Selenium may be present in the form of inorganic selenium (selenite SeO32-) or in the form of amino acids selenocysteine and selenomethionine (analogues of amino acids containing sulfur - cysteine and methionine). Plants that grow in soil with large amounts of selenium form methylselenocysteine and selenohomocysteine that can be accumulated in quantities of 5,000 ppm thus causing selenium poisoning of humans and animals that consume them.
Inorganic selenium salts and organically bound selenium is probably well absorbed, although there is little information about the mechanism and regulation of resorption. Selenocysteine is decomposed by selenocysteine β-lyase (an enzyme that require vitamin B6 for its functioning) giving selenide (Se2+), and alanine. Selenate and selenite are reduced to selenide by an enzyme glutathione reductase.
One part of inorganic selenide is directly excreted, while other forms dimethyl selenide – (CH3)2Se and trimethylselenium ion – (CH 3)3Se+. Trimethylselenium ion is the major selenium metabolite that is expelled in urine. Of the total selenium intake around 55-60% is removed in urine, perspiration around 5%, and less than 1% by breathing. If selenium poisoning occurs, then the percentage of selenium expelled by breath is significantly increased (breath of garlic).
Seleno-enzyme glutathione peroxidase reduces hydrogen peroxide to water and thus reduces the amount of peroxide that can generate free radicals. In the case of vitamin E deficiency selenium plays an important role in decreasing of precursors of lipid alkyl peroxy radicals, as if there is a lack of selenium, vitamin E plays a protective role by eliminating radicals. In this case, membrane lipid peroxidation will be prevented. Selenium plays a very important role in the restoration of vitamin E.
In addition to the liver the largest concentration of selenium in the body is in muscle. Skeletal muscles contain more selenium from the heart.
The metabolic functions of selenium (as selenocysteine) is in the catalytic site of glutathione peroxidase and thyroxine deiodinase.
Selenocysteine is incorporated into proteins in the process of protein synthesis on ribosomes. Less commonly is added to post synthetic modifications of precursor proteins. Codon for selenocysteine is UGA, which usually serves as a STOP codon signaling the end of the translation. The synthesis of proteins that contain selenocysteine codon encodes the amino acid selenocysteine more than ending translation. Such a reading of a codon is the result of a sequence of four nucleotides in untranslation region at the 3' end of the mRNA. Selenocysteine is formed from serine bound to tRNA.