1,4-Thiomorpholine-3,5-dicarboxylic acid, a novel cyclic imino acid detected in bovine brain.

Gas-liquid chromatography of enriched bovine brain extract revealed the occurrence of several sulfur-containing compounds. By co-chromatography with authentic product and by mass-spectrometric analysis, one of these compounds has been identified as 1,4-thiomorpholine-3,5-dicarboxylic acid (TMDA). The possible derivation of TMDA from lanthionine is discussed. This represents the second S-containing cyclic amino acid so far discovered in a mammalian brain whose physiological significance has not yet been explored.