Figure 2.
ROS sources and antioxidant systems. Mitochondrial respiration ETC and the membrane-bond NOX complexes are the two major ROS resources. Leakage of electrons from ETC is mediated by coenzyme Q and produces O2− through O2. There are three isoforms of SODs to defend oxidation. Cu/Zn SOD (SOD1) in the cytoplasm, MnSOD (SOD2) in the mitochondria, and Cu/Zn SOD (SOD3) in the extracellular matrix can rapidly convert O2− to H2O2. NOXs catalyze the generation of O2− from O2 and NADPH. H2O2 is converted to toxic ·OH by a metal (Fe2+ or Cu+) catalyst through the Fenton reaction. H2O2 can be converted into H2O by PRXs, GPXs, and CAT. Besides, Trxs (the cytoplasmic Trx-1 and the mitochondrial Trx-2) can reduce oxidized PRXs. Trxs themselves are also reduced to TrxR by TR using NADPH as an electron donor. GPXs oxidize reduced GSH to GSSH. GSSH is reduced back to GSH by GR accompanied by an electron from NADPH. Note. ETC: electron transport chain; NOXs: NADPH oxidase; SODs: superoxide dismutases; H2O2: hydrogen peroxide; NADPH: nicotinamide adenine dinucleotide phosphate; ·OH: hydroxyl radicals; PRXs: peroxiredoxins; GPXs: glutathione peroxidases; CAT: catalase; Trx: thioredoxin (oxidized); Trx-R: thioredoxin (reduced); TR: thioredoxin reductase; GSH: tripeptide glutathione (reduced); GSSH: glutathione disulfide (oxidized); GR: glutathione. Green dotted lines denote H2O2 diffusion. Red dotted lines denote O2− diffusion.