Mechanism of the degradation of non‐enzymatically glycated proteins under physiological conditions

Abstract

The degradation of fructosamines, formed from the non-enzymic glycation of proteins under physiological conditions, to advanced glycation end products was investigated by studying the model peptide fructosamine NE-(1-deoxy-D-fructos-1-y1)hippuryl-lysine (DHL). At pH 7.4 and 37°C in aerobic phosphate buffer, DHL degraded to form N,-carboxymethyl-hippuryl-lysine, and hippuryllysine over a 29-day incubation period. The expected NE-(3-1actato)hippuryl-lysine and 'hippuryllysylpyrraline' derivatives were not found. Superoxide radicals and hydrogen peroxide were formed during the degradation of DHL but were also both consumed during the degradation reaction. Reversal of the Amadori rearrangement was not a major fate of the fructosamine.The formation of N,-carboxymethyl-hippuryl-lysine was decreased by desferrioxamine, catalase, superoxide dismutase, catalase with superoxide dismutase, anaerobic conditions and aminoguanidine. The formation of hippuryl-lysine was decreased by desferrioxamine, catalase and catalase with superoxide dismutase, but was increased by the addition of aminoguanidine. N,-Carboxymethylserine and unmodified lysine residues are major peptide-based end products in the degradation of lysyl-fructosamine under physiological conditions. Oxygen, redox-active metal ions, catalase, superoxide dismutase and the pharmacological agent aminoguanidine are expected to be influential on the rate and fate of fructosamine degradation.Monosaccharides react non-enzymically with the N-terminal amino group and lysyl-side-chain amino groups of proteins (the Maillard reaction) under physiological conditions to form glycated proteins ( Led1 and Schleicher, 1990). Proteins known to undergo this reaction include haemoglobin, immunoglobulins, serum albumin, collagen and the lens crystallins (Bunn et al., 1978;Stevens et al., 1978;Day et al., 1979;Dolhofer et al., 1985;Bailey and Kent, 1989). The aldehyde or ketone carbonyl group of the acyclic conformer of the monosaccharide reacts with the amino group to form a Schiff s base or imine. This undergoes the Amadori rearrangement to form a ketoamine (Hodge, 1955). With glucose, the ketoamine formed is a NE-( 1 -deoxy-D-fructos-1 -yl) -amino-acid residue, a fructosamine (Mauron, 1981). The rate of formation and concentration of protein fructosamines are elevated in diabetes mellitus, due to the periodic hyperglycaemia in blood plasma and tissues associated with insulin-independent glucose uptake (lens fibre cells, capillary endothelial cells, peripheral neurones and red blood cells). The concentration of fructosamine in blood plasma and the concentration of glycated haemoglobin HbA1, are monitored clinically to assess the quality of glycaemic control in diabetic patients (Willms and Lehmann, 1990;Windeler and Kobberling, Correspondence to P. J. Thornalley,

Involvement of Hydrogen Peroxide in Collagen Cross-linking by High Glucose in Vitro and in Vivo

“…Although the experiments presented above and by others (12,33,35,36) implicate Amadori products in the formation of H 2 O 2 in vitro, several questions arise concerning the mechanism and source of H 2 O 2 formation in vivo with relationship to the Maillard reaction. Current concepts linking H 2 O 2 formation from reducing sugars involve metal-catalyzed oxidation of enediols (27,33).…”

“…A highly effective diminution of TBT increase was noted with native but not with heatinactivated catalase, thereby implicating H 2 O 2 in the Amadori product-mediated increase in heat stability of the tendons. CML was not determined in this experiment because two groups have shown unequivocally that catalase suppresses CML formation (35,36). Although these data implicate H 2 O 2 in cross-linking of collagen exposed to Amadori product in vitro, the role of H 2 O 2 for collagen cross-linking in vivo remained to be established.…”

Profound Mishandling of Protein Glycation Degradation Products in Uremia and Dialysis

“…Glycation of proteins in CRF and ESRD is expected to increase because of increased concentrations of ␣-oxoaldehyde glycating agents (9,10,27). The concentration of FL residues may be decreased in CRF and HD patients by increased oxidative stress with conversion of FL to CML residues (15). High levels of glycoxidation adducts (pentosidine and CML) in HD may reflect increased oxidative stress, relative to PD (29).…”

“…This was decreased to 0.34 mmol/mol lys (Ϫ59%) in CRF patients and to 0.39 mmol/mol lys (Ϫ53%) in HD patients but was not decreased significantly in PD patients (Figure 6a). FL residues degrade to form CML residues (15). The plasma protein content of CML residues was 0.039 mmol/mol lys in control subjects and increased two-fold in CRF and PD patients and three-fold in HD patients.…”

Overproduction of N.EPSILON.-(Carboxymethyl)lysine-Induced Neovascularization in Cultured Choroidal Explant of Aged Rat

“…CML-human serum albumin (HSA) and CML-BSA were prepared as described previously. 19,20) Amino acid analysis 18) indicated that CML contents of CML-HSA prepared were 18.01 mol CML per mol HSA.…”