The enzymic reaction of amino acids with glutathione

Abstract

A modification of the method of Nakamura and Binkley (Jour. Biol. Chem. 173,407, 1948) for estimation of cysteine in the presence of glutathione (GSH) is descr. Sheep-kidney gamma-glutamyl transferase, which catalyzes peptide-bond transfers between GSH and amino acids, was investigated. With certain qualifications, cysteine liberated by these transfer reactions can be taken as an index of the comparative reactivity of amino acids in the system. The relative reactivities of L-alpha-amino acids was detd. The most reactive amino acids in the aliphatic series were those with long, unbranched side chains. The presence of a beta-methyl group, as in L-valine and L-isoleucine, interfered with the reactivity of the amino groups of these amino acids. The ability of the amino groups of L-aspartic and L-glutamic acids to react with GSH was enhanced by amide formation as in L-asparagine and L-glutamine, respectively. With the exceptions of L-aspartic acid, L-isoleucine and L-valine, an increase in amino acid concn. brought about an increase in cysteine liberated from GSH. At higher concns. reactivity fell off. Increases in concn. of L-aspartic acid, L-isoleucine and L-valine caused a steady decrease in cysteine liberation. Chromatographic evidence has now been obtained for transpeptidation with most of the common, naturally occurring amino acids, with the exceptions of L-aspartic acid, L-proline and L-hydroxyproline.