Selenocysteine and Selenoproteins

Abstract

Selenium occurs normally in living things as a highly specific component of certain enzymes and amino acid transfer nucleic acids (tRNAs). In bacteria, biosynthesis of essential selenoenzymes has been shown to be unaffected by wide variations in sulfur levels. The naturally occurring selenoenzymes identified from bacterial sources include glycine reductase, formate dehydrogenases, a hydrogenase, nicotinic acid hydroxylase, xanthine dehydrogenase. The selenoenzyme, glutathione peroxidase, and other selenoproteins of unknown function have been isolated from animals. In certain enzymes, e.g. glycine reductase, formate dehydrogenase, hydrogenase and glutathione peroxidase, the chemical form of selenium has been identified as selenocysteine. A labile, unidentified form of selenium is present in nicotinic acid hydroxylase, and by inference, xanthine dehydrogenase.