Control Mechanisms of Gluconeogenesis and Ketogenesis

Abstract

The effects of oleate (1.0 to 1.5 mM) were studied in livers from fasted rats perfused with alanine (5 to 10 mu), L( +)lactate (8 to 12 mu), pyruvate (1.5 to 2.0 m&, or dihydroxyacetone (10 ~TLM) as gluconeogenic precursors.Rates of glucose production in control livers perfused with alanine, lactate, or pyruvate were 60, 120, and 95 pmoles per 100 g, body weight, per hour, respectively.Oleate increased these rates to 132, 250, and 194 moles 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 ,umoles 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, /3-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 phosphofructokinase (inhibition).These effects were interpreted as being caused by elevated levels of acetyl coenzyme A and citrate.The interaction between fructose-l ,6-di-P and fructose-6-P is interpreted as indicating recycling of these * This research was supported by Grant 12202-04 from the United States Public Health Service, and