Hormonal control of enzymes participating in gluconeogenesis and lipogenesis

Time course of enzyme changes after a switch from a high-fat to a low-fat diet

This study was conducted to determine the time course of metabolic changes associated with a switch from a high-fat to a low-fat diet in rats. Adult rats, maintained on a high-fat diet (42% of energy from fat) for 4-5 weeks were switched to a low-fat diet (11% of energy from fat), and the activities of several liver enzymes were followed. Three different phases could be distinguished. The early phase, complete by 2 days after the switch in diets, included an increase in the activity of glucose 6-phosphate dehydrogenase (pentose phosphate pathway), an increase in pyruvate kinase and pyruvate dehydrogenase activities (terminal end of the glycolytic pathway) and an increase in ATP-citrate lyase and fatty acid synthetase (fatty acid synthesis pathway). The early phase also included a decrease in the activity of phosphoenolpyruvate carboxykinase (PEPCK, gluconeogenesis) and a lower branched-chain amino acid dehydrogenase activity (BCAADH, branched-chain amino acid degradation). The concentration of the allosteric phosphofructokinase regulator, fructose 2,6-bisphosphate (Fru-2,6-P2, glycolysis), decreased during the early phase. An intermediate phase could also be discerned between 3 and 10 days after the switch in diets. In this phase, the decreased Fru-2,6-P2 concentration and the decreased PEPCK and BCAADH activities observed in the early phase were reversed. The late phase occurred 10 days after the dietary switch and was characterized by an increase in the activities of glucokinase (glycolytic pathway) and glycogen phosphorylase (associated with glycogenolysis) and by a decrease in glutamate dehydrogenase, PEPCK and BCAADH activities. These measurements indicate that at least 20 days are required before metabolic changes associated with a switch in diet are complete.

Dietary lipids influence insulin action

Insulin binding and insulin responsiveness are altered by dietary fat-induced changes in the fatty acid composition of the adipocyte plasma membrane. Feeding a high P/S diet increased polyunsaturated fatty acid content of major membrane phospholipids of adipocyte plasma membrane in normal and diabetic animals, increased membrane linoleic acid content, and prevented a decrease in arachidonic acid level in diabetic animals. The high P/S diet increased insulin binding in control animals. Animals fed the high P/S diet had significantly higher rates of insulin-stimulated glucose transport and lipogenesis than did animals fed the low P/S diet. Feeding a high P/S diet significantly increased the amount of glucose transported when expressed as a function of the specific amount of insulin bound. To determine if dietary fat-induced alterations in the fatty acid composition of skeletal muscle lipid alter insulin-dependent and basal muscle metabolism, contralateral epitrochlearis and extensor digitorum longus muscles were isolated and incubated in vitro. High levels of dietary omega-3 fatty acids reduced PGE2 and PGF2 alpha synthesis in extensor digitorum longus and epitrochlearis muscle. Insulin increased glucose and amino acid transport; the increase in glucose transport by insulin was significantly greater after consumption of the high omega-3 fatty acid diet. Rats fed high levels of omega-3 fatty acids showed reduced net protein degradation in the presence and absence of insulin due to decreased rates of protein degradation and synthesis. These experiments indicate that high levels of dietary omega-3 fatty acids alter muscle membrane composition, glucose transport, and metabolism of muscle protein. To determine if dietary fatty acids alter the onset of diabetes and insulin binding to liver nuclei in spontaneously diabetic rats, weanling rats were fed chow or semipurified diets containing 20% (w/w) fat of either high or low P/S ratio. Feeding a high P/S diet increased insulin binding to liver nuclei of control and diabetic animals. Although diet did not alter the onset of diabetes, insulin binding to liver nuclei is higher in animals at the onset of diabetes than in highly diabetic animals. Eight-week-old female C57 B 6J lean and ob/ob mice were fed semipurified diets containing 20% (w/w) fat of either high or low P/S ratio to investigate the effect of diet on specific binding of insulin to liver nuclei. Insulin binding was highest in nuclei from lean mice fed a high P/S diet. Specific binding of insulin to nuclei from obese mice was also increased by the high P/S diet, but to a lesser extent.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of chronic administration of T4, growth hormone and epidermal growth factor on hepatic lipogenic enzymes in hypophysectomised rats

The effect of daily injections of T4, growth hormone and epidermal growth factor on the activities of the 3 hepatic enzymes malic enzyme (E.C.1.1.1.40), 6 phosphogluconate dehydrogenase (E.C.1.1.1.44) and glucose 6-phosphate dehydrogenase (E.C.1.1.1.49) in hypophysectomised rats was examined. T4 was shown to increase the activity of malic enzyme in a dose dependent manner from a basal level of 3.55 mumol/min/liver to 48.97 mumol/min/liver at a dose of 75 micrograms/Kg/day. A smaller increase in the activity of 6 phosphogluconate dehydrogenase was also observed over the same range of T4 dosage. Growth hormone in the current study increased the total liver enzyme activity of glucose 6-phosphate dehydrogenase by 84% when given with a replacement dose of 35 micrograms T4/kg/day and 6 phosphogluconate dehydrogenase activity by 19%.

Phosphoenolpyruvate carboxykinase (guanosine 5'-triphosphate) from rat liver cytosol. Divalent cation involvement in the decarboxylation reactions

The presence of a divalent metal ion together with a catalytic amount of inosine 5'-diphosphate (IDP) is essential for the formation of pyruvate from oxalacetate catalyzed by purified rat liver cytosol phosphoenolpyruvate carboxykinase (PEPCK). With decreasing order of effectiveness, this pyruvate-forming activity was supported by micromolar levels of Cd2+, Zn2+, Mn2+, and Co2+. At the same concentrations, Mg2+ or Ca2+ was not effective. Combinations of Cd2+ with either Zn2+, Mn2+ or Co2+ were not additive with respect to the pyruvate-forming activity of PEPCK. Kinetic determination, with Cd2+ as the supporting cation, showed a 1:1 stoichiometry of interaction between each enzyme molecule and the nonconsumable substrate IDP. With 10 muM added Cd2+, the apparent Km for oxalacetate was 41 muM, and the apparent Ka for IDP was 0.25 muM. With Zn2+ or Mn2+, the apparent Ka for IDP was 0.2 or 0.13 muM, respectively. The effect of divalent transition-metal ions on PEPCK-catalyzed formation of phosphoenolpyruvate from oxalacetate was also investigated. Under steady-state conditions, the basal activity with MgITP was effectively enhanced with micromolar levels of Mn2+, Cd2+, or Co2+ included in the assay. The Vm increased 7- and 3.6-fold, and the apparent Km for MgITP changed by about a factor of 2 with the optimal concentrations of Mn2+ and Co2+, respectively. The most striking changes were in the apparent Km values for oxalacetate, which decreased to one-third and one-tenth when either Mn2+ or Co2+ was present in the assay together with Mg2+. The possible physiological importance of this kinetic effect is discussed.

Effect of experimental hyperinsulinaemia on intracellular glucose metabolism of isolated adipocytes

Glucose oxidation and lipogenesis were studied in isolated adipocytes from control and non-obese, experimentally hyperinsulinaemic rats. In cells from the hyperinsulinaemic animals oxidation of either [1(-14) C]- or [6(-14) C] glucose was increased in the presence or absence of insulin, at substrate concentrations from 0.1 to 20 mmol/l. Glucose incorporation into total triglycerides and fatty acids was also increased. These enhanced rates of glucose metabolism were due to increased activity of the glucose transport system in addition to increased activity of intracellular glucose metabolism. Therefore, these data indicate that insulin can influence long term glucose homoeostasis by augmenting the overall cellular capacity for glucose metabolism at several loci.

Adaptive character of liver glucokinase

1. Glucokinase is one of four glucose phosphorylating enzymes present in rat liver. Its distinctive features are a high K-m for glucose (high-K-m isozyme) and a rather narrow substrate specificity. In contrast, the other three enzymes, collectively called hexokinases or low-K-m isozymes, exhibit low K-m values for glucose and a wider substrate specificity. 2. Glucokinase is present in the liver os mammals (with some exceptions), amphibians and lower reptiles; It is absent from higher reptiles and birds. The presence or absence of glucokinase may represent an evolutionary adaptation to feeding habits and other physiological peculiarities. Differences in the immunological behavior and in the kinetic parameters of glucokinases from different taxa suggest the operation of divergent evolution. 3. The levels of glucokinase in rat liver depend strictly on the supply of carbohydrate in the diet. Glycogen phosphorylase and glycogen synthetase behave similarly, whereas other carbohydrate-metabolizing enzymes depend on the provision of either protein or protein plus carbohydrate. Glucokinase decays with a half-life of 33 hr when rats are starved or fed a carbohydrate-free diet, and is induced by the administration of glucose. The adaptive character is not exhibited by all mammals, indicating evolutionary discrimination within the same class and even within the same single order Rodentia. Enzyme adaptation in the liver may partially explain the condition known as 'hunger diabetes'. 4. The endocrine system plays a paramount role in glucokinase adaptation, since insulin is essential for glucose-dependent glucokinase induction and, on the other hand, glucagon, catecholamines and cyclic AMP prevent the induction. Glucocorticoids and some pituitary hormones modulate the rate of induction. The mechanisms underlying the hormonal regulation of glucokinase levels are not well known. 5. The variations in liver glucokinase correspond to changes in the amount of enzyme protein as assessed by immunochemical titration. This fact agrees with the effects of inhibitors of protein synthesis on glucokinase induction. 6. An antiserum against rat glucokinase reacts with the enzyme from mammals and turtles but not with the amphibian enzyme. It does not react with low-K-m hexokinases from different sources. 7. The saturation function for glucose is sigmoidal in mammalian and amphibian glucokinases but not in glucokinase from lower reptiles. The Hill's coefficient is very constant with values about 1.6. The K0.5 (concentration for half saturation) values in the different species studied vary between 1.5 and 8 mM. These kinetic parameters may be considered as another adaptive feature aimed to give maximal efficiency to the liver uptake of glucose at the changeable concentrations in the blood resulting from variations in the amount of dietary glucose.

Effects of palmitoyl CoA on citrate and malate transport by rat liver mitochondria

Palmitoyl CoA inhibited citrate transport from isolated rat liver mitochondria. Under conditions described, 50% inhibition was observed at about 6-8 nmol/mg of mitochondrial protein per ml. The percentage inhibition was inversely proportional to the concentration of the counter-transporting anion in the medium. Although comparable levels of palmitoyl CoA had little effect on malate exit on the dicarboxylate carrier, higher concentrations inhibited both citrate and malate transport. The specificity of the inhibition by palmitoyl CoA was investigated by examination of the effects on the tricarboxylate transport system of fatty acids, CoASH, acetyl CoA, palmitoylcarnitine, deoxycholate, and other molecules with surface active properties. None of the above compounds, at sublytic concentrations, inhibited citrate transport appreciably. The inhibition of citrate transport by low concentrations of palmitoyl CoA was rapid and could be prevented or partially reversed by addition of albumin.

Carbohydrate supply as a regulator of rat liver phosphoenolpyruvate carboxykinase activity

Administration of glucose, fructose, and glycerol to fasted rats produced a significant depression of liver phosphoenolpyruvate carboxykinase activity within 4 to 8 hours; galactose and ribose were much less effective. All the compounds yielded appreciable quantities of liver glycogen. The depression of phosphoenolpyruvate carboxykinase activity by glucose and glycerol was diminished by the concomitant administration of 2-deoxyglucose. The latter depressed glycogen formation from administered carbohydrate in muscle but not in liver. In rats made diabetic by alloxan, depression of elevated phosphoenolpyruvate carboxykinase activity by insulin was dependent upon a dietary source of carbohydrate. These results were interpreted to indicate that depression of certain gluconeogenic enzymes after carbohydrate ingestion is initiated by the metabolism of carbohydrate in some extrahepatic site.