Selenium in biology: Facts and medical perspectives

Several decades after the discovery of setenium as an essential trace eleme nt in vertebrates approximately 20 eukaryotic and more than 15 prokaryotic selenoproteins containing the 21(st) proteinogenic amino acid, selenocystei ne, have been identified, partially characterized or cloned from several sp ecies. Many of these proteins are involved in redox reactions with selenocy steine acting as an essential component of the catalytic cycle. Enzyme acti vities have been assigned to the glutathione peroxidase family, to the thio redoxin reductases, which were recently identified as selenoproteins, to th e iodothyronine deiodinases, which metabolize thyroid hormones, and to the selenophosphate synthetase 2, which is involved in selenoprotein biosynthes is. Prokaryotic selenoproteins catalyze redox reactions and formation of se lenoethers in (stress-induced) metabolism and energy production of E. coli, of the clostridial cluster XI and of other prokaryotes. Apart from the spe cific and complex biosynthesis of selenocysteine, selenium also reversibly binds to proteins, is incorporated into selenomethionine in bacteria, yeast and higher plants, or posttranslationally modifies a catalytically essenti al cysteine residue of CO dehydrogenase. Expression of individual eukaryoti c selenoproteins exhibits high tissue specificity, depends on selenium avai lability, in some cases is regulated by hormones, and if impaired contribut es to several pathological conditions. Disturbance of selenoprotein express ion or function is associated with deficiency syndromes (Keshan and Kashin- Beck disease), might contribute to tumorigenesis and atherosclerosis, is al tered in several bacterial and viral infections, and leads to infertility i n male rodents.