Biosynthesis of taurine peptides in brain cytoplasmic fraction in vitro

Brain homogenates and their cell-free soluble fraction incorporated labelled [14C]taurine, [14C]glutamic acid, [14C]aspartic acid and [14C]serine into a number of low-molecular weight peptides, among which glutamyl-, aspartyl- and seryl-taurines and their N-acetylated derivatives were identified. A partially purified cytoplasmic fraction catalyzed the formation of glutamyl-taurine. Excesses of aspartic acid and serine inhibited this reaction. Biosynthetic products were analyzed on thin-layer chromatography plates by an autoradiographic X-film technique and identified with the aid of synthetic peptides or endogenous synaptosomal peptides, whose structure was determined with mass spectrometry. gamma-Glutamyl-taurine was also formed through a group translocation mechanism from glutathione and taurine by the enzymes of the gamma-glutamyl cycle. The catalytic activity of the membraneous enzyme was identical with that of the commercial gamma-glutamyltransferase.