Studies on gamma-glutamyl transpeptidase in human and rabbit erythrocytes

Gamma-glutamyl transpeptidase transfers the gamma-glutamyl moiety of glutathione to a variety of acceptor amino acids. Through the operation of the gamma-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, we have not been able to demonstrate the presence of gamma-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 gamma-glutamyl-14C-L-methionine was followed. Using similar conditions, we have been able to demonstrate the presence of gamma-glutamyl transpeptidase in human and rabbit leukocytes and in human kidney. In contrast to a previous report, we were unable to find the accumulation of 5-oxoproline, an intermediate of the gamma-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 gamma-glutamyl transpeptidase. Thus our studies indicated that in human and rabbit erythrocytes, the gamma-glutamyl transpeptidase-cyclotransferase pathway was not operative.