(also 2,7-diamino-4-thiasuberic acid), HOOCCH(NH₂)CH₂CH₂SCH₂CH(NH₂)COOH, an amino acid containing a sulfide group. Cystathionine is an important intermediate in the biosynthesis and metabolism of amino acids containing sulfur. Of the four optically active and two racemic forms of the acid, only L-cystathionine and L-allo-cystathionine are biologically active. In mammals, L-cystathionine participates in the biosynthesis of cysteine from methionine and serine; in plants and bacteria, it participates in the biosynthesis of methionine from cysteine and homoserine.