Superoxide generation from nitric oxide synthases.

Besides nitric oxide (NO), NO synthases (NOS) also produce superoxide ((*)O(2)()), a primary reactive oxygen species involved in both cell injury and signaling. Neuronal NOS was first found to produce (*)O(2)(-) in vitro. Subsequent studies revealed (*)O(2)(-) generation as a common property of all NOS isoforms. Although NOS was originally shown to produce (*)O(2)(-) under defined conditions such as substrate or cofactor depletion, recent enzymatic studies found that the reduction of oxygen to (*)O(2)(-) is an obligatory step in NO synthesis. Tetrahydrobiopterin appears to play a key role in preventing (*)O(2)(-) release from the NOS oxygenase domain. On the other hand, the NOS reductase domain is also capable of producing significant amounts of (*)O(2)(-). Increasing evidence demonstrates that (*)O(2)(-) generation is involved in both physiological and pathological actions of NOS.