The Reduction of Cytochrome c by Milk Xanthine Oxidase

Abstract

The reduction of cytochrome c by xanthine oxidase and the competitive inhibition of this process by carbonic anhydrase and by myoglobin have been studied by kinetic and by equilibrium binding methods. Carbonic anhydrases isolated from bovine and from human erythrocytes differed strikingly in their ability to inhibit competitively the reduction of cytochrome c. The Ks for cytochrome c was a function of the concentration of xanthine oxidase, as were Ki for carbonic anhydrase and Ki for myoglobin, whereas Ks for xanthine was invariant under the same conditions. Binding studies performed by a variety of methods indicated that carbonic anhydrase does not bind to xanthine oxidase. Carbonic anhydrase was found to be a potent inhibitor of the sulfite-oxygen chain reaction initiated either by the reduction of oxygen at an electrode or by xanthine oxidase plus xanthine. The data are consistent with the conclusion that xanthine oxidase, when catalyzing the aerobic oxidation of xanthine, generates an unstable reduced form of oxygen, presumably the superoxide anion, and that this radical is the agent which directly reduces cytochrome c and initiates the sulfite-oxygen chain reaction. Carbonic anhydrase and myoglobin appear to inhibit the reduction of cytochrome c and the initiation of sulfite oxidation by reducing the steady state concentration of the superoxide anion. It is proposed that they accomplish this end by catalyzing the following dismutation reaction: O2·− + O2·− + 2H+ → O2 + H2O2