Oxo-4-methylpentanoic acid directs the metabolism of GABA into the Krebs cycle in rat pancreatic islets

OMP (oxo-4-methylpentanoic acid) stimulates by itself a biphasic secretion of insulin whereas L-leucine requires the presence of L-glutamine. L-Glutamine is predominantly converted into GABA (gamma-aminobutyric acid) in rat islets and L-leucine seems to promote its metabolism in the 'GABA shunt' [Fernandez-Pascual, Mukala-Nsengu-Tshibangu, Martin del Rio and Tamarit-Rodriguez (2004) Biochem. J. 379,721-729]. In the present study, we have investigated how 10mM OMP affects L-glutamine metabolism to uncover possible differences with L-leucine that might help to elucidate whether they share a common mechanism of stimulation of insulin secretion. In contrast with L-leucine, OMP alone stimulated a biphasic insulin secretion in rat perifused islets and decreased the islet content of GABA without modifying its extracellular release irrespective of the concentration of L-glutamine in the medium. GABA was transaminated to L-leucine whose intracellular concentration did not change because it was efficiently transported out of the islet cells. The L-[U-C-14]-Glutamine (at 0.5 and 10.0 mM) conversion to (CO2)-C-14 was enhanced by 10 mM OMP within 30% and 70% respectively. Gabaculine (250 mu M), a GABA transaminase inhibitor, suppressed OMP-induced oxygen consumption but not L-leucineor glucose-stimulated respiration. It also suppressed the OMP-induced decrease in islet GABA content and the OMP-induced increase in insulin release. These results support the view that OMP promotes islet metabolism in the 'GABA shunt' generating 2-oxo-glutarate, in the branched-chain a-amino acid transaminase reaction, which would in turn trigger GABA deamination by GABA transaminase. OMP, but not L-leucine, suppressed islet semialdehyde succinic acid reductase activity and this might shift the metabolic flux of the 'GABA shunt' from gamma-hydroxybutyrate to succinic acid production.