Superoxide

Superoxide, or the superoxide anion, is the chemical species O2−, a radical anion of molecular oxygen. It is formed by the one-electron reduction of O2 and is paramagnetic.

In biological systems, superoxide arises from enzymes such as NADPH oxidase during the respiratory burst of phagocytes, as well as from mitochondria and other oxidases. In chemistry and lab settings, it can be generated from oxygen in the presence of reactive metals or from salts of the superoxide anion, such as potassium superoxide (KO2) and sodium superoxide (NaO2).

Reactivity and fate: In aqueous solutions, superoxide is relatively unstable and tends to undergo dismutation, spontaneously or catalyzed by the enzyme superoxide dismutase (SOD), to form hydrogen peroxide and oxygen: 2 O2− + 2 H+ → H2O2 + O2. It can also react with nitric oxide to form peroxynitrite, a reactive nitrogen species. Its chemistry involves interactions with metals and various biological substrates, contributing to redox processes in cells.

Biological significance: At low concentrations, superoxide can participate in signaling pathways. However, excessive production contributes to oxidative stress, cellular damage, inflammation, and aging-related processes. Antioxidant defenses include SOD, catalase, and other enzymes that convert reactive oxygen species to less harmful species.

Measurement and context: Detecting superoxide in biological systems relies on methods such as cytochrome c reduction, electron spin resonance, and fluorescent probes, each with limitations related to specificity and artifacts. Overall, superoxide is a key reactive oxygen species with both signaling and damaging potential, depending on context and concentration.