leucine (lōˈsēn) [key], organic compund, one of the 20 amino acids commonly found in animal proteins. Only the l -stereooisomer appears in mammalian protein. It is one of several essential amino acids needed in the diet; the human body cannot synthesize it from simpler metabolites. Young adults need about 31 mg of this amino acid per day per kilogram (14 mg per lb) of body weight. Leucine can be degraded into simpler compounds by the enzymes of the body. Leucine contributes to the structure of proteins into which it has been incorporated by the tendency of its side chain to participate in hydrophobic interactions. Certain proteins that bind to DNA (see nucleic acid) and may help regulate its activities, posses regions in which leucines are arranged in configurations called leucine zippers. Leucine was isolated from cheese in an impure form in 1819 and from muscle and wool in the crystalline state in 1820. It was named after the Greek word leukos [white], evidently because at that time the purification of a subtance from nature to a white, crystalline state was considered noteworthy. The stucture of leucine was established by laboratory synthesis in 1891. See isoleucine.