FIG. 3.

Sources of ROS. A majority of ROS is produced by the mitochondria mainly through the aberrant reduction of O₂ at complexes I and III during oxidative phosphorylation. The highly reactive and unstable superoxide anion (O₂⁻) is scavenged by SOD-1 to form H₂O₂, also a type of ROS. H₂O₂ is more stable and is responsible for direct protein modifications. Another source of intracellular ROS is NADPH oxidase, which produces O₂^- through the oxidation of NADPH. SOD, superoxide dehydrogenase; H₂O₂, hydrogen peroxide. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars