= Superoxide dismutase
A \b[superoxide dismutase] is an <enzyme> that catalyzes <disproportionation> of <superoxide>:
$$
2\,\mathrm{O_2^{\bullet-}}+2\,\mathrm{H^+}
\longrightarrow\mathrm{O_2}+\mathrm{H_2O_2}.
$$
One superoxide equivalent is oxidized and the other reduced. The enzyme activity is EC1.15.1.1; different metal-containing protein families perform it.<Enzyme Nomenclature by the IUBMB>[EC1.15.1.1, reaction, systematic name and metal-containing forms]{ref}
For an original stoichiometric example, consuming $10\,\mathrm{\mu mol}$ superoxide by this reaction produces $5\,\mathrm{\mu mol}$ oxygen and $5\,\mathrm{\mu mol}$ <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.<Citation Classic commentary on Superoxide dismutase by Joe M. McCord>[single-page research recollection and original-paper citation]{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.
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