THIOL-DEPENDENT METAL-CATALYZED OXIDATION OF COPPER, ZINC SUPEROXIDE-DISMUTASE

Superoxide dismutase (SOD) is a key enzyme in the antioxidant system o f the cells. When exposed to a metal-catalyzed oxidation (MCO) system composed of Fe3+, O-2, and thiol as an electron donor copper, zinc SOD (CuZnSOD) was susceptible to oxidative modification and damage as ind icated by the loss of activity, fragmentation and aggregation of pepti de as well as by the formation of carbonyl groups. Oxidative damage to CuZnSOD was inhibited by diethylenetriaminepentaacetic acid as well a s by free radical scavengers and spin-trapping agents. The results of the present study indicate that hydrogen peroxide may be generated fro m a thiol/Fe3+/O-2 system and that hydroxyl free radicals, produced by metal-catalyzed Fenton reactions, may be the ultimate species mediati ng the SOD damage. Incubation with the MCO system resulted in the rele ase of Cu ions from CuZnSOD. Incubation with the thiol-MCO did not sig nificantly increase the formation of 2-oxohistidine in CuZnSOD. The la ck of formation of 2-oxohistidine, as well as the pronounced preventiv e effect of spin-traps on the thiol-MCO-mediated damage to CuZnSOD, in dicates that inactivation might actually be predominantly due to globa l oxidation rather than a site-specific oxidation. The thiol-MCO-media ted damage to SOD may result in the perturbation of cellular antioxida nt defense mechanisms and subsequently lead to a pro-oxidant condition . (C) 1998 Elsevier Science B.V. All rights reserved.