Human hemolysate glycated proteome

Specific and Reversible Enrichment of Early-Stage Glycated Proteome Based on Thiazolidine Chemistry and Palladium-Mediated Cleavage

Comprehensive analysis of protein glycation is important for better understanding of its formation mechanism and biological significance. The current preconcentration methods of glycated proteome mainly depend on the reversible combination of boronic acid and cis-dihydroxy group by pH adjustment, but it has inherent limitations (e.g., poor specificity and time-consuming). Herein, for the first time, a novel enrichment method for glycated peptides is proposed based on the reversible chemical reaction between aldehyde and 1,2-aminothiol groups, in which oxidized glycated peptides are captured onto the magnetic nanoparticles via thiazolidine chemistry and then released by palladium-mediated cleavage. The method is rapid, with excellent selectivity (even at a 1:1000 molar ratio of glycated peptides/nonglycated peptides) and high sensitivity (1 fmol/μL). As a good evidence, 1549 glycated peptides were identified from glycated human serum with 94.6% specificity, providing a powerful technique for high-throughput analysis of glycated peptides.

Structural analysis of glycated human hemoglobin using native mass spectrometry

Glycated hemoglobin (GHb) is the indicator of the long-term glycemic index of an individual. GHb is formed by the irreversible modification of N-terminal α-amino group of β globin chain with glucose via Amadori rearrangement. Cation exchange chromatography exploits the difference in surface charges between GHb and native hemoglobin (HbA₀ ) for their separation and quantification. However, glucose condensation is specific to primary amino groups. Therefore, structural characterization of GHb synthesized in vivo is essential as multiple glycation may interfere with GHb assessment. The stoichiometric composition of different glycated hemoglobin from a 19% GHb sample was deduced using native mass spectrometry. We observed a comparable population of α and β glycated tetramers for mono-glycated HbA₀ . Surprisingly, doubly and triply glycated HbA₀ also showed mono-glycated α and β globins. Thus, we propose that glycation of hemoglobin (HbA) occurs symmetrically across α and β globins with preference to unmodified globin first. Correlation between conventional and mass spectrometry-based quantification of GHb showed a reliable estimation of the glycemic index of individuals carrying HbA₀ . Mutant HbAs have different retention time than HbA₀ due to the differences in their surface charge. Thus, their glycated analog may elute at different retention time compared to GHb. Consequently, our method would be ideal for assessing the glycemic index of an individual carrying mutant HbA.

Glycation profile of minor abundant erythrocyte proteome across varying glycemic index in diabetes mellitus

PURPOSE

Long-term glycemic index in patients with diabetes mellitus (DM) is measured by glycated hemoglobin (HbA_1c) besides blood glucose. In DM, the primary amino groups of proteins get glycated via non-enzymatic post-translational modification. This study aims at identifying and characterizing site-specific glycation of erythrocyte proteome across varying glycemic index in patients with DM.

EXPERIMENTS

We isolated the glycated erythrocyte proteome devoid of hemoglobin from control and diabetic samples using boronate affinity chromatography. Proteomic analysis was performed using nanoLC/ESI-MS proteomics platform. The site-specific modification on different proteins was deciphered using a customized database.

RESULTS

We report 37 glycated proteins identified and characterized from samples with HbA_1c of 6%, 8%, 12%, and 16%. Our results show that both extent and site-specific modification of proteins increased with increasing HbA_1c. The observed residue-specific modifications of catalase, peroxiredoxin, carbonic anhydrase, lactate dehydrogenase B and delta-aminolevulinic acid dehydratase were correlated with the literature report on their functional disorder in DM.

CONCLUSIONS

and clinical relevance: 37 glycated erythrocyte proteins apart from hemoglobin were characterized from DM patient samples with varying HbA_1c values. We correlated the site-specific glycation and associated functional disorder of five representative proteins. However, the clinical correlation with the observed modifications needs further investigation.

Rapid Non-Enzymatic Glycation of the Insulin Receptor under Hyperglycemic Conditions Inhibits Insulin Binding In Vitro: Implications for Insulin Resistance

The causes of insulin resistance are not well-understood in either type 1 or type 2 diabetes. Insulin (INS) is known to undergo rapid non-enzymatic covalent conjugation to glucose or other sugars (glycation). Because the insulin receptor (IR) has INS-like regions associated with both glucose and INS binding, we hypothesize that hyperglycemic conditions may rapidly glycate the IR, chronically interfering with INS binding. IR peptides were synthesized spanning IR- associated INS-binding regions. Glycation rates of peptides under hyperglycemic conditions were followed over six days using matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. INS conjugated to horse-radish peroxidase was used to determine INS binding to IR peptides in glycated and non-glycated forms. Several IR peptides were glycated up to 14% within days of exposure to 20-60 mM glucose. Rates of IR-peptide glycation were comparable to those of insulin. Glycation of four IR peptides significantly inhibits INS binding to them. Glycation of intact IR also decreases INS binding by about a third, although it was not possible to confirm the glycation sites on the intact IR. Glycation of the IR may therefore provide a mechanism by which INS resistance develops in diabetes. Demonstration of glycation of intact IR in vivo is needed.

Development of Diagnostic Fragment Ion Library for Glycated Peptides of Human Serum Albumin: Targeted Quantification in Prediabetic, Diabetic, and Microalbuminuria Plasma by Parallel Reaction Monitoring, SWATH, and MSE

Human serum albumin is one of the most abundant plasma proteins that readily undergoes glycation, thus glycated albumin has been suggested as an additional marker for monitoring glycemic status. Hitherto, only Amadori-modified peptides of albumin were quantified. In this study, we report the construction of fragment ion library for Amadori-modified lysine (AML), N(ε)-(carboxymethyl)lysine (CML)-, and N(ε)-(carboxyethyl)lysine (CEL)-modified peptides of the corresponding synthetically modified albumin using high resolution accurate mass spectrometry (HR/AM). The glycated peptides were manually inspected and validated for their modification. Further, the fragment ion library was used for quantification of glycated peptides of albumin in the context of diabetes. Targeted Sequential Window Acquisition of all THeoretical Mass Spectra (SWATH) analysis in pooled plasma samples of control, prediabetes, diabetes, and microalbuminuria, has led to identification and quantification of 13 glycated peptides comprised of four AML, seven CML, and two CEL modifications, representing nine lysine sites of albumin. Five lysine sites namely K549, K438, K490, K88, and K375, were observed to be highly sensitive for glycation modification as their respective m/z showed maximum fold change and had both AML and CML modifications. Thus, peptides involving these lysine sites could be potential novel markers to assess the degree of glycation in diabetes.

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.

Characterization of the glycated human cerebrospinal fluid proteome

Protein glycation is a nonenzymatic modification that involves pathological functions in neurological diseases. Despite the high number of studies showing accumulation of advanced end glycation products (AGEs) at clinical stage, there is a lack of knowledge about which proteins are modified, where those modifications occur, and to what extent. The goal of this study was to achieve a comprehensive characterization of proteins modified by early glycation in human cerebrospinal fluid (CSF). Approaches based on glucose diferential labeling and mass spectrometry have been applied to evaluate the glycated CSF proteome at two physiological conditions: native glucose level and in vitro high glucose content. For both purposes, detection of glycated proteins was carried out by HCD-MS2 and CID-MS3 modes after endoproteinase Glu-C digestion and boronate affinity chromatography. The abundance of glycation was assessed by protein labeling with (13)C(6)-glucose incubation. The analysis of native glycated CSF identified 111 glycation sites corresponding to 48 glycated proteins. Additionally, the in vitro high glucose level approach detected 265 glycation sites and 101 glycated proteins. The comparison of glycation levels under native and 15 mM glucose conditions showed relative concentration increases up to ten folds for some glycated proteins. This report revealed for the first time a number of key glycated CSF proteins known to be involved in neuroinflammation and neurodegenerative disorders. Altogether, the present study contains valuable and unique information, which should further help to clarify the pathological role of glycation in central nervous system pathologies. This article is part of a Special Issue entitled: Translational Proteomics.