Glycation isotopic labeling with 13C-reducing sugars for quantitative analysis of glycated proteins in human plasma

Exploring glycated sites in human serum albumin: impact of sample processing techniques on detection and analysis

Glycation and the subsequent formation of advanced glycation end products (AGEs) disrupt and impair the physiological functions of proteins. This study presents a comprehensive glycation site mapping of human serum albumin (HSA) utilizing liquid chromatography-tandem mass spectrometry (LC-MS/MS). Both in vitro glycation experiments and patient samples were investigated, exploring various enzymes, processing techniques, and their impacts on glycation site detection. A pilot study was conducted, analyzing sixteen serum samples, which spanned from healthy individuals to severe diabetic patients (with HbA1c values ranging from 5.7% to 18.1%). The aim was to comprehend the progression of glycation on various sites of HSA with increasing levels of glycation. Their glycated albumin levels (GA) spanned from 19.7% to 62.3%. Trypsin-mediated proteolytic digestion unveiled 12 glycation sites through direct in-solution digestion of whole serum. However, isolating albumin from serum enabled the identification of a higher number of glycation sites in each sample compared to direct serum digestion. Boronate affinity chromatography facilitated the segregation of less glycated albumin (LGA) from the more glycated albumin (MGA) fraction. Subsequent proteolytic digestion of both LGA and MGA samples revealed similar glycation sites. The MGA fraction exhibited a greater number of identified glycation sites, thereby elucidating which sites are particularly prone to glycation in highly glycated albumin samples. Changes in relative glycation levels were noted in the tryptic digests of albumin samples following the sample enrichment steps, as opposed to direct in-solution digestion of whole serum. Two enzymes, trypsin and Glu-C, were evaluated for efficacy in sequence coverage and glycation site analysis of HSA, with trypsin demonstrating superior efficiency over Glu-C.

Protein Haptenation and Its Role in Allergy

Humans are exposed to numerous electrophilic chemicals either as medicines, in the workplace, in nature, or through use of many common cosmetic and household products. Covalent modification of human proteins by such chemicals, or protein haptenation, is a common occurrence in cells and may result in generation of antigenic species, leading to development of hypersensitivity reactions. Ranging in severity of symptoms from local cutaneous reactions and rhinitis to potentially life-threatening anaphylaxis and severe hypersensitivity reactions such as Stephen-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), all these reactions have the same Molecular Initiating Event (MIE), i.e. haptenation. However, not all individuals who are exposed to electrophilic chemicals develop symptoms of hypersensitivity. In the present review, we examine common chemistry behind the haptenation reactions leading to formation of neoantigens. We explore simple reactions involving single molecule additions to a nucleophilic side chain of proteins and complex reactions involving multiple electrophilic centers on a single molecule or involving more than one electrophilic molecule as well as the generation of reactive molecules from the interaction with cellular detoxification mechanisms. Besides generation of antigenic species and enabling activation of the immune system, we explore additional events which result directly from the presence of electrophilic chemicals in cells, including activation of key defense mechanisms and immediate consequences of those reactions, and explore their potential effects. We discuss the factors that work in concert with haptenation leading to the development of hypersensitivity reactions and those that may act to prevent it from developing. We also review the potential harnessing of the specificity of haptenation in the design of potent covalent therapeutic inhibitors.

NanoLC-timsTOF-Assisted Analysis of Glycated Albumin in Diabetes-Affected Plasma and Tears

Glycation is a spontaneous and nonenzymatic glycosylation. Glycated albumin (GA), which serves as an important biomarker in plasma in the diagnosis and characterization of diabetes, can be passively filtered from the plasma to tears. Tears are important targets for research in clinical diagnostics due to the ability to collect this biofluid noninvasively and repeatably. Therefore, the analysis of GA in tear film provides information for monitoring diabetes progression independent of blood pathologies. Due to the limited volume (1-5 μL) of natural tear film, we developed a small volume assay using a nano liquid chromatography-trapped ion mobility spectrometry-time-of-flight MS (nanoLC-timsTOF) platform for the analysis of glycated albumin in human plasma and tear films affected by diabetes. The peptides containing lysine 525, which is the main glycation site in GA, were relatively quantified and represented as the GA level. The results of the measurements showed that GA levels were significantly higher in diabetes-affected plasma and tears compared to controls with a p-value < 0.01. A strong correlation of glycated albumin levels was observed for the plasma and tear film in diabetes samples (Pearson coefficient 0.92 with a p-value 0.0012). Moreover, the number of GA glycation sites was significantly higher in diabetes-affected plasma and tear comparatively to controls. Among all the glycation sites in plasma albumin, the GA level quantified by lysine 136/137 had a strong correlation with more commonly used lysine 525, suggesting that lysine 136 /137 is an alternative diabetes biomarker in plasma. Overall, our findings demonstrate GA in tears as a biomarker for monitoring diabetes progression, highlighting new possibilities for quick and noninvasive diabetes detection and monitoring.

Protein Advanced Glycation End Products and Their Implications in Pancreatic Cancer

Protein advanced glycation end products (AGE) formed by nonenzymatic glycation can disrupt the normal structure and function of proteins, and stimulate the receptor for AGEs (RAGE), triggering intricate mechanisms that are etiologically related to various chronic diseases, including pancreatic cancer. Many common risk factors of pancreatic cancer are the major sources for the formation of protein AGEs and glycative stress in the human body. Abnormal accumulation of protein AGEs can impair the cellular proteome and promote AGE-RAGE driven pro-inflammatory signaling cascades, leading to increased oxidative stress, protease resistance, protein dysregulation, transcription activity of STAT, NF-κB, and AP-1, aberrant status in ubiquitin-proteasome system and autophagy, as well as other molecular events that are susceptible for the carcinogenic transformation towards the development of neoplasms. Here, we review studies to highlight our understanding in the orchestrated molecular events in bridging the impaired proteome, dysregulated functional networks, and cancer hallmarks initiated upon protein AGE formation and accumulation in pancreatic cancer.

Antiglycation, antifibrillation and antioxidative effects of para coumaric acid and vitamin D; an in-vitro and in-silico comparative-cum-synergistic approach

Diabetes Mellitus is a metabolic disorder that results in impaired utilization of carbohydrates, lipids, and proteins. Severe hyperglycemia is its principal clinical symptom. Human serum albumin (HSA) is used as a model protein since it is viewed as a sign of glycaemic management because it is more likely to get glycated in diabetic people than other proteins. Para-coumaric acid (pCA), a phenolic acid, and Vitamin D (vit-D) are used as protective agents. In the present work, we deduce a synergistic-cum-comparative effect of pCA and vit-D, expecting some improvement in the efficacy of pCA when combined with vit-D. Methods employed are DPPH radical scavenging activity, In-vitro glycation of HSA, UV-vis spectroscopy, fluorescence analysis, and circular dichroism measurement. After treatment, increasein the absorbance and fluorescence intensity were reduced along with normalization of CD value. . The glycation-mediated fibrillation assessed by Congo-Red and Thioflavin T (ThT) were found to be diminishedwhen HSA was treated with equimolar concentration of p-CA and vit-D- treatment. Fructosamine adduct formation and lysine modificationwas also decreased, while inhibition to hemolysis and lipid peroxidation was found to increase upon treatment. The reactive oxygen species generation detection was also performed in lymphocytes treated with various protein samples. Docking results further confirmed theblocking some glycation-prone amino acids by both compounds. The study shows that the combination in the ratio of 1:1 has provided higher overall protection comparable to aminoguanidine (AG), the molecule which is utilized as a positive control.

Discovery and validation of glucose-sensitive peptide biomarkers from human serum albumin to diagnose type 2 diabetes mellitus

Glycated albumin (GA), which represents the global glycation level of albumin, has emerged as a biomarker for diagnosing prediabetes and diabetes. In our previous study, we developed a peptide-based strategy and found three putative peptide biomarkers from the tryptic peptides of GA to diagnose type 2 diabetes mellitus (T2DM). However, the trypsin cleavage sites at the carboxyl side of lysine (K) and arginine (R) are consistent with the nonenzymatic glycation modification site residues, which considerably increases the number of missed cleavage sites and half-cleaved peptides. To solve this problem, the endoproteinase Glu-C was used to digest GA from human serum to screen putative peptides to diagnose T2DM. In the discovery phase, we found eighteen and fifteen glucose-sensitive peptides from purified albumin and human serum incubated with ¹³C glucose in vitro, respectively. In the validation phase, eight glucose-sensitive peptides were screened and validated in 72 clinical samples (28 healthy controls and 44 patients with diabetes) using label-free LC-ESI-MRM. Three putative sensitive peptides (VAHRFKDLGEE, FKPLVEEPQNLIKQNCE and NQDSISSKLKE) from albumin exhibited good specificity and sensitivity based on receiver operating characteristic analysis. In summary, three peptides were found as promising biomarkers for the diagnosis and assessment of T2DM based on mass spectrometry.

Protein glycation in diabetes mellitus

Diabetes mellitus is the ninth leading cause of mortality worldwide. It is a complex disease that manifests as chronic hyperglycemia. Glucose exposure causes biochemical changes at the proteome level as reflected in accumulation of glycated proteins. A prominent example is hemoglobin A1c (HbA1c), a glycated protein widely accepted as a diabetic indicator. Another emerging biomarker is glycated albumin which has demonstrated utility in situations where HbA1c cannot be used. Other proteins undergo glycation as well thus impacting cellular function, transport and immune response. Accordingly, these glycated counterparts may serve as predictors for diabetic complications and thus warrant further inquiry. Fortunately, modern proteomics has provided unique analytic capability to enable improved and more comprehensive exploration of glycating agents and glycated proteins. This review broadly covers topics from epidemiology of diabetes to modern analytical tools such as mass spectrometry to facilitate a better understanding of diabetes pathophysiology. This serves as an attempt to connect clinically relevant questions with findings of recent proteomic studies to suggest future avenues of diabetes research.

Comprehensive profiling and kinetic studies of glycated lysine residues in human serum albumin

Lysine residues of proteins slowly react with glucose forming Amadori products. In hyperglycemic conditions, such as diabetes mellitus, this non-enzymatic glycation becomes more pervasive causing severe medical complications. The structure and conformation of a protein predisposes lysine sites to differing reactivity influenced by their steric availability and amino acid microenvironment. The goal of our study was to identify these sites in albumin and measure glycation affinities of lysine residues. We applied a bottom-up approach utilizing a combination of three LC-MS instruments: timsTOF, Orbitrap, and QTRAP. To prove applicability to samples of varying glycemic status, we compared in vitro glycated and non-glycated HSA, as well as diabetic and non-diabetic individual samples. The analysis of lysine glycation affinities based on peptide intensities provide a semi-quantitative approach, as the results depend on the mass spectrometry platform used. We found that glycation levels based on multiple reaction monitoring (MRM) quantitation better reflect individual glycemic status and that the glycation percentage for each site is in linear relation to all other sites. To develop an approach which more accurately reflects glycation affinity, we developed a kinetics model which uses results from stable isotope dilution HPLC-MRM methodology. Through glycation of albumin at different glucose concentrations, we determine the rate constants of glycation for every lysine residue by simultaneous comparative analysis.

Open Search of Peptide Glycation Products from Tandem Mass Spectra

Identification of chemically modified peptides in mass spectrometry (MS)-based glycation studies is a crucial yet challenging task. There is a need to establish a mode for matching tandem mass spectrometry (MS/MS) data, allowing for both known and unknown peptide glycation modifications. We present an open search approach that uses classic and modified peptide fragment ions. The latter are shifted by the mass delta of the modification. Both provide key structural information that can be used to assess the peptide core structure of the glycation product. We also leverage redundant neutral losses from the modification side chain, introducing a third ion class for matching referred to as characteristic fragment ions. We demonstrate that peptide glycation product MS/MS spectra contain multidimensional information and that most often, more than half of the spectral information is ignored if no attempt is made to use a multi-step matching algorithm. Compared to regular and/or modified peptide ion matching, our triple-ion strategy significantly increased the median interpretable fraction of the glycation product MS/MS spectra. For reference, we apply our approach for Amadori product characterization and identify all established diagnostic ions automatically. We further show how this method effectively applies the open search concept and allows for optimized elucidation of unknown structures by presenting two hitherto undescribed peptide glycation modifications with a delta mass of 102.0311 and 268.1768 Da. We characterize their fragmentation signature by integration with isotopically labeled glycation products, which provides high validity for non-targeted structure identification.

Proteomic Analysis of Methylglyoxal Modifications Reveals Susceptibility of Glycolytic Enzymes to Dicarbonyl Stress

Methylglyoxal (MGO) is a highly reactive cellular metabolite that glycates lysine and arginine residues to form post-translational modifications known as advanced glycation end products. Because of their low abundance and low stoichiometry, few studies have reported their occurrence and site-specific locations in proteins. Proteomic analysis of WIL2-NS B lymphoblastoid cells in the absence and presence of exogenous MGO was conducted to investigate the extent of MGO modifications. We found over 500 MGO modified proteins, revealing an over-representation of these modifications on many glycolytic enzymes, as well as ribosomal and spliceosome proteins. Moreover, MGO modifications were observed on the active site residues of glycolytic enzymes that could alter their activity. We similarly observed modification of glycolytic enzymes across several epithelial cell lines and peripheral blood lymphocytes, with modification of fructose bisphosphate aldolase being observed in all samples. These results indicate that glycolytic proteins could be particularly prone to the formation of MGO adducts.

Comprehensive overview of human serum albumin glycation in diabetes mellitus

The presence of excess glucose in blood is regarded as a sweet hurt for patients with diabetes. Human serum albumin (HSA) is the most abundant protein in human plasma, which undergoes severe non-enzymatic glycation with glucose in patients with diabetes; this modifies the structure and function of HSA. Furthermore, the advanced glycation end products produced by glycated HSA can cause pathological damage to the human body through various signaling pathways, eventually leading to complications of diabetes. Many potential glycation sites on HSA have different degrees of sensitivity to glucose concentration. This review provides a comprehensive assessment of the in vivo glycation sites of HSA; it also discusses the effects of glycation on the structure and function of HSA. Moreover, it addresses the relationship between HSA glycation and diabetes complications. Finally, it focuses on the value of non-enzymatic glycation of HSA in diabetes-related clinical applications.

Molecular characterization of sequence-driven peptide glycation

Peptide glycation is an important, yet poorly understood reaction not only found in food but also in biological systems. The enormous heterogeneity of peptides and the complexity of glycation reactions impeded large-scale analysis of peptide derived glycation products and to understand both the contributing factors and how this affects the biological activity of peptides. Analyzing time-resolved Amadori product formation, we here explored site-specific glycation for 264 peptides. Intensity profiling together with in-depth computational sequence deconvolution resolved differences in peptide glycation based on microheterogeneity and revealed particularly reactive peptide collectives. These peptides feature potentially important sequence patterns that appear in several established bio- and sensory-active peptides from independent sources, which suggests that our approach serves system-wide applicability. We generated a pattern peptide map and propose that in peptide glycation the herein identified molecular checkpoints can be used as indication of sequence reactivity.

Integrated Strategy for Discovery and Validation of Glycated Candidate Biomarkers for Hemodialysis Patients with Cardiovascular Complications

Glycation plays a pathogenic role in many age-related degenerative pathological conditions, such as diabetes, end-stage renal diseases, and cardiovascular diseases. Mass spectrometry-based qualitative and quantitative analysis methods have been greatly developed and contribute to our understanding of protein glycation. However, it is still challenging to sensitively and accurately quantify endogenous glycated proteome in biological samples. Herein, we proposed an integrated and robust quantitative strategy for comprehensive profiling of early-stage glycated proteome. In this strategy, a filter-assisted sample preparation method was applied to reduce sample loss and improve reproducibility of sample preparation, contributing to high-throughput analysis and accurate quantification of endogenous glycated proteins with low abundance. Standard glycated peptides were spiked and performed the subsequent process together with complex samples both in label-free quantification and multiple reaction monitoring (MRM) analysis, contributing to the improvement of quantitative accuracy. In parallel, a novel approach was developed for the synthesis of heavy isotope-labeled glycated peptides used in MRM analysis. By this way, a total of 1128 endogenous glycated peptides corresponding to 203 serum proteins were identified from 60 runs of 10 pairs of hemodialysis patients with and without cardiovascular complications, and 234 glycated peptides corresponding to 63 proteins existed in >70% runs, among which 17 peptides were discovered to be differentially glycated (P < 0.05, fold-change > 1.5 or <0.67). Furthermore, we validated the glycation difference of four target peptides in 46 serum samples using MRM analysis, which were consistent with our results of label-free quantification.

Dietary inclusion of Antarctic krill meal during the finishing feed period improves health and fillet quality of Atlantic salmon (Salmo salar L.)

There is an urgent need to find alternative feed resources that can further substitute fishmeal in Atlantic salmon diets without compromising health and food quality, in particular during the finishing feeding period when the feed demand is highest and flesh quality effects are most significant. This study investigates efficacy of substituting a isoprotein (35 %) and isolipid (35 %) low fishmeal diet (FM, 15 %) with Antarctic krill meal (KM, 12 %) during 3 months with growing finishing 2·3 kg salmon (quadruplicate sea cages/diet). Final body weight (3·9 (se 0·04) kg) was similar in the dietary groups, but the KM group had more voluminous body shape, leaner hearts and improved fillet integrity, firmness and colour. Ectopic epithelial cells and focal Ca deposits in intestine were only detected in the FM group. Transcriptome profiling by microarray of livers showed dietary effects on several immune genes, and a panel of structural genes were up-regulated in the KM group, including cadherin and connexin. Up-regulation of genes encoding myosin heavy chain proteins was the main finding in skeletal muscle. Morphology examination by scanning electron microscopy and secondary structure by Fourier transform IR spectroscopy revealed more ordered and stable collagen architecture of the KM group. NEFA composition of skeletal muscle indicated altered metabolism of n-3, n-6 and SFA of the KM group. The results demonstrated that improved health and meat quality in Atlantic salmon fed krill meal were associated with up-regulation of immune genes, proteins defining muscle properties and genes involved in cell contacts and adhesion, altered fatty acid metabolism and fat deposition, and improved gut health and collagen structure.

Comprehensive Glycomic Analysis Reveals That Human Serum Albumin Glycation Specifically Affects the Pharmacokinetics and Efficacy of Different Anticoagulant Drugs in Diabetes

Long-term hyperglycemia in patients with diabetes leads to human serum albumin (HSA) glycation, which may impair HSA function as a transport protein and affect the therapeutic efficacy of anticoagulants in patients with diabetes. In this study, a novel mass spectrometry approach was developed to reveal the differences in the profiles of HSA glycation sites between patients with diabetes and healthy subjects. K199 was the glycation site most significantly changed in patients with diabetes, contributing to different interactions of glycated HSA and normal HSA with two types of anticoagulant drugs, heparin and warfarin. An in vitro experiment showed that the binding affinity to warfarin became stronger when HSA was glycated, while HSA binding to heparin was not significantly influenced by glycation. A pharmacokinetic study showed a decreased level of free warfarin in the plasma of diabetic rats. A preliminary retrospective clinical study also revealed that there was a statistically significant difference in the anticoagulant efficacy between patients with diabetes and patients without diabetes who had been treated with warfarin. Our work suggests that larger studies are needed to provide additional specific guidance for patients with diabetes when they are administered anticoagulant drugs or drugs for treating other chronic diseases.

Signature Ions Triggered Electron-Transfer/Higher-Energy Collisional Dissociation (EThcD) for Specific and Confident Glycation Site Mapping in Therapeutic Proteins

Higher-energy collisional dissociation (HCD) is a well-established fragmentation technique in liquid chromatography tandem mass spectrometry (LC-MS/MS) and is used to study protein post translational modifications (PTMs) during peptide mapping. However, labile PTMs like glycosylation, glycation, sulfonylation, or phosphorylation tend to fragment earlier than peptide backbones under HCD. This leads to complicated MS/MS spectra, compromising data quality and downstream data interpretation. Electron-transfer/higher-energy collision dissociation (EThcD) has been used to analyze PTMs, but important components might be missed because of the increased duty cycle. To address this issue, modification-specific fragment ions formed in HCD experiments could be utilized to trigger EThcD analysis only for modified peptides. The trigger for EThcD was established by applying HCD with a high normalized collision energy, generating multiple informative Amadori derived lysine signature ions from a glycated peptide. These signature ions were then applied to trigger targeted EThcD for lysine glycation identification. This improved approach can further expand the characterization efforts of highly labile PTMs in therapeutic proteins.

Glycated Plasma Proteins as More Sensitive Markers for Glycemic Control in Type 1 Diabetes

PURPOSE

Glycated hemoglobin (HbA1c) is used clinically for diagnosis and therapeutic management of diabetes. However, HbA1c reflects average blood glucose level over a long period. The aim of this study is to look for short period, more sensitive protein markers that correlate better with glycemic level.

EXPERIMENTAL DESIGN

The glycated proteome of human plasma from type 1 diabetic individuals with good and poor (n = 20 each) glycemic control are analyzed using an online two-dimensional proteomics approach. Selected glycated peptides are further validated for their potential as candidate biomarkers using parallel reaction monitoring.

RESULTS

305 glycated peptides are quantified and 290 are significantly increased in samples with poor glycemic control. 76 of the 88 selected glycated peptides have receiver operating characteristic area under curve (AUC) values greater than 0.8. Six validated glycated peptides with high AUC show high correlation with HbA1c and have higher fold changes between poor and good glycemic control than HbA1c. The parent proteins have half-lives shorter than HbA1c.

CONCLUSIONS AND CLINICAL RELEVANCE

Using an advanced proteomics platform for protein glycation analysis, glycated peptides and proteins are identified that are promising as more sensitive, shorter term indicators of glycemic control in diabetic patients than the commonly used HbA1c.

GlyStruct: glycation prediction using structural properties of amino acid residues

Background

Glycation is a one of the post-translational modifications (PTM) where sugar molecules and residues in protein sequences are covalently bonded. It has become one of the clinically important PTM in recent times attributed to many chronic and age related complications. Being a non-enzymatic reaction, it is a great challenge when it comes to its prediction due to the lack of significant bias in the sequence motifs.

Results

We developed a classifier, GlyStruct based on support vector machine, to predict glycated and non-glycated lysine residues using structural properties of amino acid residues. The features used were secondary structure, accessible surface area and the local backbone torsion angles. For this work, a benchmark dataset was extracted containing 235 glycated and 303 non-glycated lysine residues. GlyStruct demonstrated improved performance of approximately 10% in comparison to benchmark method of Gly-PseAAC. The performance for GlyStruct on the metrics, sensitivity, specificity, accuracy and Mathew’s correlation coefficient were 0.7013, 0.7989, 0.7562, and 0.5065, respectively for 10-fold cross-validation.

Conclusion

Glycation has emerged to be one of the clinically important PTM of proteins in recent times. Therefore, the development of computational tools become necessary to predict glycation, which could help medical professionals administer drugs and manage patients more effectively. The proposed predictor manages to classify glycated and non-glycated lysine residues with promising results consistently on various cross-validation schemes and outperforms other state of the art methods.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2547-x) contains supplementary material, which is available to authorized users.

Identification of differences in the formation of plasma glycated proteins between dogs and humans under diabetes-like glucose concentration conditions

Dogs have been proposed as a translational model and used for studying aging, diabetes, and diabetes-related complications in humans. However, no studies have ever compared the glycation of plasma proteins between dogs and humans under similar experimental conditions. Thus, the aim of this study was to fill this gap by comparing the plasma protein glycation patterns of dogs and humans in an ex-vivo system. Canine and human plasma samples were incubated with glucose at concentrations comparable to those observed in diabetic patients. The final glucose plasma concentration resulted in similar glucose:albumin ratios in both species. Glycated proteins were evaluated by measuring the content of fructosamine, protein carbonyls, and the formation of advanced glycation end-products (AGEs). The concentrations of fructosamine and protein carbonyls in canine and human plasma increased in a glucose concentration-dependent manner (P < 0.0001). Of note, the relative increment of fructosamine and protein carbonyl content and AGE formation was always higher in human than in dog plasma. Our results reveal that the plasma glycation processes in dogs and humans are not similar. These novel findings could contribute to improve our understating about canine and human diabetes as well as other condition associated in the glycation of proteins.

Determining the glycation site specificity of human holo-transferrin

Understanding the effect of glycation on the function of transferrin, the systemic iron transporter, is fundamental to fully grasp the mechanisms leading to the loss of iron homeostasis observed in diabetes mellitus (DM). The spontaneous reaction with protein amino groups is one of the main causes of glucose toxicity, but the site specificity of this reaction is still poorly understood. Here in, an in vitro approach was used to study human holo-transferrin glycation in detail. Lysine residues 103, 312 and 380 proved to be the most reactive sites, and overall glycation specificity was found to be remarkably different from that described for apo-transferrin. A computational biochemistry approach was subsequently applied to rationalize lysine reactivity. Even though pK_a values, solvent accessible surface area, hydrogen bonds or the presence of nearby charged/polar residues could be related to lysine reactivity, these parameters do not suffice to describe glycation site specificity in holo-transferrin. Furthermore, analysis of the most reactive residues suggests that the correct lysine side chain orientation may play a fundamental role in reactivity. Nevertheless, in holo-transferrin, glycation occurs away from the iron-binding sites and, despite the observed iron release, the modification of apo-transferrin should play a more relevant role for the loss of iron-binding capacity observed in the blood serum of DM patients.

Online 2D-LC-MS/MS Platform for Analysis of Glycated Proteome

Glycated proteins are emerging as good indicators for diabetes and age related diseases. However, the platform for analysis of glycated proteome has been relatively less well established. We here introduce an online 2D-LC-HCD-MS/MS platform for comprehensive glycated peptide quantification. This platform includes a boronate affinity column in the first dimension for enrichment, reversed phase nanoLC column in the second dimension for separation, a benchtop Orbitrap mass spectrometer with HCD-MS/MS for peptide sequencing, and MaxQuant bioinformatics tool for identification and quantification of glycated peptides. This online 2D-LC-HCD-MS/MS platform has high enrichment efficiency with 85% of identified peptides in the enriched fraction as glycated, high sensitivity for detection of glycated peptides with LOD and LOQ at 1.2 and 2.4 pg, respectively, and high reproducibility with interday CVs < 20% for 80% of the glycated peptides. The number of glycated peptides quantified in in vitro glycated human plasma increased more than 3-fold using this platform in comparison to that obtained using 1D-LC-HCD-MS/MS platform without boronate affinity enrichment. Application of this online platform to human plasma identified 376 glycated peptides from 10 μg of protein digests. This highly sensitive and reproducible online 2D platform is promising for glycated protein analysis of complex clinical samples.

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.

An overview of in vitro and in vivo glycation of albumin: a potential disease marker in diabetes mellitus

Non-enzymatic glycation of macromolecules, especially proteins leading to their oxidation is increased in diabetes mellitus due to hyperglycaemia and play an important role in associated complications of the disease. Protein glycation mostly occurs in intra chain lysine residues resulting in the formation of early stage Amadori products which are finally converted to advance glycation end products (AGEs). This review deals with the structural studies of in vitro and in vivo glycated human serum albumin (HSA). The aim of this review is to explain the disturbance in secondary and tertiary structure of albumin upon glucosylation and the immunogenic potential of modified albumin. Amadori-albumin may have enough potential to provoke the immunoregulatry cells and generate autoantibodies in diabetic patients. Role of Amadori-albumin in the induction of autoantibodies in type2 diabetes especially in chronic kidney disease (CKD) patients has been discussed. This review also considers various studies that investigate the effects of glycation on the structural and immunological properties of HSA. The use of glycated albumin (GA) as a short to intermediate term marker for glycaemic control in diabetes is also focused.

Quantification of prospective type 2 diabetes mellitus biomarkers by stable isotope dilution with bi-labeled standard glycated peptides

A new analytical approach is proposed for quantification of multiple glycated peptides in human plasma tryptic digests.

Importance of post-translational modifications on the function of key haemostatic proteins

Post-translational modifications (PTMs) such as glycosylation and phosphorylation play an important role on the function of haemostatic proteins and are critical in the setting of disease. Such secondary level changes to haemostatic proteins have wide ranging effects on their ability to interact with other proteins. This review aimed to summarize the knowledge of the common PTMs associated with haemostatic proteins and the implications of such modifications on protein function. Haemostatic proteins that represent the main focus for studies specific to PTMs are von Willebrand factor, tissue factor, factor VIII, antithrombin and fibrinogen. These proteins are susceptible to PTMs by glycosylation, phosphorylation, sulphation, citrullination and nitration, respectively, with a significant impact on their function. During synthesis, vWF must undergo extensive PTMs, with N-linked glycosylation being the most common. Increased phosphorylation of tissue factor results in increased affinity for platelets to the vessel endothelium. Citrullination of antithrombin leads to an increased anticoagulant function of this protein and therefore an anticoagulant state that inhibits clot formation. On the contrary, nitration of fibrinogen has been shown to result in a prothrombotic state, whilst sulphation is required for the normal function of Factor VIII. From this review, it is evident that PTMs of haemostatic proteins as a change in protein structure at a secondary level greatly influences the behaviour of the protein at a tertiary level.

Glycation Reactivity of a Quorum-Sensing Signaling Molecule

Reported herein is that (4S)-4,5-dihydroxy-2,3-pentanedione (DPD) can undergo a previously undocumented non-enzymatic glycation reaction. Incubation of DPD with viral DNA or the antibiotic gramicidin S resulted in significant biochemical alterations. A protein-labeling method was consequently developed that facilitated the identification of unrecognized glycation targets of DPD in a prokaryotic system. These results open new avenues toward tracking and understanding the fate and function of the elusive quorum-sensing signaling molecule.

Enhanced Approaches for Identifying Amadori Products: Application to Peanut Allergens

The dry roasting of peanuts is suggested to influence allergic sensitization as a result of the formation of advanced glycation end products (AGEs) on peanut proteins. Identifying AGEs is technically challenging. The AGEs of a peanut allergen were probed with nano-scale liquid chromatography-electrospray ionization-mass spectrometry (nanoLC-ESI-MS) and tandem mass spectrometry (MS/MS) analyses. Amadori product ions matched to expected peptides and yielded fragments that included a loss of three waters and HCHO. As a result of the paucity of b and y ions in the MS/MS spectrum, standard search algorithms do not perform well. Reactions with isotopically labeled sugars confirmed that the peptides contained Amadori products. An algorithm was developed on the basis of information content (Shannon entropy) and the loss of water and HCHO. Results with test data show that the algorithm finds the correct spectra with high precision, reducing the time needed to manually inspect data. Computational and technical improvements allowed for better identification of the chemical differences between modified and unmodified proteins.

Identification of Glucose-Binding Pockets in Human Serum Albumin Using Support Vector Machine and Molecular Dynamics Simulations

Human Serum Albumin (HSA) has been suggested to be an alternate biomarker to the existing Hemoglobin-A1c (HbA1c) marker for glycemic monitoring. Development and usage of HSA as an alternate biomarker requires the identification of glycation sites, or equivalently, glucose-binding pockets. In this work, we combine molecular dynamics simulations of HSA and the state-of-art machine learning method Support Vector Machine (SVM) to predict glucose-binding pockets in HSA. SVM uses the three dimensional arrangement of atoms and their chemical properties to predict glucose-binding ability of a pocket. Feature selection reveals that the arrangement of atoms and their chemical properties within the first 4Å from the centroid of the pocket play an important role in the binding of glucose. With a 10-fold cross validation accuracy of 84 percent, our SVM model reveals seven new potential glucose-binding sites in HSA of which two are exposed only during the dynamics of HSA. The predictions are further corroborated using docking studies. These findings can complement studies directed towards the development of HSA as an alternate biomarker for glycemic monitoring.

Redox proteomics: Methods for the identification and enrichment of redox-modified proteins and their applications

PTMs are defined as covalent additions to functional groups of amino acid residues in proteins like phosphorylation, glycosylation, S-nitrosylation, acetylation, methylation, lipidation, SUMOylation as well as oxidation. Oxidation of proteins has been characterized as a double-edged sword. While oxidative modifications, in particular of cysteine residues, are widely involved in the regulation of cellular homeostasis, oxidative stress resulting in the oxidation of biomolecules along with the disruption of their biological functions can be associated with the development of diseases, such as cancer, diabetes, and neurodegenerative diseases, respectively. This is also the case for advanced glycation end products, which result from chemical reactions of keto compounds such as oxidized sugars with proteins. The role of oxidative modifications under physiological and pathophysiological conditions remains largely unknown. Recently, novel technologies have been established that allow the enrichment, identification, and characterization of specific oxidative PTMs (oxPTMs). This is essential to develop strategies to prevent and treat diseases that are associated with oxidative stress. Therefore this review will focus on (i) the methods and technologies, which are currently applied for the detection, identification, and quantification of oxPTMs including the design of high throughput approaches and (ii) the analyses of oxPTMs related to physiological and pathological conditions.

Effect of selective breeding on collagen properties of Atlantic salmon (Salmo salar L.)

Collagen content and properties of skeletal muscle were studied among selected (FP) and unselected (WP) Atlantic salmon lines that were reared together to avoid any environmental effects. The FP group had significantly higher body weight at harvesting, softer texture and lower connective tissue yield compared with the WP group. The relative collagen fractions (acid, pepsin, insoluble) were similar, but the FP group had a greater abundance of amino acids involved in collagen triple helix conformation and stabilisation (Gly, Pro, Hyp and Hyl), whilst the Lys content was greater for the WP group, indicating a more aggregated collagen. The connective tissue denaturation temperature was lower for the FP group, coinciding with a lower degree of collagen self-assembly and intermolecular-crosslinks. It is concluded that selective breeding has resulted in lower connective tissue stability of Atlantic salmon fillets.

Glycation of nail proteins: from basic biochemical findings to a representative marker for diabetic glycation-associated target organ damage

Background

Although assessment of glycated nail proteins may be a useful marker for monitoring of diabetes, their nature and formation are still poorly understood. Besides a detailed anatomical analysis of keratin glycation, the usefulness of glycated nail protein assessment for monitoring diabetic complications was investigated.

Methods

216 patients (94 males, 122 females; mean age ± standard deviation: 75.0 ± 8.7 years) were enrolled. Glycation of nail and eye lens proteins was assessed using a photometric nitroblue tetrazolium-based assay. Following chromatographic separation of extracted nail proteins, binding and nonbinding fractions were analyzed using one-dimensional gel electrophoresis. Using a hand piece containing a latch-type-bur, a meticulous cutting of the nail plate into superficial and deep layers was performed, followed by a differential analysis of fructosamine.

Results

Using SDS PAGE, four and two bands were identified among the nonglycated and glycated nail fraction respectively. Significantly lower fructosamine concentrations were found in the superficial nail layer (mean: 2.16 ± 1.37 μmol/g nails) in comparison with the deep layer (mean: 4.36 ± 2.55 μmol/g nails) (P<0.05). A significant higher amount of glycated eye lens proteins was found in diabetes mellitus patients (mean: 3.80 ± 1.57 μmol/g eye lens) in comparison with nondiabetics (mean: 3.35 ± 1.34 μmol/g eye lens) (P<0.05). A marked correlation was found between glycated nail and glycated eye lens proteins [y (glycated nail proteins) = 0.39 + 0.99 x (eye lens glycated proteins); r² = 0.58, P<0.001]. The concentration of glycated eye lens proteins and the HbA1c level were found to be predictors of the concentration of glycated nail proteins.

Conclusions

Glycation of nail proteins takes place in the deep layer of finger nails, which is in close contact with blood vessels and interstitial fluid. Glycation of nail proteins can be regarded as a representative marker for diabetic glycation-associated target organ damage.

Comparison of modification sites in glycated crystallin in vitro and in vivo

Glycation of α-crystallin is responsible for age- and diabetic-related cataracts, which are the main cause of blindness worldwide. We optimized the method of identification of lysine residues prone to glycation using the combination of LC-MS, isotopic labeling, and modified synthetic peptide standards with the glycated lysine derivative (Fmoc-Lys(i,i-Fru,Boc)-OH). The in vitro glycation of bovine lens α-crystallin was conducted by optimized method with the equimolar mixture of [(12)C6]- and [(13)C6]D-glucose. The in vivo glycation was studied on human lens crystallin. The glycated protein was subjected to proteolysis and analyzed using LC-MS. The results of in vitro and in vivo glycation of α-crystallin reveal a different distribution of the modified lysine residues. More Amadori products were detected as a result of the in vitro reaction due to forced glycation conditions. The developed method allowed us to identify the glycation sites in crystallin from eye lenses obtained from patients suffering from the cataract. We identified K166 in the A chain and K166 in the B chain of α-crystallin as major glycation sites during the in vitro reaction. We found also two in vivo glycated lysine residues: K92 in the B chain and K166 in the A chain, which are known as locations for Amadori products. These modification sites were confirmed by the LC-MS experiment using two synthetic standards. This study demonstrates the applicability of the LC-MS methods combined with the isotopic labeling and synthetic peptide standards for analysis of post-translational modifications in the biological material.

A chemical and computational approach to comprehensive glycation characterization on antibodies

Non-enzymatic glycation is a challenging post-translational modification to characterize due to the structural heterogeneity it generates in proteins. Glycation has become increasingly recognized as an important product quality attribute to monitor, particularly for the biotechnology sector, which produces recombinant proteins under conditions that are amenable to protein glycation. The elucidation of sites of glycation can be problematic using conventional collision-induced dissociation (CID)-based mass spectrometry because of the predominance of neutral loss ions. A method to characterize glycation using an IgG1 monoclonal antibody (mAb) as a model is reported here. The sugars present on this mAb were derivatized using sodium borohydride chemistry to stabilize the linkage and identified using CID-based MS(2) mass spectrometry and spectral search engines. Quantification of specific glycation sites was then done using a targeted MS(1) based approach, which allowed the identification of a glycation hot spot in the heavy chain complementarity-determining region 3 of the mAb. This targeted approach provided a path forward to developing a structural understanding of the propensity of sites to become glycated on mAbs. Through structural analysis we propose a model in which the number and 3-dimensional distances of carboxylic acid amino acyl residues create a favorable environment for glycation to occur.

The glycation site specificity of human serum transferrin is a determinant for transferrin's functional impairment under elevated glycaemic conditions

Human serum transferrin is susceptible to modification under elevated glycaemic conditions, such as those encountered in diabetes mellitus. The study of transferrin glycation shows that key amino acid residues undergo glycation, inducing structural alterations that compromise its function as an iron-binding protein.

Identification and comparative quantitation of glycation by stable isotope labeling and LC-MS

Glycation is a common modification of proteins both in vitro and in vivo. To aid identification and comparative quantitation, a method of stable isotope labeling followed by LC-MS analysis was proposed. The samples were reduced using sodium borohydride or sodium borodeuteride. Reduction of the Schiff base between the amine group and the reducing sugars resulted in a molecular weight increase of 2Da using sodium borohydride or a molecular weight increase of 3Da using sodium borodeuteride. The molecular weight difference of 1Da between peptides containing glycated lysine residue reduced using sodium borohydride or sodium borodeuteride was used to identify glycated peptides and to calculate the glycation difference between samples. The method was used to investigate glycation of a recombinant human IgG1 antibody under native and denaturing conditions. The result demonstrated a good correlation between glycation propensity of lysine residues and their solvent exposure levels.

Glycated proteome: from reaction to intervention

Glycation, a nonenzymatic reaction between reducing sugars and proteins, is a proteome wide phenomenon, predominantly observed in diabetes due to hyperglycemia. Glycated proteome of plasma, kidney, lens, and brain are implicated in the pathogenesis of various diseases, including diabetic complications, neurodegenerative diseases, cancer, and aging. This review discusses the strategies to characterize protein glycation, its functional implications in different diseases, and intervention strategies to protect the deleterious effects of protein glycation.

Non-enzymatic glycation and glycoxidation protein products in foods and diseases: an interconnected, complex scenario fully open to innovative proteomic studies

The Maillard reaction includes a complex network of processes affecting food and biopharmaceutical products; it also occurs in living organisms and has been strictly related to cell aging, to the pathogenesis of several (chronic) diseases, such as diabetes, uremia, cataract, liver cirrhosis and various neurodegenerative pathologies, as well as to peritoneal dialysis treatment. Dozens of compounds are involved in this process, among which a number of protein-adducted derivatives that have been simplistically defined as early, intermediate and advanced glycation end-products. In the last decade, various bottom-up proteomic approaches have been successfully used for the identification of glycation/glycoxidation protein targets as well as for the characterization of the corresponding adducts, including assignment of the modified amino acids. This article provides an updated overview of the mass spectrometry-based procedures developed to this purpose, emphasizing their partial limits with respect to current proteomic approaches for the analysis of other post-translational modifications. These limitations are mainly related to the concomitant sheer diversity, chemical complexity, and variable abundance of the various derivatives to be characterized. Some challenges to scientists are finally proposed for future proteomic investigations to solve main drawbacks in this research field.

A new strategy for early diagnosis of type 2 diabetes by standard-free, label-free LC-MS/MS quantification of glycated peptides

The early diagnosis of diabetes, one of the top three chronic incurable diseases, is becoming increasingly important. Here, we investigated the applicability of an (18)O-labeling technique for the development of a standard-free, label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the early diagnosis of type 2 diabetes mellitus (T2DM). Rather than attempting to identify quantitative differences in proteins as biomarkers, glycation of the highest abundance protein in human plasma, human serum albumin (HSA), was monitored through quantitative analysis of HSA characteristic peptides using the (18)O-labeling technique. Eight glucose-sensitive peptides and one glucose-insensitive peptide were discovered. The glucose-insensitive peptide served as the internal standard, and a standard-free, label-free LC-MS/MS method was developed. This method was then used to select putative biomarkers for T2DM in a clinical trial with 389 human plasma samples. As a result, three of the eight glucose-sensitive peptides (FKDLGEENFK, LDELRDEGK, and KVPQVSTPTLVEVSR) were selected and could be used as potential biomarkers for the early diagnosis of T2DM.

A study on human serum albumin influence on glycation of fibrinogen

Although in vivo glycation proceeds in complex mixture of proteins, previous studies did not take in consideration the influence of protein-protein interaction on Maillard reaction. The aim of our study was to test the influence of human serum albumin (HSA) on glycation of fibrinogen. The isotopic labeling using [(13)C6] glucose combined with LC-MS were applied as tool for identification possible glycation sites in fibrinogen and for evaluation the effect of HSA on the glycation level of selected amino acids in fibrinogen. The obtained data indicate that the addition of HSA protects the fibrinogen from glycation. The level of glycation in presence of HSA is reduced by 30-60% and depends on the location of glycated residue in sequence of protein.

Review: Glycation of human serum albumin

Glycation involves the non-enzymatic addition of reducing sugars and/or their reactive degradation products to amine groups on proteins. This process is promoted by the presence of elevated blood glucose concentrations in diabetes and occurs with various proteins that include human serum albumin (HSA). This review examines work that has been conducted in the study and analysis of glycated HSA. The general structure and properties of HSA are discussed, along with the reactions that can lead to modification of this protein during glycation. The use of glycated HSA as a short-to-intermediate term marker for glycemic control in diabetes is examined, and approaches that have been utilized for measuring glycated HSA are summarized. Structural studies of glycated HSA are reviewed, as acquired for both in vivo and in vitro glycated HSA, along with data that have been obtained on the rate and thermodynamics of HSA glycation. In addition, this review considers various studies that have investigated the effects of glycation on the binding of HSA with drugs, fatty acids and other solutes and the potential clinical significance of these effects.