High Quality Content by WIKIPEDIA articles! Selenomethionine is an amino acid containing selenium. The L-enantiomer of selenomethionine, known as Se-met and Sem, is a common natural food source of selenium. In vivo, selenomethionine is randomly incorporated instead of methionine and is readily oxidized. Its antioxidant activity arises from its ability to deplete reactive species. Selenium and sulfur are chalcogen elements that share many chemical properties and the substitution of methionine to selenomethionine may have no effect on protein structure and function. However, the incorporation of selenomethionine into tissue proteins and keratin in horses causes alkali disease. Alkali disease is characterized by emaciation, loss of hair, deformation and shedding of hooves, loss of vitality and erosion of the joints of long bones.