Studies on γ glutamyl transpeptidase in human and rabbit erythrocytes

γ Glutamyl transpeptidase transfers the γ glutamyl moiety of glutathione to a variety of acceptor amino acids. Through the operation of the γ glutamyl cyclotransferase cycle, this enzyme has been implicated in the transport of amino acids into cells, especially the cells of the proximal tubules of kidney. It has been reported to be present in rabbit erythrocytes. However, using white cell free preparations, the authors have not been able to demonstrate the presence of γ glutamyl transpeptidase in human or rabbit erythrocytes either by measuring the utilization of GSH or by following the formation of the product. ¹⁴C L methionine was used as acceptor amino acid, and the formation of γ glutamyl ¹⁴C L methionine was followed. Using similar conditions, the authors have been able to demonstrate the presence of γ glutamyl transpeptidase in human and rabbit leukocytes and in human kidney. In contrast to a previous report, the authors were unable to find the accumulation of 5 oxoproline, an intermediate of the γ glutamyl cyclotransferase pathway in human red cells incubated in Krebs Ringer solution. Immunologic studies demonstrated that human red cell membranes contained no protein antigenically similar to kidney γ glutamyl transpeptidase. Thus these studies indicated that in human and rabbit erythrocytes, the γ glutamyl transpeptidase cyclotransferase pathway was not operative.