Figure 1.

Schematic illustration of cellular maintenance of redox homeostasis. Mitochondria electron-transport chain (ETC), membrane-bound NADPH oxidase (NOX) complex and endoplasmic reticulum (ER) are the three major intracellular sources of reactive oxygen species (ROS). Anion superoxide (O₂⁻) is the principal form of ROS and can be rapidly converted into hydrogen peroxide (H₂O₂) by superoxide dismutases (SODs) or can alternatively, form peroxynitrite (ONOO⁻) through reacting with the nitric oxide (NO^·). H₂O₂ can be catalyzed to HO^· in the presence of Fe²⁺ or Cu²⁺ ions or be converted to H₂O and O₂ catalyzed by catalase, glutathione peroxidase (Gpx) or peroxiredoxins (Prx). To maintain the redox homeostasis, the living cells engage powerful scavenger antioxidant enzyme systems to eliminate the intracellular ROS, major ROS-scavenging enzymes are shown in green. NOS, nitric oxide synthase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione