Studies of glycogen synthesis and the Krebs cycle by mass isotopomer analysis with [U-13C]glucose in rats

Starved rats were infused intragastrically via indwelling duodenal cannulae with glucose at a rate of 30 mg/min/kg. The infusate contained U-13C]glucose at an enrichment of 32 or 17%. At the end of the infusion, after 160 min, glucose and lactate were isolated from arterial and portal blood and from liver, and liver glycogen was isolated and hydrolyzed to glucose. The enrichment in glucose and lactate and the isotopomer distribution in glucose of masses from 180 to 186 were determined by gas chromatography-mass spectrometry (GC-MS). From analysis of these data we determined (a) gluconeogenesis proceeds at half the basal rate in the presence of a large infused glucose load, (b) one-quarter of the hepatic pyruvate pool is derived from nonglucose carbon, (c) half of the labeled molecules in liver glycogen are of mass 186 from the infused glucose and half are of masses 181-183, (d) the contribution of the indirect path from pyruvate when corrected for synthesis from unlabeled pyruvate ranges from 55 to 65%, (e) the rate of pyruvate carboxylase averages 90% that of citrate synthase, and (f) the rate of exchange of oxaloacetate with fumarate is about three times the rate of flux in the Krebs cycle (four times in the "forward" direction), and the enrichment in carbon 1 of oxaloacetate was 2.3 times that in carbon 4. In the Appendix a method to obtain the isotopomer distribution of newly formed glucose and glycogen glucose is described. An algorithm to correct for the contribution of natural abundance of 13C and the presence of 12C in commercial U-13C]glucose is presented. A novel mathematical analysis to obtain the parameters of the Krebs cycle from the isotopomer distribution is developed in the Appendix. Equations to calculate the relative rates of pyruvate carboxylase (y), and the equilibration of oxaloacetate with fumarate from the isotopomer distribution are derived. Mass isotopomer analysis provides a novel and powerful tool for the study of carbohydrate metabolism and the operation of the Krebs cycle.