valine (vălˈēn) [key], organic compound, one of the 22 α-amino acids commonly found in animal proteins. Only the l -stereoisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet, as the human body cannot synthesize it from simpler metabolites. Young adults need about 23 mg of this amino acid per day per kilogram (10 mg per lb) of body weight. Valine can be degraded into simpler compounds by the enzymes of the body; an inherited defect in one of the enzymes involved in this process results in a rare disorder called maple syrup urine disease. Valine contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions. The structure of valine was established in 1906, after it had been first isolated from albumin in 1879. See isoleucine.