On the significance of γ-glutamyl transpeptidation in peptide biosynthesis

Abstract

In a study of the significance of the gamma-glutamyl transpeptidation hypothesis for the biosynthesis of peptides in various tissue preparations, it was found that free glycine was always incorporated into protein more efficiently than glycine from gamma- glutamylglycine and no favorable influence of the gamma-glutamyl peptide was observed on the incorporation of radioactive glycine. The incorporation of glycine, valine or phenylalanine into protein was not increased by the addition of glutamic acid or gamma-glutamyl compounds. Chromatographic fractionation on Dowex-50 of the supernatant fluids of tissue preparations incubated with gamma-glutamyl glycine showed that this peptide is rapidly hydrolyzed by liver and kidney homogenates, but is remarkably stable in the spleen. The chromatogram showed no evidence of the formation of the corresponding radioactive gamma-glutamyl peptides from incubations with radioactive glycine, valine or phenylalanine in the presence or absence of unlabeled gamma-glutamylglycine. The formation of acidic metabolites, including gamma-glutamyl peptides, from neutral substances was studied using chromatography with Amberlite IR-4B resin. An upper level of 5% was obtained as the possible extent of formation of gamma-glutamyl peptides. Attempts to confirm the formation of gamma-glutamylvaline and g?mma-glutamylphenylo.lanine, employing radioactive amino acids, by the sheep kidney system of Hanes and modifications of this system with rat kidney were unsuccessful. A micro synthesis of gamma-glutamyl-(1:2-C14 glycine is described.