The incomplete reduction of oxygen to water during respiration results in the formation of the superoxide anions, which are spontaneously or enzymatically converted to H2O2. Hydrogen peroxide itself is not very reactive, but can be further reduced to the extremely damaging hydroxyl radicals. The reactive oxygen species are also produced during the beta-oxidation of fatty acids, and upon exposure to radiation, light, metals, and redox drugs. Therefore, all aerobic cells are equipped with H2O2-removing enzymes, which include catalase, glutathione peroxidase, haem-containing peroxidase, and peroxiredoxin. Furthermore, substantial evidence suggests that H2O2 is produced transiently in response to the activation of many cell surface receptors and serves as an intracellular messenger (32). Timely elimination of intracellular messengers after completion of their mission is critical for receptor singnaling. This would seem especially true for H2O2, which is readily converted to deleterious hydroxyl radicals.