Thiol-dependent metal-catalyzed oxidation of copper, zinc superoxide dismutase.

Superoxide dismutase (SOD) is a key enzyme in the antioxidant system of the cells. When exposed to a metal-catalyzed oxidation (MCO) system composed of Fe3+, O2, and thiol as an electron donor copper, zinc SOD (CuZnSOD) was susceptible to oxidative modification and damage as indicated by the loss of activity, fragmentation and aggregation of peptide as well as by the formation of carbonyl groups. Oxidative damage to CuZnSOD was inhibited by diethylenetriaminepentaacetic acid as well as by free radical scavengers and spin-trapping agents. The results of the present study indicate that hydrogen peroxide may be generated from a thiol/Fe3+/O2 system and that hydroxyl free radicals, produced by metal-catalyzed Fenton reactions, may be the ultimate species mediating the SOD damage. Incubation with the MCO system resulted in the release of Cu ions from CuZnSOD. Incubation with the thiol-MCO did not significantly increase the formation of 2-oxohistidine in CuZnSOD. The lack of formation of 2-oxohistidine, as well as the pronounced preventive effect of spin-traps on the thiol-MCO-mediated damage to CuZnSOD, indicates that inactivation might actually be predominantly due to global oxidation rather than a site-specific oxidation. The thiol-MCO-mediated damage to SOD may result in the perturbation of cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant condition.