Selenium-dependent enzymes

Selenium, molecular weight 78.96, resembles sulfur in many of its chemical properties and occurs in inorganic forms as H2Se, H2Se2O3, H2SeO3, and H2SeO4 which are the analogues of hydrogen sulfide, thiosulfate, sulfite, and sulfate, respectively. The commonly available radionuclide, 75Se, is a gamma emitter (half-life 122 days) that is used extensively as a tracer in biochemical studies and as a radiopharmaceutical agent for diagnostic purposes. Organoselenium compounds, in general, are less stable and more reactive than the corresponding sulfur analogues and these properties may account for the toxicity of selenium when it is incorporated indiscriminately in place of sulfur in cellular constituents. On the other hand living systems may have exploited the greater reactivity of certain types of organoselenium compounds in those instances where selenium is specifically required as a component of an enzyme or other macromolecule. Several enzymic processes that do not distinguish selenium from sulfur and therefore may be important in selenium toxicity were discussed in some detail in two earlier reviews on selenium biochemistry (1, 2) and this aspect of the problem is not treated here. Rather, the information currently available on the properties and catalytic functions of the four known selenium-dependent enzymes is summarized. These enzymes are formate dehydrogenases of Escherichia coli and several anaerobic bacteria, clostridial glycine reductase, mammalian and avian glutathione peroxidase, and nicotinic acid hydroxylase of Clostridium barkeri. Additional selenoproteins whose catalytic activities are as yet unidentified are mentioned.