n. [ Gr. leyko`s white. ] (Physiol. Chem.) a naturally occurring alpha-amino acid ((CH3)2CH.CH2.CH(NH2)-COOH), one of the building units of almost all proteins of living organisms, both animal and vegetable. It is one of the essential amino acids (not synthesized by the human body, a required component for proper nutrition), and is hydrophobic in character when bound in proteins. In isolated form it is a white, crystalline, zwitterionic substance formed, e. g. by the decomposition of proteins by pancreatic digestion, by the action of boiling dilute mineral acid, or by putrefaction. Chemically it is to be considered as amido-caproic acid. It occurs as two optical isomers, the L- and D-forms. The L-form, L-leucine, is the natural form, present in most proteins. [ 1913 Webster +PJC ]