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Keywords:

  • brain;
  • α-ketoglutarate;
  • glutamate;
  • mitochondrial metabolite transport;
  • citric acid cycle

Abstract

Some models of brain energy metabolism used to interpret in vivo 13C nuclear magnetic resonance spectroscopic data assume that intramitochondrial α-ketoglutarate is in rapid isotopic equilibrium with total brain glutamate, most of which is cytosolic. If so, the kinetics of changes in 13C-glutamate can be used to predict citric acid cycle flux. For this to be a valid assumption, the brain mitochondrial transporters of glutamate and α-ketoglutarate must operate under physiologic conditions at rates much faster than that of the citric acid cycle. To test the assumption, we incubated brain mitochondria under physiologic conditions, metabolizing both pyruvate and glutamate and measured rates of glutamate, aspartate, and α-ketoglutarate transport. Under the conditions employed (66% of maximal O2 consumption), the rate of synthesis of intramitochondrial α-ketoglutarate was 142 nmol/min·mg and the combined initial rate of α-ketoglutarate plus glutamate efflux from the mitochondria was 95 nmol/min·mg. It thus seems that much of the α-ketoglutarate synthesized within the mitochondria proceeds around the citric acid cycle without equilibrating with cytosolic glutamate. Unless the two pools are in such rapid exchange that they maintain the same percent 13C enrichment at all points, 13C enrichment of glutamate alone cannot be used to determine tricarboxylic acid cycle flux. The α-ketoglutarate pool is far smaller than the glutamate pool and will therefore approach steady state faster than will glutamate at the metabolite transport rates measured. © 2004 Wiley-Liss, Inc.