The reduction of cytochrome c by xanthine oxidase and the competitive inhibition of this process by carbonic anhydrase and myoglobin were studied using kinetic and equilibrium binding methods. Carbonic anhydrases from bovine and human erythrocytes showed different inhibitory potencies. The $K_s$ for cytochrome c and $K_i$ for carbonic anhydrase and myoglobin were influenced by xanthine oxidase concentration, while $K_s$ for xanthine was constant. Binding studies showed that carbonic anhydrase does not bind to xanthine oxidase. Carbonic anhydrase was found to inhibit the sulfite-oxygen chain reaction initiated by xanthine oxidase. The data suggest that xanthine oxidase generates an unstable reduced form of oxygen, likely the superoxide anion, which directly reduces cytochrome c and initiates the chain reaction. Carbonic anhydrase and myoglobin inhibit cytochrome c reduction by reducing the steady-state concentration of superoxide anion through a dismutation reaction: $O_2\cdot^- + O_2\cdot^- + 2H^+ \rightarrow O_2 + H_2O_2$. The reduction of cytochrome c by milk xanthine oxidase requires oxygen. Compounds like menadione can act as electron carriers. Oxygen mediates electron transfer from xanthine oxidase to cytochrome c, but hydrogen peroxide is not a key intermediate. Tiron and tris inhibited the oxygen-dependent reduction of cytochrome c. Proteins like myoglobin and carbonic anhydrase inhibited cytochrome c reduction competitively without affecting xanthine oxidation. These proteins have low $K_i$ values and inhibit cytochrome c reduction by competing with cytochrome c for the reducing species generated by xanthine oxidase. The study shows that myoglobin and carbonic anhydrase inhibit cytochrome c reduction without binding to xanthine oxidase. Binding studies did not detect direct interactions between xanthine oxidase and carbonic anhydrase. Carbonic anhydrase inhibited the sulfite-oxygen chain reaction initiated by xanthine oxidase and electrode reduction. Myoglobin did not inhibit the electrode-initiated sulfite oxidation, possibly due to coating the electrode with unfolded protein. The results support the hypothesis that univalently reduced oxygen is the reductant in the reduction of cytochrome c and the initiation of the sulfite-oxygen chain reaction. Carbonic anhydrase and myoglobin catalyze the dismutation of superoxide anion, reducing its concentration and inhibiting cytochrome c reduction. The study also shows that carbonic anhydrase and myoglobin can act as effective inhibitors of the sulfite-oxygen chain reaction. The findings suggest that these proteins protect cells from oxidative damage by catalyzing the dismutation of oxygen radicals.The reduction of cytochrome c by xanthine oxidase and the competitive inhibition of this process by carbonic anhydrase and myoglobin were studied using kinetic and equilibrium binding methods. Carbonic anhydrases from bovine and human erythrocytes showed different inhibitory potencies. The $K_s$ for cytochrome c and $K_i$ for carbonic anhydrase and myoglobin were influenced by xanthine oxidase concentration, while $K_s$ for xanthine was constant. Binding studies showed that carbonic anhydrase does not bind to xanthine oxidase. Carbonic anhydrase was found to inhibit the sulfite-oxygen chain reaction initiated by xanthine oxidase. The data suggest that xanthine oxidase generates an unstable reduced form of oxygen, likely the superoxide anion, which directly reduces cytochrome c and initiates the chain reaction. Carbonic anhydrase and myoglobin inhibit cytochrome c reduction by reducing the steady-state concentration of superoxide anion through a dismutation reaction: $O_2\cdot^- + O_2\cdot^- + 2H^+ \rightarrow O_2 + H_2O_2$. The reduction of cytochrome c by milk xanthine oxidase requires oxygen. Compounds like menadione can act as electron carriers. Oxygen mediates electron transfer from xanthine oxidase to cytochrome c, but hydrogen peroxide is not a key intermediate. Tiron and tris inhibited the oxygen-dependent reduction of cytochrome c. Proteins like myoglobin and carbonic anhydrase inhibited cytochrome c reduction competitively without affecting xanthine oxidation. These proteins have low $K_i$ values and inhibit cytochrome c reduction by competing with cytochrome c for the reducing species generated by xanthine oxidase. The study shows that myoglobin and carbonic anhydrase inhibit cytochrome c reduction without binding to xanthine oxidase. Binding studies did not detect direct interactions between xanthine oxidase and carbonic anhydrase. Carbonic anhydrase inhibited the sulfite-oxygen chain reaction initiated by xanthine oxidase and electrode reduction. Myoglobin did not inhibit the electrode-initiated sulfite oxidation, possibly due to coating the electrode with unfolded protein. The results support the hypothesis that univalently reduced oxygen is the reductant in the reduction of cytochrome c and the initiation of the sulfite-oxygen chain reaction. Carbonic anhydrase and myoglobin catalyze the dismutation of superoxide anion, reducing its concentration and inhibiting cytochrome c reduction. The study also shows that carbonic anhydrase and myoglobin can act as effective inhibitors of the sulfite-oxygen chain reaction. The findings suggest that these proteins protect cells from oxidative damage by catalyzing the dismutation of oxygen radicals.