Fig. 1.

Intracellular reactive oxygen species (ROS) generation and the antioxidant scavenging system. ROS is produced from mitochondria, peroxisome, nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and xanthine oxidase. Among these sources, the mitochondrial electron transport chain is the primary source for ROS production. Superoxide dismutase 2 (SOD2) catalyzes the conversion of superoxide anion (O₂^–) into hydrogen peroxide (H₂O₂) in the mitochondria. The H₂O₂ is then detoxified in the mitochondria or moves to the cytoplasm. Cytoplasmic superoxide anion is generated from NADPH oxidase and xanthine oxidase, and subsequently converted into H₂O₂ by superoxide dismutase 1 (SOD1). H₂O₂ is detoxified by glutathione (GSH)/glutaredoxin (GRx)/glutathione reductase (GR), catalase (CAT), peroxiredoxin (Prx), thioredoxin (Trx)/thioredoxin reductase (TrxR), and glutathione peroxidase (GPx). Oxidized methionine (Ox-Met) is reduced by methionine sulfoxide reductase (Msr) to methionine (Met). Nitric oxide (NO) reacts with superoxide anion to form peroxynitrite (ONOO^–), which is detoxified by Prx, GSH, and GPx.