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Control Mechanisms of Gluconeogenesis and Ketogenesis

I. effects of oleate on gluconeogenesis in perfused rat liver
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      The effects of oleate (1.0 to 1.5 mM) were studied in livers from fasted rats perfused with alanine (5 to 10 mM), L(+)-lactate (8 to 12 mM), pyruvate (1.5 to 2.0 mM), or dihydroxyacetone (10 mM) as gluconeogenic precursors. Rates of glucose production in control livers perfused with alanine, lactate, or pyruvate were 60, 120, and 95 µmoles per 100 g, body weight, per hour, respectively. Oleate increased these rates to 132, 250, and 194 µmoles per 100 g, body weight, per hour with the three substrates. With lactate as substrate, glucose formation accounted for 60% and 78% of the lactate uptake in the absence and presence of oleate. With pyruvate as substrate, about half of the pyruvate taken up by the liver was converted to glucose, both in the presence and absence of oleate. With dihydroxyacetone as substrate, the rate of glucose production was 250 µmioles per 100 g, body weight, per hour, and was not increased further by oleate. This indicates that the rate-controlling step of gluconeogenesis is prior to the formation of triose phosphates.
      Oleate increased the state of reduction of the pyridine nucleotide systems in both mitochondrial and cytosolic spaces as shown by increases in the ratios of lactate to pyruvate, β-hydroxybutyrate to acetoacetate, and malate to oxalacetate. Total tissue levels of the reduced forms of the pyridine nucleotides also increased after oleate administration.
      Measurements of metabolic intermediates of the gluconeogenic pathway were made in order to identify sites of interaction in the enzyme sequence. The significance of these interactions was analyzed by the crossover theorem. Glyceraldehyde-3-P dehydrogenase appeared as a control site only with alanine as substrate. It is proposed that this reaction was stimulated by the elevated NADH:NAD+ ratio maintained in the cytosol during enhanced fatty acid oxidation. With lactate and pyruvate as substrates, control sites were observed at pyruvate carboxylase (activation) and phos-phofructokinase (inhibition). These effects were interpreted as being caused by elevated levels of acetyl coenzyme A and citrate. The interaction between fructose-1,6-di-P and fructose-6-P is interpreted as indicating recycling of these intermediates, which is at least as rapid as the increased glucose flux observed after oleate addition. This results in a wasteful use of ATP, but probably represents no more than 10 to 20% of the oxygen consumption. Control of pyruvate carboxylation is considered to represent the most physiologically meaningful interaction between fatty acid oxidation and gluconeogenesis.

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