The study investigates the reduction of cytochrome c by xanthine oxidase and the competitive inhibition of this process by carbonic anhydrase and myoglobin. The researchers found that the inhibitory constants (Ks) for cytochrome c, carbonic anhydrase, and myoglobin were influenced by the concentration of xanthine oxidase, while the Ks for xanthine remained invariant. Binding studies using various methods showed that carbonic anhydrase does not bind to xanthine oxidase. Carbonic anhydrase was found to inhibit the sulfite-oxygen chain reaction initiated by either the reduction of oxygen at an electrode or by xanthine oxidase plus xanthine. The data suggest that xanthine oxidase generates an unstable reduced form of oxygen, likely superoxide anion, which directly reduces cytochrome c and initiates the sulfite-oxygen chain reaction. Carbonic anhydrase and myoglobin inhibit this process by reducing the steady-state concentration of the superoxide anion through a dismutation reaction. The authors propose that these proteins catalyze the reaction: O2·- + O2·- + 2H+ → O2 + H2O2 This mechanism explains the observed inhibitory effects and supports the hypothesis that the reducing species in the xanthine oxidase system is univalently reduced oxygen.The study investigates the reduction of cytochrome c by xanthine oxidase and the competitive inhibition of this process by carbonic anhydrase and myoglobin. The researchers found that the inhibitory constants (Ks) for cytochrome c, carbonic anhydrase, and myoglobin were influenced by the concentration of xanthine oxidase, while the Ks for xanthine remained invariant. Binding studies using various methods showed that carbonic anhydrase does not bind to xanthine oxidase. Carbonic anhydrase was found to inhibit the sulfite-oxygen chain reaction initiated by either the reduction of oxygen at an electrode or by xanthine oxidase plus xanthine. The data suggest that xanthine oxidase generates an unstable reduced form of oxygen, likely superoxide anion, which directly reduces cytochrome c and initiates the sulfite-oxygen chain reaction. Carbonic anhydrase and myoglobin inhibit this process by reducing the steady-state concentration of the superoxide anion through a dismutation reaction. The authors propose that these proteins catalyze the reaction: O2·- + O2·- + 2H+ → O2 + H2O2 This mechanism explains the observed inhibitory effects and supports the hypothesis that the reducing species in the xanthine oxidase system is univalently reduced oxygen.