Free radical generation by selenium compounds and their prooxidant toxicity

Selenium (Se) and many of its compounds are among the most toxic of nutrients. Selenium toxicity was first described in range animals in the western United States in the 1930's which consumed "selenium accumulator" plants of the genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya. Selenites and selenates from the soil accumulate in these plants primarily as methylated selenium compounds and plants evolve dimethyldiselenide and dimethylselenide. Dietary selenium, primarily as selenomethionine and selenocysteine for humans fulfill the dietary requirement for selenoenzymes and proteins. In humans and animals excessive dietary selenium may be toxic. In vitro, selenium compounds such as selenite, selenium dioxide and diselenides react with thiols, such as glutathione, producing superoxide and other reactive oxygen species. This catalytic reaction of selenium compounds with thiols likely accounts for selenium toxicity to cells ex vivo and in vivo where the major glutathione producing organ, the liver, is also the major target organ of selenium toxicity. Selenium enzymes and selenoethers that do not readily form a selenide (RSe-) anion and compounds such as Ebselen where selenium is sequestered, are not toxic. Methylation of selenium by both plants and animals serves to detoxify selenium by generating methylselenides. Alternatively, full reduction of Se to elemental selenium (Se0) as done by some bacteria and the formation of heavy metal selenides such as Ag2Se or Hg2Se, results in a non-catalytic non-toxic form of selenium. This catalytic prooxidant attribute of some selenium compounds appears to account for its toxicity when such activity exceeds plant and animal methylation reactions and antioxidant defenses. This prooxidant activity may also account for cellular apoptosis and may provide a useful pharmaceutical application for selenium compounds as antibacterial, antiviral, antifungal and anticancer agents.