The univalent reduction of oxygen by reduced flavins and quinones.

FMN and flavodoxin, when reduced by the action of ferredoxin-TPN+ oxidoreductase, have been shown to cause both univalent and divalent reductions of oxygen. Superoxide radicals, so generated, were detected by their abilities to oxidize epinephrine to adrenochrome and to reduce cytochrome c. Superoxide dismutase inhibited these actions of the superoxide radical. The ratio of the univalent reduction of oxygen, to the sum of the univalent plus divalent reductions of oxygen, was determined as a function of pH and of oxygen concentration. These data were compared with the results of similar measurements made previously, with ferredoxins and xanthine oxidase. FMN and flavodoxin carry out the univalent reduction of oxygen much less effectively than either xanthine oxidase or the ferredoxins. Furthermore, the univalent reduction of oxygen by xanthine oxidase and by the ferredoxins showed similar responses to changes in pH and oxygen concentration; whereas the behavior of the flavodoxin and FMN mediated systems was distinctly different. These results suggest that the reduction of oxygen by native milk xanthine oxidase may well be a function of its non-heme iron centers. The fully reduced form of menadione, generated by the action of Diaphorase, caused a predominantly divalent reduction of oxygen; whereas the semiquinone forms of this carrier, generated by the action of the ferredoxin-TPN+ oxidoreductase, caused primarily a univalent reduction of oxygen.