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Mechanism of Ethanol Suppression of Gluconeogenesis

INHIBITION OF PHOSPHOENOLPYRUVATE SYNTHESIS FROM GLUTAMATE AND α-KETOGLUTARATE
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      A series of experiments was performed to test the proposal of others that the oxidation of ethanol to acetate is NAD+-linked, and that, when transferred into the mitochondria, the cytoplasmic reducing equivalents produced during ethanol oxidation inhibit the oxidation of glucogenic precursors required for the formation of P-enolpyruvate.
      Particulate-free dialyzed extracts of rabbit livers oxidized various concentrations of ethanol to acetate with experimental ratios of the increase in NADH to that of acetate of 2.02 to 2.19. Although other hepatic enzymes known to oxidize aldehydes were active in these preparations, there was no measurable disappearance of ethanol not accounted for by net reduction of NAD+ to NADH and net production of acetate. These data are taken to indicate that the oxidation of ethanol to acetate by the liver is predominantly, if not exclusively, NAD+-linked.
      With β-hydroxybutyrate as source of competing reducing equivalents, simulating the transfer of reducing equivalents into the mitochondria during ethanol oxidation, P-enolpyruvate synthesis from α-ketoglutarate or glutamate by rabbit liver mitochondria was tested under conditions in which electron transfer was rate-limiting (with 0 to 20 µm dinitrophenol), or in which electron transfer was not limiting (with 50 µm dinitrophenol). In the presence of 0 to 20 µm dinitrophenol, β-hydroxybutyrate had little effect on respiratory rate, but decreased the synthesis of P-enolpyruvate from α-ketoglutarate or glutamate, and increased the mitochondrial NADH:NAD+ ratio. With 50 µm dinitrophenol, β-hydroxybutyrate oxidation increased respiration but had no effect on P-enolpyruvate synthesis or the NADH:NAD+ ratio.
      The data presented are taken to support our previously stated hypothesis that the rate of gluconeogenesis by the liver is regulated to a large degree by the rate of flow of carbon from glucogenic amino acids into and through the citric acid cycle.

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