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.
