PYRUVATE CYCLES IN THE METABOLISM OF GLUCOGENIC CELLS

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Cyclic reactions involving pyruvate have been established in the important biosynthetic pathways of gluconeogenesis and lipogenesis. Pyruvate cycles may also contribute to thermogenesis in the hyperthyroid state, and could play a role in the regulation of amino acid catabolism. We will continue to map the fluxes in the various pyruvate cycles in hepatocytes from rats under different dietary regimes, including fasted, ad libitum fed, and fasted-refed high carbohydrate. We will also use hepatocytes from streptozotocin diabetic rats, and from hyperthyroid rats. The sites of hormonal control of the steps in the cycles will be explored using inhibitor titration techniques, flux analysis using radiosotopes, intermediate analysis, and with the use of some newly developed substrates, monoesters of impermeable intermediates. Effects of glucagon, epinephrine and other adrenergic agonists, insulin, and vasopressin will be studied. We have found that glucose control of gluconeogenesis from lactate occurs in vitro under physiological conditions. The factors which reverse this inhibitory effect of glucose will be more fully characterized; these factors include fatty acid and lactate levels, and glucagon, but apparently not epinephrine. In adiabetes, these factors may override normal glucose control of gluconeogenesis. The rate-limiting steps in pyruvate cycling and in gluconeogenesis include pyruvate transport, pyruvate carboxylase, phosphoenolpyruvate carboxykinase and pyruvate kinase. In metabolic acidosis, provision of bicarbonate may be rate-limiting for both pyruvate carboxylase and carbamyl phosphate synthetase, restricting both gluconeogenesis and ureogenesis. Hypoglycemia from such effects in the very young could contribute to the causes of sudden infant death.