The superoxide radical is produced from the one electron reduction of oxygen. The major source of this radical is from the oxygen reduction pathway in the electron transport chain of mitochondria and the endoplasmic reticulum. However, within the vascular wall, superoxides can be produced from NAD(P)H oxidase (2). The activated form of NAD(P)H oxidase is responsible for the single electron reduction of molecular oxygen to form superoxide (Equation 2).

A vascular isoform of this enzyme has been identified. It is constitutively active and is a major source of vascular superoxide production. Two cytochrome b558 subunits, p22/,/lM and gp9Fw, of NADPH oxidase have been shown to be important for electron transport and the reduction of molecular oxygen to superoxide (2). Other sources of superoxide production include xanthine oxidase and nitric oxide synthase (NOS). Xanthine oxidase converts hypoxanthine to xanthine and produces superoxide as a by-product. NOS generates superoxide rather than NO under conditions in which L-arginine or tetrahydrobiopterin are limiting (26).

Vascular superoxide production sites include the endothelium, vascular smooth muscle cells and the fibroblasts within the adventitia (2). Unfortunately, the superoxide radical is dangerous to biological systems because it is easily converted to a more reactive oxygen species. This radical can generate the hydroxyl radical via a series of reactions known as the metal-catalyzed HaberWeiss reaction or the superoxide radical-driven Fenton reaction (Equations 3, 4, and 5).

In addition, the superoxide radical can react with nitric oxide to give peroxynitrite (ONOO") (Equation 6). The products of both reactions (Equations 5 and 6) are very reactive.

Fortunately, the superoxide radical is removed in vivo by superoxide dismutase (SOD) (Equation 7).

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