13 C NMR studies of gluconeogenesis in rat liver cells: Utilization of labeled glycerol by cells from euthyroid and hyperthyroid rats

The gluconeogenic pathway from 2-13Cglycerol and 1,3-13Cglycerol has been followed in suspensions of isolated rat hepatocytes at 25 degrees C by 13C NMR at 90.5 MHz. The flow of label through the major pathway from glycerol to L-glycerol 3-phosphate and into glucose was followed in cells from control and triiodothyronine-treated rats. Treatment increased the rates of glucose formation and glycerol consumption 2-fold and decreased the alphaGP level to 40%. We calculate that approximately 60% of the flux is through the mitochondrial glycerol phosphate dehydrogenase in cells from triiodothyronine-treated rats, compared with approximately 15% in cells from the controls. Equal distribution of label between the trioses of glucose was obtained and, because the C3-C4 spin-spin coupling gives the distribution of labeled carbons in the same molecule, it was possible to measure the amount of triose from unlabeled fructose incorporated into the glucose labeled at carbons 1, 3, 4, and 6. About 10% of the hexoses had flowed through the pentose cycle and back into the hexose pathway in cells from fasted rats. From the distribution of label at glucose carbons not labeled via the major pathway and from the carbon spin-spin splitting patterns observed, we conclude that transketolase is reversible whereas transaldolase is essentially irreversible in the nonoxidative pentose branch.