Mass spectrometric study on the protein chemical modification of uremic patients in advanced Maillard reaction

Assessment of Absorption of Glycated Nail Proteins in Patients with Diabetes Mellitus and Diabetic Retinopathy

Background and objectives: Glycation occurs in a variety of human tissues and organs. Knowledge about the relationship between predictive biochemical factors such as absorption of glycated nail proteins and severity of type 2 diabetes mellitus (DM) and diabetic retinopathy (DR) remains limited. Materials and Methods: The study group consisted of patients with type 2 DM and DR (n = 32) and a control group (n = 28). Each patient underwent a comprehensive ophthalmic examination. The glycation process in nail clippings was evaluated in stages of in vitro glycation and deglycation stages. ATR–FTIR spectroscopy was used to calculate the infrared absorption in the region of interest. The absorption of solutions with nail clippings was evaluated by NanoDrop spectrophotometry. Absorption spectra differences before and after the exposure to fructosamine 3-kinase were compared between DM patients with DR and the control group. Results: The absorption of glycated nail protein greater than 83.00% increased the chance of developing DM and DR (OR = 15.909, 95% CI 3.914–64.660, p < 0.001). Absorption of glycated nail protein by ATR–FTIR spectroscopy in patients with DM and DR in vitro glycation was statistically significantly higher than in the control group; also absorption of solution with nails by NanoDrop spectroscopy was statistically significantly higher than in controls in vitro glycation and in vitro deglycation. After exposure to fructosamine 3-kinase, absorption of nail protein in DM + severe/proliferative DR group was statistically significantly lower in comparison with DM + mild/moderate group DR. Conclusions: Evaluation of glycated nail protein could be applied to evaluate the risk of having DM and for long-term observation of DM control.

Measurement of Protein Glycation, Glycated Peptides, and Glycation Free Adducts

Protein glycation adducts, early glycation adducts, such as N∊-fructosyl-lysine, and advanced glycation end products (AGEs) are uremic toxins. Glycation adducts are found in plasma and tissue proteins (glycation adduct residues), in peptides (glycation adduct peptide residues), and glycated amino acids (glycation free adducts). The latter two analyte groups arise from proteolysis of glycated proteins and glycation of peptides and amino acids. Quantitation of glycation adducts in uremia is difficult because of the presence of many different AGEs at low concentrations in different forms in the presence of many potential interferences. Application of liquid chromatography with tandem mass spectrometric (LC-MS/MS) detection to plasma, urine, and dialysate samples of uremic patients has provided a comprehensive and quantitative analysis of glycation adducts in uremia. Glycation free adducts accumulate markedly in the plasma of uremic patients and are eliminated in the peritoneal dialysate. Multiple glycation adducts, and also protein oxidation and nitration adducts, may be quantified concurrently. Glycation free adducts are the major form of glycation adduct eliminated in dialysate. LC-MS/MS may now be used to quantify concentrations, extents of protein modification, clearances, and excretion rates of glycation adducts in uremia.

Review of pentosidine and pyrraline in food and chemical models: formation, potential risks and determination

Pyrraline and pentosidine are advanced Maillard reaction products derived from the reaction of glucose with the lysine amino group on proteins. They have been implicated in uremia, diabetes, and related complications, including inflammation, retinopathy, and nephropathy. This review focuses on the formation mechanism, human potential risks, and detections of pentosidine and pyrraline and lays the foundation for further study of pentosidine and pyrraline. © 2017 Society of Chemical Industry.

Formation and elimination of pyrraline in the Maillard reaction in a saccharide-lysine model system

BACKGROUND

Pyrraline, a causative factor for various kinds of disease, is also used as a food contaminant to evaluate the formation of advanced glycation end-products (AGEs) in diet foods. In this study, model systems consisting of lysine and different saccharides were heated at different times, temperatures and initial molar ratios of saccharide to lysine under microwave heating conditions in order to investigate the formation of pyrraline.

RESULTS

Increase in initial molar ratio of saccharide to lysine could significantly promote the formation of pyrraline. Specifically, the pyrraline formation rate was influenced by the structure of saccharides involved in the reaction, and decreased in the following order: lactose > fructose > glucose > sucrose; the highest pyrraline was generated in lactose-lysine models. The maximum pyrraline was formed at 140 °C. Moreover, saccharides and lysine had different effects on the stability of pyrraline. Among the reactants, lysine was the major factor for the instability of pyrraline; a dipyrraline and a crosslink by pyrraline reacting with lysine could be formed.

CONCLUSION

Pyrraline formation by the saccharide-lysine model system was a dynamic reaction, consisting not only of the pyrraline formation, but also pyrraline elimination with some formation of crosslinks. © 2015 Society of Chemical Industry.

Pathophysiological importance of aggregated damaged proteins

Reactive oxygen species (ROS) are formed continuously in the organism even under physiological conditions. If the level of ROS in cells exceeds the cellular defense capacity, components such as RNA/DNA, lipids, and proteins are damaged and modified, thus affecting the functionality of organelles as well. Proteins are especially prominent targets of various modifications such as oxidation, glycation, or conjugation with products of lipid peroxidation, leading to the alteration of their biological function, nonspecific interactions, and the production of high-molecular-weight protein aggregates. To ensure the maintenance of cellular functions, two proteolytic systems are responsible for the removal of oxidized and modified proteins, especially the proteasome and organelles, mainly the autophagy-lysosomal systems. Furthermore, increased protein oxidation and oxidation-dependent impairment of proteolytic systems lead to an accumulation of oxidized proteins and finally to the formation of nondegradable protein aggregates. Accordingly, the cellular homeostasis cannot be maintained and the cellular metabolism is negatively affected. Here we address the current knowledge of protein aggregation during oxidative stress, aging, and disease.

Formation of Maillard reaction products during heat treatment of carrots

As indicators of the early stage of the Maillard reaction in carrots, N-(furoylmethyl) amino acids (FMAAs) formed during acid hydrolysis of the corresponding Amadori products were analyzed using RP-HPLC with UV detection. N(ε)-FM-Lys (furosine), FM-Gly, FM-Ala, FM-Val, FM-Ile, FM-Leu, and FM-GABA were identified using synthesized standard material by means of mass spectrometry. Furthermore, N(ε)-carboxymethyllysine (CML) and pyrraline were analyzed as indicators for advanced stages of glycation. For commercial samples with high water content, the formation of Amadori compounds predominates, whereas the advanced stage of Maillard reaction plays only a minor part. Carrot juices, baby food, and tinned carrots showed quite low rates of amino acid modification up to 5%. For dehydrated carrots, significantly higher values for Amadori products were measured, corresponding to a lysine derivatization of up to 58% and nearly 100% derivatization of GABA. Drying experiments revealed great differences in reactivity between the amino acids studied. Whereas furosine reached constant values quite quickly, some FMAAs showed a continuous increase with heating time, indicating that selected FMAAs can be used as a hallmark for the early Maillard reaction to control processing conditions.

Analysis and biological properties of amino acid derivates formed by Maillard reaction in foods

Maillard reaction products (MRPs), especially early stage MRPs and melanoidins, are currently gaining a lot of attention due to their reported health-promoting properties and their potential to be used as functional food ingredients. It is often not clear which specific biological function is assigned to which MRP, due to the large amount of MRPs formed during the reaction and difficulties in their purification and identification. This paper provides an overview of amino acid derivatives such as Amadori compounds, carboxymethyllysine, pyrraline, cross-linking products and melanoidins, which can be formed by Maillard reaction in foods, their biological properties and the analytical tools commonly employed for their determination.

Non-enzymatic model glycation reactions--a comprehensive study of the reactivity of a modified arginine with aldehydic and diketonic dicarbonyl compounds by electrospray mass spectrometry

Non-enzymatic glycation (Maillard reaction) of long-lived proteins is a major contributor to the pathology of diabetes, and possibly aging and Alzheimer's disease. Among the amino residues in proteins arginine plays an important role, and its modification by sugar moieties generates the so-called advanced glycation end products (AGEs). Moreover, alpha-dicarbonyl compounds have been found as the main participants in those modifications. Four alpha-dicarbonyl compounds, aldehydic and ketonic, were reacted with the modified amino acid N(alpha)-acetyl-L-arginine (AcArg), in an attempt to establish structure/activity relationships for the reactivity of alpha-dicarbonyls with the amine compound. Electrospray ionization mass spectrometry (ESI-MS), combined with tandem mass spectrometry (MS/MS), was used to identify and characterize reagents, intermediates and reaction products. The fragmentation patterns of precursor ions showed similarities in all reaction systems studied, in which fragmentation of the amino acid residue prevails, especially for the dehydrated and/or multiple dehydrated precursor ions. For the non-hydrated ion species, fragmentation of the arginyl guanidino group was mainly observed. Specific information regarding the nature of the ions formed, in which the dicarbonyl electrophile character played an important role, was obtained. As an example, singly and doubly hydrated acetyl-argpyrimidine ions were detected for the methylglyoxal reaction only. For symmetrical dicarbonyls, glyoxal and diacetyl, the importance of steric contributions with respect to the energetic ones is discussed. Furthermore, the dehydrated acetyl-tetrahydropyrimidine ions for methylglyoxal and phenylglyoxal reactions revealed fragment ion compositions including the protonated molecules of acetyl-argpyrimidine, -hydroimidazolone and -5-methylimidazolone. An explanation for the acetyl-argpyrimidine formation from the acetyl-hydroimidazolone formation reaction is proposed. Aspects such as the amount of acetyl-hydroimidazolone formed, the response of the hydration equilibria of the dicarbonyl forms to the new unhydrated dicarbonyls introduced by the reversal of the acetyl-hydroimidazolone formation reaction and the stability of the dicarbonyl intermediate involved in the acetyl-argpyrimidine formation are proposed, as being responsible to control the formation of acetyl-argpyrimidine.

Measurement of Nε-(Carboxymethyl)lysine and Nε-(Carboxyethyl)lysine in Human Plasma Protein by Stable-Isotope-Dilution Tandem Mass Spectrometry

Background: N ε-(Carboxymethyl)lysine (CML) and Nε-(carboxyethyl)lysine (CEL) are two stable, nonenzymatic chemical modifications of protein lysine residues resulting from glycation and oxidation reactions. We developed a tandem mass spectrometric method for their simultaneous measurement in hydrolysates of plasma proteins.

Methods: CML and CEL were liberated from plasma proteins by acid hydrolysis after addition of deuterated CML and CEL as internal standards. Chromatographic separation was performed by gradient-elution reversed-phase chromatography with a mobile phase containing 5 mmol/L nonafluoropentanoic acid as ion-pairing agent. Mass transitions of 205.1→84.1 and 219.1→84.1 for CML and CEL, respectively, and 209.1→88.1 and 223.1→88.1 for their respective internal standards were monitored in positive-ion mode.

Results: CML and CEL were separated with baseline resolution with a total analysis time of 21 min. The lower limit of quantification was 0.02 μmol/L for both compounds. Mean recoveries from plasma samples to which CML and CEL had been added were 92% for CML and 98% for CEL. Within-day CVs were &lt;7.2% for CML and &lt;8.2% for CEL, and between-day CVs were &lt;8.5% for CML and &lt;9.0% for CEL. In healthy individuals (n = 10), mean (SD) plasma concentrations of CML and CEL were 2.80 (0.40) μmol/L (range, 2.1–3.4 μmol/L) and 0.82 (0.21) μmol/L (range, 0.5–1.2 μmol/L), respectively. In hemodialysis (n = 17) and peritoneal dialysis (n = 9) patients, plasma concentrations of CML and CEL were increased two- to threefold compared with controls, without significant differences between dialysis modes [7.26 (1.36) vs 8.01 (3.80) μmol/L (P = 0.89) for CML, and 1.84 (0.39) vs 1.71 (0.42) μmol/L (P = 0.53) for CEL].

Conclusions: This stable-isotope-dilution tandem mass spectrometry method is suitable for simultaneous analysis of CML and CEL in hydrolysates of plasma proteins. Its robustness makes it suitable for assessing the value of these compounds as biomarkers of oxidative stress resulting from sugar and lipid oxidation.

N(carboxymethyl)lysine as a biomarker for microvascular complications in type 2 diabetic patients

AIMS

Hyperglycemia is linked to vascular dysfunction in patients with diabetes mellitus, either directly or through advanced glycation end product (AGE) formation. Experimental evidence has indicated the possible involvement of AGEs in the genesis of vascular complications. We investigated whether serum levels of AGEs and of the glycoxidation compound carboxymethyl-lysine (CML) were increased and correlated with vascular complications in type II diabetes mellitus.

METHODS

Serum levels of AGEs and CML-human serum protein (CML-HSP) were measured by a specific immunoassay in 51 men and 26 women aged 58 +/- 6.1 years (mean +/- SD) who had been treated for type II diabetes mellitus for 11 +/- 8 years, and in a non-diabetic control group consisting of 39 men and 21 women aged 55.5 +/- 7.5 years. Patients with macroalbuminuria or abnormal creatinine clearance were excluded from the study.

RESULTS

The serum levels of AGEs were significantly increased in patients with type II diabetes compared to controls (P<0.001). Blood levels of CML-HSP were significantly increased in diabetic patients compared to normal subjects [35.3 +/- 27.4 and 9.3 +/- 7.2 (mean +/- SD) pmol/mg of protein, respectively; P<0.0001]. In diabetic patients with retinopathy or microalbuminuria (urinary albumin excretion: UAE > 30 mg/24 h), CML-HSP levels were significantly higher (P<0.02), and even more elevated in patients with both complications.

CONCLUSION

In patients with type II diabetes, CML-HSP levels that are at variance with the HbA(1c) index for blood glucose may be a biomarker of glycoxidation, and related to the development of microvascular complications.

Gramicidin S: a peptide model for protein glycation and reversal of glycation using nucleophilic amines

Nonenzymatic glycation of proteins has been implicated in various diabetic complications and age-related disorders. Proteins undergo glycation at the N-terminus or at the epsilon-amino group of lysine residues. Glycation of proteins proceeds through the stages of Schiff base formation, conversion to ketoamine product and advanced glycation end products. Gramicidin S, which has two ornithine residues, was used as a model system to study the various stages of glycation of proteins using electrospray ionization mass spectrometry. The proximity of two ornithine residues in the peptide favors the glycation reaction. Formation of advanced glycation end products and diglycation on ornithine residues in gramicidin S were observed. The formation of Schiff base adduct is reversible, whereas the Amadori rearrangement to the ketoamine product is irreversible. Nucleophilic amines and hydrazines can deglycate the Schiff base adduct of glucose with peptides and proteins. Hydroxylamine, isonicotinic acid hydrazide and aminoguanidine effectively removed glucose from the Schiff base adduct of gramicidin S. Hydroxylamine is more effective in deglycating the adduct compared with isonicotinic acid hydrazide and aminoguanidine. The observation that the hydrazines are effective in deglycating the Schiff base adduct even in the presence of high concentrations of glucose, may have a possible therapeutic application in preventing complications of diabetes mellitus. Hydrazines may be used to distinguish between the Schiff base and the ketoamine products formed at the initial stages of glycation.

Chromatographic assay of glycation adducts in human serum albumin glycated in vitro by derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl-carbamate and intrinsic fluorescence

Glycation of proteins leads to the formation of advanced glycation endproducts (AGEs) of diverse molecular structure and biological function. Serum albumin derivatives modified to minimal and high extents by methylglyoxal and glucose in vitro have been used in many studies as model AGE proteins. The early and advanced glycation adduct contents of these proteins were investigated using the 6-aminoquinolyl-N-hydroxysuccinimidyl-carbamate (AQC) chromatographic assay of enzymic hydrolysates. AGEs derived from methylglyoxal, glyoxal and 3-deoxyglucosone, the hydroimidazolones N(delta)-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine (MG-H1), N(delta)-(5-hydro-4-imidazolon-2-yl)ornithine (G-H1) and N(delta)-[5-(2,3,4-trihydroxybutyl)-5-hydro-4-imidazolon-2-yl]ornithine (3DG-H1), bis(lysyl)imidazolium cross-links methylglyoxal-derived lysine dimer (MOLD), glyoxal-derived lysine dimer (GOLD), 3-deoxyglucosone-derived lysine dimer (DOLD), monolysyl adducts N(epsilon)-(1-carboxyethyl)lysine (CEL), N(epsilon)-carboxymethyl-lysine (CML) and pyrraline, other AGEs, N(delta)-(4-carboxy-4,6-dimethyl-5,6-dihydroxy-1,4,5,6-tetrahydropyrimidin-2-yl)ornithine (THP), argpyrimidine and pentosidine, and fructosyl-lysine were determined. AGEs with intrinsic fluorescence (argpyrimidine and pentosidine) were assayed without derivatization. Human serum albumin (HSA) glycated minimally by methylglyoxal in vitro contained mainly MG-H1 with minor amounts of THP and argpyrimidine. Similar AGEs were found in prothrombin glycated minimally by methylglyoxal and in N(alpha)-t-butyloxycarbonyl-arginine incubated with methylglyoxal. HSA glycated highly by methylglyoxal contained mainly argpyrimidine, MG-H1 and THP, with minor amounts of CEL and MOLD. HSA glycated minimally by glucose in vitro contained mainly fructosyl-lysine and CML, with minor amounts of THP, MG-H1, G-H1, 3DG-H1, argpyrimidine and DOLD. HSA glycated highly by glucose contained these AGEs and pyrraline, and very high amounts ( approximately 8 mol/mol of protein) of fructosyl-lysine. Most AGEs in albumin glycated minimally by methylglyoxal and glucose were identified. Significant proportions of arginine and lysine-derived AGEs in albumin modified highly by methylglyoxal, and lysine-derived AGEs in albumin modified highly by glucose, remain to be identified.

Identification of N(omega)-carboxymethylarginine, a new advanced glycation endproduct in serum proteins of diabetic patients: possibility of a new marker of aging and diabetes

A new advanced glycation end product (AGE), N(omega)-carboxymethyl-arginine (CMA), was found in acid-soluble skin collagen of a newborn bovine prepared by in vitro glycation with 1 M glucose incubation at 37 degrees C for about 30 days [ 1 ]. CMA production was increased with incubation time in parallel, and after 30 days incubation the yield was 100 times higher than that of pentosidine [ 1 ]. This result suggested the importance of CMA as a major AGE in collagen. We have detected and measured the CMA level in human serum proteins by electrospray ionization/liquid chromatography/mass spectrometry (ESI/LC/MS), using CMA standard concentration curve. In this report, we first show the existence of CMA in vivo, and its serum level is significantly elevated in diabetic serum proteins, compared to age-matched control serum proteins. These results provide strong evidence that CMA is a new diagnostic marker of glycation in diabetes.

Helical peptide models for protein glycation: proximity effects in catalysis of the Amadori rearrangement

INTRODUCTION

Non-enzymatic glycation of proteins has been implicated in various diabetic complications and age-related disorders. Proteins undergo glycation at the N-terminus or at the epsilon-amino group of lysine residues. The observation that only a fraction of all lysine residues undergo glycation indicates the role of the immediate chemical environment in the glycation reaction. Here we have constructed helical peptide models, which juxtapose lysine with potentially catalytic residues in order to probe their roles in the individual steps of the glycation reaction.

RESULTS

The peptides investigated in this study are constrained to adopt helical conformations allowing residues in the i and i+4 positions to come into spatial proximity, while residues i and i+2 are far apart. The placing of aspartic acid and histidine residues at interacting positions with lysine modulates the steps involved in early peptide glycation (reversible Schiff base formation and its subsequent irreversible conversion to a ketoamine product, the Amadori rearrangement). Proximal positioning of aspartic acid or histidine with respect to the reactive lysine residue retards initial Schiff base formation. On the contrary, aspartic acid promotes catalysis of the Amadori rearrangement. Presence of the strongly basic residue arginine proximate to lysine favorably affects the pK(a) of both the lysine epsilon-amino group and the singly glycated lysine, aiding in the formation of doubly glycated species. The Amadori product also formed carboxymethyl lysine, an advanced glycation endproduct (AGE), in a time-dependent manner.

CONCLUSIONS

Stereochemically defined peptide scaffolds are convenient tools for studying near neighbor effects on the reactivity of functional amino acid sidechains. The present study utilizes stereochemically defined peptide helices to effectively demonstrate that aspartic acid is an efficient catalytic residue in the Amadori arrangement. The results emphasize the structural determinants of Schiff base and Amadori product formation in the final accumulation of glycated peptides.

Advanced glycation end products: a highly complex set of biologically relevant compounds detected by mass spectrometry

Structural information on 'AGE-peptides,' a class of substances belonging to advanced glycation end products (AGE) and originating by proteolysis of glycated proteins, was gained through various analytical approaches on the mixture produced by proteinase K digestion of in vitro glycated bovine serum albumin. Both matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) were employed, and the results were compared with those from conventional spectroscopic methods (UV, fluorescence, gel permeation). The data acquired by the various techniques all depict the digestion mixtures as highly complex, with components exhibiting molecular mass in the range 300-3500 Da. In the analysis of HPLC/ESI-MS data, identification of AGE-peptides was facilitated by 3D mapping. Structural information was gained by means of multiple mass spectrometric experiments.

Chromatographic quantification of argpyrimidine, a methylglyoxal-derived product in tissue proteins: comparison with pentosidine

Methylglyoxal (MG), an alpha-dicarbonyl compound, can be produced in vivo by several metabolic pathways and the Maillard reaction. It reacts rapidly with proteins to form advanced glycation end products or AGEs. We previously isolated and characterized a blue fluorescent product of the reaction between MG and arginine, which we named argpyrimidine. We found that argpyrimidine was stable to acid hydrolysis, which allowed us to hydrolyze tissue proteins with 6 N HCl and quantify argpyrimidine by high-performance liquid chromatography. Here we report argpyrimidine concentrations in human lens and serum proteins as determined by HPLC. We have also measured pentosidine, a fluorescent AGE derived from pentose sugars, and compared the concentrations of pentosidine and argpyrimidine. We found two- to threefold higher argpyrimidine concentrations in diabetic serum proteins than in nondiabetic controls (9.3 +/- 6.7 vs 4.4 +/- 3.4 pmol/mg). We found a significant correlation (P = 0.0001) between serum protein argpyrimidine and glycosylated hemoglobin. Argpyrimidine concentrations were approximately seven times greater in brunescent cataractous lenses than in aged noncataractous lenses. Pentosidine concentrations in serum and lens proteins were much lower than argpyrimidine concentrations; in general, argpyrimidine levels were 10--25 times higher than pentosidine. Results from our study confirm that MG-mediated arginine modifications occur in vivo and provide a method for assessing protein-arginine modification by MG in aging and diabetes.

Maillard reactions in lens proteins: methylglyoxal-mediated modifications in the rat lens

The nonenzymatic Maillard reaction is thought to contribute to aging and cataract formation in the lens. As levels of methylglyoxal (MG) and glutathione (GSH) affect the reaction, we examined the relationship of these factors and determined the effect of a glyoxalase I inhibitor on the Maillard reaction. Rat lens cultures were maintained for up to 3 days in TC-199 medium with or without 20 m m glyceraldehyde (GLD) and 250 microm S-[N-hydroxy-N-(4-chlorophenyl) carbamoyl] glutathione diethyl ester (HCCG diester). We measured GSH, MG, D-lactate, glyoxalase I activity, immunoreactive MG-derived advanced glycation endproducts (MG-AGEs) and imidazolysine in organ cultured rat lenses. In vitro experiments with isolated rat lens proteins revealed that HCCG alone inhibited glyoxalase I activity in a dose-dependent manner. In organ cultured rat lens protein, GLD increased MG levels 24-fold, and the addition of HCCG diester further increased it by about two-fold. GSH levels fell sharply in the presence of GLD and this was prevented to some extent by the presence of HCCG diester. D-lactate production in the lens was suppressed by HCCG diester treatment. Dialysed lens proteins retained glyoxalase I activity, indicating that the enzyme was unaltered during incubation. MG-AGEs and imidazolysine levels were significantly higher (P<0.05) in GLD-treated lenses, but a combination of HCCG diester and GLD lowered immunoreactive MG-AGEs and imidazolysine levels compared to GLD alone. HCCG had no significant effect on MG-AGE formation in lens proteins incubated with GLD or MG. We conclude that exogenous GLD enhances MG and MG-AGE levels in the rat lens and that this increase is accompanied by a loss in GSH. In addition, inhibition of glyoxalase I promotes MG accumulation.