Generation of Superoxide Free Radical during the Autoxidation of Thiols

The autoxidation of dithiothreitol, 2-mercaptoethanol, reduced glutathione, and ethyl mercaptan has been seen to involve the univalent reduction of oxygen. The superoxide radicals, so generated, were detected by their ability to reduce p-nitroblue tetrazolium to a formazan and by the inhibition of this reaction by superoxide dismutase. The rate of autoxidation of these thiols and the sensitivity of the reduction of nitroblue tetrazolium to inhibition by superoxide dismutase were both augmented, as the pH was raised from 7.8 to 10.2. These results, lead to the proposal that thiols reduce nitroblue tetrazolium by at least two different pathways, one of which involves O2- and is therefore inhibitable by superoxide dismutase.