isoleucine (Īˌsəlōˈsēn) [key], organic compound, one of the 20 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; human beings cannot synthesize it from simpler metabolites. Young adults need about 20 mg of this amino acid per day per kg (or about 8 mg per lb) of body weight. Isoleucine can be degraded into simpler compounds by the enzymes of the body. In a rare, inherited disorder called maple syrup urine disease, a nonfunctional enzyme in the common pathway of isoleucine, leucine, and valine degradation causes the buildup of certain metabolites in the urine, resulting in the characteristic odor from which the disease derives its name. Once isoleucine is incorporated into protein, it contributes to the structure of protein by the tendency of its side chain (composed only of carbon and hydrogen) to seek an environment consisting of similar side chains, like those of leucine, valine, tryptophan, and phenylalanine, and to exclude water. This hydrophobic property is analogous to that which prevents oil from dissolving in water. The tendency for these hydrophobic residues to associate with one another is evidently quite important in determining the bending and folding (tertiary structure) of the peptide chain characteristically seen in every protein. Isoleucine was isolated from beet sugar molasses in 1904.