The Utility of Superoxide Dismutase in Studying Free Radical Reactions

The superoxide dismutase (dismutase) which has already been found to be useful in detecting the role of O₂·^- in certain enzymic reactions has here been shown to be applicable to the study of nonenzymic reactions, as well. The aerobic oxidation of sulfate, which proceeds by a free radical chain mechanism, has been investigated with the aid of superoxide dismutase. The "spontaneous" oxidation of sulfite, which is catalyzed by traces of heavy metals, was unaffected by superoxide dismutase in the absence of ethylenediaminetetraacetate yet was inhibited by 1 ng per ml levels of the dismutase in the presence of EDTA. Dimethyl sulfoxide (DMSO) has been found to be a powerful catalyst of the aerobic oxidation of sulfite. This is a complex reaction with a mechanism which appears to change with changes in the relative concentrations of sulfite and of DMSO. When the ratio of [sulfite] to [DMSO] was high, sulfite oxidation was inhibited by dismutase. In contrast, when the ratio of [sulfite] to [DMSO] was low, dismutase stimulated the oxidation of sulfite. The effects of variation of the concentrationn of sulfite, DMSO, and oxygen on the DMSO-induced sulfite oxidation have been explored. DMSO was found to contain approximately 0.2% of an impurity which was a very effective radical scavenger. This impurity could not be eliminated by treatment with charcoal and vacuum distillation, but it could be destroyed by treatment with H₂O₂. The reduction of cytochrome c by sulfite and the effects of EDTA, DMSO, oxygen, and dismutase on this reduction have also been studied. The results, which have permitted clarifying deductions concerning these very complex reactions, could not have been achieved, were the superoxide dismutase not available.