Competitive Inhibition by Myoglobin of the Reduction of Cytochrome c by Xanthine Oxidase*

During a study of the reduction of cytochrome c by milk xanthine oxidase, it was observed that relatively impure preparations of cytochrome c were reduced more slowly than highly purified preparations. This difference was found to relate to the presence, in impure preparations, of a potent inhibitor of the reduction of cytochrome c by xanthine oxidase. This inhibitor was of interest because its action was formally competitive with respect to ferricytochrome c, whereas it had no effect on the reduction of oxygen by this enzyme. Identification of the inhibitor as myoglobin and characterization of the inhibition form the subject of this report.