Glycation, receptor-mediated cell activation and vascular complications of diabetes

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.

Oxidized LDL and Fructosamine Associated with Severity of Coronary Artery Atherosclerosis in Insulin Resistant Pigs Fed a High Fat/High NaCl Diet

Background

Insulin-resistant subjects develop more severe and diffuse coronary artery atherosclerosis than insulin-sensitive controls but the mechanisms that mediate this atherosclerosis phenotype are unknown.

Research Objective

To determine the metabolic parameters that associate with the severity of coronary atherosclerosis in insulin resistant pigs fed a high fat/high NaCl diet.

Key Methods

The primary endpoint was severity of coronary atherosclerosis in adult pigs (Sus scrofa, n = 37) fed a high fat diet that also contained high NaCl (56% above recommended levels) for 1 year.

Principal Findings

Twenty pigs developed severe and diffuse distal coronary artery atherosclerosis (i.e., severe = intimal area as a percent medial area > 200% in at least 2 coronary artery cross sections and diffuse distal = intimal area as a percent medial area ≥ 150% over 3 sections separated by 2 cm in the distal half of the coronary artery). The other 17 pigs had substantially less coronary artery atherosclerosis. All 37 pigs had blood pressure in a range that would be considered hypertensive in humans and developed elevations in total and LDL and HDL cholesterol, weight gain, increased backfat, and increased insulin resistance (Bergman Si) without overt diabetes. Insulin resistance was not associated with atherosclerosis severity. Five additional pigs fed regular pig chow also developed increased insulin resistance but essentially no change in the other variables and little to no detectible coronary atherosclerosis. Most importantly, the 20 high fat/high NaCl diet -fed pigs with severe and diffuse distal coronary artery atherosclerosis had substantially greater increases (p< 0.05) in oxidized LDL (oxLDL) and fructosamine consistent with increased protein glycation.

Conclusion

In pigs fed a high fat/high NaCl diet, glycated proteins are induced in the absence of overt diabetes and this degree of increase is associated with the development of severe and diffuse distal coronary artery atherosclerosis.

Elevated Level of Carbonyl Compounds Correlates with Insulin Resistance in Type 2 Diabetes

Introduction: Recent periodicals direct that reactive carbonyl compounds are formed due to existing oxidative stress in type 2 diabetes mellitus, which further nonenzymatically react with proteins and lipids to form irreversible advanced glycation end products (AGE) and advanced lipoxidation end products (ALE). In type 2 diabetes mellitus, insulin resistance plays a pivotal role in hyperglycaemia. In this study, we tried to find the relation between insulin resistance and carbonyl stress. Materials and Methods: Forty-seven patients of type 2 diabetes mellitus (age 51 ± 5.06 years) were selected and fasting plasma glucose, serum insulin, total carbonyl compounds, HbA1c, thiobarbituric acid reacting substances (TBARS) and Trolox equivalent antioxidant capacity (TEAC) were estimated using standard protocols. Homeostatic model assessment of insulin resistance (HOMA-IR) was evaluated from fasting plasma glucose and serum insulin levels. Results: We found highly significant correlations of carbonyl compounds with HOMA-IR, fasting plasma glucose and glycated haemoglobin (HbA1c). Correlations of lipid peroxidation end product, TBARS were not so significant. Conclusion: Findings from this study indicate that the level of carbonyl compounds can be a biomarker of insulin resistance in type 2 diabetes mellitus. Keywords: Carbonyl stress, HOMA-IR, Oxidative stress

Advanced glycation end products in renal failure

Cellular and extracellular proteins suffer significant damage in vivo by glycation. Physiological proteolysis of proteins damaged by glycation forms glycation free adducts that are released into plasma for urinary excretion. Inefficient elimination of these free adducts in uremia leads to their accumulation. In mild renal insufficiency, plasma glycation free adducts accumulated as renal clearance declined. In patients with end-stage renal disease, plasma glycation free adducts were increased up to 18-fold on peritoneal dialysis and up to 40-fold on hemodialysis. Glycation free adduct concentrations in peritoneal dialysate increased with dialysate dwell time, achieving concentrations in the dialysate higher than in plasma--suggesting that glycation adduct formation may occur in the peritoneal cavity and active transport into the peritoneal cavity may occur. In hemodialysis, plasma glycation free adducts equilibrated rapidly across the dialysis membrane, with both plasma and dialysate concentrations decreasing during a dialysis session. Therefore, protein glycation free adducts normally excreted efficiently in urine show profound mishandling and accumulation in chronic renal failure. Their accumulation may impair vascular cell function and contribute to morbidity and mortality in renal disease.

Glycation free adduct accumulation in renal disease: the new AGE

Glycation adducts formed in the later stages of protein glycation reactions, advanced glycation endproducts (AGEs), are a class of uraemic toxin. Protein glycation was viewed originally as a post-translational modification that accumulated mostly on extracellular proteins. We now know that AGE residues are also formed on short-lived cellular and extracellular proteins. Cellular proteolysis forms AGE free adducts from these proteins, which are released into plasma for urinary excretion. AGE free adducts are also absorbed from food. AGE free adducts are the major molecular form by which AGEs are excreted in urine. They normally have high renal clearance, but this declines markedly in chronic renal failure patients, leading to profound increases in plasma AGE free adducts. Accumulation of plasma AGE free adducts is increased further in end stage renal disease patients on peritoneal dialysis and haemodialysis by increased AGE formation. The impact of AGEs absorbed from food is probably most marked for undialysed patients with mild uraemia. The toxicity of AGEs has been associated with resistance of the extracellular matrix to proteolysis and AGE receptor-mediated responses. AGE free adducts may also contribute to vascular disease in uraemia. They represent an important new age for glycation research in nephrology.

Peptide mapping of human serum albumin modified minimally by methylglyoxal in vitro and in vivo

Methylglyoxal is a potent glycating agent and important precursor of advanced glycation end products (AGEs) in physiological systems. Unlike glucose, methylglyoxal is predominantly an arginine-directed glycating agent. Methylglyoxal reacts with proteins to form mainly the arginine-derived hydroimidazolone AGE, Ndelta-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine (MG-H1), argpyrimidine, the lysine-derived AGEs, N(epsilon)-(1-carboxyethyl)lysine (CEL), and methylglyoxal-derived lysine dimer (MOLD). Sites within proteins susceptible to modification by methylglyoxal have not been identified. Here we show that modification of human serum albumin by methylglyoxal forms mainly hydroimidazolone MG-H1 residues. The location of MG-H1 residues was identified by mass spectrometric peptide mapping. This method identified a hot spot of hydroimidazolone formation at Arg-410, with other minor MG-H1 modifications at Arg-114, Arg-186, Arg-218, and Arg-428. Other extracellular and intracellular proteins are modified by methylglyoxal in physiological systems. Modification of arginine residues by methylglyoxal may be particularly damaging because arginine residues have a high frequency of occurrence in ligand and substrate recognition sites in receptor and enzyme active sites.

Increased protein glycation in cirrhosis and therapeutic strategies to prevent it

Glycation of liver proteins by reactive aldehydes formed from the metabolism of ethanol and lipid peroxidation has been implicated in the development of both alcoholic and nonalcoholic liver cirrhosis. Modified proteins are targeted to the proteasome for proteolysis. Release of glycation-free adducts into the circulation may provide a diagnostic "signature" of hepatic protein damage. We quantitatively screened protein glycation, oxidation, and nitrosation adduct residues and free adducts in portal, hepatic, and peripheral venous blood plasma of cirrhotic patients; we also screened the hepatic and peripheral venous blood plasma of control subjects by liquid chromatography-mass spectrometry. There was a remarkable 14-16-fold increase of glyoxal-derived, hydroimidazolone-free adduct in portal and hepatic venous plasma of cirrhotic patients with respect to normal controls. There was only a twofold increase of glycation adduct residues in plasma proteins in cirrhotic patients, which was attributed mainly to decreased albumin turnover. Therapeutic strategies to decrease dicarbonyl compounds may be beneficial, such as dicarbonyl scavengers, glutathione repleting agents, and high-dose thiamine therapy.