Selenocysteine Metabolism in Mammals

This chapter discusses selenocysteine metabolism in mammals. Several microbial and mammalian proteins, particularly enzymes, contain selenium as an essential component. Four of them have been shown to contain a selenocysteine residue in their polypeptide chains: selenoprotein A of glycine reductase complex from Clostridium sticklandii, formate dehydrogenase and hydrogenase of Methanococcus vannielii, and glutathione peroxidase of rat liver and bovine erythrocyte. Clostridial thiolase contains selenomethionine. Selenocysteine can be synthesized from O-acetylserine and H2Se by O-acetylserine(thiol)-lyase of selenium accumulator and non-accumulator plants and of Paracoccus denitrificans. Elemental selenium is reduced to H2Se by selenocysteine or other reductants such as dithiothreitol, which is added to prepare selenocysteine from selenocysteine in the anaerobic reaction system. L-selenocysteine is converted stoichiometrically to H2Se and L-alanine in the presence of an excess amount of dithiothreitol in the enzyme reaction mixture. However, this does not necessarily mean that the enzymatic primary product is H2Se. Evidence has been obtained to show that selenocysteine is synthesized from selenomethionine derived from a diet in the analogous pathway to the sulfur counterparts, although other pathways of selenocysteine synthesis may be possible.