13C NMR study of gluconeogenesis from labeled alanine in hepatocytes from euthyroid and hyperthyroid rats.
Metabolism of [3-13C]alanine in the presence and absence of beta-hydroxybutyrate or ethanol has been followed at 25 degrees C by 13C NMR at 90.5 MHz in primary hepatocytes from untreated rats and rats treated with triiodothyronine and not allowed to eat for 24 hr. The phosphoenolpyruvate/pyruvate futile cycle was followed in situ by comparing the concentration of 13C at the scrambled alanine C2 position with that at glucose C5. In the absence of ethanol, the flux through pyruvate kinase was 60% of the gluconeogenic flux in hepatocytes from hyperthyroid rats, compared with 25% in the controls. Incubation with ethanol reduced the pyruvate kinase flux in the hyperthyroid state to that measured in the controls. Under all conditions, the relative concentration of label at the aspartate C2 and C3 sites was 1:2, whereas at the corresponding carbons in glutamate, randomization was almost complete. These observations, which require flux of unscrambled label into aspartate, are consistent with intramitochondrial synthesis of aspartate only if there is incomplete mixing of the intramitochondrial oxaloacetate pool. The 13C enrichment measured in the ketone bodies is increased by the presence of exogenous beta-hydroxybutyrate. The greater labeling that we observe at C2 of beta-hydroxybutyrate compared with C4 under this condition is explained by the flow through 3-hydroxy-3-methylglutaryl-coenzyme A synthase.