REVERSIBILITY OF THE MITOCHONDRIAL ISOCITRATE DEHYDROGENASE REACTION IN THE PERFUSED-RAT-LIVER - EVIDENCE FROM ISOTOPOMER ANALYSIS OF CITRIC-ACID CYCLE INTERMEDIATES

The reversal of the mitochondrial isocitrate dehydrogenase reaction wa s investigated in rat livers perfused with [U-C-13(5)]glutamate or [U- C-13(5)]glutamine. The mass isotopomer distribution of citric acid cyc le intermediates extracted from the livers was determined by gas chrom atography-mass spectrometry. Citrate was enriched in an isotopomer con taining five C-13. The formation of this isotopomer can only be explai ned by the reversal of the isocitrate dehydrogenase reaction. Calculat ion of kinetic parameters from the mass isotopomer data reveals a rapi d interconversion of isocitrate and alpha-ketoglutarate. This intercon version results in an isotopic exchange between carbon 6 of citrate an d mitochondrial CO2 that can affect the calculation of citric acid cyc le kinetic parameters. Thus, the reversal of the isocitrate dehydrogen ase reaction should be included in isotope labeling models of the citr ic acid cycle.