FIG. 1.

Reactive oxygen species. Cells generate aerobic energy by reducing molecular oxygen (O₂) to water. During the metabolism of oxygen, superoxide (⁻O₂) is occasionally formed. Superoxide is rapidly dismutated to hydrogen peroxide (H₂O₂), which is converted by glutathione peroxidase or catalase to water. MPD (myeloperoxidase) converts H₂O₂ in neutrophils to hypochlorous acid (HOCl), a strong oxidant that acts as a bactericidal agent in phagocytic cells. During a Fenton reaction, H₂O₂ is converted in a spontaneous reaction catalyzed by Fe²⁺ to the highly reactive hydroxyl radical ·OH.