An amino acid classified as nonessential with respect to its growth effect in rats. Not among the naturally occurring hydrolytic products of proteins, see Schmidt, Chemistry of the Amino Acids and Proteins (Springfield, Ill., 2nd ed., 1944) p 1090. Synthesis from ethyl α-isonitrosoacetone dicarboxylate: Harington, Randall, Biochem. J. 25, 1917 (1931); Levene, Schormüller, J. Biol. Chem. 106, 595 (1934).