Figure 2.

Reactions involving ROS, antioxidant systems and energy metabolism. ${\text{O}}_2^{•-}$ and H₂O₂ are produced from oxygen through reactions that may oxidize NADPH (dotted arrows, e.g., catalysis by NADPH oxidases). ${\text{O}}_2^{•-}$ is dismutated to H₂O₂ by SOD, and ${\text{O}}_2^{•-}$ and H₂O₂ may be converted to HO^• by Fenton reactions. CAT, PRDX and GPX scavenge H₂O₂. The catalytic cycles of PRDX and TXN are represented, where SH and SS, respectively, indicate reduced and oxidized (disulfide) thiols. Boxes and dashed arrows indicate the external processes with which the metabolites are associated. For example, XCT/CD44 mediates cysteine import, which may then be incorporated into proteins, such as TXN and PRDX, or may be metabolized to yield GSH. Colors indicate proteins or processes from the same pathway.