An amino acid whose homocysteine and serine moieties are both l in configuration. Intermediate in transsulfuration whereby the mammal transfers the sulfur of methionine via homocysteine to cysteine: Hope, Proc. 4th Int. Congr. Biochem. 13, 63 (1960). Preparation: du Vigneaud et al., J. Biol. Chem. 143, 59 (1942); Weiss, Stekol, J. Am. Chem. Soc. 73, 2497 (1951); Sch"oberl, T"auber, Ann. 599, 23 (1956). Isolation from human brain: Tallau et al., J. Biol. Chem. 230, 707 (1958). Abs config: G. Jung et al., Eur. J. Biochem. 35, 436 (1973); C.-S. Chen et al., Biochim. Biophys. Acta 538, 534 (1978). Review: J. P. Greenstein, M. Winitz, Chemistry of the Amino Acids vol. 3 (John Wiley, N. Y., 1961) p 2682.