Hormonal regulation of fatty acid synthetase, acetyl-CoA carboxylase and fatty acid synthesis in mammalian adipose tissue and liver.

The major objectives of this study were to define the roles of adrenal glucocorticoids and glucagon in the long-term regulation of fatty acid synthetase and acetyl-CoA carboxylase of mammalian adipose tissue and liver. Particular emphasis was given to elucidation of the mechanisms whereby these hormones produce their regulatory effects on enzymatic activity. To dissociate mental manipulation, nutritional conditions were ridgidly controlled in the experiments described. Administration of glucocorticoids to adult rats led to a marked reductionin activities of fatty acid synthetase and carboxylase in adipose in adipose tissue but no change occurred in liver. Adrenalectomy produced an increase in activities of these lipogenic enzymes in adipose tissure, but, again, no change was noted in liver. The decrease in enzymatic activities in adipose tissue with glucocorticoid administration correlated well with a decrease in fatty acid synthesis, determined in vivo by the 3-H2O method. The mechanisms whereby glucocorticoids led to a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the decrease in fatty acid synthetase activity observed in adipose tissue was shown to reflect a decrease in content of enzyme, and not a change in catalytic efficiency. The mechanism underlying the decrease in enzyme content is a decrease in synthesis of the enzyme. The relation of the effects of glucocorticoids to the effects of certain other hormones involved in regulation of lipogenesis was investigated in hypophysectomized and in diabetic animals. Thus, the observation that the glucocorticoid effect on synthetase and carboxylase occurred in adipose tissue of hypophysectomized rats indicated that alterations in levels of other pituitary-regulated hormones were not necessary for the effect. That glucocorticoids play some role in regulation of synthetase and carboxylase in liver, at lease in the diabetic state, was shown by the observation that the low activities of these enzymes in diabetic animals could be restored to normal by adrenalectomy. An even more pronounced restorative effect was apparent in adipose tissue of adrenalectomized, diabetic animals. Administration of glucagon during the refeeding of starved rats resulted in a marked reduction in the induction of fatty acid synthetase, acetyl-CoA carboxylase and in the rate of incorporation of 3-H from 3-H2O into fatty acids in liver, but no change in these parameters occurred in adipose tissue. Administration of theophylline resulted in intermediate reduction in liver. The mechanisms whereby glucagon led tto a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the changes in fatty acid synthetase activity were shown to reflect reductions in content of enzyme. The mechanism underlying these reductions in content is reduced synthesis of enzyme.     

Comparative studies on lipogenesis and cholesterogenesis in lipemic BHE rats and normal Wistar rats.

The BHE strain of rat is characterized by early hyperinsulinemia and maturity onset hyperlipemia and hyperglycemia. Since we have previously shown that insulin is required for the coordinate regulation of a number of lipogenic enzymes in rat liver, a comparative study of the hepatic activities of the rate-limiting enzymes of lipid synthesis and the in vivo rates of fatty acid and cholesterol synthesis in the liver and the adipose tissue has been conducted in BHE and Wistar rats. In the liver, BHE rats had 25–28% higher acetyl-CoA carboxylase and fatty acid synthetase activities as measured in vitro but a 100% greater rate of fatty acid synthesis in vivo as compared to Wistar animals. These results strongly suggest that factors other than the amount of acetyl-CoA carboxylase, such as allosteric effectors, must be operating in vivo, thereby facilitating the carboxylase to function at its maximal capacity in BHE rats. Such a regulation of fatty acid biosynthesis by allosteric modifiers of acetyl-CoA carboxylase is already known, although the mechanism of this regulation is not fully understood. BHE rats also exhibited a twofold greater rate of fatty acid synthesis in the adipose tissue compared to the Wistar rats. Thus, increased lipogenic capacity and increased lipogenesis in BHE rats are consistent with early hyperinsulinemia in this strain. Furthermore, BHE rats had 71% more 3-hydroxy-3-methylglutaryl CoA reductase activity with a 97% greater rate of cholesterol synthesis as compared to Wistar rats. In contrast, cholesterol 7α-hydroxylase activity was only 20% greater in BHE rats compared to Wistar rats, suggesting that the BHE rat does not have the capacity to degrade cholesterol to bile acids at a rate commensurate with the increased rate of cholesterol synthesis. This difference in synthesis versus degradation might account for the hypercholesterolemia which occurs in BHE rats, but not in Wistar rats.     

Fatty acid inhibition of hormonal induction of acetyl-coenzyme A carboxylase in hepatocyte monolayers

Primary cultures of adult rat hepatocytes were utilized to ascertain the impact of free fatty acids on the insulin plus dexamethasone induction of acetyl-CoA carboxylase. Lipogenesis was induced threefold by the combination of insulin and dexamethasone. The rise in fatty acid synthesis was accompanied by a comparable increase in the rate-determining enzyme acetyl-CoA carboxylase. Dexamethasone was required for the insulin induction of acetyl-CoA carboxylase. Under the permissive action of glucocorticoid, 10(-7) M insulin maximally increased enzyme activity. Half-maximum stimulation occurred with 5 X 10(-9) M insulin. Media containing 0.2 mM palmitate, oleate, linoleate, arachidonate, or docosahexaenoate significantly suppressed the hormonal induction of acetyl-CoA carboxylase. The extent of suppression was only 30-35% and did not vary with chain length or degree of unsaturation. Carboxylase activity was not suppressed further by raising the concentration of linoleate to 0.5 mM; however, 0.5 mM palmitate depleted the cells of ATP and abolished acetyl-CoA carboxylase activity. Therefore, based upon the inhibitory characteristics of the various fatty acids and the lack of a concentration dependency of the fatty acid inhibition, it would appear that fatty acid inhibition of the induction of acetyl-CoA carboxylase activity may not be a direct, physiological regulatory mechanism.     

Acute effects in vivo of anti-insulin serum on rates of fatty acid synthesis and activities of acetyl-coenzyme A carboxylase and pyruvate dehydrogenase in liver and epididymal adipose tissue of fed rats.

Plasma insulin concentrations in fed rats were altered acutely by administration of glucose or anti-insulin serum. Rates of fatty acid synthesis in adipose tissue and liver were estimated from the incorporation of 3H from 3H2O. In the adipose tissue dehydrogenase and acetyl-CoA carboxylase were evident. In liver, although changes in rates of fatty acid synthesis were found, the initial activity of pyruvate dehydrogenase did not alter, but small parallel changes in acetyl-CoA carboxylase activity were observed.     

The interaction between heme and protein in cytochrome c1

The hormonal regulation of two regulatory enzymes of fatty acid synthesis acetyl-CoA carboxylase (EC 6.4.1.2) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49), has been investigated in human diploid fibroblasts. There was a 35% increase in acetyl-CoA carboxylase activity, 72 h following addition of 10 microU/ml insulin to the culture medium. Addition of 1 microgram/ml of 3,3'5-triiodothyronine for 72 h resulted in an increase in acetyl-CoA carboxylase activity to 166% of the controls. The simultaneous addition of 1 microgram/ml triiodothyronine and 10 mU/ml insulin caused the enzyme activity to rise to 240% of the controls. A dose-dependent reduction in acetyl-CoA carboxylase activity was brought about by 1 X 10(-4) to 1 X 10(-3) M dibutyryl cyclic AMP. The earliest effect of dibutyryl cyclic AMP was observed within 24 h. Glucose-6-phosphate dehydrogenase followed qualitatively the same pattern of response, whereas the constitutive enzyme, lactate dehydrogenase (EC 1.1.1.27), did not show significant changes in these experiments. The data demonstrate common features of hormonal regulation of lipogenesis in human fibroblasts with liver and adipose tissue and substantiate the growing evidence that thyroid hormones are of major importance for the regulation of this process.     

Effect of chorionic gonadotropin, triamcinolone, progesterone and estrogen on enzymes of placenta and liver in rats

The activity of several enzymes of regulatory importance for the pathways of glycolysis, gluconeogenesis and lipogenesis was investigated in the placenta and liver of pregnant rats and in the liver of non-pregnant female rats. The rats received daily hormonal treatments on Days 15 to 17 of pregnancy and enzyme activities were measured on Day 18. Chorionic gonadotropin induced minor changes in enzyme activity, apart from a decrease in the activity of hepatic enzymes of lipogenesis in non-pregnant rats. Triamcinolone induced a marked increase in enzymes of gluconeogenesis and a decrease in the activity of pyruvate k kinase in the liver of pregnant and non-pregnant rats; in contrast, inverse changes in activity, these enzymes were observed in the placenta. This response in the placenta was considered to arise not from direct hormone effect, but from the accompanying hyperglycemia and hyperinsulinemia. Triamcinolone also increased the activity of hepatic acetyl-CoA carboxylase in pregnant and non-pregnant rats, whereas it reduced the activity of this enzyme in the placent. Estrogen produced changes similar to those of triamcinolone in the liver and placenta, except that it depressed the activity of acetyl-CoA carboxylase in both tissues. Progesterone had little effect on placental and hepatic enzymes. In general, the changes induced by these hormones in the placenta affected fewer enzymes than in the liver, were less extensive in magnitude and not necessarily in the same direction as in the liver. This indicates that the regulatory placental enzymes are subject to specific control mechanisms not necessarily influenced by direct hormone action.     

Development of gluconeogenesis in neonatal rat liver. Effect of triamcinolone

1. The normal development of the key enzymes of gluconeogenesis in rat liver, glucose 6-phosphatase, hexose diphosphatase, phosphopyruvate carboxylase and pyruvate carboxylase, was measured during the neonatal period. 2. Glucose 6-phosphatase, hexose diphosphatase and pyruvate carboxylase are all present in the late foetal liver, but all the enzymes show an increase in activity after birth. 3. Phosphopyruvate carboxylase is not present in liver extracts from foetal rats, but activity appears immediately after birth and increases rapidly over the first day and then more slowly to reach its maximum at the fourth postnatal day. 4. The fluorinated synthetic glucocorticoid, triamcinolone, was administered to foetal rats at various gestation times by intraperitoneal injection in utero and the animals were killed at intervals between 4 and 48hr. later. 5. The administration of triamcinolone results in slight depression of glucose 6-phosphatase, and a more significant depression of hexose diphosphatase to about one-half its normal activity in foetal rat liver. 6. Triamcinolone injection is without effect on pyruvate carboxylase activity and does not result in premature appearance of phosphopyruvate carboxylase in foetal rat liver. 7. Pyruvate kinase and aspartate amino-transferase activities in foetal rat liver are both depressed by triamcinolone treatment, whereas phosphofructokinase activity is elevated. 8. Tyrosine amino-transferase activity in foetal rat liver is markedly elevated in animals exposed to triamcinolone for 10hr. or more, but the effect is only observed in animals close to term. 9. The results are discussed in relation to mechanisms involved in the initial synthesis of tissue-specific enzymes in developing tissues, and it is concluded that glucocorticoids do not initiate the synthesis of the gluconeogenic enzymes.     

Coordinate control of rat liver lipogenic enzymes by insulin

Recent evidence has established that insulin is required for the dietary induction of rat liver fatty acid synthetase [Proc. Nat. Acad. Sci. USA69, 3516 (1972)]. Since other hepatic lipogenic enzymes as well as fatty acid synthetase exhibit coordinate adaptation to nutritional changes [Advan. Enzyme Regul.10, 187(1972)], the role of insulin in the dietary induction of these enzymes has been investigated. When a high-carbohydrate, fat-free diet was fed to diabetic rats previously fasted for 48 hr, insulin was shown to be required for the dietary induction of acetyl-CoA carboxylase, citrate cleavage enzyme, malic enzyme, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, fatty acid synthetase, and glucokinase. Activity of serine dehydrase, selected as a model gluconeogenic enzyme, was increased in diabetic rats, whereas insulin treatment reduced the activity of this enzyme during the course of refeeding. The behavior of serine dehydrase was consistent with its gluconeogenic role. The activity of the cytosol isocitrate dehydrogenase did not change during refeeding in the diabetic or insulin-treated diabetic rat. Glucagon, the physiological antagonist of insulin, inhibited the increase in activity of each of the lipogenic enzymes requiring insulin for induction. Our results indicate that insulin is required for the coordinate regulation of the lipogenic enzymes of mammalian liver.     

Initiation of lipogenic enzyme activities in rat mammary glands.

The activities of acetyl-CoA carboxylase, ATP citrate-lyase and fatty acid synthetase remained low until parturition at 22 days of gestation and increased significantly within 1 day post partum. Administration of progesterone on days 20 and 21 and at parturition abolished the increases for at least 48 h after parturition. Removal of the pups of normal rats prevented the increases in activities of acetyl-CoA carboxylase and ATP citrate-lyase, but not of fatty acid synthetase, and administration of prolactin corticosterone or insulin did not stimulate activity. Tissue from suckled glands in which the ducts had been ligated at parturition showed no increase in the activities of acetyl-CoA carboxylase and ATP citrate-lyase within 24 h, whereas fatty acid synthetase activity was similar to that in the sham-operated contralateral glands. Foetoplacentectomy on day 18 increased the activity of fatty acid synthetase but not of acetyl-CoA carboxylase and ATP citrate-lyase; suckling of these dams by foster pups increased both acetyl-CoA carboxylase and ATP citrate-lyase.     

Effect of diets rich in medium-chain and long-chain triglycerides on lipogenic-enzyme gene expression in liver and adipose tissue of the weaned rat.

The activity and mRNA concentrations of two lipogenic enzymes, fatty-acid synthase and acetyl-CoA carboxylase were measured in the liver and white adipose tissue of rats weaned to a carbohydrate-rich diet containing either long-chain or medium-chain fatty acids, and compared to those of rats weaned on a diet containing less than 1% (total energy) fat (high-carbohydrate diet). In the liver, the diet containing long-chain fatty acids inhibited the increase of both lipogenic-enzyme mRNA concentrations and activities seen at weaning on the high-carbohydrate diet but did not prevent the decrease in phosphoenolpyruvate carboxykinase mRNA and activity. In contrast, the diet containing medium-chain fatty acids induced a slower but finally similar increase in lipogenic-enzyme mRNA concentrations and activities. In adipose tissue, a similar trend was observed, although the inhibitory effect of the diet containing long-chain fatty acids was considerably less marked than in liver. It is concluded that medium-chain and long-chain fatty acids have not the same inhibitory potency of the gene expression of lipogenic enzymes, and that long-chain fatty acids have a more marked effect in the liver.     

Physiological regulation of acetyl-CoA carboxylase gene expression: effects of diet, diabetes, and lactation on acetyl-CoA carboxylase mRNA

We measured acetyl-CoA carboxylase mRNA levels in various tissues of the rat under different nutritional and hormonal states using a cDNA probe. We surveyed physiological conditions which are known to alter carboxylase activity, and thus fatty acid synthesis, to determine whether changes in the levels of carboxylase mRNA are involved. The present studies include the effects of fasting and refeeding, diabetes and insulin, and lactation on carboxylase mRNA levels. Northern blot analysis of liver RNA revealed that fasting followed by refeeding animals a fat-free (high carbohydrate) diet dramatically increased the amount of carboxylase mRNA compared to the fasted condition. These changes in the level of mRNA correspond to changes in the activity and amount of acetyl-CoA carboxylase. Acetyl-CoA carboxylase mRNA levels in epididymal fat tissue decreased upon fasting and increased to virtually normal levels after 72 h of refeeding, closely resembling the liver response. The amount of acetyl-CoA carboxylase mRNA decreased markedly in epididymal fat tissue of diabetic rats as compared to nondiabetic animals. However, 6 h after injection of insulin the mRNA level returned to that of the nondiabetic animals. Gestation and lactation also affected the levels of carboxylase mRNA in both liver and mammary gland. Maximum induction in both tissues occurred 5 days postpartum. These studies suggest that these diverse physiological conditions affect fatty acid synthesis in part by altering acetyl-CoA carboxylase gene expression.     

Effects of ethanol feeding on hepatic lipid synthesis

Rats were fed a high-fat, liquid diet containing either 36% of total calories as ethanol or an isocaloric amount of sucrose, for a period up to 35 days. At different time intervals we measured the effects of ethanol administration on the activities of a number of key enzymes involved in hepatic lipid synthesis. At the start of the experimental period the activities of acetyl-CoA carboxylase and fatty acid synthase, measured in liver homogenates, increased in the control as well as in the ethanol-fed group. After 35 days these enzyme activities were still elevated but there were no significant differences between the two groups. In hepatocytes isolated from controls as well as from ethanol-fed rats, short-term incubations with ethanol induced an increase in the rate of fatty acid synthesis and in the activities of acetyl-CoA carboxylase and fatty acid synthase. However, no alterations in the regulation of these enzymes by short-term modulators of lipogenesis were apparent in hepatocytes isolated from alcohol-treated animals. The results do not indicate a major role for the enzymes of de novo fatty acid synthesis in the development of the alcoholic fatty liver. The amount of liver triacylglycerols increased in ethanol-fed rats during the entire treatment period, whereas the hepatic levels of phosphatidylcholine and phosphatidylethanolamine were not affected by ethanol ingestion. Ethanol administration for less than 2 weeks increased the activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and microsomal phosphocholine cytidylyltransferase, whereas the cytosolic activity of phosphocholine cytidylyltransferase was slightly decreased. Upon prolonged ethanol administration the activities of these enzymes were slowly restored to control values after 35 days, suggesting development of some kind of adaptation. It is interesting that, although the activities of phosphatidate phosphohydrolase and diacylglycerol acyltransferase were restored to the levels found in the control rats, this effect was not accompanied by a stabilization or decrease of the concentration of hepatic triacylglycerols.     

Hormonal regulation of adipose-tissue acetyl-coenzyme A carboxylase by changes in the polymeric state of the enzyme. The role of long-chain fatty acyl-coenzyme A thioesters and citrate

1. Acetyl-CoA carboxylase activity was measured in extracts of rat epididymal fat-pads either on preparation of the extracts (initial activity) or after incubation of the extracts with citrate (total activity). In the presence of glucose or fructose, brief exposure of pads to insulin increased the initial activity of acetyl-CoA carboxylase; no increase occurred in the absence of substrate. Adrenaline in the presence of glucose and insulin decreased the initial activity. None of these treatments led to a substantial change in the total activity of acetyl-CoA carboxylase. A large decrease in the initial activity of acetyl-CoA carboxylase also occurred with fat-pads obtained from rats that had been starved for 36h although the total activity was little changed by this treatment. 2. Conditions of high-speed centrifugation were found which appear to permit the separation of the polymeric and protomeric forms of the enzyme in fat-pad extracts. After the exposure of the fat-pads to insulin (in the presence of glucose), the proportion of the enzyme in the polymeric form was increased, whereas exposure to adrenaline (in the presence of glucose and insulin) led to a decrease in enzyme activity. 3. These changes are consistent with a role of citrate (as activator) or fatty acyl-CoA thioesters (as inhibitors) in the regulation of the enzyme by insulin and adrenaline; no evidence that the effects of these hormones involve phosphorylation or dephosphorylation of the enzyme could be found. 4. Changes in the whole tissue concentration of citrate and fatty acyl-CoA thioesters were compared with changes in the initial activity of acetyl-CoA carboxylase under a variety of conditions of incubation. No correlation between the citrate concentration and the initial enzyme activity was evident under any condition studied. Except in fat-pads which were exposed to insulin there was little inverse correlation between the concentration in the tissue of fatty acyl-CoA thioesters and the initial activity of acetyl-CoA carboxylase. 5. It is suggested that changes in the concentration of free fatty acyl-CoA thioesters (which may not be reflected in whole tissue concentrations of these metabolites) may be important in the regulation of the activity of acetyl-CoA carboxylase. The possibility is discussed that the concentration of free fatty acyl-CoA thioesters may be controlled by binding to a specific protein with properties similar to albumin.     

Zonation of hepatic lipogenic enzymes identified by dual-digitonin-pulse perfusion.

The zonal distribution within rat liver of acetyl-CoA carboxylase, ATP citrate-lyase and fatty acid synthase, the principal enzymes of fatty acid synthesis, was investigated by using dual-digitonin-pulse perfusion. Analysis of enzyme mass by immunoblotting revealed that, in normally feeding male rats, the periportal/perivenous ratio of acetyl-CoA carboxylase mass was 1.9. The periportal/perivenous ratio of ATP citrate-lyase mass was 1.4, and fatty acid synthase exhibited the largest periportal/perivenous mass gradient, having a ratio of 3.1. This pattern of enzyme distribution was observed in male rats only; in females, the periportal/perivenous ratio of enzyme mass was nearly equal. The periportal/perivenous gradients for acetyl-CoA carboxylase, ATP citrate-lyase and fatty acid synthase observed in fed (and fasted) males were abolished when animals were fasted (48 h) and refed (30 h) with a high-carbohydrate/low-fat diet. As determined by enzyme assay of eluates obtained from the livers of normally feeding male rats, there is also periportal zonation of acetyl-CoA carboxylase activity, expressed either as units per mg of eluted protein or units per mg of acetyl-CoA carboxylase protein, suggesting the existence of gradients in both enzyme mass and specific activity. From these results, we conclude that the enzymes of fatty acid synthesis are zonated periportally in the liver of the normally feeding male rat.     

Adaptation to a high protein, carbohydrate-free diet induces a marked reduction of fatty acid synthesis and lipogenic enzymes in rat adipose tissue that is rapidly reverted by a balanced diet

We have previously shown that in vivo lipogenesis is markedly reduced in liver, carcass, and in 4 different depots of adipose tissue of rats adapted to a high protein, carbohydrate-free (HP) diet. In the present work, we investigate the activity of enzymes involved in lipogenesis in the epididymal adipose tissue (EPI) of rats adapted to an HP diet before and 12 h after a balanced diet was introduced. Rats fed an HP diet for 15 days showed a 60% reduction of EPI fatty acid synthesis in vivo that was accompanied by 45%-55% decreases in the activities of pyruvate dehydrogenase complex, ATP-citrate lyase, acetyl-CoA carboxylase, glucose-6-phosphate dehydrogenase, and malic enzyme. Reversion to a balanced diet for 12 h resulted in a normalization of in vivo EPI lipogenesis, and in a restoration of acetyl-CoA carboxylase activity to levels that did not differ significantly from control values. The activities of ATP-citrate lyase and pyruvate dehydrogenase complex increased to about 75%-86% of control values, but the activities of glucose-6-phosphate dehydrogenase and malic enzyme remained unchanged 12 h after diet reversion. The data indicate that in rats, the adjustment of adipose tissue lipogenic activity is an important component of the metabolic adaptation to different nutritional conditions.     

Regulation of palmitic acid synthesis in cultured glial cells: effects of glucocorticoid on fatty acid synthetase, acetyl-CoA carboxylase, fatty acid and sterol synthesis.

Abstract— C-6 glial cells in culture were utilized to define the role of glucocorticoid in the regulation of palmitic acid synthesis and the important lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase. Particular emphasis was given to fatty acid synthetase which exhibited more than a 50% reduction in specific activity when cells were exposed to hydrocortisone (10 μg/ml) for 1 week. Coordinate changes in acetyl-CoA carboxylase activity and in palmitic acid (and sterol) synthesis from acetate accompanied the alterations in fatty acid synthetase. Immunochemical techniques were utilized to show that the decrease in synthetase activity involved an alteration in enzyme content, not in catalytic efficiency. The changes in content of fatty acid synthetase were caused by alterations in enzyme synthesis. Glucocorticoids may regulate fatty acid synthetase in C-6 glial cells by a mechanism similar to that suggested for adipose tissue. The inhibition of palmitic acid synthesis may be relevant to other effects of glucocorticoids on developing brain.     

Integrative metabolic regulation of peripheral tissue fatty acid oxidation by the SRC kinase family member Fyn

Mice null for Fyn (a member of the Src family of nonreceptor tyrosine kinases) display a reduced percentage of adipose mass associated with decreased adipocyte cell size. In parallel, there is a substantial reduction in fasting plasma glucose, insulin, triglycerides, and free fatty acids concomitant with decreased intrahepatocellular and intramyocellular lipid accumulation. Importantly, the Fyn null mice exhibit improved glucose tolerance resulting from increased peripheral tissue (adipose and skeletal muscle) insulin sensitivity with a very small effect in the liver. Moreover, whole-body, adipose, and skeletal muscle fatty acid uptake and oxidation are increased along with AMP kinase activation and acetyl-CoA carboxylase inhibition. Together, these data demonstrate crosstalk between Src-family kinase activity and fatty acid oxidation and show that the loss of Fyn markedly improves peripheral tissue insulin sensitivity by relieving a selective negative modulation of AMP kinase activity in adipose tissue and skeletal muscle.     

Polymer-protomer transition of acetyl-CoA carboxylase as a regulator of lipogenesis in rat liver

Data are presented which indicate that the transition of acetyl-CoA carboxylase between the active polymeric and inactive protomeric conformations defined for the purified enzyme also occurs with the enzyme in vivo, depends upon the nutritional state of the animal, and is an important physiological phenomenon in the acute regulation of liver fatty acid synthesis. This conclusion utilized the observation that the protomeric form of purified acetyl-CoA carboxylase is inactivated by the binding of avidin to the biotinyl prosthetic group; the catalytically active filamentous form of the enzyme is resistant to avidin. Acetyl-CoA carboxylase activity was 75% avidin-resistant (polymeric) in the liver of meal-fed rats that had completed the consumption of a high glucose meal. This avidin resistance gradually decreased to 20% during the 21-h interval between meals. Peak resistance to avidin of liver carboxylase was attained within 30 min of initiating meal ingestion. The rise in carboxylase resistance to avidin could not be mimicked by insulin injection alone, but could be greatly attenuated by the addition of fat to the glucose meal. The amount of avidin-resistant acetyl-CoA carboxylase was closely associated with the concentration of hepatic malonyl-CoA and the subsequent rate of fatty acid synthesis.     

Diurnal variations of lipogenic enzymes, their substrate and effector levels, and lipogenesis from tritiated water in rat liver

The activities of glucose-6-phosphate dehydrogenase, malic enzyme, fatty acid synthetase and acetyl-CoA carboxylase (extracted with or without phosphatase inhibitor) in rat liver did not vary significantly during 24 h. The hepatic levels of glucose 6-phosphate and malate increased coordinately 3-6 h after the beginning (1900 h) of food intake and were high until morning, whereas the levels of acetyl-CoA and citrate peaked at 1900 h and then decreased. However, it is remarkable that the in vivo incorporation of 3H from tritiated water into fatty acids in liver increased with the level of malonyl-CoA after food intake. Comparing the substrate and effector levels with the Km and Ka values for the enzymes, the levels of acetyl-CoA, malonyl-CoA and citrate appear to limit the enzyme activities. It is suggested that, after food intake, the physiological activity of acetyl-CoA carboxylase was increased with the substrate increase and/or with the catalytic activation with citrate, and consequently, the fatty acid synthetase activity was also increased, whereas the enzyme activities measured under optimum conditions were not.     

Fatty liver and kidney syndrome in chicks. II. Biochemical role of biotin.

The role of biotin-dependent enzymes in the fatty liver and kidney syndrome of young chicks was studied. Under conditions of a marginal deficiency of dietary biotin, the level of biotin in the liver has differing effects on the activities of two biotin-dependent enzymes, pyruvate carboxylase and acetyl-CoA carboxylase. The activity of acetyl-CoA carboxylase is increased, but when the dietary deficiency of biotin produces biotin levels which are below 0-8 mug/g of liver, the activity of pyruvate carboxylase may be insufficient to completely metabolize pyruvate via gluconeogenesis. There is an increase in liver size and in the activities of enzymes involved in alternate pathways for the removal of pyruvate. Blood lactate accumulates and there is increased synthesis of fatty acids, and an accumulation of palmitoleic acid; these steps are accomplished by increased activities of at least the following enzymes: acetyl-CoA carboxylase, malate dehydrogenase (decarboxylating) (NADP+) and the desaturase enzyme. When the biotin level is below 0-35 mug/g of liver and the chick is subjected to a stress, physiological defence mechanisms of the chick may be inadequate to maintain homeostasis and they finally collapse, resulting in accumulation of triacylglycerol in the liver and blood; the chick is unable to maintain blood glucose levels and death occurs, often only a few hours after the imposition of the stress.