MODULATORS OF GLUTAMINASE ACTIVITY

Abstract— The activating effect of GTP on particulate preparations of glutaminase from rat brain or rat kidney was competitively inhibited by cyclic guanosine 3′,5′-monophosphate (_c-GMP). Similarly, the effect of ATP was inhibited by cyclic adenosine 3′,5′-monophosphate (_c-AMP). In the absence of an added activator, the glutaminase activity of brain or kidney particles, if measurable, was also inhibited by _c-GMP and _c-AMP. GTP was a stronger activator than ATP, and _c-GMP was a stronger inhibitor than _c-AMP, for the enzyme from either tissue. The K₁, of _c-GMP was about 40 mM, that of _c-AMP was about 60 mM, as determined with a brain preparation. Soluble preparations of glutaminase from pig brain and pig kidney were activated, in decreasing order of efficiency, by riboflavin phosphate, GTP, ATP and orthophosphate. The relative potencies were similar for the soluble enzymes from brain and kidney and also in the particulate and soluble forms of the brain enzyme. The apparent K_m, values for the soluble brain enzyme were about the same (9–10 mM) whether riboflavin phosphate, GTP, ATP or 100 mM orthophosphate were used as activators. The Km increased with concentrations of orthophosphate lower than 100 mM, but was not significantly affected by changes in the concentrations of the other activators. Results for the kidney enzyme were similar, but the K_m, tended to be somewhat lower. The estimation of the apparent K_A with either the brain or kidney enzyme preparation indicated that affinities and activating efficiency were related. The activating effects of carboxylic acids on the soluble brain enzyme were similar to those on the particulate brain enzyme in terms of some correlation to the number of carboxylic groups per molecule and the potentiation by orthophosphate, but in the soluble kidney enzyme these effects were less marked or absent.