Rat renal gamma-glutamyltransferase activity: a reaction of glutamine synthetase

The experimental observations of Welbourne (Am. J. Physiol. 226, 544-548 (1974)) led him to propose that a gamma-glutamyltransferase activity contributes significantly to basal renal ammonia production. He measured the transferase activity in vitro as gamma-glutamylhydroxamate or ammonia production from glutamine and hydroxylamine. We report that in crude homogenates of rat kidney, gamma-glutamyltransferase activity requires the presence of a divalent cation, arsenate, and ADP. These conditions are also required for the maximal expression of the gamma-glutamyltransferase reaction catalyzed by glutamine synthetase. Glutamine synthetase and gamma-glutamyltransferase activities exhibit an identical distribution during differential centrifugation. In addition, the two activities comigrate sucrose gradient velocity centrifugation, and exhibit identical heat inactivation and L-methionine sulfoximine inhibition profiles. Furthermore, both activities are found to be localized primarily in the outer stripe region of the renal medulla. Based on these observations, it is concluded that the primary gamma-glutamyltransferase activity as assayed in rat renal tissue is a partial reaction of glutamine synthetase, an enzyme which does not catalyze the production of ammonia and gamma-glutamyl peptides from glutamine.