 Superoxide dismutase

A superoxide dismutase is an enzyme that catalyzes disproportionation of superoxide:One superoxide equivalent is oxidized and the other reduced. The enzyme activity is EC1.15.1.1; different metal-containing protein families perform it.^[ref]

For an original stoichiometric example, consuming $10\mu \mathrm{mol}$ superoxide by this reaction produces $5\mu \mathrm{mol}$ oxygen and $5\mu \mathrm{mol}$ hydrogen peroxide. Removing superoxide therefore does not mean that every reactive product has become water. Other reactions, including those catalyzed by catalase, handle the peroxide.

The discovery illustrates an indirect assay's explanatory power. A researcher's later first-person account describes how a protein preparation inhibited cytochrome c reduction in a xanthine oxidase system. The explanation shifted from a supposed protein–protein blockage to catalytic removal of a freely diffusible superoxide intermediate. The same account relates this to the 1969 isolation and definition of the enzyme activity.^[ref]

In a model readout, decreasing a reporter slope from 0.060 to 0.030 absorbance units per minute gives 50-percent inhibition of that reporter change. It does not imply that 50 percent of the enzyme molecules were active or that a universal enzyme amount has been measured. Coupled-assay conditions and calibration determine the activity inference.