Leu"cin (lū"s&ibreve;n), n. [Gr. leyko`s white.] (Physiol. Chem.) A white, crystalline, nitrogenous substance formed in the decomposition of albuminous matter by pancreatic digestion, by the action of boiling dilute sulphuric acid, and by putrefaction. It is also found as a constituent of various tissues and organs, as the spleen, pancreas, etc., and likewise in the vegetable kingdom. Chemically it is to be considered as amido-caproic acid.