Table 1.

Brief description of some reactive oxygen species (ROS).

Oxidant	Description
•O2 −, superoxide anion	One-electron reduction state of O2, formed in many autoxidation reactions and by the electron transport chain. Rather unreactive but can release Fe2+ from iron-sulfur proteins and ferritin. Undergoes dismutation to form H2O2 spontaneously or by enzymatic catalysis and is a precursor for metal-catalyzed •OH formation.
H2O2, hydrogen peroxide	Two-electron reduction state, formed by dismutation of •O2 − or by direct reduction of O2. Lipid soluble and thus able to diffuse across membranes.
•OH, hydroxyl radical	Three-electron reduction state, formed by Fenton reaction and decomposition of peroxynitrite. Extremely reactive and will attack most cellular components.
ROOH, organic hydroperoxide	Formed by radical reactions with cellular components such as lipids and nucleobases.
RO•, alkoxy, and ROO•, peroxy radicals	Oxygen centred organic radicals. Lipid forms participate in reactions. Produced in the presence of oxygen by radical addition to double bonds or hydrogen abstraction.
HOCl, hypochlorous acid	Formed from H2O2 by myeloperoxidase. Lipid soluble and highly reactive. Will readily oxidize protein constituents, including thiol groups, amino groups, and methionine.
ONOO−, peroxynitrite	Formed in a rapid reaction between •O2 − and NO•. Lipid soluble and similar in reactivity to hypochlorous acid. Protonation forms peroxynitrous acid, which can undergo homolytic cleavage to form hydroxyl radical and nitrogen dioxide.