Figure 2: The schematic diagram of H2O2 source and effects. SOD can catalyze the dismutation of the superoxide radical to H2O2 and molecular oxygen. Then it can be converted to H2O by CAT and GPX. H2O2 also can be converted either to water or OH. Low dose of H2O2 can induce adaptation, regulating diverse biological processes. High dose of H2O2 together with its conversion to OH can induce maladaptation and cause cytotoxic effects. NOX: NADPH oxidase; NOS: uncoupled NO synthases; XO: xanthine oxidase; H2O2: Hydrogen peroxide; OH: hydroxyl radical; SOD: superoxide dismutase; CAT: catalase; GPX: glutathione peroxidase.