During the last decade considerable progress have been made in characterizing biochemistry and genetics of selenium. It is demonstrated that selenium in nature can exist in two forms: selenomethionine and selenocysteine. Selenomethionine is found in plants and selenocysteine in animals and human organism. Selenocysteine differs from cysteine by a single atom and has similar chemical properties, but stronger nucleophilicity of selenocysteine makes it much more reactive. Selenocysteine insertion into proteins is dictated by the codon UGA, and requires the presence of a conserved stem-loop structure known as the selenoprotein insertion sequence element. In eukaryotes this structure is located in the 3’-untranslated region of selenoprotein mRNA. Proteins containing selenocysteine are called selenoproteins. In the cells selenoproteins are involved in many enzymatic reactions. In the human genome, 25 genes for selenoproteins have been identified and over 340 selenoproteins are known. Even selenium is toxic element but it is essential component or cofactor of enzymes. The deficiency of selenium in diet disturbs synthesis of selenoproteins and causes some disorders. Se lenium also has an anticarcinogenic eftect that is thought to be induced by the production of methylselenol, a selenometabolite that affects gene expression and modifies cell cycling and immune function.
The rec ommended di etary al lowance for selenium is 55 ^g/d for normal adult. Diet is the predominant source of selenium in humans. Selenium is found in grain and meat, most concentrated in Brazilian nuts, kidney, liver and others. The amount of sel enium in dietary depends on the selenium concentration in soil.
