Mass spectrometric characterization of glycated beta-lactoglobulin peptides derived from galacto-oligosaccharides surviving the in vitro gastrointestinal digestion

Improved bioavailability and antioxidation of β-carotene-loaded biopolymeric nanoparticles stabilized by glycosylated oat protein isolate

The limited bioavailability of β-carotene hinders its potential application in functional foods, despite its excellent antioxidant properties. Protein-based nanoparticles have been widely used for the delivery of β-carotene to overcome this limitation. However, these nanoparticles are susceptible to environmental stress. In this study, we utilized glycosylated oat protein isolate to prepare nanoparticles loaded with β-carotene through the emulsification-evaporation method, aiming to address this challenge. The results showed that β-carotene was embedded into the spherical nanoparticles, exhibiting relatively high encapsulation efficiency (86.21 %) and loading capacity (5.43 %). The stability of the nanoparticles loaded with β-carotene was enhanced in acidic environments and under high ionic strength. The nanoparticles offered protection to β-carotene against gastric digestion and facilitated its controlled release (95.76 % within 6 h) in the small intestine, thereby leading to an improved in vitro bioavailability (65.06 %) of β-carotene. This improvement conferred the benefits on β-carotene nanoparticles to alleviate tert-butyl hydroperoxide-induced oxidative stress through the upregulation of heme oxygenase-1 and NAD(P)H quinone dehydrogenase 1 expression, as well as the promotion of nuclear translocation of nuclear factor-erythroid 2-related factor 2. Our study suggests the potential for the industry application of nanoparticles based on glycosylated proteins to effectively deliver hydrophobic nutrients and enhance their application.

Food protein glycation: A review focusing on stability and in vitro digestive characteristics of oil/water emulsions

Recently, increasing studies have shown that the functional properties of proteins, including emulsifying properties, antioxidant properties, solubility, and thermal stability, can be improved through glycation reaction under controlled reaction conditions. The use of glycated proteins to stabilize hydrophobic active substances and to explore the gastrointestinal fate of the stabilized hydrophobic substances has also become the hot spot. Therefore, in this review, the effects of glycation on the structure and function of food proteins and the physical stability and oxidative stability of protein-stabilized oil/water emulsions were comprehensively summarized and discussed. Also, this review sheds lights on the in vitro digestion characteristics and edible safety of emulsion stabilized by glycated protein. It can further serve as a research basis for understanding the role of structural features in the emulsification and stabilization of glycated proteins, as well as their utilization as emulsifiers in the food industry.

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.

Consequential fate of bisphenol-attached PVC microplastics in water and simulated intestinal fluids

The ever-increasing prevalence of microplastics and different bisphenols made the presence of bisphenol-attached microplastics a critical concern. In this study, experiments were performed to examine desorption behaviors and cytotoxicity performance of contaminated microplastics in aquatic surroundings and intestinal environment after ingestion by organisms (cold-/warm-blooded). The kinetic study shows that the rate of desorption for bisphenols can be enhanced threefold under simulated warm intestinal conditions. The Freundlich isotherms indicate multiple-layer desorption of the bisphenols on the heterogeneous surfaces of polyvinyl chloride (PVC) microplastics. Hysteresis was detected in the adsorption/desorption of bisphenols in a water environment, but no adsorption/desorption hysteresis was observed in the simulated intestinal conditions of warm-blooded organisms. Due to enhanced bioaccessibility, the desorption results imply that the environmental risk of contaminated PVC microplastics may be significantly increased after ingestion at a high bisphenols dosage. Although with different IC₅₀, the five bisphenols released under the intestinal conditions of warm-blooded organisms can cause higher proliferation reduction in fish and human cell lines than the bisphenols released in water. This study helps elucidate the consequential fate and potential cytotoxicity of contaminated microplastics and the possible implications of the microplastics as a critical vector for bisphenols to increase the potential health risks.

Glycation of whey proteins: Technological and nutritional implications

Whey proteins are globular proteins that have received much attention due to their high nutritional value and characteristic functional properties. In addition to being part of the protein system in milk, they constitute the main proteins in whey and whey protein products. Interaction of whey proteins with reducing sugars and carbohydrates via Maillard reaction have been extensively studied in milk and in model systems. Glycation of individual whey proteins results in variable increases in their solubility, thermal stability, antioxidant activity, and emulsion and foam stabilization. Limited glycation of whey protein products particularly whey protein isolates (WPI) using polysaccharides has been studied with the aim to produce conjugates with modified functional properties and acceptable sensory properties. An overview is presented here on the effect of glycation on individual whey proteins and whey protein products and the potential uses of the glycated whey proteins.

Maillard Proteomics: Opening New Pages

Protein glycation is a ubiquitous non-enzymatic post-translational modification, formed by reaction of protein amino and guanidino groups with carbonyl compounds, presumably reducing sugars and α-dicarbonyls. Resulting advanced glycation end products (AGEs) represent a highly heterogeneous group of compounds, deleterious in mammals due to their pro-inflammatory effect, and impact in pathogenesis of diabetes mellitus, Alzheimer's disease and ageing. The body of information on the mechanisms and pathways of AGE formation, acquired during the last decades, clearly indicates a certain site-specificity of glycation. It makes characterization of individual glycation sites a critical pre-requisite for understanding in vivo mechanisms of AGE formation and developing adequate nutritional and therapeutic approaches to reduce it in humans. In this context, proteomics is the methodology of choice to address site-specific molecular changes related to protein glycation. Therefore, here we summarize the methods of Maillard proteomics, specifically focusing on the techniques providing comprehensive structural and quantitative characterization of glycated proteome. Further, we address the novel break-through areas, recently established in the field of Maillard research, i.e., in vitro models based on synthetic peptides, site-based diagnostics of metabolism-related diseases (e.g., diabetes mellitus), proteomics of anti-glycative defense, and dynamics of plant glycated proteome during ageing and response to environmental stress.

Deciphering how Cpl-7 cell wall-binding repeats recognize the bacterial peptidoglycan

Endolysins, the cell wall lytic enzymes encoded by bacteriophages to release the phage progeny, are among the top alternatives to fight against multiresistant pathogenic bacteria; one of the current biggest challenges to global health. Their narrow range of susceptible bacteria relies, primarily, on targeting specific cell-wall receptors through specialized modules. The cell wall-binding domain of Cpl-7 endolysin, made of three CW_7 repeats, accounts for its extended-range of substrates. Using as model system the cell wall-binding domain of Cpl-7, here we describe the molecular basis for the bacterial cell wall recognition by the CW_7 motif, which is widely represented in sequences of cell wall hydrolases. We report the crystal and solution structure of the full-length domain, identify N-acetyl-D-glucosaminyl-(β1,4)-N-acetylmuramyl-L-alanyl-D-isoglutamine (GMDP) as the peptidoglycan (PG) target recognized by the CW_7 motifs, and characterize feasible GMDP-CW_7 contacts. Our data suggest that Cpl-7 cell wall-binding domain might simultaneously bind to three PG chains, and also highlight the potential use of CW_7-containing lysins as novel anti-infectives.

Generation of Soluble Advanced Glycation End Products Receptor (sRAGE)-Binding Ligands during Extensive Heat Treatment of Whey Protein/Lactose Mixtures Is Dependent on Glycation and Aggregation

Heating of protein- and sugar-containing materials is considered the primary factor affecting the formation of advanced glycation end products (AGEs). This study aimed to investigate the influence of heating conditions, digestion, and aggregation on the binding capacity of AGEs to the soluble AGE receptor (sRAGE). Samples consisting of mixtures of whey protein and lactose were heated at 130 °C. An in vitro infant digestion model was used to study the influence of heat treatment on the digestibility of whey proteins. The amount of sRAGE-binding ligands before and after digestion was measured by an ELISA-based sRAGE-binding assay. Water activity did not significantly affect the extent of digestibility of whey proteins dry heated at pH 5 (ranging from 3.3 ± 0.2 to 3.6 ± 0.1% for gastric digestion and from 53.5 ± 1.5 to 64.7 ± 1.1% for duodenal digestion), but there were differences in cleavage patterns of peptides among the samples heated at different pH values. Formation of sRAGE-binding ligands depended on the formation of aggregates and was limited in the samples heated at pH 5. Moreover, the sRAGE-binding activity of digested sample was changed by protease degradation and correlated with the digestibility of samples. In conclusion, generation of sRAGE-binding ligands during extensive heat treatment of whey protein/lactose mixtures is limited in acidic heating condition and dependent on glycation and aggregation.

PL3 Amidase, a Tailor-made Lysin Constructed by Domain Shuffling with Potent Killing Activity against Pneumococci and Related Species

The emergence and spread of antibiotic-resistant bacteria is pushing the need of alternative treatments. In this context, phage therapy is already a reality to successfully fight certain multiresistant bacteria. Among different phage gene products, murein hydrolases responsible of phage progeny liberation (also called lysins or endolysins) are weapons that target specific peptidoglycan bonds, leading to lysis and death of susceptible bacteria when added from the outside. In the pneumococcal system, all but one phage murein hydrolases reported to date share a choline-binding domain that recognizes cell walls containing choline residues in the (lipo)teichoic acids. Some purified pneumococcal or phage murein hydrolases, as well as several chimeric proteins combining natural catalytic and cell wall-binding domains (CBDs) have been used as effective antimicrobials. In this work we have constructed a novel chimeric N-acetylmuramoyl-L-alanine amidase (PL3) by fusing the catalytic domain of the Pal amidase (a phage-coded endolysin) to the CBD of the LytA amidase, the major pneumococcal autolysin. The physicochemical properties of PL3 and the bacteriolytic effect against several pneumococci (including 48 multiresistant representative strain) and related species, like Streptococcus pseudopneumoniae, Streptococcus mitis, and Streptococcus oralis, have been studied. Results have shown that low doses of PL3, in the range of 0.5–5 μg/ml, are enough to practically sterilize all choline-containing strains tested. Moreover, a single 20-μg dose of PL3 fully protected zebrafish embryos from infection by S. pneumoniae D39 strain. Importantly, PL3 keeps 95% enzymatic activity after 4 weeks at 37°C and can be lyophilized without losing activity, demonstrating a remarkable robustness. Such stability, together with a prominent efficacy against a narrow spectrum of human pathogens, confers to PL3 the characteristic to be an effective therapeutic. In addition, our results demonstrate that the structure/function-based domain shuffling approach is a successful method to construct tailor-made endolysins with higher bactericidal activities than their parental enzymes.

Substrate recognition and catalysis by LytB, a pneumococcal peptidoglycan hydrolase involved in virulence

Streptococcus pneumoniae is a major cause of life-threatening diseases worldwide. Here we provide an in-depth functional characterization of LytB, the peptidoglycan hydrolase responsible for physical separation of daughter cells. Identified herein as an N-acetylglucosaminidase, LytB is involved also in colonization and invasion of the nasopharynx, biofilm formation and evasion of host immunity as previously demonstrated. We have shown that LytB cleaves the GlcNAc-β-(1,4)-MurNAc glycosidic bond of peptidoglycan building units. The hydrolysis occurs at sites with fully acetylated GlcNAc moieties, with preference for uncross-linked muropeptides. The necessity of GlcN acetylation and the presence of a single acidic moiety (Glu585) essential for catalysis strongly suggest a substrate-assisted mechanism with anchimeric assistance of the acetamido group of GlcNAc moieties. Additionally, modelling of the catalytic region bound to a hexasaccharide tripentapeptide provided insights into substrate-binding subsites and peptidoglycan recognition. Besides, cell-wall digestion products and solubilisation rates might indicate a tight control of LytB activity to prevent unrestrained breakdown of the cell wall. Choline-independent localization at the poles of the cell, mediated by the choline-binding domain, peptidoglycan modification, and choline-mediated (lipo)teichoic-acid attachment contribute to the high selectivity of LytB. Moreover, so far unknown chitin hydrolase and glycosyltransferase activities were detected using GlcNAc oligomers as substrate.

A novel chimeric phage lysin with high in vitro and in vivo bactericidal activity against Streptococcus pneumoniae

OBJECTIVES

Streptococcus pneumoniae is becoming increasingly antibiotic resistant worldwide and new antimicrobials are urgently needed. Our aim was new chimeric phage endolysins, or lysins, with improved bactericidal activity by swapping the structural components of two pneumococcal phage lysozymes: Cpl-1 (the best lysin tested to date) and Cpl-7S.

METHODS

The bactericidal effects of four new chimeric lysins were checked against several bacteria. The purified enzymes were added at different concentrations to resuspended bacteria and viable cells were measured after 1 h. Killing capacity of the most active lysin, Cpl-711, was tested in a mouse bacteraemia model, following mouse survival after injecting different amounts (25-500 μg) of enzyme. The capacity of Cpl-711 to reduce pneumococcal biofilm formation was also studied.

RESULTS

The chimera Cpl-711 substantially improved the killing activity of the parental phage lysozymes, Cpl-1 and Cpl-7S, against pneumococcal bacteria, including multiresistant strains. Specifically, 5 μg/mL Cpl-711 killed ≥7.5 log of pneumococcal R6 strain. Cpl-711 also reduced pneumococcal biofilm formation and killed 4 log of the bacterial population at 1 μg/mL. Mice challenged intraperitoneally with D39_IU pneumococcal strain were protected by treatment with a single intraperitoneal injection of Cpl-711 1 h later, resulting in about 50% greater protection than with Cpl-1.

CONCLUSIONS

Domain swapping among phage lysins allows the construction of new chimeric enzymes with high bactericidal activity and a different substrate range. Cpl-711, the most powerful endolysin against pneumococci, offers a promising therapeutic perspective for the treatment of multiresistant pneumococcal infections.

Antioxidant activity of bovine alpha lactalbumin Maillard products and evaluation of their in vitro gastro-duodenal digestive proteolysis

Maillard conjugation alters alpha lactalbumin antioxidant capacity, including during in vitro digestion, with different effect in adults versus infants.

Investigation of the use of Maillard reaction inhibitors for the production of patatin-carbohydrate conjugates

Selected Maillard reaction inhibitors, including aminoguanidine, cysteine, pyridoxamine, and sodium bisulfite, were evaluated for their effect on the production of carbohydrate conjugated proteins with less cross-linking/browning. Patatin (PTT), a major potato protein, was glycated with galactose, xylose, galactooligosaccharides, xylooligosaccharides, galactan, and xylan under controlled conditions. The effectiveness of the inhibitors to control the glycation reaction was assessed by monitoring the glycation extent, the protein cross-linking, and the formation of dicarbonyl compounds. Sodium bisulfite was the most effective inhibitor for PTT-galactose and PTT-xylan reaction systems (reaction control ratios of 210.0 and 12.8). On the other hand, aminoguanidine and cysteine led to the highest reaction control ratios for the PTT-xylose/xylooligosaccharide (160.0 and 143.0) and PTT-galactooligosaccharides/galactan (663.0 and 71.0) reaction systems, respectively. The use of cysteine and aminoguanidine as inhibitors led to 1.7-99.4% decreases in the particle size distribution of the PTT conjugates and to 0.4-9.3% increases in their relative digestibility, per 5% blocked lysine.

Allergenicity of potato proteins and of their conjugates with galactose, galactooligosaccharides, and galactan in native, heated, and digested forms

The effect of glycation of potato proteins on their immunoreactivity was studied by using a pool of human sera with specific IgE to potato proteins. Patatin conjugates were more immunoreactive than protease inhibitors ones. To better understand this behavior, the changes in patatin structure upon glycation and heat treatment were investigated. Patatin demonstrated an increase in total immunoreactivity when glycated with galactose and galactooligosaccharides. However, galactan conjugation to patatin resulted in a decrease in immunoreactivity by restricting IgE's access to the epitopes. Although the heat treatment resulted in a decrease in patatin's immunoreactivity through aggregation, it was less effective when patatin conjugates were used due to the decrease in aggregation and the secondary structural changes. Upon digestion, native patatin exhibited the largest decrease in immunoreactivity resulting from the disruption of both conformational and sequential epitopes. Patatin conjugates were less digested and had higher IgE-immunoreactivity as compared to the digested patatin.

Digestion proteomics: tracking lactoferrin truncation and peptide release during simulated gastric digestion

Peptide release profiles from lactoferrin were tracked and characterised during simulated gastric digestion using a novel quantitative proteomic approach.

The impact of the Maillard reaction on the in vitro proteolytic breakdown of bovine lactoferrin in adults and infants

The impact of the Maillard reaction on proteolysis of the bioactive bovine lactoferrin is comparedin vitrobetween adults and infants for the first time, coupling proteomics to elucidate bioactive peptide formation.

Characterisation of Maillard reaction products derived from LEKFD--a pentapeptide found in β-lactoglobulin sequence, glycated with glucose--by tandem mass spectrometry, molecular orbital calculations and gel filtration chromatography coupled with continuous photodiode array

Maillard reaction peptides (MRPs) contribute to taste, aroma, colour, texture and biological activity. However, peptide degradation or the cross-linking of MRPs in the Maillard reaction has not been investigated clearly. A peptide of LEKFD, a part of β-lactoglobulin, was heated at 110 °C for 24h with glucose and the reaction products were analysed by HPLC with ODS, ESI-MS, ESI-MS/MS and HPLC with gel-filtration column and DAD detector. In the HPLC fractions, an imminium ion of LEK*FD, a pyrylium ion or a hydroxymethyl furylium ion of LEK*FD, and KFD and EK were detected by ESI-MS. Therefore, those products may be produced by the Maillard reaction. The molecular orbital of glycated LEKFD at the lysine epsilon-amino residue with Schiff base form was calculated by MOPAC. HPLC with gel-filtration column showed cross-linking and degradation of peptides.

Improving the lethal effect of cpl-7, a pneumococcal phage lysozyme with broad bactericidal activity, by inverting the net charge of its cell wall-binding module

Phage endolysins are murein hydrolases that break the bacterial cell wall to provoke lysis and release of phage progeny. Recently, these enzymes have also been recognized as powerful and specific antibacterial agents when added exogenously. In the pneumococcal system, most cell wall associated murein hydrolases reported so far depend on choline for activity, and Cpl-7 lysozyme constitutes a remarkable exception. Here, we report the improvement of the killing activity of the Cpl-7 endolysin by inversion of the sign of the charge of the cell wall-binding module (from -14.93 to +3.0 at neutral pH). The engineered variant, Cpl-7S, has 15 amino acid substitutions and an improved lytic activity against Streptococcus pneumoniae (including multiresistant strains), Streptococcus pyogenes, and other pathogens. Moreover, we have demonstrated that a single 25-μg dose of Cpl-7S significantly increased the survival rate of zebrafish embryos infected with S. pneumoniae or S. pyogenes, confirming the killing effect of Cpl-7S in vivo. Interestingly, Cpl-7S, in combination with 0.01% carvacrol (an essential oil), was also found to efficiently kill Gram-negative bacteria such as Escherichia coli and Pseudomonas putida, an effect not described previously. Our findings provide a strategy to improve the lytic activity of phage endolysins based on facilitating their pass through the negatively charged bacterial envelope, and thereby their interaction with the cell wall target, by modulating the net charge of the cell wall-binding modules.

Effect of Fructose and glucose on glycation of β-lactoglobulin in an intermediate-moisture food model system: analysis by liquid chromatography-mass spectrometry (LC-MS) and data-independent acquisition LC-MS (LC-MS(E))

To evaluate the effect of glucose and fructose on the glycation of β-lactoglobulin (β-Lg) in intermediate-moisture food (IMF), model systems consisting of β-Lg, glucose/fructose/sorbitol, glycerol, and water were established. All systems were stored at 25 and 35 °C for 2 months. The progress of the Maillard reaction and the mass change of β-Lg were investigated by the browning assay and gel electrophoresis, respectively. Meanwhile, liquid chromatography-mass spectrometry (LC-MS) and data-independent acquisition LC-MS (LC-MS(E)) were used to monitor the glycation extent and the glycated sites of β-Lg. The results indicated that glucose had a higher reaction activity of glycation than fructose, but both sugars had similar preference on the glycation site for β-Lg. The ranking order from high to low for the 9 detected glycated sites was L 1, K 91 > K 47 > K 70, K 77, K 83, K 100 > K 75 > K 135 for both sugars.

Thermal stability of Cpl-7 endolysin from the streptococcus pneumoniae bacteriophage Cp-7; cell wall-targeting of its CW_7 motifs

Endolysins comprise a novel class of selective antibacterials refractory to develop resistances. The Cpl-7 endolysin, encoded by the Streptococcus pneumoniae bacteriophage Cp-7, consists of a catalytic module (CM) with muramidase activity and a cell wall-binding module (CWBM) made of three fully conserved CW_7 repeats essential for activity. Firstly identified in the Cpl-7 endolysin, CW_7 motifs are also present in a great variety of cell wall hydrolases encoded, among others, by human and live-stock pathogens. However, the nature of CW_7 receptors on the bacterial envelope remains unknown. In the present study, the structural stability of Cpl-7 and the target recognized by CW_7 repeats, relevant for exploitation of Cpl-7 as antimicrobial, have been analyzed, and transitions from the CM and the CWBM assigned, using circular dichroism and differential scanning calorimetry. Cpl-7 stability is maximum around 6.0-6.5, near the optimal pH for activity. Above pH 8.0 the CM becomes extremely unstable, probably due to deprotonation of the N-terminal amino-group, whereas the CWBM is rather insensitive to pH variation and its structural stabilization by GlcNAc-MurNAc-l-Ala-d-isoGln points to the cell wall muropeptide as the cell wall target recognized by the CW_7 repeats. Denaturation data also revealed that Cpl-7 is organized into two essentially independent folding units, which will facilitate the recombination of the CM and the CWBM with other catalytic domains and/or cell wall-binding motifs to yield new tailored chimeric lysins with higher bactericidal activities or new pathogen specificities.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Detection of two minor phosphorylation sites for bovine κ-casein macropeptide by reversed-phase liquid chromatography-tandem mass spectrometry

This work addresses the characterization of phosphopeptides in bovine κ-casein macropeptide by reversed-phase liquid chromatography-electrospray ionization-tandem mass spectrometry (RPLC-ESI-MS(2)). Two different mass spectrometers, equipped with an ion trap (IT) or a quadrupole time-of-flight (Q-TOF) analyzer, were used to perform an accurate phosphorylation site assignment. A total of 8 phosphopeptides from 26 identified peptides were characterized. MS(2) spectra of phosphopeptides were dominated by the neutral loss of a phosphoric acid molecule (H(3)PO(4)) and sufficient informative fragment ions resulting from peptide backbone cleavages enabling the elucidation of the phosphopeptide sequence. A higher number of sequence informative b and y ions were detected using a Q-TOF instead of an IT analyzer. In addition to the well-established phosphorylation sites at Ser(149) and Ser(127), this study also revealed the presence of two minor phosphorylation sites at Thr(145) and Ser(166). These findings indicate that RPLC-ESI-MS(2) on a Q-TOF analyzer is a useful technique for identifying low-abundance phosphorylation sites in caseins.

Application of liquid chromatography-tandem mass spectrometry for the characterization of galactosylated and tagatosylated beta-lactoglobulin peptides derived from in vitro gastrointestinal digestion

This article describes a comprehensive characterization of bovine beta-lactoglobulin peptides glycated with an aldohexose (galactose) or a ketohexose (tagatose), derived from in vitro gastrointestinal digestion, by liquid chromatography coupled to positive electrospray ion trap tandem mass spectrometry. In addition to the dissociation pathway previously described for aldohexoses-derived Amadori compounds, i.e. formation of the pyrylium ([M+H](+)-54) and furylium ions ([M+H](+)-84) via the oxonium ion ([M+H](+)-18), another and more direct fragmentation route involving the formation of the imminium ion ([M+H](+)-150) is also reported following extensive glycation rates of beta-lactoglobulin with both carbohydrates. These results indicated that the analysis of digested proteins by LC-ESI-MS(2) on a three-dimensional ion trap monitoring neutral losses is an efficient and direct method to identify peptides glycated not only through the Amadori rearrangement but also via the Heyns rearrangement. Nevertheless, as the predominating MS(2) fragmentation pattern of the glycated peptides is derived from the sugar moiety, the sequence-informative b- and y-ions resulting from peptide backbone cleavage were undetected. To overcome this drawback, and taking advantage of multi-stage fragmentation capabilities of ion traps, the indicative Amadori and Heyns-derived imminium ions were successfully used in MS(3) analyses to identify the peptide backbone, as well as the specific glycation site. In addition, further MS(4) analyses were needed to carry out the characterization of doubly glycated peptides.