“…The superoxide anion (O 2 •-) is produced intracellularly by transferring electrons leaked from the electron transport chain in mitochondria and from NADPH cytochrome P450 reductase in the endoplasmic reticulum to oxygen, as well as by the action of several enzymes including NADPH oxidase, lipooxygenase, cyclooxygenase, flavoenzymes (e.g., xanthine oxidase), and uncoupled nitric oxide synthase (47). In particular, the superoxide anion is also formed endogenously by the reaction of selenium compounds such as selenite, selenium dioxide, diselenides, and selenocysteine, and with thiols such as reduced GSH or L-cysteine (3,11,48). The generation of superoxide anions by selenium has been confirmed by treating cells with exogenous GSH or selenite; this has also been observed by adding isolated mitochondria to selenium-containing compounds, including selenite, selenocysteine, selenocystamine, and selenodioxide, to GSH, or to both (10,49,50).…”