Figure 7: A) chemical reaction for measuring cytochrome c reduction. In the process of converting xanthine to urate, xanthine oxidase generates superoxide anion. In the absence of SOD, superoxide anion reduces cytochrome c (arrow down). In the presence of SOD, the dismutation reaction to hydrogen peroxide is greatly accelerated (arrow up). The addition of superoxide dismutase leads to a competition for superoxide between cytochrome c and SOD. B) Equilibrium of the cytochrome c reduction reaction in the presence (open symbols) or absence (closed symbols) of superoxide dismutase..