Abstract
In the last decade, it has become clear that virtually all mammalian cells produce reactive oxygen species. It was generally believed that these were by-products of cellular respiration and metabolism, and that they exerted toxic effects, including DNA damage and lipid oxidation. Recent evidence has demonstrated that this concept is incorrect, and that reactive oxygen species are produced in a controlled fashion and likely have critical signaling functions. Likewise, antioxidant defenses play a crucial role in modulating the ambient steady-state levels of reactive oxygen species. Biological or pharmacological manipulation of endogenous antioxidants can have a profound effect on cellular function. Emerging evidence suggests that hydrogen peroxide (H2O2) plays a particularly important role in signal transduction. H2O2 is uncharged and is freely diffusible within and between cells. Compared with other reactive oxygen species, it is also quite stable. A major source of H2O2 is a membrane-bound NADH/NADPH oxidase, the activity of which is regulated by hormones, growth factors, and physical forces. The primary product of this enzyme system is superoxide (O2·−), which is rapidly dismutated to H2O2 by the superoxide dismutases. Removal of H2O2 is regulated by two important enzymes, …
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