Organoselenium in Nature

Selenium, among the naturally occurring elements, is nowadays considered the most relevant for the redox homeostasis of living systems. In this chapter, its role in plants, bacteria, and humans is scholarly discussed. Some plants have the possibility to accumulate this element, thus becoming a natural source for animals and humans, in which selenium is embedded in selenoproteins, as the 21(st) amino acid, selenocysteine (l-Sec). The main classes of selenoenzymes (glutathione peroxidase, thioredoxin reductase, and iodothyronine deiodinases) are reported here and the molecular mechanism that characterizes their physiological action is discussed.