Superoxide in apoptosis. Mitochondrial generation triggered by cytochrome c loss.

Activation of apoptosis is associated with generation of reactive oxygen species. The present research shows that superoxide is produced by mitochondria isolated from apoptotic cells due to a switch from the normal 4-electron reduction of O2 to a 1-electron reduction when cytochrome c is released from mitochondria. Bcl-2, a protein that protects against apoptosis and blocks cytochrome c release, prevents superoxide production when it is overexpressed. The switch in electron transfer provides a mechanism for redox signaling that is concomitant with cytochrome c-dependent activation of caspases. The block of cytochrome c release provides a mechanism for the apparent antioxidant function of Bcl-2.