Metabolism of glucose into glutamate via the hexose monophosphate shunt and its inhibition by 6-aminonicotinamide in rat brain in vivo

Abstract

The treatment of rats for 4 h with 6-aminonicotinamide (60 mg kg –1 ) resulted in an 180-fold increase in the concentration of 6-phospho-gluconate in their brains; glucose increased 2.6-fold and glucose 6-phosphate, 1.7-fold. Moreover, lactate decreased by 20%, glutamate by 8% and y -aminobutyrate by 12 %, and aspartate increased by 10%. No significant changes were found in glutamine and citrate. In blood, 6-phosphogluconate increased 5-fold; glucose, 1.4-fold and glucose 6-phosphate, 1.8-fold. The metabolism of glucose in the rat brain, via both the Embden–Meyerhof pathway and the hexose monophosphate shunt, was investi­gated by injecting [U- 14 C]glucose or [2- 14 C]glucose, and that via the hexose monophosphate shunt alone by injecting [3, 4- 14 C]glucose. The total radioactive yield of amino acids in the rat brain was 5.63 μmol at 20 min after injection of [U- 14 C]glucose, or 5.82 μmol after injection of [2- 14 C]glucose; by contrast, it was 0.62 μmol after injection of [3, 4- 14 C]glucose. The treatment of rats with 6-aminonicotinamide showed significant decreases in these values, owing to decreases in the radioactive yields of glutamate, glutamine, aspartate, y -aminobutyrate, and alanine+glycine+serine. Glutamate isolated from the brain contained approximately 43% of its radioactivity in carbon 1 after injection of [3, 4- 14 C]glucose, in contrast to 13% and 18% after injection of [U- 14 C]glucose and [2- 14 C]glucose, respectively, in both the control and treated rats. The calculations based on these findings showed that approximately 69% of the 14 C-labelled glutamate was formed from [ 14 C]acetyl coenzyme A (acetyl CoA) and the residual 31% by 14 CO 2 fixation of pyruvate after injection of [3, 4- 14 C]glucose in both control and treated rats. The results gave direct evidence that glutamate and y -aminobutyrate in the brain were formed by metabolism of glucose via the hexose monophosphate shunt as well as via the Embden–Meyerhof pathway. From the radioactive yields of glutamate formed via [ 14 C]acetyl CoA it was estimated that approximately 7.8% of the total glucose utilized was channelled via the hexose monophosphate shunt. Assuming that [ 14 C]glutamate formed by carbon-dioxide fixation of pyruvate was also dependent on the metabolism of glucose through the hexose mono­phosphate shunt, the estimated value was approximately 9.5% of the total glucose converted into glutamate. The results of the present investigation, taken in conjunction with other findings, suggest that the utilization of glucose via the hexose monophosphate shunt is functionally important in the rat brain.