Toxicity superoxide ion

If, however, O2 in a cell gains only one electron, the product is the toxic superoxide ion ... 

In 1968 it was discovered that respiring cells are protected from superoxide by the action of an enzyme called superoxide dismutase (SOD). Superoxide dismu-tase (EC 1.15.1.1) converts the toxic superoxide ion to hydrogen peroxide, which is then rapidly decomposed to water and dioxygen by the action of catalase (EC 1.11.1.6), an extremely active enzyme whose function has been known for several decades ... 

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