In the mammalian cell, molecular oxygen is essential for energy production through oxidative phosphorylation. Cell death will rapidly ensue in the absence of oxygen. Under normal circumstances, most of the molecular oxygen undergoes tetravalent reduction to water by the following intracellular cytochrome oxidase system:
However, under certain circumstances, univalent reduction can occur with transfer of only one electron (e− ), resulting in the release of highly reactive free radical intermediates. This presents an immediate and severe threat to the integrity of the cell. With this highly lethal potential for self-destruction, it is clear that some form of natural protection is required in mammals with oxidative metabolism. This protection exists in the form of endogenous antioxidants or enzymes, such as glutathione peroxidase, superoxide dismutase, and catalase, which are found in varying concentrations in most mammalian cells, and which, under normal physiologic conditions, protect the cell from attack by free radicals.
Hinder RA, Stein HJ. Oxygen-Derived Free Radicals. Arch Surg. 1991;126(1):104-105. doi:10.1001/archsurg.1991.01410250112019