3-Aminopicolinate inhibits phosphoenolpyruvate carboxykinase in hepatocytes and increases release of gluconeogenic precursors from peripheral tissues

Pathway of gluconeogenesis from D- and L-glycerate in rat hepatocytes

Synthesis of glucose from 10 mM D-glycerate by hepatocytes isolated from rats fasted 24 h was inhibited by 3-aminopicolinate (3AP), 3-mercaptopicolinate (3MP), and aminoxyacetate (AOA). The inhibition by AOA can be exerted over that caused by either of the picolinates. The effects of 3AP and 3MP were not additive, and cannot be ascribed totally to inhibition of reducing equivalent transfer by their inhibitory effect on phosphoenolpyruvate carboxykinase. The time course and dependence on substrate concentration of gluconeogenesis from L-glycerate was similar to those from D-glycerate, even though the cellular content of D-glycerate was markedly less when L-glycerate was the substrate. Gluconeogenesis from L-glycerate was not significantly affected by 3AP or 3MP, but was inhibited by AOA. The degree of inhibition by AOA of glucose synthesis from L-glycerate was greater than that from D-glycerate. Studies employing [14C]paraformaldehyde revealed that hydroxypyruvate may be converted to dihydroxyacetone in the hepatocyte. A hitherto unknown inhibition site of 3AP and 3MP is inferred and a dual pathway of D- and L-glycerate gluconeogenesis consistent with these results is proposed.

Evidence for lactate utilization for fetal lung glycogen synthesis

Fetal rabbit lungs from 23 day gestation animals were used to investigate the potential role of lactate as a substrate for fetal lung glycogen synthesis. Fetal lactate dehydrogenase activity was approximately twice that found in the adult lung, while the activity of phosphoenolpyruvate carboxykinase was elevated fourfold over the adult value. Pyruvate carboxylase activities were similar in both fetal and adult lungs. Studies employing fetal lung explants in organ culture indicated that the presence of both glucose and lactate may be necessary for glycogen accumulation in the developing fetal lung. These data support the hypothesis that lactate is an important precursor for fetal lung glycogen.

Alanine and inter-organ relationships in branched-chain amino and 2-oxo acid metabolism. Review

Branched-chain amino acid metabolism in skeletal muscle promotes the production of alanine, an important precursor in hepatic gluconeogenesis. There is controversy concerning the origin of the carbon skeleton of alanine produced in muscle, specifically whether it is derived from carbohydrate via glycolysis (the glucose-alanine cycle) or from amino acid precursors (viz. glutamate, valine, isoleucine, methionine, aspartate, asparagine) via a pathway involving phosphoenolpyruvate (PEP) carboxykinase and pyruvate kinase, or NADP-malate dehydrogenase (malic enzyme). The relevant literature is reviewed and it is concluded that neogenic flux from amino acids is unlikely to be of major quantitative importance for provision of the carbon skeleton of alanine either in vitro or in vivo. Evidence is presented that branched-chain amino acid oxidation in muscle is incomplete and that the branched-chain 2-oxo acids and the products of their partial oxidation (including glutamine) are released. The role of these metabolites is discussed in the context of fuel homeostasis in starvation.