IN-VIVO ACTIVITY OF GLUTAMINASE IN THE BRAIN OF HYPERAMMONEMIC RATS MEASURED BY N-15 NUCLEAR-MAGNETIC-RESONANCE

The in vivo activity of phosphate-activated glutaminase (PAG) was measured in the brain of hyperammonaemic rat by N-15 n.m.r. Brain glutamine was N-15-enriched by intravenous infusion of (NH4+)-N-15 until the concentration of 5-N-15 glutamine reached 6.1 mu mol/g. Further glutamine synthesis was inhibited by intra-peritoneal injection of methionine-DL-sulphoximine, an inhibitor of glutamine synthetase, and the infusate was changed to (NH4+)-N-14 during observation of decrease in brain 5-N-15 glutamine due to PAG and other glutamine utilization pathways. Progressive decrease in brain 5-N-15 glutamine, PAG-catalysed production of (NH4+)-N-15 and its subsequent assimilation into glutamate by glutamate dehydrogenase were monitored in vivo by N-15 n.m.r. Brain 5-N-15 glutamine (N-15 enrichment of 0.35-0.50) decreased at a rate of 1.2 mu mol/h per g of brain. The in vivo PAG activity, determined from the observed rate and the quantity of (NH4+)-N-15 produced and subsequently assimilated into glutamate and aspartate, was 0.9-1.3 mu mol/h per g. This activity is less than 1.1% of the reported activity in vitro measured in rat brain homogenate at a 10 mM concentration of the activator P-i. Inhibition by ammonia (brain level 1.4 mu mol/g) alone does not account for the observed low activity in vivo. The result strongly suggests that, in intact brain, PAG activity is maintained at a low level by a suboptimal in situ concentration of P-i and the strong inhibitory effect of glutamate. The observed PAG activity in vivo is lower than the reported in vivo activity of glutamate decarboxylase which converts glutamate into gamma-aminobutyrate (GABA). The result suggests that PAG-catalysed hydrolysis of glutamine is not the sole provider of glutamate used for GABA synthesis.