Selenomethionine is not known to have a physiological function separate from that of methionine.
Selenocysteine is present in animal selenoproteins that have been characterized (see below) and is the form of selenium that accounts for the biological activity of the element. In contrast to selenomethionine, there is no evidence that selenocysteine substitutes for cysteine in humans.
Selenium functions largely through an association with proteins, known as selenoproteins (Stadtman, 1991), and disruption of their synthesis is lethal for embryos (Bösl et al., 1997). A selenoprotein is a protein that contains selenium in stoichiometric amounts. Fourteen selenoproteins have been characterized to date in animals. The four known selenium-dependent glutathione peroxidases designated as GSHPx 1 through 4 defend against oxidative stress (Flohe, 1988). Selenoproteins P and W are postulated to do so as well (Arteel et al., 1998; Burk et al., 1995; Saito et al., 1999; Sun et al., 1999). Three selenium-dependent iodothyronine deiodinases regulate thyroid hormone metabolism (Berry and Larsen, 1992). Three thioredoxin reductases have been identified (Sun et al., 1999). Their functions include reduction of intramolecular disulfide bonds and regeneration of ascorbic acid from its oxidized metabolites (May et al., 1998). The selenium-dependent isoform of selenophosphate synthetase participates in selenium metabolism (Guimaraes et al., 1996). Other selenoproteins have not yet been characterized to the same extent with respect to function (Behne et al., 1997). Thus, the known biological functions of selenium include defense against oxidative stress, regulation of thyroid hormone action, and regulation of the redox status of vitamin C and other molecules.
Absorption of selenium is efficient and is not regulated. More than 90 percent of selenomethionine, the major dietary form of the element, is absorbed by the same mechanism as methionine itself (Swanson et al., 1991). Although little is known about selenocysteine absorption, it appears to be absorbed very well also.
An inorganic form of selenium, selenate (SeO42−), is absorbed almost completely, but a significant fraction of it is lost in the urine before it can be incorporated into tissues. Another inorganic form