OXIDATIVE DAMAGE OF GLYCATED PROTEIN IN THE PRESENCE OF TRANSITION-METAL ION

Glycation of proteins was induced by incubation of high concentrations of D-glucose (1 M) and proteins (4 mg/ml) at 40-degrees-C for 7 days. The oxidative damage of glycated protein mediated by addition of traces of metal ion in phosphate buffer (1/15 M, pH 7.2) under aerated conditions was investigated. Free amino groups (mainly epsilon-NH2 groups of lysine residues) of protein decreased during the glycation of protein but were regenerated in the oxidation of glycated protein induced with metal ion. Also in this process, an alpha-dicarbonyl compound was generated. Glycated protein was degraded to lower molecular weight fragments in the presence of Cu(II) but not in the absence of Cu(II). Moreover, the histidine residue of protein was selectively degradated in this oxidative process. The oxidative fragmentations of glycated protein were inhibited by EDTA or catalase, but not by urea, DMSO (scavengers of hydroxy radicals) or superoxide dismutase. A possible mechanism of oxidative fragmentation of glycated protein catalyzed by the metal ion is discussed.