Isotopomer Analysis of Citric Acid Cycle and Gluconeogenesis in Rat Liver

Abstract

We conducted an extensive mass isotopomer analysis of citric acid cycle and gluconeogenic metabolites isolated from livers of overnight fasted rats perfused with 4 mM glucose, 0.2 mM octanoate, 1 mM [U-[IMG]C[IMG]]lactate, and 0.2 mM [U-[IMG]C[IMG]]pyruvate, in the anterograde or retrograde mode. In both perfusion modes, two distinct isotopomer patterns were observed: (i) those of phosphoenolpyruvate, glucose, malate, and aspartate and (ii) those of citrate, [IMG]-ketoglutarate, glutamate, and glutamine. Key citric acid cycle parameters and, hence, rates of gluconeogenesis, calculated (Lee, W.-N. P. (1989) J. Biol. Chem. 264, 13002-13004 and Lee, W.-N. P. (1993) J. Biol. Chem. 268, 25522-25526) from our mass isotopomer data did not only vary, but lead to conclusions inconsistent with Lee's citric acid cycle model. Compared to lactate and pyruvate uptake, which sets an upper limit to glucose production, rates of gluconeogenesis calculated (i) with the phosphoenolpyruvate and citrate data were similar, but those calculated (ii) with the glutamate data amounted to only 60%, which is unlikely. All these conclusions are independent of the perfusion modes. We provide evidence that the following processes contribute to the observed labeling discrepancy: (i) the reversibility of the isocitrate dehydrogenase reaction and (ii) an active citrate cleavage pathway for the transfer of the oxaloacetate carbon skeleton from mitochondria to the cytosol. Also, a good fit of our labeling data was obtained with a model of citric acid cycle and gluconeogenesis which we developed to incorporate the above reactions (Fernandez, C. A., and Des Rosiers, C. (1995) J. Biol. Chem. 270, 10037-10042). The following conclusions can be drawn from the calculated reaction rates: (i) about half of the lactate conversion to glucose occurs via the citrate cleavage pathway, (ii) the flux through the reversal of the isocitrate dehydrogenase reaction is almost as fast as that through the citrate synthase reaction, and (iii) the flux through citrate synthase and [IMG]-ketoglutarate dehydrogenase is 1.6- and 3.2-fold that through pyruvate carboxylase, respectively.