Proteomic characterization of intermediate and advanced glycation end-products in commercial milk samples

Comparative analysis of proteomics and transcriptomics reveals novel mechanism underlying the antibacterial activity and immune-enhancing properties of horse milk

Background

Horse milk is a highly valuable organic food that is a promising alternative to cow milk, exhibiting plenty of healthy and immune benefits to human. However, identification of proteins associated human wellness and underlying molecular mechanism in horse milk remain unclear.

Methodology

Label-free mass spectrometry-based protein quantification technology was employed to investigate protein composition of animal milk, including cow, goat, camel and horse milk. Prokaryotic expression and disk diffusion assay were applied to acquire and evaluate in vitro antimicrobial activity of candidate proteins. RAW264.7 macrophage model cell line was used to validate effect of proteins on cytotoxicity, apoptosis and immune induction. ROS probe detected cell ROS change and RT-qPCR verified expression of immune response genes induced by proteins. Microscopy was used to observe the effects of protein on the morphological characteristics of bacteria, further transcriptome analysis was performed to investigate transcriptional changes of bacteria induced by candidate proteins.

Results

A total of 1,335 proteins was identified in cow, goat, camel and horse milk. GO enrichment analysis showed that the proteins related to protein degradation were highly expressed in horse milk compared to other three types of milk, contributing to easier assimilation and palatability. KEGG analysis showed that horse milk contained abundant antimicrobial associated proteins relevant to pathogenic bacterial resistance, leading to the decreased risk of pathogenic diseases. A higher accumulation of proteins associated with caffeine metabolism, amino acid biosynthesis, and glycolysis/gluconeogenesis in horse milk contributes to its distinctive flavor. Notably, highly expressed proteins in horse milk were closely linked to immune signaling pathways, functioning as immune modulators. Importantly, we identified four highly expressed antimicrobial associated proteins in horse milk including LPO, B2M, CD14 and PGL, among them, PGL functioned dually by in vitro antibacterial activity and immune activation. Further transcriptome analysis demonstrated that PGL exerted significant transcriptional changes to bacteria. Enrichment analysis showed PGL could inhibit growth of P. aeruginosa and E. coli by repressing the biosynthesis of secondary metabolites.

Conclusion

Comparative proteomics revealed immune enhancement and nutrient composition of horse milk compared to cow, goat and camel milk. Identification of PGL showed antibacterial activity and potential medicinal value.

Protein-bound and free glycation compounds in human milk: A comparative study with minimally processed infant formula and pasteurized bovine milk

The role of the Maillard reaction and the accumulation of non-enzymatic glycation compounds in human milk have been scarcely considered. In this study, we investigated the proteins most susceptible to glycation, the identity of the corresponding modified residues and the quantitative relationship between protein-bound and free glycation compounds in raw human milk and, for comparison, in minimally processed infant formula and pasteurized bovine milk. In human milk, total protein-bound lysine modifications were up to 10% of the counterparts in infant formula, while Nε-carboxymethyllysine reached up to 27% of the concentration in the other two products. We demonstrated that the concentration of free pyrraline and methylglyoxal-hydroimidazolone were of the same order of magnitude in the three milk types. Our results delineate how the occurrence of some glycation compounds in human milk can be an unavoidable part of the breastfeeding and not an exclusive attribute of infant formulas and pasteurized bovine milk.

Effects of spray drying and freeze drying on the protein profile of whey protein concentrate

Whey protein concentrate (WPC) is consumed for its high protein content. The structure and biological functionality of whey proteins in WPC powders may be affected by the drying technique applied. However, the specific impact of spray drying and freeze drying on the overall protein profile of whey protein derived from sweet whey streams at scale is unknown. Herein, we examine the effects of commercial-scale freeze drying and spray drying on WPC to determine which method better preserves bioactive whey proteins, with the goal of helping the dairy industry create high-value products that meet the growing consumer demand for functional dairy products. WPCs were produced from pasteurized liquid whey using either a commercial spray dryer or freeze dryer. A variety of analytical techniques, including enzyme-linked immunosorbent assay, polyacrylamide gel electrophoresis, and bottom-up proteomics using liquid chromatography-tandem mass spectroscopy were used to identify, quantify, and compare the retention of bioactive proteins in WPC before and after spray drying and freeze drying. In addition, the extent of denaturation was studied via solubility testing, differential scanning calorimetry, and hydrophobicity assessment. There was little to no difference in the retention or denaturation of key bioactive proteins between spray-dried and freeze-dried WPC powders. There was a higher percentage of select Maillard modifications in freeze-dried and spray-dried powders than in the control. The lack of significant differences between spray drying and freeze drying identified herein indicates that freeze drying does not meaningfully improve retention of bioactive proteins compared with spray drying when performed after multiple pasteurization steps. PRACTICAL APPLICATION: This study aimed to provide insight into the impacts of spray drying versus freeze drying on whey proteins. Overall, our results indicate that for commercial dairy processing that involves multiple rounds of pasteurization, freeze drying does not meaningfully improve the retention of bioactive proteins compared with spray drying. These findings may help the food and dairy industry make informed decisions regarding the processing of its whey protein products to optimize nutritional value.

Estimation of Advanced Glycation End Products in Selected Foods and Beverages by Spectrofluorimetry and ELISA

Advanced glycation end products (AGEs) are formed within the body as a part of normal metabolism and are also the by-products of cooking food. The elevated levels of AGEs in the body are considered pathogenic. The modern diets contain high levels of AGEs which are getting incorporated into the body AGEs pool and contribute to post-diabetic and age-related complications. The objective of the present study is to estimate the cross-linked AGEs (AGE-fluorescence) and the more stable carboxymethyl-lysine (CML) by spectrofluorimetry and ELISA in 58 kinds of foods in India. It was evident from the results that the foods cooked at higher temperatures showed high levels of AGEs. Among the studied foods, the highest fluorescence was observed in Biscuits 2 (362 AU), and the highest level of carboxymethyl lysine (CML) was found in Soya milk (659.3 ng/g). However, there was less correlation between the AGE-fluorescence and the CML content of the food samples. Processed food such as tomato sauce, chilli sauce, and cheese, along with western foods like chicken nuggets, pizza, and biscuits like Biscuits 2, are known to contain high levels of AGEs. In the present study a preliminary database of AGE-fluorescence and CML content of 58 foods was developed, which is the first attempt among Indian foods. Furthermore, elaborated database can be developed including maximum consumed foods in India which will help in suggesting a better diet for the diabetic population.

Phenotypical and biochemical characterization of tomato plants treated with triacontanol

Biostimulants are heterogeneous products designed to support plant development and to improve the yield and quality of crops. Here, we focused on the effects of triacontanol, a promising biostimulant found in cuticle waxes, on tomato growth and productivity. We examined various phenological traits related to vegetative growth, flowering and fruit yield, the metabolic profile of fruits, and the response of triacontanol-treated plants to salt stress. Additionally, a proteomic analysis was conducted to clarify the molecular mechanisms underlying triacontanol action. Triacontanol application induced advanced and increased blooming without affecting plant growth. Biochemical analyses of fruits showed minimal changes in nutritional properties. The treatment also increased the germination rate of seeds by altering hormone homeostasis and reduced salt stress-induced damage. Proteomics analysis of leaves revealed that triacontanol increased the abundance of proteins related to development and abiotic stress, while down-regulating proteins involved in biotic stress resistance. The proteome of the fruits was not significantly affected by triacontanol, confirming that biostimulation did not alter the nutritional properties of fruits. Overall, our findings provide evidence of the effects of triacontanol on growth, development, and stress tolerance, shedding light on its mechanism of action and providing new insights into its potential in agricultural practices.

Food allergen detection by mass spectrometry: From common to novel protein ingredients

Food allergens are molecules, mainly proteins, that trigger immune responses in susceptible individuals upon consumption even when they would otherwise be harmless. Symptoms of a food allergy can range from mild to acute; this last effect is a severe and potentially life-threatening reaction. The European Union (EU) has identified 14 common food allergens, but new allergens are likely to emerge with constantly changing food habits. Mass spectrometry (MS) is a promising alternative to traditional antibody-based assays for quantifying multiple allergenic proteins in complex matrices with high sensitivity and selectivity. Here, the main allergenic proteins and the advantages and drawbacks of some MS acquisition protocols, such as multiple reaction monitoring (MRM) and data-dependent analysis (DDA) for identifying and quantifying common allergenic proteins in processed foodstuffs are summarized. Sections dedicated to novel foods like microalgae and insects as new sources of allergenic proteins are included, emphasizing the significance of establishing stable marker peptides and validated methods using database searches. The discussion involves the in-silico digestion of allergenic proteins, providing insights into their potential impact on immunogenicity. Finally, case studies focussing on microalgae highlight the value of MS as an effective analytical tool for ensuring regulatory compliance throughout the food control chain.

The effect of lactose and its isomerization product lactulose on functional and structural properties of glycated casein

Lactulose is an isomer of lactose, formed under thermal processing of milk. Alkaline conditions favor the isomerization of lactose. As reducing sugar, lactose and lactulose could participate in the Maillard reaction and cause protein glycation in milk products. In this study, the influence of lactose and lactulose on the functional and structural properties of glycated casein was investigated. The results demonstrated that compared with lactose, lactulose led to severer changes in molecular weight, more disordered spatial structure and decrease of tryptophan fluorescence intensity of casein. Besides, the glycation degree and advanced glycation end products (AGEs) results suggested that lactulose exhibited stronger glycation ability than lactose due to the higher proportion of open chain in solution. Furthermore, higher glycation degree induced by lactulose resulted in lower solubility, surface hydrophobicity, digestibility and emulsifying capacity of casein-glycoconjugates compared with lactose. The results of this study are essential for tracking the effects of harmful Maillard reaction products on the quality of milk and dairy products.

Investigation of the Structure and Allergic Potential of Whey Protein by Both Heating Sterilization and Simulation with Molecular Dynamics

As the main allergens in milk, whey proteins are heat-sensitive proteins and are widespread in dairy products and items in which milk proteins are involved as food additives. The present work sought to investigate the effect of heating sterilization on the allergenicity of α-lactalbumin (α-LA) and β-lactoglobulin (β-LG), the main composite and allergen in whey protein isolate (WPI), by combining molecular dynamics with experimental techniques for detecting the spatial structure and IgE binding capacity. The structure of WPI was basically destroyed at heat sterilization conditions of 95 °C for 5 min and 65 °C for 30 min by SDS-PAGE analysis and spectroscopic analysis. In addition, α-lactalbumin (α-LA) may be more sensitive to temperature, resulting in exposure to allergic epitopes and increasing the allergic potential, while the binding capacity of β-lactoglobulin (β-LG) to IgE was reduced under 65 °C for 30 min. By the radius of gyration (Rg) and root-mean-square deviation (RMSD) plots calculated in molecular dynamics simulations, α-LA was less structurally stable at 368 K, while β-LG remained stable at higher temperatures, indicating that α-LA was more thermally sensitive. In addition, we observed that the regions significantly affected by temperatures were associated with the capacity of allergic epitopes (α-LA 80-101 and β-LG 82-93, 105-121) to bind IgE through root-mean-standard fluctuation (RMSF) plots, which may influence the two major allergens. We inferred that these regions are susceptible to structural changes after sterilization, thus affecting the allergenicity of allergens.

Ultra-High Temperature Treatment and Storage of Infant Formula Induces Dietary Protein Modifications, Gut Dysfunction, and Inflammation in Preterm Pigs

SCOPE

Ready-to-feed liquid infant formula is increasingly used for preterm infants when human milk is unavailable. These formulas are sterilized by ultra-high temperature treatment, but heating and storage may reduce bioactivity and increase formation of Maillard reaction products with potential negative consequences for immature newborns.

METHODS AND RESULTS

Using preterm pigs as a model for sensitive newborn infants, the study tests the intestinal responses of feeding experimental liquid formula within 5 days. A pasteurized formula (PAST) with the same nutrient composition but less protein modifications serves as control to ultra-high temperature-treated formula without (UHT) and with prolonged storage (SUHT). Relative to PAST, UHT contains lower levels of lactoferrin and IgG. Additional storage (40 °C, 60 days, SUHT) reduces antimicrobial capacity and increases non-reducible protein aggregates and Maillard reaction products (up to 13-fold). Pigs fed SUHT have more diarrhea and show signs of intestinal inflammation (necrotizing enterocolitis) compared with pigs fed PAST and UHT. These clinical effects are accompanied by accumulation of Maillard reaction products, protein cross-links, and inflammatory responses in the gut.

CONCLUSION

The results demonstrate that feeding UHT infant formulas, particularly after prolonged storage, adversely affects gut maturation and function in preterm pigs used as a model of preterm infants.

Cross-linking reactions in food proteins and proteomic approaches for their detection

Various protein cross-linking reactions leading to molecular polymerization and covalent aggregates have been described in processed foods. They are an undesired side effect of processes designed to reduce bacterial load, extend shelf life, and modify technological properties, as well as being an expected result of treatments designed to modify raw material texture and function. Although the formation of these products is known to affect the sensory and technological properties of foods, the corresponding cross-linking reactions and resulting protein polymers have not yet undergone detailed molecular characterization. This is essential for describing how their generation can be related to food processing conditions and quality parameters. Due to the complex structure of cross-linked species, bottom-up proteomic procedures developed to characterize various amino acid modifications associated with food processing conditions currently offer a limited molecular description of bridged peptide structures. Recent progress in cross-linking mass spectrometry for the topological characterization of protein complexes has facilitated the development of various proteomic methods and bioinformatic tools for unveiling bridged species, which can now also be used for the detailed molecular characterization of polymeric cross-linked products in processed foods. We here examine their benefits and limitations in terms of evaluating cross-linked food proteins and propose future scenarios for application in foodomics. They offer potential for understanding the protein cross-linking formation mechanisms in processed foods, and how the inherent beneficial properties of treated foodstuffs can be preserved or enhanced.

Deciphering Changes in the Structure and IgE-Binding Ability of Ovalbumin Glycated by α-Dicarbonyl Compounds under Simulated Heating

As a complex reaction, biological consequences of the Maillard reaction (MR) on dietary proteins need to be deciphered. Despite previous studies on the structural and antigenic properties of ovalbumin (OVA) by MR, associated changes induced by specific MR intermediates and their downstream products are largely unknown. This study focused on the impacts of glycation by α-dicarbonyl compounds (α-DCs), intermediates of MR and precursors of advanced glycation end-products (AGEs), on the structural and IgE-binding properties of ovalbumin (OVA) under simulated heating. Methylglyoxal (MGO), glyoxal (GO), and butanedione (BU) were selected as typical α-DCs to generate glycated OVA with different AGE-modifications (AGE-Ms). The results showed that reactions between OVA and α-DCs generated OVA-AGE with various degrees of modification and conformational unfolding, and the reactivity of α-DCs followed the order GO > MGO > BU. Depending on the precursor type, the levels of 10 specific AGEs were verified, and the amounts of total AGEs increased with heating temperature and α-DC dosage. Compared to native OVA, glycated OVA showed reduced IgE-binding levels but with sRAGE-binding ligands, the extent of which was associated with the contents of total AGEs and Nε-carboxymethyllysine, and changes in certain protein conformational structures. High-resolution mass spectrometry further identified different AGE-Ms on the Lys and Arg residues of OVA, confirming variations in the glycation sites and their associations with the immunoreactive epitopes of OVA under different conditions.

Early AGEing and metabolic diseases: is perinatal exposure to glycotoxins programming for adult-life metabolic syndrome?

Perinatal early nutritional disorders are critical for the developmental origins of health and disease. Glycotoxins, or advanced glycation end-products, and their precursors such as the methylglyoxal, which are formed endogenously and commonly found in processed foods and infant formulas, may be associated with acute and long-term metabolic disorders. Besides general aspects of glycotoxins, such as their endogenous production, exogenous sources, and their role in the development of metabolic syndrome, we discuss in this review the sources of perinatal exposure to glycotoxins and their involvement in metabolic programming mechanisms. The role of perinatal glycotoxin exposure in the onset of insulin resistance, central nervous system development, cardiovascular diseases, and early aging also are discussed, as are possible interventions that may prevent or reduce such effects.

Effect of methylglyoxal on the alteration in structure and digestibility of α-lactalbumin, and the formation of advanced glycation end products under simulated thermal processing

α-Dicarbonyl compounds (α-DCs) are a class of compounds generated during the thermal processing of food. Due to the high reactivity, α-DCs were endowed with the ability to react with food components thus lowering nutrition value and even leading to a potential risk for food safety. In this study, methylglyoxal (MG), the most abundant α-DCs, was selected to investigate the alteration effects on the structure and digestibility of α-lactalbumin (αLA) under thermal processing (60-100°C). The results showed that the modification degree of αLA by MG increased with the rise of processing temperature, accompanied by the significant changes in molecular weight, intrinsic fluorescence, and secondary structures of αLA. High-resolution mass spectrometry analysis identified that lysine (Lys) and arginine (Arg) are the modification sites, and Nε-(carboxyethyl)-L-lysine is the main modification type. Since the Lys and Arg are also the cleavage sites of trypsin, the digestibility of MG modified αLA (MG-αLA) by trypsin correspondingly decreased with an increase of processing temperature. The reacted Lys and Arg residues, and the protein-bound AGEs were quantified, and the contents were found to be highly dependent on the temperature.

Adverse Effects of Thermal Food Processing on the Structural, Nutritional, and Biological Properties of Proteins

Thermal processing is one of the most important processing methods in the food industry. However, many studies have revealed that thermal processing can have detrimental effects on the nutritional and functional properties of foods because of the complex interactions among food components. Proteins are essential nutrients for humans, and changes in the structure and nutritional properties of proteins can substantially impact the biological effects of foods. This review focuses on the interactions among proteins, sugars, and lipids during thermal food processing and the effects of these interactions on the structure, nutritional value, and biological effects of proteins. In particular, the negative effects of modified proteins on human health and strategies for mitigating these detrimental effects from two perspectives, namely, reducing the formation of modified proteins during thermal processing and dietary intervention in vivo, are discussed.

Comprehensive Profiling of the Native and Modified Peptidomes of Raw Bovine Milk and Processed Milk Products

Bovine milk contains a variety of endogenous peptides, partially formed by milk proteases that may exert diverse bioactive functions. Milk storage allows further protease activities altering the milk peptidome, while processing, e.g., heat treatment can trigger diverse chemical reactions, such as Maillard reactions and oxidations, leading to different posttranslational modifications (PTMs). The influence of processing on the native and modified peptidome was studied by analyzing peptides extracted from raw milk (RM), ultra-high temperature (UHT) milk, and powdered infant formula (IF) by nano reversed-phase liquid chromatography coupled online to electrospray ionization (ESI) tandem mass spectrometry. Only unmodified peptides proposed by two independent software tools were considered as identified. Thus, 801 identified peptides mainly originated from α_S- and β-caseins, but also from milk fat globular membrane proteins, such as glycosylation-dependent cell adhesion molecule 1. RM and UHT milk showed comparable unmodified peptide profiles, whereas IF differed mainly due to a higher number of β-casein peptides. When 26 non-enzymatic posttranslational modifications (PTMs) were targeted in the milk peptidomes, 175 modified peptides were identified, i.e., mostly lactosylated and a few hexosylated or oxidized peptides. Most modified peptides originated from α_S-caseins. The numbers of lactosylated peptides increased with harsher processing.

Profiling of Low-Molecular-Weight Carbonyls and Protein Modifications in Flavored Milk

Thermal treatments of dairy products favor oxidations, Maillard reactions, and the formation of sugar or lipid oxidation products. Additives including flavorings might enhance these reactions or even induce further reactions. Here we aimed to characterize protein modifications in four flavored milk drinks using samples along the production chain—raw milk, pasteurization, mixing with flavorings, heat treatment, and the commercial product. Therefore, milk samples were analyzed using a bottom up proteomics approach and a combination of data-independent (MS^E) and data-dependent acquisition methods (DDA). Twenty-one small carbonylated lipids were identified by shotgun lipidomics triggering 13 protein modifications. Additionally, two Amadori products, 12 advanced glycation end products (AGEs), and 12 oxidation-related modifications were targeted at the protein level. The most common modifications were lactosylation, formylation, and carboxymethylation. The numbers and distribution of modification sites present in raw milk remained stable after pasteurization and mixing with flavorings, while the final heat treatment significantly increased lactosylation and hexosylation in qualitative and quantitative terms. The processing steps did not significantly affect the numbers of AGE-modified, oxidized/carbonylated, and lipid-carbonylated sites in proteins.

Influence of seasonal variation and processing on protein glycation and oxidation in regular and hay milk

Climate and feeding influence the composition of bovine milk, which is further affected by thermal treatment inducing oxidation and Maillard reactions. This study aimed to evaluate season- and processing-related changes in the modified proteome of milk from two different feeding systems. Therefore, tryptic digests of regular and hay milk were analyzed by targeting 26 non-enzymatic modifications using LC-MS. Forty-five glycated, 48 advanced glycation endproduct (AGE-) modified, and 20 oxidized/carbonylated peptides representing 44 proteins were identified with lactosylation, formyllysine, and carboxymethyllysine being most common. The numbers and quantities of glycation- and oxidation-related modifications were similar between regular and hay milk and among seasons. The effects of pasteurization and ultra-high temperature (UHT) treatment were comparable for both milk types. In particular UHT treatment increased the numbers of identified modifications and the relative quantities of lactosylated peptides. The number of identified AGE-modified and oxidized residues increased slightly after UHT-treatment, but the contents were stable.

Influence of ultrasound pretreatment on the subsequent glycation of dietary proteins

The influence of ultrasound treatment on the subsequent glycation process of proteins is controversial. Glycation behaviors of bovine serum albumin (BSA), β-lactoglobulin (β-Lg) and β-casein (β-CN) after ultrasound pretreatment (UP) were compared by both evaluating glycation kinetics and analyzing structural changes of proteins. UP resulted in both unfolding and aggregation behavior in protein samples, which altered the accessibility of the Lys and Arg. Five cycles of UP up-regulated the glycation degree of BSA and β-Lg, possibly due to the unfolding behavior induced by UP, which exposed additional glycation sites. In contrast, 30 cycles of UP induced a dramatic increase (by 97.9 nm) in particle size of BSA, thus burying portions of glycation sites and suppressing the glycation process. Notably, UP had minimal influence on glycation kinetics of β-CN, due to its intrinsic disordered structure. Based on proteomics analysis, the preference of Lys and Arg during glycation was found to be changed by UP in BSA and β-Lg. Four, 3 and 3 unique carboxyethylated lysine residues were identified in glycated BSA after 0, 5 and 30 cycles of UP, respectively. This study suggests that the protein glycation can be affected by UP, depending on the ultrasonication duration and native structure of the protein.

Limitation of Maillard Reactions in Lactose-Reduced UHT Milk via Enzymatic Conversion of Lactose into Galactooligosaccharides during Production

Lactose-hydrolyzed (LH) ultrahigh temperature (UHT) processed milk is more prone to Maillard reactions and formation of advanced glycation end products (AGEs) during processing and storage than conventional (CON) UHT milk because of the presence of free galactose and glucose. Commercially available β-d-galactosidases with transgalactosylating activity can incorporate galactose into galactooligosaccharides (GOSs) and potentially limit Maillard reactions in this lactose-reduced GOS-containing milk. The aim of this study was to examine the extent of Maillard reactions in a lactose-reduced GOS milk compared to LH and CON milk after UHT processing. The GOS milk had significant lower levels of lysine- and arginine-derived AGEs compared to LH milk, while their concentrations were similar to those found in CON milk. The total concentration of measured Arg-derived AGEs was similar to the total concentration of Lys-derived AGEs in the three types of milk, indicating that Arg is an important source of AGEs in milks. Interestingly, the GOS milk generated threefold higher concentrations (up to 330 ± 6 μM) of 3-deoxyglucosone (3-DG, a C6 α-dicarbonyl). These results demonstrate that GOS milk could be a potential alternative for LH milk for lactose-intolerant individuals, although further studies are needed to understand the increased formation of 3-DG in GOS-containing milk.

Untargeted Proteomics-Based Profiling for the Identification of Novel Processing-Induced Protein Modifications in Milk

Nonenzymatic post-translational protein modifications (nePTMs) affect the nutritional, physiological, and technological properties of proteins in food and in vivo. In contrast to the usual targeted analyses, the present study determined nePTMs in processed milk in a truly untargeted proteomic approach. Thus, it was possible to determine to which extent known nePTM structures explain protein modifications in processed milk and to detect and identify novel products. The method combined ultrahigh-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry with bioinformatic data analysis by the software XCMS. The nePTMs detected by untargeted profiling of a β-lactoglobulin-lactose model were incorporated in a sensitive scheduled multiple reaction monitoring method to analyze these modifications in milk samples and to monitor their reaction kinetics during thermal treatment. Additionally, we identified the structures of unknown modifications. Lactosylation, carboxymethylation, formylation of lysine and N-terminus, glycation of arginine, oxidation of methionine, tryptophan, and cysteine, oxidative deamination of N-terminus, and deamidation of asparagine and glutamine were the most important reactions of β-lactoglobulin during milk processing. The isomerization of aspartic acid was observed for the first time in milk products, and N-terminal 4-imidazolidinone was identified as a novel nePTM.

Comparison of Free and Bound Advanced Glycation End Products in Food: A Review on the Possible Influence on Human Health

Debate on the hazards of advanced glycation end products (AGEs) in food has continued for many years as a result of their uncertain bioavailability and ability to bind to their receptors (RAGEs) in vivo. There are increasing evidence that free and bound AGEs have many differences in gastrointestinal digestion, intestinal absorption, binding with RAGEs, in vivo circulation, and renal clearance. Therefore, this paper compares these aspects between free and bound AGEs by summarizing the available knowledge. On the basis of the current knowledge, we conclude that it is time to differentiate free AGEs from bound AGEs in food in future studies, because they vary in many aspects that are closely related to their influence on human health. Several perspectives were proposed at the end of this review for further exploring the difference between free and bound AGEs in food.

Bioremediation of a Common Product of Food Processing by a Human Gut Bacterium

Dramatic increases in processed food consumption represent a global health threat. Maillard reaction products (MRPs), which are common in processed foods, form upon heat-induced reaction of amino acids with reducing sugars and include advanced glycation end products with deleterious health effects. To examine how processed foods affect the microbiota, we fed gnotobiotic mice, colonized with 54 phylogenetically diverse human gut bacterial strains, defined sugar-rich diets containing whey as the protein source or a matched amino acid mixture. Whey or ϵ-fructoselysine, an MRP in whey and many processed foods, selectively increases Collinsella intestinalis absolute abundance and induces Collinsella expression of genomic loci directing import and metabolism of ϵ-fructoselysine to innocuous products. This locus is repressed by glucose in C. aerofaciens, whose abundance decreases with whey, but is not repressed in C. intestinalis. Identifying gut organisms responding to and degrading potentially harmful processed food components has implications for food science, microbiome science, and public health.

Modification of Glutenin and Associated Changes in Digestibility Due to Methylglyoxal during Heat Processing

Glutenin is the main protein of flour and is a very important source of protein nutrition for humans. Methylglyoxal (MGO) is an important product of the Maillard reaction that occurs during the hot-processing of flour products, and it reacts with glutenin to facilitate changes in glutenin properties. Here, the effects of MGO on glutenin digestion during the heating process were investigated using a simulated MGO-glutenin system. MGO significantly reduced the digestibility of glutenin. The structure of MGO-glutenin and physicochemical properties were studied to understand the mechanism of the decrease of digestibility. These data suggest that changes in digestibility were caused by decreases in surface hydrophobicity and increases in disulfide bonds. MGO induces strong aggregation of glutenin after heating that led to the masking of cleavage sites for proteases. Moreover, carbonyl oxidation induced by MGO leads to intermolecular cross-linking of glutenin that increasingly masks or even destroys cleavage sites, further decreasing digestibility.

Bridging the Knowledge Gap for the Impact of Non-Thermal Processing on Proteins and Amino Acids

Proteins represent one of the major food components that contribute to a wide range of biophysical functions and dictate the nutritional, sensorial, and shelf-life of food products. Different non-thermal processing technologies (e.g., irradiation, ultrasound, cold plasma, pulsed electric field, and high-pressure treatments) can affect the structure of proteins, and thus their solubility as well as their functional properties. The exposure of hydrophobic groups, unfolding followed by aggregation at high non-thermal treatment intensities, and the formation of new bonds have been reported to promote the modification of structural and functional properties of proteins. Several studies reported the reduction of allergenicity of some proteins after the application of non-thermal treatments. The composition and concentration of free amino acids could be changed after non-thermal processing, depending on the processing time and intensity. The present review discusses the effects of different non-thermal treatments on protein properties in detail, and highlights the opportunities and disadvantages of these technologies in relation to protein functionality.

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.

Diversity of advanced glycation end products in the bovine milk proteome

Milk processing relies on thermal treatments warranting microbiologically safe products with extended shelf life. However, elevated temperatures favor also Maillard reactions yielding the structurally diverse advanced glycation end products (AGEs). AGEs may alter protein functions and immunogenicity and also decrease the nutritional value of milk products. Furthermore, dietary AGEs contribute to the circulating AGE pool with potentially harmful effects. Here, 14 types of protein-derived AGEs present in raw milk or produced during processing/storage of regular and lactose-free milk products were identified by nanoRP-UPLC-ESI-MS/MS. In total, 132 peptides (118 modification sites in 62 proteins) were modified by at least one studied AGE. Amide-AGEs were the most abundant group with formyllysine being the main type. Most lysine- and arginine-derived AGEs and their modification sites have not been reported before. The number of AGE modification sites increased with the harsher processing conditions of regular milk, but remained stable during storage. This was further supported by quantitative data.

Products of Early and Advanced Glycation in the Soy Milk Proteome

SCOPE

Thermal processing of soy milk kills pathogens and denatures anti-nutrition factors warranting microbiological safety, better digestibility, and longer storage. Additionally, Maillard reactions are triggered, yielding glycated proteins (Amadori/Heyns products) and a heterogeneous group of advanced glycation end-products (AGEs). These modifications alter the nutritional value, antigenicity, and digestibility of proteins. They also raise concerns about potentially toxic effects. This study aims at characterizing these modifications in proteins from different soy milk products.

METHODS AND RESULTS

Here, glycation and AGE-modification sites in the proteome of ultrahigh-temperature-treated natural soy milk, soy milk sweetened with hexose (fructose)-containing sweeteners (SSM), and sucrose as well as soy-based infant formulas (SIFs) from different manufacturers are reported for the first time. A bottom-up proteomic approach based on nano reversed-phase high-perfomance liquid chromatography-electrospray ionization-tandem mass spectrometry (nanoRP-HPLC-ESI-MS/MS) (collision-induced dissociation (CID) and electron transfer dissociation modes) identified 229 glycated peptides and 128 AGE-modified peptides resembling 53 proteins. The glycation sites are mainly derived from hexoses, whereas N^δ -carboxyethylarginine and methylglyoxal-derived hydroimidazolone are the main AGEs in soy milk.

CONCLUSION

The qualitative and quantitative data obtained here indicate that early glycation increases with harsher processing conditions (SIFs) and the addition of hexose-containing sweeteners (SSMs), whereas the latter sweeteners (but not the harsher processing) triggered more AGE modifications.

Identification and quantification of proteins at adsorption layer of emulsion stabilized by pea protein isolates

This study adopted the method of quantitative proteomics to analyze the adsorbed proteins in oil-in-water emulsions stabilized by pea protein isolate (PPI). Adsorbed proteins were precipitated by an optimized precipitation method and precipitates were labeled and subjected to a reversed-phase high performance liquid chromatography coupled to tandem mass spectrometry (RPLC-ESI-MS/MS) for protein identification and quantification. In total, 77 proteins were identified, of which 49 proteins with significant differences were observed. There were 25 upregulated proteins (fold change > 1) and 24 downregulated proteins (fold change < 1). The interfacial adsorption abilities of these proteins were compared according to the classification of protein families. The results showed that all isoforms of vicilins exhibited high adsorption abilities at the oil-water interface. Compared with vicilin, convicilin showed opposite adsorption capacity. Different legumin families showed significantly different affinities on the oil-water interface. In contrast to albumin-1, albumin-2 was preferentially adsorbed to the interface. The amino acid sequence alignment and hydropathy profile analysis of these proteins showed that the proteins well-balanced between hydrophobic and hydrophilic amino acid groups displayed high interfacial activity. In contrast, a long hydrophilic or hydrophobic fragment could adversely influence protein interfacial activity. This study provides an insight into the interfacial behaviors of proteins by supplying detailed quantitative information of interfacial layer.

Identification of candidate biomarkers of the exposure to PCBs in contaminated cattle: A gene expression- and proteomic-based approach

Dioxins and polychlorinated biphenyls (PCBs) are widespread and persistent contaminants. Through a combined gene expression/proteomic-based approach, candidate biomarkers of the exposure to such environmental pollutants in cattle subjected to a real eco-contamination event were identified. Animals were removed from the polluted area and fed a standard ration for 6 months. The decontamination was monitored by evaluating dioxin and PCB levels in pericaudal fat two weeks after the removal from the contaminated area (day 0) and then bimonthly for six months (days 59, 125 and 188). Gene expression measurements demonstrated that CYP1B1 expression was significantly higher in blood lymphocytes collected in contaminated animals (day 0), and decreased over time during decontamination. mRNA levels of interleukin 2 showed an opposite quantitative trend. MALDI-TOF-MS polypeptide profiling of serum samples ascertained a progressive decrease (from day 0 to 188) of serum levels of fibrinogen β-chain and serpin A3-7-like fragments, apolipoprotein (APO) C-II and serum amyloid A-4 protein, along with an augmented representation of transthyretin isoforms, as well as APOC-III and APOA-II proteins during decontamination. When differentially represented species were combined with serum antioxidant, acute phase and proinflammatory protein levels already ascertained in the same animals (Cigliano et al., 2016), bioinformatics unveiled an interaction network linking together almost all components. This suggests the occurrence of a complex PCB-responsive mechanism associated with animal contamination/decontamination, including a cohort of protein/polypeptide species involved in blood redox homeostasis, inflammation and lipid transport. All together, these results suggest the use in combination of such biomarkers for identifying PCB-contaminated animals, and for monitoring the restoring of their healthy condition following a decontamination process.

Evaluation of six sample preparation procedures for qualitative and quantitative proteomics analysis of milk fat globule membrane

Proteomic analysis of membrane proteins is challenged by the proteins solubility and detergent incompatibility with MS analysis. No single perfect protocol can be used to comprehensively characterize the proteome of membrane fraction. Here, we used cow milk fat globule membrane (MFGM) proteome analysis to assess six sample preparation procedures including one in-gel and five in-solution digestion approaches prior to LC-MS/MS analysis. The largest number of MFGM proteins were identified by suspension trapping (S-Trap) and filter-aided sample preparation (FASP) methods, followed by acetone precipitation without clean-up of tryptic peptides method. Protein identifications with highest average coverage was achieved by Chloroform/MeOH, in-gel and S-Trap methods. Most distinct proteins were identified by FASP method, followed by S-Trap. Analyses by Venn diagram, principal-component analysis, hierarchical clustering and the abundance ranking of quantitative proteins highlight differences in the MFGM fraction by the all sample preparation procedures. These results reveal the biased proteins/peptides loss occurred in each protocol. In this study, we found several novel proteins that were not observed previously by in-depth proteomics characterization of MFGM fraction in milk. Thus, a combination of multiple procedures with orthologous properties of sample preparation was demonstrated to improve the protein sequence coverage and expression level accuracy of membrane samples.

Current (Food) Allergenic Risk Assessment: Is It Fit for Novel Foods? Status Quo and Identification of Gaps

Food allergies are recognized as a global health concern. In order to protect allergic consumers from severe symptoms, allergenic risk assessment for well-known foods and foods containing genetically modified ingredients is installed. However, population is steadily growing and there is a rising need to provide adequate protein-based foods, including novel sources, not yet used for human consumption. In this context safety issues such as a potential increased allergenic risk need to be assessed before marketing novel food sources. Therefore, the established allergenic risk assessment for genetically modified organisms needs to be re-evaluated for its applicability for risk assessment of novel food proteins. Two different scenarios of allergic sensitization have to be assessed. The first scenario is the presence of already known allergenic structures in novel foods. For this, a comparative assessment can be performed and the range of cross-reactivity can be explored, while in the second scenario allergic reactions are observed toward so far novel allergenic structures and no reference material is available. This review summarizes the current analytical methods for allergenic risk assessment, highlighting the strengths and limitations of each method and discussing the gaps in this assessment that need to be addressed in the near future.

Protein carbonylation sites in bovine raw milk and processed milk products

During thermal treatment of milk, proteins are oxidized, which may reduce the nutritional value of milk, abolish protein functions supporting human health, especially important for newborns, and yield potentially harmful products. The side chains of several amino acids can be oxidized to reactive carbonyls, which are often used to monitor oxidative stress in organisms. Here we mapped protein carbonylation sites in raw milk and different brands of pasteurized, ultra high temperature (UHT) treated milk, and infant formulas (IFs) after digesting the precipitated proteins with trypsin. Reactive carbonyls were derivatized with O-(biotinylcarbazoylmethyl)hydroxylamine to enrich the modified peptides by avidin-biotin affinity chromatography and analyze them by nanoRP-UPLC-ESI-MS. Overall, 53 unique carbonylated peptides (37 carbonylation sites, 15 proteins) were identified. Most carbonyls were derived from dicarbonyls (mainly glyoxal). The number of carbonylation sites increased with the harsher processing from raw milk (4) to pasteurized (16) and UHT milk (16) and to IF (24).

Skin advanced glycation end-products evaluation in infants according to the type of feeding and mother's smoking habits

OBJECTIVES

This study was conducted to assess whether formula-fed infants had increased skin advanced glycation end-products compared with breastfed ones. We also evaluated the effect of maternal smoke during pregnancy and lactation on infant skin advanced glycation end-products accumulation.

METHODS

Advanced glycation end-product-linked skin autofluorescence was measured in 101 infants.

RESULTS

In infants born from non-smoking mothers, advanced glycation end-products were higher in formula-fed subjects than in breastfed subjects (0.80 (0.65-0.90) vs 1.00 (0.85-1.05), p < 0.001). Advanced glycation end-products in breastfed infants from smoking mothers were higher than in those from non-smoking mothers (0.80 (0.65-0.90) vs 1.00 (0.90-1.17), p = 0.009).

CONCLUSION

Formula-fed infants had increased amounts of advanced glycation end-products compared with the breastfed ones, confirming that breast milk represents the best food for infants. Breastfed infants from mothers smoking during pregnancy and lactation had increased skin advanced glycation end-products, suggesting that smoke-related advanced glycation end-products transfer throughout breast milk. Moreover, advanced glycation end-products may already increase during gestation, possibly affecting fetal development. Thus, we reinforced that smoking must be stopped during pregnancy and lactation.

Influence of storage and heating on protein glycation levels of processed lactose-free and regular bovine milk products

Thermal treatment preserves the microbiological safety of milk, but also induces Maillard reactions modifying for example proteins. The purpose of this study was evaluating the influence of consumer behaviors (storage and heating) on protein glycation degrees in bovine milk products. Lactosylation and hexosylation sites were identified in ultra-high temperature (UHT), lactose-free pasteurized, and lactose-free UHT milk (ULF) and infant formula (IF) using tandem mass spectrometry (electron transfer dissociation). Overall, 303 lactosylated and 199 hexosylated peptides were identified corresponding to 170 lactosylation (31 proteins) and 117 hexosylation sites (25 proteins). In quantitative terms, storage increased lactosylation up to fourfold in UHT and IF and hexosylation up to elevenfold in ULF and threefold in IF. These levels increased additionally twofold when the stored samples were heated (40°C). In conclusion, storage and heating appear to influence protein glycation levels in milk at similar or even higher degrees than industrial processing.

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.

Flavonoid Interaction with a Chitinase from Grape Berry Skin: Protein Identification and Modulation of the Enzymatic Activity

In the present study, an antibody raised against a peptide sequence of rat bilitranslocase (anti-peptide Ab) was tested on microsomal proteins obtained from red grape berry skin. Previously, this antibody had demonstrated to recognize plant membrane proteins associated with flavonoid binding and transport. Immuno-proteomic assays identified a number of proteins reacting with this particular antibody, suggesting that the flavonoid binding and interaction may be extended not only to carriers of these molecules, but also to enzymes with very different functions. One of these proteins is a pathogenesis-related (PR) class IV chitinase, whose in vitro chitinolytic activity was modulated by two of the most representative flavonoids of grape, quercetin and catechin, as assessed by both spectrophotometric and fluorimetric assays in grape microsomes and commercial enzyme preparations. The effect of these flavonoids on the catalysis and its kinetic parameters was also evaluated, evidencing that they determine a hormetic dose-dependent response. These results highlight the importance of flavonoids not only as antioxidants or antimicrobial effectors, but also as modulators of plant growth and stress response. Implications of the present suggestion are here discussed in the light of environment and pesticide-reduction concerns.

Purification and characterization of a Cys-Gly hydrolase from the gastropod mollusk, Patella caerulea

A magnesium-dependent cysteinyl-glycine hydrolyzing enzyme from the gastropod mollusk Patella caerulea was purified to electrophoretic homogeneity through a simple and rapid purification protocol. The molecular masses of the native protein and the subunit suggest that the enzyme has a homohexameric structure. Structural data in combination with kinetic parameters determined with Cys-Gly and compared with Leu-Gly as a substrate, indicate that the purified enzyme is a member of the peptidase family M17. The finding that an enzyme of the peptidase family M17 is responsible also in mollusks for the breakdown of Cys-Gly confirms the important role of this peptidase family in the glutathione metabolism.

Targeted proteomics: Current status and future perspectives for quantification of food allergens

Allergen levels in fresh and processed foods can vary dynamically. As different sources of foods can cause different types of allergic reactions, the food industry and regulatory bodies urgently require reliable detection and absolute quantitation methods for allergen detection in complex food products to effectively safeguard the food-allergic population. Recent advances of targeted proteomic technologies namely multiple-reaction monitoring (MRM) mass spectrometry (MS) coupled with isotope-labeled internal standard, also known as AQUA peptides offers absolute quantitation of food allergens even at 10ppb level in a multiplex fashion. However, development of successful AQUA-MRM assay relies on a number of pre and post MS criteria. In this review, we briefly describe how allergen levels could potentially change in plant and animal based foods, necessitating the development of a high throughput multiplexed allergen quantification methodology for successful AQUA-MRM assay. We also propose some future strategies that could provide better management of food allergy.

BIOLOGICAL SIGNIFICANCE

Given the rapid increases of food allergenicity, it has become imperative to know absolute allergen levels in foods. This essential information could be the most effective means of protecting humans suffering from allergies. In this review, we emphasize the significance of the absolute quantitation of food allergens using AQUA-MRM approach and discuss the likely critical steps for successful assay development.