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The regulation of triglyceride synthesis and fatty acid synthesis in rat epididymal adipose tissue. Effects of altered dietary and hormonal conditions.

ABSTRACT: 1. Epididymal adipose tissues obtained from rats that had been previously starved, starved and refed a high fat diet for 72h, starved and refed bread for 144h or fed a normal diet were incubated in the presence of insulin+glucose or insulin+glucose+acetate. 2. Measurements were made of the whole-tissue concentrations of hexose phosphates, triose phosphates, glycerol 1-phosphate, 3-phosphoglycerate, 6-phosphogluconate, adenine nucleotides, acid-soluble CoA, long-chain fatty acyl-CoA, malate and citrate after 1h of incubation. The release of lactate, pyruvate and glycerol into the incubation medium during this period was also determined. 3. The rates of metabolism of glucose in the hexose monophosphate pathway, the glycolytic pathway, the citric acid cycle and into glyceride glycerol, fatty acids and lactate+pyruvate were also determined over a 2h period in similarly treated tissues. The metabolism of acetate to CO(2) and fatty acids in the presence of glucose was also measured. 4. The activities of acetyl-CoA carboxylase, fatty acid synthetase and isocitrate dehydrogenase were determined in adipose tissues from starved, starved and fat-refed, and alloxan-diabetic animals and also in tissues from animals that had been starved and refed bread for up to 96h. Changes in these activities were compared with the ability of similar tissues to incorporate [(14)C]glucose into fatty acids in vitro. 5. The activities of acetyl-CoA carboxylase and fatty acid synthetase roughly paralleled the ability of tissues to incorporate glucose into fatty acids. 6. Rates of triglyceride synthesis and fatty acid synthesis could not be correlated with tissue concentrations of long-chain fatty acyl-CoA, citrate or glycerol 1-phosphate. In some cases changes in phosphofructokinase flux rates could be correlated with changes in citrate concentration. 7. The main lesion in fatty acid synthesis in tissues from starved, starved and fat-refed, and alloxan-diabetic rats appeared to reside at the level of pyruvate utilization and to be related to the rate of endogenous lipolysis. 8. It is suggested that pyruvate utilization by the tissue may be regulated by the metabolism of fatty acids within the tissue. The significance of this in directing glucose utilization away from fatty acid synthesis and into glyceride-glycerol synthesis is discussed.

SUBMITTER: Saggerson ED

PROVIDER: S-EPMC1179344 | biostudies-other | 1970 Sep

REPOSITORIES: biostudies-other

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Project description:1. Adipose tissues from rats fed a balanced diet were incubated in the presence of glucose (20mm) with the following additions: insulin, anti-insulin serum, insulin+acetate, insulin+pyruvate, insulin+lactate, insulin+phenazine methosulphate, insulin+oleate+albumin, insulin+adrenaline+albumin, insulin+6-N-2'-O-dibutyryl 3':5'-cyclic AMP+albumin. 2. Measurements were made of the whole tissue concentrations of adenine nucleotides, hexose phosphates, triose phosphates, glycerol 1-phosphate, 3 phosphoglycerate, 6-phosphogluconate, long-chain fatty acyl-CoA, acid-soluble CoA, citrate, isocitrate, malate and 2-oxoglutarate, and of the release into the incubation medium of lactate, pyruvate and glycerol after 1h of incubation. 3. Fluxes of [(14)C]glucose carbon through the major pathways of glucose metabolism were calculated from the yields of (14)C in various products after 2h of incubation. Fluxes of [(14)C]acetate, [(14)C]pyruvate or [(14)C]lactate carbon in the presence of glucose were also determined. 4. Measurements were also made of the whole-tissue concentrations of metabolites in tissues taken directly from Nembutal-anaesthetized rats. 5. Whole tissue mass-action ratios for phosphofructokinase, phosphoglucose isomerase and the combined (aldolasextriose phosphate isomerase) reaction were similar in vivo and in vitro. The reactants of phosphofructokinase appeared to be far from mass-action equilibrium. In vitro, the reactants of hexokinase also appeared to be far from mass-action equilibrium. 6. Correlation of observed changes in glycolytic flux with changes in fructose 6-phosphate concentration suggested that phosphofructokinase may show regulatory behaviour. The enzyme appeared to be activated in the presence of oleate or adrenaline and to be inhibited in the presence of lactate or pyruvate. 7. Evidence is presented that the reactants of lactate dehydrogenase and glycerol 1-phosphate dehydrogenase may be near to mass-action equilibrium in the cytoplasm. 8. No satisfactory correlations could be drawn between the whole-tissue concentrations of long-chain fatty acyl-CoA, citrate and glycerol 1-phosphate and the observed rates of triglyceride and fatty acid synthesis. Under the conditions employed, the concentration of glycerol 1-phosphate appeared to depend mainly on the cytoplasmic [NAD(+)]/[NADH] ratios. 9. Calculated hexose monophosphate pathway flux rates roughly correlated with fatty acid synthesis rates and with whole tissue [6-phosphogluconate]/[glucose 6-phosphate] ratios. The relative rates of production of NADPH for fatty acid synthesis by the hexose monophosphate pathway and by the ;malic enzyme' are discussed. It is suggested that all NADH produced in the cytoplasm may be used in that compartment for reductive synthesis of fatty acids, lactate or glycerol 1-phosphate.

Project description:1. Methods are described for the extraction and assay of acetyl-CoA and of total acid-soluble and total acid-insoluble CoA derivatives in rat epididymal adipose tissue. 2. The concentration ranges of the CoA derivatives in fat pads incubated in vitro under various conditions were: total acid-soluble CoA, 0.20-0.59mm; total acid-insoluble CoA, 0.08-0.23mm; acetyl-CoA, 0.03-0.14mm. 3. An investigation was made of some postulated mechanisms of control of fatty acid and triglyceride synthesis in rat epididymal fat pads incubated in vitro. The concentrations of intermediates of possible regulatory significance were measured at various rates of fatty acid and triglyceride synthesis produced by the addition to the incubation medium (Krebs bicarbonate buffer containing glucose) of insulin, adrenaline, albumin, palmitate or acetate. 4. The whole-tissue concentrations of glucose 6-phosphate, l-glycerol 3-phosphate, citrate, acetyl-CoA, total acid-soluble CoA and total acid-insoluble CoA were assayed after 30 or 60min. incubation. The rates of fatty acid and triglyceride synthesis, calculated from the incorporation of [U-(14)C]glucose into fatty acids and glyceride glycerol respectively, and the rates of glucose uptake, lactate plus pyruvate output and glycerol output were measured over a 60min. incubation. 5. The rate of triglyceride synthesis could not be correlated with the concentrations of either l-glycerol 3-phosphate or long-chain fatty acyl-CoA (measured as total acid-insoluble CoA). Factor(s) other than the whole-tissue concentrations of these recognized precursors appear to be involved in the determination of the rate of triglyceride synthesis. 6. No relationship was found between the rate of fatty acid synthesis and the whole-tissue concentrations of the intermediates, citrate or acetyl-CoA, or with the two proposed effectors of acetyl-CoA carboxylase, citrate (as activator) or long-chain fatty acyl-CoA (as inhibitor). The control of fatty acid synthesis appears to reside in additional or alternative factors.

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The regulation of triglyceride synthesis and fatty acid synthesis in rat epididymal adipose tissue. Effects of altered dietary and hormonal conditions.

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