevated the average bitterness index of identified peptides by generating high-Q values peptides (1714-1790 Cal/mol), including KDLFDPIIQ, LIDDHFLFDKPVSPL, and QLIDDHFLFDKPVSPLLL. Proteolysis during enzymatic tenderization acted as a "pre-digestion" step and significantly elevated the degree of hydrolysis of beef protein (by 4.5-17.3%) in subsequent simulated gastrointestinal digestion. Peptidomics analysis of digests revealed large variations in the peptide composition, which was positively correlated with the degree of proteolysis during enzymatic tenderization. Enzymatic tenderization with proteinase K- (for 0.5 h) or bromelain-treated samples largely increased the survival rate (by 65.5 or 82.8%) of peptides during simulated digestion, possibly because of the "secondary enzyme-substrate interaction" effect. This work could provide a new sight into the possible influence of enzymatic tenderization on meat nutrition.

Interplay between Residual Protease Activity in Commercial Lactases and the Subsequent Digestibility of β-Casein in a Model System

One of the conventional ways to produce lactose-hydrolyzed (LH) milk is via the addition of commercial lactases into heat-treated milk in which lactose is hydrolyzed throughout storage. This post-hydrolysis method can induce proteolysis in milk proteins due to protease impurities remaining in commercial lactase preparations. In this work, the interplay between lactose hydrolysis, proteolysis, and glycation was studied in a model system of purified β-casein (β-CN), lactose, and lactases using peptidomic methods. With a lactase presence, the proteolysis of β-CN was found to be increased during storage. The protease side-activities mainly acted on the hydrophobic C-terminus of β-CN at Ala, Pro, Ile, Phe, Leu, Lys, Gln, and Tyr positions, resulting in the formation of peptides, some of which were N-terminal glycated or potentially bitter. The proteolysis in β-CN incubated with a lactase was shown to act as a kind of “pre-digestion”, thus increasing the subsequent in vitro digestibility of β-CN and drastically changing the peptide profiles of the in vitro digests. This model study provides a better understanding of how the residual proteases in commercial lactase preparations affect the quality and nutritional aspects of β-CN itself and could be related to its behavior in LH milk.

Digestibility of glycated milk proteins and the peptidomics of their in vitro digests

BACKGROUND

Milk proteins are widely used in food production and are often glycated by reducing sugar. Although many studies have reported the digestibility of glycated milk protein, most have focused on measuring degree of hydrolysis (DH), showing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) image of digests. Detailed information on the changes in peptide composition of digests has seldom been revealed. Therefore, in addition to measuring the DH and showing the SGS-PAGE images of digests, we also analyzed the peptidomics in digests using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) and Mascot database in this work to further reveal the influence of glycation on protein nutrition.

RESULTS

Compared with β-lactoglobulin and bovine serum albumin (BSA), DH of β-casein was suppressed to a lesser extent by glycation in both gastric and intestinal stages. Aggregates of glycated BSA were less sensitive to the action of digestive enzymes throughout gastrointestinal digestion according to SDS-PAGE images. Changes in the peptide composition of digests induced by glycation were distinctly displayed, showing both absence of peptides and occurrence of new peptides, based on the results obtained from LC-ESI-MS/MS.

CONCLUSIONS

Glycation can greatly change the peptide composition in digests of milk protein. The nutritional impact of the change in the peptide composition requires further investigation, and the impact of MRPs in unabsorbed digests on the gut flora should be an interesting field for further studies. © 2018 Society of Chemical Industry.

Proteomics and microstructure profiling of goat milk protein after homogenization

This study investigated the protein changes in goat milk during the homogenization process using label-free quantification. We quantified 310 and 315 proteins in the control group (CG) and homogenized group (HG), respectively, and 16 proteins were significantly different between the 2 groups. For HG, the goat milk protein particle sizes were smaller and more evenly distributed and exhibited an increase in the regular arrangement of the secondary structures. Proteomics analysis verified that xanthine dehydrogenase and asparaginase-like 1 expression in CG were higher than in HG, whereas the opposite was observed for fructose-bisphosphate aldolase, κ-casein, and β-casein. Significant changes were found in the homogenization-treated goat milk proteome that were related to goat milk glycolysis/gluconeogenesis metabolism. This work provides updated information on the current proteome characteristics of homogenized goat milk, which may be important for applying the protein component of goat milk to human nutrition and health.

Discovery and Quantification of Nonhuman Proteins in Human Milk

The question whether and which nonhuman peptides or proteins are present in human milk was raised many decades ago. However, due to cross-reactivity or nonspecific antibody recognition, the accuracy of detection by immunochemical methods has been a concern. Additionally, the relative low-abundance of nonhuman peptides/proteins in the complex milk sample makes them a challenging target to detect. Here, by deep proteome profiling, we detected several nonhuman peptides, which could be grouped as nonhuman proteins. We next estimated their concentration in human milk by combining data-dependent shotgun proteomics and parallel reaction monitoring. First, we fractionated human milk at the protein level and were able to detect 1577 human proteins. Additionally, we identified 109 nonhuman peptides, of which 71 were grouped into 36 nonhuman proteins. In the next step, we targeted 37 nonhuman peptides and nine of them could be repeatedly quantified in human milk samples. Peptides/proteins originating from bovine milk products were the dominant nonhuman proteins observed, notably bovine caseins (α-S1-, α-S2-, β-, κ-caseins) and β-lactoglobulin. The method we present here can be expanded to investigate more about nonhuman peptides and proteins in human milk and give a better understanding of how human milk plays a role in allergy prevention.

Symposium review: Characterization of the bovine milk protein profile using proteomic techniques

Identification and characterization of the comprehensive bovine milk proteome has historically been limited due to the dichotomy of protein abundances within milk. The high abundance of a select few proteins, including caseins, α-lactalbumin, β-lactoglobulin, and serum albumin, has hindered intensive identification and characterization of the vast array of low-abundance proteins in milk due to limitations in separation techniques and protein labeling capacity. In more recent years, the development and advancement of proteomics techniques have yielded valuable tools for characterization of the protein profile in bovine milk. More extensive fractionation and enrichment techniques, including the use of combinations of precipitation techniques, immunosorption, gel electrophoresis, chromatography, ultracentrifugation, and hexapeptide-based binding enrichment, have allowed for better isolation of lower abundance proteins for further downstream liquid chromatography-tandem mass spectrometry approaches. The different milk subfractions isolated during these processes can also be analyzed as individual entities to assess the protein profile unique to the different fractions-for instance, investigation of the skim milk-associated proteome versus the milk fat globule membrane-associated proteome. Updates to high-throughput methods, equipment, and software have also allowed for greater interpretation and visualization of the data. For instance, labeling techniques have enabled analysis of multiplexed samples and more accurate comparison of specific protein abundances and quantities across samples, and integration of gene ontology analysis has allowed for a more in-depth and visual representation of potential relationships between identified proteins. Inclusively, these developments in proteomic techniques have allowed for a rapid increase in the number of milk-associated proteins identified and a better grasp of the relationships and potential functionality of the proteins within the milk proteome.

Enhancing bioactive peptide release and identification using targeted enzymatic hydrolysis of milk proteins

Milk proteins have been extensively studied for their ability to yield a range of bioactive peptides following enzymatic hydrolysis/digestion. However, many hurdles still exist regarding the widespread utilization of milk protein-derived bioactive peptides as health enhancing agents for humans. These mostly arise from the fact that most milk protein-derived bioactive peptides are not highly potent. In addition, they may be degraded during gastrointestinal digestion and/or have a low intestinal permeability. The targeted release of bioactive peptides during the enzymatic hydrolysis of milk proteins may allow the generation of particularly potent bioactive hydrolysates and peptides. Therefore, the development of milk protein hydrolysates capable of improving human health requires, in the first instance, optimized targeted release of specific bioactive peptides. The targeted hydrolysis of milk proteins has been aided by a range of in silico tools. These include peptide cutters and predictive modeling linking bioactivity to peptide structure [i.e., molecular docking, quantitative structure activity relationship (QSAR)], or hydrolysis parameters [design of experiments (DOE)]. Different targeted enzymatic release strategies employed during the generation of milk protein hydrolysates are reviewed herein and their limitations are outlined. In addition, specific examples are provided to demonstrate how in silico tools may help in the identification and discovery of potent milk protein-derived peptides. It is anticipated that the development of novel strategies employing a range of in silico tools may help in the generation of milk protein hydrolysates containing potent and bioavailable peptides, which in turn may be used to validate their health promoting effects in humans. Graphical abstract The targeted enzymatic hydrolysis of milk proteins may allow the generation of highly potent and bioavailable bioactive peptides.

Effect of Storage on Lactase-Treated β-Casein and β-Lactoglobulin with Respect to Bitter Peptide Formation and Subsequent in Vitro Digestibility

Using active lactose to hydrolyze lactose during storage is a common process to produce lactose-hydrolyzed (LH) milk. Proteolysis induced by residual proteases in commercial lactase was studied in a system using purified β-casein or β-lactoglobulin during a 60-day storage period at 22 or 38 °C. The proteolysis of β-casein by residual proteases occurred more extensively than that of β-lactoglobulin. Peptidomic analysis by LC-ESI-MS/MS revealed that Ile, Leu, Tyr, and Phe residues near the C-terminus of β-casein were the main sites of cleavage by the residual proteases, generating assumed bitter peptides. In the subsequent in vitro digestion study, proteolysis during storage was shown to greatly affect the subsequent digestibility of β-casein, leading to an elevated degree of hydrolysis and the formation of new digested peptides. This study highlights the potential influence of residual proteases in commercial lactase on the storage quality and digestibility of LH milk containing active lactase.