Figure 1.

Generation of reactive oxygen species. Oxygen readily accepts free electrons produced by normal oxidative metabolism, generating the unstable superoxide anions (O₂^•−), which are scavenged by superoxide dismutase (SOD) and transformed into hydrogen peroxide (H₂O₂) or rapidly converted into peroxynitrite (ONOO⁻) when reacting with bioactive nitric oxide (^•NO). The H₂O₂ converts to water by catalase. In the Harber–Weiss reaction, hydroxyl radicals (^•OH) are generated in the presence of H₂O₂ and iron ions.