NADPH-cytochrome P-450 reductase catalyzes one-electron reduction of aminochrome to the corresponding ortho-semiquinone, which was found to be unstable as indicated by the occurrence of NADPH oxidation and oxygen consumption. The addition of superoxide dismutase and catalase, alone or together, to the incubation mixture, during reduction of aminochrome catalyzed by NADPH-cytochrome P-450 reductase, did not prevent the autoxidation of ortho-semiquinone, but instead they increased NADPH oxidation. These results contrast with the almost complete inhibition of autoxidation (NADH oxidation) of ortho-hydroquinone during reduction of aminochrome catalyzed by DT-diaphorase in the presence of both superoxide dismutase and catalase. However, the effect of superoxide dismutase and catalase on oxygen consumption was found to differ from the effect on NADH or NADPH oxidation, since these enzymes, alone or together, inhibited the oxygen consumption during the reduction of aminochrome catalyzed by both NADPH-cytochrome P-450 reductase and DT-diaphorase. These results support the proposed role of NADPH-cytochrome P-450 reductase in neurodegeneration as a consequence of activation of aminochrome to reactive oxygen species. In addition, they also support the protective and antioxidant role of DT-diaphorase, together with superoxide dismutase and catalase, by competing with NADPH-cytochrome P-450 reductase to reduce aminochrome to ortho-hydroquinone and prevent the formation of reactive oxygen species. A possible mechanism is proposed.