γ‐Glutamyl Transpeptidase in Rat Ascites Tumor Cell LY‐5

γ‐Glutamyl transpeptidase activity was detected in rat ascites tumor cells (LY‐5) suspended in Hanks' balanced saline solution using l ‐γ‐glutamyl‐ p ‐nitroanilide as a substrate. Whole‐cell suspension of the tumor cells exhibited full activity of the enzyme without detectable cell disruption under the conditions examined. Various amino acids, transported through specific membrane carriers, did not affect the accessibility of the substrate for the enzyme. An inhibitor of sodium‐dependent transport systems of amino acids caused no significant change in the rate of enzyme catalysis. Like glutathione or S ‐methylglutathione, S ‐acetyldextran (mol. wt 215000) derivative of glutathione, which is believed to be unable to penetrate into intact cells, caused marked inhibition of the rate of p ‐nitroaniline release from the synthetic substrate by the tumor cells. These data indicated that the active site of the enzyme faced to the outer surface of the cells. γ‐Glutamyl transpeptidase of the tumor cells was successfully affinity‐labeled by 6‐diazo‐5‐oxo‐ l ‐norleucine, a glutamine analog, without causing detectable change in the viability of the cells under the conditions examined. The rate of transport of alanine, leucine, glycine and glutamine into cells was not affected by the inactivation of this enzyme with the affinity label. Thus, the activity of γ‐glutamyl transpeptidase located on the outer surface of tumor cell membrane does not seem to be requisite for the transport process of amino acids.