superoxide dismutase (SOD) A widely distributed enzyme that removes the superoxide radical (O₂·), with the formation of molecular oxygen and hydrogen peroxide, in the following reaction: O₂· + O₂· + 2H⁺ → O₂ + H₂O₂

The hydrogen peroxide formed is removed by the action of catalase. The superoxide anion, which damages tissues, is a free radical formed by the partial reduction of molecular oxygen; it is produced during the metabolic breakdown of various toxins (including drugs and chemical poisons) and as part of the immune response to the presence of bacteria and virus-infected cells.

superoxide dismutase (SOD) Enzyme that converts the superoxide radical to hydrogen peroxide; sometimes promoted as having useful antioxidant properties, but would be ineffective taken by mouth since it is a protein and will be digested.