The Generation of Superoxide Radical during the Autoxidation of Ferredoxins*

Clostridial and spinach ferredoxins, reduced enzymatically by the action of ferredoxin-TPN+ oxidoreductase, have been shown to carry out the univalent reduction of oxygen. The superoxide radicals, so generated, were detected by their ability to cause the oxidation of epinephrine to adrenochrome. Superoxide dismutase prevented the production of adrenochrome in these reaction mixtures, while having no effect on the rate of oxidation of TPNH. The ratio of the univalent reduction of oxygen to the swn of the univalent plus divalent reductions of oxygen was computed as a function of pH and of the concentrations of oxygen. This percentage of univalent flux increased with rising pH and with increasing concentrations of oxygen as has previously been observed in the case of milk xanthine oxidase. Under comparable conditions of pH and of oxygen concentration, milk xanthine oxidase generated 02four times more rapidly than did clostridial ferredoxin.