Regulation of glutaminase by exogenous glutamate, ammonia and 2-oxoglutarate in synaptosomal enriched preparation from rat brain.

Phosphate activated glutaminase in synaptosomal enriched preparation from rat brain is very sensitive to inhibition by low concentration of glutamate, ammonia and 2-oxoglutarate when added to the incubation medium at pH 7.6. By increasing the concentration of either of these compounds up to 0.5 mM a pronounced initial inhibition is followed by little or no further effect when the concentration is increased beyond this level. By lowering the pH of the reaction mixture to 7.0, the inhibition by glutamate is almost abolished and that of ammonia reduced. Glutamate inhibits mainly the N-ethylmaleimide-sensitive fraction of glutaminase which previously is suggested to be localized to the outer phase of the mitochondrial inner membrane whereas ammonia inhibits both the N-ethylmaleimide-sensitive and -insensitive fraction. Evidence has been produced to show that the inhibition by 2-oxoglutarate is caused by glutamate formation by aminotransferase reactions. Since 2-oxoglutarate is produced by the tricarboxylic acid cycle, the operation of this cycle may regulate the glutaminase reaction by controlling glutamate formation via the aminotransferase reactions.