In phagocytes, superoxide is produced in large quantities by the enzyme NADPH oxidase for use in oxygen-dependent killing mechanisms of invading pathogens.
NADPH oxidase is an enzyme that effectively reduces O2 to superoxide (O2–•), which can be used by the immune system to kill bacteria and fungi.
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Familial ALS: Apocynin extended the lives of mutant mice and reduced glial cell toxicity of cultured cells lines with a defective Superoxide Dismutase-1 (SOD1) gene -- a genetic defect found in some people with hereditary amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease).
During UV irradiation free radicals, mainly superoxide/hydroperoxyl (ROS/RNS) are produced, that can react with the ketoamines (Amadori products) and other intermediates of the Maillard reaction.
Part of the digestion mechanism involves an enzyme-mediated respiratory burst, which produces reactive oxygen-derived compounds, including superoxide (which is produced by NADPH oxidase).
Reactive nitrogen species (RNS) are a family of antimicrobial molecules derived from nitric oxide (·NO) and superoxide (O2·−) produced via the enzymatic activity of inducible nitric oxide synthase 2 (NOS2) and NADPH oxidase respectively.
Tetrathionate has also been found to serve as a terminal electron acceptor for Salmonella Typhimurium, whereas existing thiosulfate in the lumen of mammals is oxidized by reactive oxygen species released by the immune system (mainly NADPH oxidase produced superoxide) to form tetrathionate.