γ-Glutamyl transpeptidase from kidney bean fruit I. Purification and mechanism of action

Abstract

1. 1.|A γ-glutamyl transpeptidase from kidney bean fruit has been further purified and its properties have been studied. The purified material seems to consist of one component, as judged by its behavior in the ultracentrifuge and its elution pattern from an acrylamide-gel column. The preparation catalyzes the transfer of the γ-glutamyl moiety of glutathione, γ-glutamylaniline, and γ- glutamyl-p- nitroaniline to suitable acceptor molecules, usually amino acids. 2. 2.|The same preparation was found to catalyze the hydrolytic cleavage of the γ-glutamyl peptide linkage of these compounds. Evidence, based on purification, gel filtration and activation, is presented that the two activities are associated with the same enzyme. 3. 3.|From kinetic studies, it is concluded that a basic nitrogen of an imidazole group and a phenolic group of tyrosine constitute a part of the catalytic site of the enzyme molecule. On the basis of these findings a mechanism for the transfer and the hydrolytic reactions is proposed based on a nucleophilic attack of imidazole nitrogen on substrates. 4. 4.|It is suggested that the enzyme is involved in the degradation of glutathione.