PRESENCE OF DOPA AND AMINO-ACID HYDROPEROXIDES IN PROTEINS MODIFIED WITH ADVANCED GLYCATION END-PRODUCTS (AGES) - AMINO-ACID OXIDATION-PRODUCTS AS A POSSIBLE SOURCE OF OXIDATIVE STRESS-INDUCED BY AGE PROTEINS

Glycation and subsequent Maillard or browning reactions of glycated pr oteins, leading to the formation of advanced glycation end products (A GEs), are involved in the chemical modification of proteins during nor mal aging and have been implicated in the pathogenesis of diabetic com plications. Oxidative conditions accelerate the browning of proteins b y glucose, and AGE proteins also induce oxidative stress responses in cells bearing AGE receptors. These observations have led to the hypoth esis that, glycation-induced pathology results from a cycle of oxidati ve stress, increased chemical modification of proteins via the Maillar d reaction, and further AGE-dependent oxidative stress. Here we show t hat the preparation of AGE-collagen by incubation with glucose under o xidative conditions in vitro leads not only to glycation and formation of the glycoxidation product N-epsilon-(carboxymethyl)lysine (CML), b ut also to the formation of amino acid oxidation products on protein, including m-tyrosine, dityrosine, dopa, and valine and leucine hydrope roxides. The formation of both CML and amino acid oxidation products w as prevented by anaerobic, anti-oxidative conditions. Amino acid oxida tion products were also formed when glycated collagen, prepared under anti-oxidative conditions, was allowed to incubate under aerobic condi tions that led to the formation of CML. These experiments demonstrate that amino acid oxidation products are formed in proteins during glyco xidation reactions and suggest that reactive oxygen species formed by redox cycling of dopa or by the metal-catalysed decomposition of amino acid hydroperoxides, rather than by redox activity or reactive oxygen production by AGEs on protein, might contribute to the induction of o xidative stress by AGE proteins.