Diabetes induces an impairment in the proteolytic activity against oxidized proteins and a heterogeneous effect in nonenzymatic protein modificationsin the cytosol of rat liver and kidney

It is assumed that increased oxidative stress contributes to the developmen t of complications in diabetes. In this study, several markers of protein s tructural modifications directly induced by free radicals were investigated in the liver and kidney cytosolic fractions of rats with streptozotocin-in duced diabetes. Sulfydryl residue and side-chain amino group analyses, as w ell as immunoblotting and chromatographic measurements of protein-bound car bonyl, suggest that protein oxidative modification is not increased by diab etes, with the exception of sulfydryl groups in renal cytosol. The levels o f the glycation-derived carbonyl N epsilon-fructosyl-lysine are significant ly increased by diabetes. Furthermore, unchanged proteolytic activity again st in vivo-oxidized proteins, significant decreases both in activity agains t H2O2-modified proteins and in proteasome activity, measured by the degrad ation of a specific fluorogenic substrate, suggest that the unchanged oxida tive protein modification in the diabetic state cannot be attributed to an increased cytosolic proteolytic activity in these tissues. These results pr ovide evidence against a generalized increase in protein oxidative damage a nd demonstrate a diabetes-induced alteration in cytosolic proteolytic pathw ays, suggesting that proteasome activity may be impaired in these organs.