Superoxide is a radical, i.e. a chemical species with an unpaired electron. The production of superoxide by the mitochondrial respiratory chain occurs continuously during normal aerobic metabolism. It has been estimated that one to two percent of all the electrons traveling down the mitochondrial respiratory chain never make it to the end, but instead form superoxide. In addition to the mitochondrial respiratory chain, there are other endogenous sources of superoxide production. In the presence of small amounts of iron or copper, hydroxyl radicals may be formed.
Hydroxyl radicals are extremely reactive and can cause severe damage to cells and tissues.
The recent, though as yet unconfirmed, evidence that excess iron intake may be linked to increased risk of heart disease and cancer may, in part, be explained by the role iron plays in the conversion of superoxide to hydroxyl radicals, resulting in oxidative damage to arteries and genes. The ROS formed by the mechanisms explained above can cause oxidative damage to various biological molecules. Lipid peroxidation occurs by a radical chain reaction, i.e. once started, it spreads rapidly and affects a great number of lipid molecules. Oxidative damage to DNA also occurs at a high rate under normal metabolic conditions. Many of these lesions are known to cause mutations.