Figure 1.

Elevated oxygen triggers intracellular ROS accumulation. The ubisemiquinone intermediates present in complexes I and III of the respiratory chain are the primary source of univalent reduction of oxygen into superoxide (O₂ ^.-) (indicated by stars). At over-oxygenation conditions, electrons leak from complexes I and III generating superoxide increases. Superoxide is converted immediately to hydrogen peroxide (H₂O₂) by manganese superoxide dismutase (MnSOD) or copper/zinc superoxide dismutase (Cu/ZnSOD). At normoxic conditions, the catalases and peroxidase systems minimize the accumulation of H₂O₂ but at higher oxygen concentrations; these antioxidant defenses can be overwhelmed and the accumulating H₂O₂ diffuse freely from the mitochondria (dashed lines) reaching targets that can be damaged such as dehydratases and DNA. Abbreviations: CoQ, coenzyme Q10; Cyt C, cytochrome C; GPX, glutathione peroxidase; IM, inner membrane; IMS, intermembrane space; OM, outer membrane.