FIG. 1.

Cells maintain redox homeostasis through a balance of generation and elimination of reactive oxygen species (ROS). Superoxide (O₂⁻), the principal form of ROS, comes from the byproducts of electron leakage of mitochondrial electron transport chain (Mito-ETC), endoplasmic reticulum (ER), and membrane-located NAD(P)H oxidase complex (NOX). O₂⁻ can be rapidly converted to hydrogen peroxide (H₂O₂) by superoxide dismutase (SOD); H₂O₂ can be catalyzed to release hydroxyl radicals (HO^•) in the presence of Fe²⁺ or Cu²⁺ ions. H₂O₂ can be converted by myeloperoxidase (MPD) to hypochlorous acid (HOCl), a stronger oxidant. H₂O₂ is converted to H₂O+O2 by catalase or glutathione peroxidase (GPX). Nitric oxide (NO^•), once generated from arginine catalyzed by nitric oxide synthase (NOS), is rapidly converted to peroxynitrite (ONOO⁻) by reacting with O₂⁻. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).