The Utility of Superoxide Dismutase in Studying Free Radical Reactions

Abstract The rate of reduction of cytochrome c by the ferredoxin-TPN reductase of spinach or by chicken liver xanthine dehydrogenase is markedly augmented by a variety of electron carriers. Menadione, flavin mononucleotide, flavin adenine dinucleotide, N-methylphenazinium methosulfate, methylene blue, and 2,6-dichlorobenzenone indophenol were all effective. When cytochrome c reduction was mediated aerobically by menadione, flavin mononucleotide, or flavin adenine dinucleotide, superoxide dismutase was an inhibitor. When cytochrome c reduction was mediated by these same electron carriers in the absence of molecular oxygen, superoxide dismutase was without effect. When cytochrome c reduction was mediated by N-methylphenazinium methosulfate, methylene blue, or 2,6-dichlorobenzenone indophenol, superoxide dismutase was without effect whether or not oxygen was present. These results indicate that of these electron carriers only the reduced forms of menadione and of the flavins preferentially reduce O2 to the superoxide radical in the presence of cytochrome c. In the absence of cytochrome c, it was shown that reduced methylene blue is likewise capable of the univalent reduction of oxygen. In this case O-2 was detected by its ability to oxidize epinephrine to adrenochrome.