The subcellular distribution of short-chain fatty acyl-CoA synthetase activity in rat tissues

Short- and medium-chain fatty acids in energy metabolism: the cellular perspective

Short- and medium-chain fatty acids (SCFAs and MCFAs), independently of their cellular signaling functions, are important substrates of the energy metabolism and anabolic processes in mammals. SCFAs are mostly generated by colonic bacteria and are predominantly metabolized by enterocytes and liver, whereas MCFAs arise mostly from dietary triglycerides, among them milk and dairy products. A common feature of SCFAs and MCFAs is their carnitine-independent uptake and intramitochondrial activation to acyl-CoA thioesters. Contrary to long-chain fatty acids, the cellular metabolism of SCFAs and MCFAs depends to a lesser extent on fatty acid-binding proteins. SCFAs and MCFAs modulate tissue metabolism of carbohydrates and lipids, as manifested by a mostly inhibitory effect on glycolysis and stimulation of lipogenesis or gluconeogenesis. SCFAs and MCFAs exert no or only weak protonophoric and lytic activities in mitochondria and do not significantly impair the electron transport in the respiratory chain. SCFAs and MCFAs modulate mitochondrial energy production by two mechanisms: they provide reducing equivalents to the respiratory chain and partly decrease efficacy of oxidative ATP synthesis.

An acetate switch regulates stress erythropoiesis

Endocrine erythropoietin (Epo), which is synthesized in the kidney or liver of adult mammals, controls erythrocyte production and is regulated by the stress-responsive transcription factor Hypoxia Inducible Factor 2 (HIF-2). We previously reported that the lysine acetyltransferase Cbp is required for HIF-2α acetylation and efficient HIF-2 dependent Epo induction during hypoxia. We now show these processes require acetate-dependent acetyl CoA synthetase 2 (Acss2). In Hep3B hepatoma cells and in Epo-generating organs of hypoxic or acutely anemic mice, acetate levels increase and Acss2 is required for HIF-2α acetylation, Cbp/HIF-2α complex formation and recruitment to the Epo enhancer, and efficient Epo induction. In acutely anemic mice, acetate supplementation augments stress erythropoiesis in an Acss2-dependent manner. In acquired and genetic chronic anemia mouse models, acetate supplementation also increases Epo expression and resting hematocrits. Thus, a mammalian stress-responsive acetate switch controls HIF-2 signaling and Epo induction during pathophysiological states marked by tissue hypoxia.

Hypothesis: butyrate is not an HDAC inhibitor, but a product inhibitor of deacetylation

The short-chain fatty acid butyrate is classically referred to as an inhibitor of histone deacetylases (HDACi), however evidence from direct assays is both sparse and contradictory. This paper assesses the strength of the historical evidence, potential gaps, inadequacies and simplifications in the butyrate-as-HDACi hypothesis. An alternate model to explain the action of butyrate is proposed wherein butyrate acts as a product inhibitor of deacetylation. The model makes testable predictions which may enable future determination of the mode of action of this and other SCFAs.

Comparison of the alkalizing effects of bicarbonate precursors in calves with experimentally induced metabolic acidosis

The aims of this study were to confirm whether commercial acetated Ringer's solution, which contains 28 mM of sodium acetate, is superior to commercial lactated Ringer's solution in alkalizing effects in calves with experimentally induced metabolic acidosis. Twenty calves with experimentally induced mild acidosis were intravenously administered isotonic saline, DL-lactated, L-lactated or acetated Ringer's solution at a dose of 80 ml/kg body weight (BW). The acetated Ringer's solution induced a significantly greater increase in venous HCO(3)(-) and base excess concentrations than the other fluids during the early phases of extracellular fluid replacement in mild metabolic acidosis. Therefore, the alkalizing effect of commercial acetated Ringer's solution is superior to commercial DL- and L-lactated Ringer's solution in treatment of mild metabolic acidosis in calves.

Competition between acetate and oleate for the formation of malonyl-CoA and mitochondrial acetyl-CoA in the perfused rat heart

We previously showed that, in the perfused rat heart, the capacity of n-fatty acids to generate mitochondrial acetyl-CoA decreases as their chain length increases. In the present study, we investigated whether the oxidation of a long-chain fatty acid, oleate, is inhibited by short-chain fatty acids, acetate or propionate (which do and do not generate mitochondrial acetyl-CoA, respectively). We perfused rat hearts with buffer containing 4 mM glucose, 0.2 mM pyruvate, 1 mM lactate, and various concentrations of either (i) [U-(13)C]acetate, (ii) [U-(13)C]acetate plus [1-(13)C]oleate, or (iii) unlabeled propionate plus [1-(13)C]oleate. Using mass isotopomer analysis, we determined the contributions of the labeled substrates to the acetyl moiety of citrate (a probe of mitochondrial acetyl-CoA) and to malonyl-CoA. We found that acetate, even at low concentration, markedly inhibits the oxidation of [1-(13)C]oleate in the heart, without change in malonyl-CoA concentration. We also found that propionate, at a concentration higher than 1 mM, decreases (i) the contribution of [1-(13)C]oleate to mitochondrial acetyl-CoA and (ii) malonyl-CoA concentration. The inhibition by acetate or propionate of acetyl-CoA production from oleate probably results from a competition for mitochondrial CoA between the CoA-utilizing enzymes.

Comparative effects of Ringer's acetate and lactate solutions on intraoperative central and peripheral temperatures

STUDY OBJECTIVES

To compare the effects of Ringer's lactate (LR) and Ringer's acetate (AR) solutions on core body and peripheral temperatures during isoflurane or sevoflurane anesthesia.

DESIGN

Prospective, randomized study.

SETTING

Operating rooms of a university hospital.

PATIENTS

60 ASA physical status I and II patients undergoing general surgery.

INTERVENTIONS

Following induction with 5 mg/kg of thiamylal and 0.1 mg/kg of vecuronium, patients were randomly assigned to one of four groups (15 patients per group). They received inhalation anesthetics (66% nitrous oxide [N2O] and 1.0% to 2.0% isoflurane or 1.3% to 2.6% sevoflurane) and LR or AR.

MEASUREMENTS AND MAIN RESULTS

Tympanic membrane (central) temperatures, forearm temperatures, and fingertip temperatures were recorded during surgery every 30 minutes. Tympanic membrane temperatures in the patients given AR were significantly higher than those given LR during isoflurane anesthesia 5 and 30 minutes after induction of anesthesia. However, this was not the case for sevoflurane anesthesia. There were no significant differences in forearm and fingertip temperatures or fingertip bloodflow among the four groups.

CONCLUSION

There was no significant difference between AR and LR in the preservation of heat during either sevoflurane or isoflurane anesthesia. However, AR may be superior to LR for maintaining central temperature during the early period of isoflurane anesthesia.

Ethanol inhibits acetate metabolism in rat hepatocytes

The metabolism of acetate at concentrations of 1, 2.5, 5 and 10 mM was investigated in freshly isolated hepatocytes from 48 hr fasted, female rats in the absence and presence of 10 mM ethanol. The maximal capacity for acetate metabolism was 0.85 mumol/(10(8) cells.min). Ethanol caused a 20% decrease in the apparent Vmax for acetate metabolism and an increase in the apparent Km for acetate from 3.0 to 4.6 mM. At physiological concentration of acetate (approximately 1 mM) and in the absence of an inhibitory effect of ethanol, the capacity for acetate metabolism was 15-20% of the rate of acetate formation from ethanol and the inhibitory effect of ethanol further reduced it to 10-15%. The results thus explain the well-known but hitherto not understood fact that only a small fraction of acetate produced in the liver during ethanol oxidation is further metabolized by the liver, while the majority is exported for oxidation in other tissues. Finally, a new method for calculation of liver acetate uptake in the presence of ethanol is presented.

Periportal zonation of the cytosolic acetyl-CoA synthetase of male rat liver

Several important metabolic functions of the mammalian liver have been shown to be located in zones with respect to the complex microcirculation of the organ. The zonal distribution of the cytosolic component of the acetyl-CoA synthetase activity has been investigated using the dual-digitonin-pulse-perfusion technique, which allows highly zone-selective sampling of cytosol from the periportal and perivenous zone of rat liver. Approximately 80% of the cytosolic enzymes are eluted from the hepatocytes in the periportal and perivenous sub-zones affected by digitonin, while less than 1% of the glutamate dehydrogenase activity (a marker enzyme of the mitochondrial compartment) is eluted. A twofold higher activity of the cytosolic form of acetyl-CoA synthetase is found in the periportal zone compared to the perivenous zone in fed male rats. Following a fasting/refeeding transition, this activity gradient is abolished in a manner similar to that observed for the enzyme acetyl-CoA carboxylase. Since the latter enzyme is utilizing the product of acetyl-CoA synthetase, acetyl-CoA, the similarity in the observed regulation suggests a functional coupling between cytosolic acetate activation and fatty-acid synthesis.

5'-Nucleotidase activity and adenosine production in rat liver mitochondria

The controversial subject of mitochondrial 5'-nucleotidase in the liver was studied employing density gradient fractionation combined with a method for analyzing the distribution profiles of marker enzymes based on multiple regression analysis. Triton WR-1339 was used to improve the separation of mitochondria from lysosomes by the gradient centrifugation technique. Adenosine production was examined further using acetate to increase intramitochondrial AMP, and thus adenosine production, in incubations with gradient centrifugation-purified mitochondria. Distribution analysis of the crude homogenate showed that 5'-nucleotidase activity exists in the mitochondrial fraction. To increase the resolution of this approach with respect to mitochondria, a crude mitochondrial fraction was also studied. In this case the relative mitochondrial activity decreased but 5'-nucleotidase activity was still clearly detectable. The mitochondrial 5'-nucleotidase exhibited a Km of 94 microM and a Vmax of 31 nmol/min per mg protein for AMP. The kinetic data for the Mg2+, ATP, ADP and AOPCP sensitivity of the enzyme showed that it differs from the plasma membrane, lysosome and cytosol 5'-nucleotidases. AOPCP was only a moderate inhibitor, and ATP was a more potent inhibitor than ADP at a 1 mM concentration. The enzyme also showed a requirement of Mg2+. Acetate caused the conversion of intramitochondrial adenylates to AMP and the formation of adenosine. Adenosine concentration increased in the extramitochondrial space in a time-dependent manner, but only trace amounts of nucleotides were detected. The data show that 5'-nucleotidase activity producing adenosine exists in rat liver mitochondria and a concentration-dependent adenosine output from mitochondria by diffusion or facilitated diffusion is also suggested.

Compartmentation of acetyl CoA studied by analysis of tricarboxylic acid cycle acids and 3-hydroxybutyrate in bile of rats given [2,2,2-2H3]ethanol

Acetate, 3-hydroxybutyrate, pyruvate, lactate, citrate, 2-oxoglutarate, succinate, fumarate and malate were analysed in rat bile by gas chromatography and gas chromatography/mass spectrometry of their O-melthyloxime-t-butyldimethylsilyl derivatives. The concentration of acetate increased to about 1.8 mmol/l after administration of [2,2,2-2H3]ethanol. Acetate was formed from ethanol to an extent of about 82% and retained all of the 2H at C-2, whereas 15% of the 2H had been lost in the tricarboxylic acid cycle intermediates and 24% in 3-hydroxybutyrate. Thus the exchange of 2H for 1H takes place after formation of acetyl CoA. For citrate and 3-hydroxybutyrate, 41% and 11% respectively was formed from [2,2,2-2H3]ethanol. These results indicate that different pools of acetyl CoA are used for the synthesis of ketone bodies and citrate, with the latter being derived from ethanol to a much larger extent. Smaller fractions of 2-oxoglutarate (16%) and succinate (5%) were derived from [2,2,2--2H3]ethanol, indicating significant contributions from amino acids.

Ca2+-induced accumulation of pyrophosphate in mitochondria during acetate metabolism

The mechanism of pyrophosphate (PPi) accumulation in rat liver during acetate metabolism was investigated. Perfusion of the liver with acetate in the presence of noradrenaline and glucagon induced marked accumulation of PPi (2 mumol/g of liver, 200 times that of control). In contrast, perfusion with glutamine, which generates PPi only in the cytosol, caused little accumulation of PPi, even in the presence of the two hormones. The site of PPi accumulation was shown to be the mitochondria by the finding that isolated mitochondria from the liver perfused with acetate and the hormones contained 50 nmol of PPi/mg of protein. The addition of an uncoupler to mitochondria with accumulated PPi caused gradual decrease in their PPi content, with concomitant release of a stoichiometric amount of Ca2+. Similar accumulation of PPi was observed when isolated mitochondria were incubated with acetate and Ca2+. These results show that an increase in cytosolic Ca2+ caused by the co-administration of the two hormones induced uptake of the ion into mitochondria, and that PPi accumulated in mitochondria only when it was generated in the organelles with an elevated concentration of Ca2+. High mitochondrial concentrations of Ca2+ are considered to inhibit inorganic pyrophosphatase through the formation of a stable complex, CaPPi-. Mitochondria with accumulated PPi had normal respiratory activities, and their adenine nucleotide concentrations were increased 2-fold rather than being decreased, the increases also being considered to be caused by their high concentration of Ca2+.

Adenine nucleotide transport and adenosine production in isolated rat heart mitochondria during acetate metabolism

In view of its vasodilatory effect on the coronary circulation (probably mediated by adenosine) and its metabolic compartmentalization (intramitochondrial activation to form acetyl-CoA), the metabolic effects of acetate were studied in isolated rat heart mitochondria. Acetate caused conversion of adenylates to AMP and the formation of adenosine. Adenylate efflux was inhibited by carboxyatractyloside but not by N-ethylmaleimide. The intramitochondrial accumulation of AMP was enhanced by carboxyatractyloside during acetate metabolism and the formation of extramitochondrial adenosine inhibited. A carboxyatractyloside-sensitive unidirectional AMP influx with a Km of 50 microM and Vmax of 11 nmol/min per mg mitochondrial protein was also observed. The mitochondrial adenosine content was high and constant during the experiments. The steep apparent concentration gradient of adenosine indicates that most of the mitochondrial adenosine is tightly bound to protein. Adenosine formation was proportional to the extramitochondrial AMP concentration, showing that the 5'-nucleotidase activity of cardiac mitochondrial preparations is extramitochondrial in origin. The data suggest that the mitochondrial ATP/ADP carrier is capable of transporting AMP and that intramitochondrial AMP is recycled during acetate metabolism in the myocardium partially by means of the ATP/ADP translocator, leading to an increase in extramitochondrial AMP and adenosine formation.

A simple colorimetric assay method for pyrophosphate in the presence of a 1000-fold excess of orthophosphate: application of the method to the study of pyrophosphate metabolism in mitochondria

A sensitive colorimetric method for the assay of inorganic pyrophosphate with excess of orthophosphate is described. The principle of this method lies in the formation of phosphomolybdate and PPi-molybdate complexes with subsequent extraction of the phosphomolybdate complex by organic solvents and reduction of the PPi-molybdate complex by dithiothreitol and Eikonogen. The sensitivity of the method was from 5 to 120 nmol of PPi in a 2.0-ml sample.

Acetate-induced changes in cardiac energy metabolism and hemodynamics in the rat

The hemodynamic and metabolic effects of acetate were studied in rats in vivo and in the isolated perfused heart. Hemodynamic parameters, myocardial phosphagens, inorganic phosphate, and adenosine were measured in vivo. Acetate uptake, coronary flow, O2 consumption, parameters of the cellular energy state, and hypoxanthine compounds and their washout were measured in heart perfusion experiments. Heart rate (HR), cardiac output, and the peak derivative of the left ventricular pressure rise (dP/dtmax) increased significantly during acetate infusion in vivo, but mean arterial pressure, systolic arterial pressure, and systemic vascular resistance decreased. Heart muscle ATP concentrations decreased after 7 min of acetate infusion. In vivo cardiac work load (HR.(peak left ventricular pressure] showed a positive correlation with tissue adenosine concentration and a negative correlation with phosphorylation potential. Acetate uptake in the perfused hearts was about 2.5 mumol/min per gram wet weight. Acetate perfusion increased O2 consumption and coronary flow concomitantly with a decrease in tissue ATP concentration. Tissue AMP and perfusate effluent adenosine concentration and adenosine output increased significantly, perfusate adenosine showing a non-linear positive correlation with coronary flow. The results demonstrate that acetate induces considerable changes in hemodynamics and metabolism in the heart.

The medical and metabolic consequences of administration of sodium acetate

1. The standard total parenteral nutrition, peritoneal dialysis, hemodialysis and many surgical fluids in use today contain 36 to 45 mM D,L-lactate or 2 to 140 mM acetate whereas the normal blood level of D-lactate is 0.02 mM L-lactate 0.5 to 5 mM and acetate 0.1 nM. The reasons for the continued use in patients of such unphysiological concentrations of these anions appear to be historic. 2. Administration of similar concentrations of these anions to the rat causes widespread metabolic disturbances which mimic many of the untoward complications associated with current parenteral and dialysis therapy. Understanding of the mechanisms attendant upon the metabolism of these anions may serve as a guide for designing improved parenteral fluids for human patients. 3. Elevation of blood D-lactate to 5 mM is associated with cerebral dysfunction in human patients. 4. Acetate stimulates the release of the inflammatory leukokine, interleukin-1 from human monocytes. Use of 35 to 45 mM acetate in peritoneal dialysis fluids led to peritoneal fibrosis. Patients exposed to acetate containing hemodialysis fluids have 12-fold elevation in their plasma interleukin-1 levels. 5. Administration of 20 mM sodium acetate to rats leads to a number of metabolic disturbances similar to those seen in human dialysis patients: (a) Acetate elevates blood glucose in the rat and may contribute to the exacerbation of the carbohydrate intolerance seen in uremic patients. (b) Acetate increases the levels of hepatic malonyl CoA, the rate controlling substrate of fatty acid synthesis and may exacerbate the hypertriglyceridemia characteristic of dialysis patients. (c) Acetate administration in the rat leads to a decrease in the cytosolic phosphorylation potential, reduction of the redox state of the free cytosolic NAD couple and paradoxical oxidation of the mitochondrial NAD couple in a pattern analogous to that produced by uncouplers of oxidative phosphorylation and may account in part for the elevation of temperature reported in patients undergoing hemodialysis with acetate. (d) Acetate administration in the rat leads to an increase in intracellular phosphorylated intermediates, adenine nucleotides, inorganic phosphate, inorganic pyrophosphate, calcium and magnesium. On cessation of acetate metabolism, the inorganic phosphate and calcium accumulated intracellularly leave the intracellular space. In patients undergoing hemodialysis, the blood phosphate returns to predialysis levels, within 6 hr after the completion of treatment, leaving significant numbers of patients with chronic hyperphosphatemia and the multiple complications attendant to that state.(ABSTRACT TRUNCATED AT 400 WORDS)

The accumulation of pyrophosphate by rat hepatocytes

Hepatocytes that were isolated from 48 hr starved rats and incubated in Krebs-Henseleit bicarbonate buffer containing 10 microM A23187, 10mM l-lactate, 1mM pyruvate and 2mM l-lysine were found to contain 0.064 mumol of inorganic pyrophosphate/g wet wgt cells. Addition of either 20mM acetate or butyrate, which caused the formation of pyrophosphate in both the cytosol and the mitochondrial matrix or only the matrix, resulted in an increase of 0.915 and 1.91 mumol pyrophosphate/g wet wgt cells, respectively. The accumulation of pyrophosphate was shown to be non-linear with time and dependent on the calcium concentration of the incubation media. In contrast, incubations containing a combination of 10 mM NH4Cl and 5 mM ornithine, which resulted in the formation of pyrophosphate only in the cytosol, had a pyrophosphate content of 0.032 mumol/g wgt cells. When isolated hepatocytes that had been incubated with acetate or butyrate were subjected to digitonin fractionation, all of the recoverable pyrophosphate was present in the particulate fraction. It is concluded that pyrophosphate accumulates in isolated rat hepatocytes only in the presence of calcium and a calcium ionophore, only within the mitochondrial matrix and only when pyrophosphate is formed within the mitochondrial matrix.

Increased ketogenesis in hyperthyroid rats metabolizing ethanol

The levels of various metabolites were measured in freeze-clamped samples of liver from triiodothyronine-treated and control rats to which either saline or ethanol (2.5 g/kg body weight) had been administered 2 hours earlier. It was found that ethanol led to a sharp increase in the hepatic acetate concentration in both hyperthyroid and euthyroid rats whereas lactate and pyruvate concentrations were lowered in both groups. The lactate/pyruvate ratio rose significantly in euthyroid animals that had received ethanol but the ratio remained relatively low in hyperthyroid rats. The adenine nucleotide phosphorylation potential, already low in hyperthyroid rats, was further lowered by ethanol. However, the most remarkable difference between the responses of euthyroid and hyperthyroid rats to ethanol was in the hepatic concentrations of ketone bodies, particularly 3-hydroxybutyrate. In control animals, administration of ethanol did not affect either the acetoacetate or 3-hydroxybutyrate concentration but, although the level of ketone bodies in the livers of hyperthyroid rats that had not received ethanol was the same as that of controls, there was a greater than fivefold increase in the 3-hydroxybutyrate level when ethanol was given. While this increase in ethanol-dependent ketogenesis is not explicable at this stage, hyperthyroidism did not increase the activity of cytoplasmic acetyl-CoA synthetase, an enzyme that is probably involved in the formation of ketone bodies from ethanol-derived acetate.

The effect of short chain fatty acid administration on hepatic glucose, phosphate, magnesium and calcium metabolism

Intra peritoneal administration of the short chain fatty acids, acetate, propionate and butyrate, in amounts calculated to reach 20 mM in total body water were given to fed and 48 hour starved male Wistar rats. One half hour after administration, the livers were freeze-clamped and the hepatic contents of various intermediary metabolites were measured. The liver content of total glycolytic intermediates was elevated by short chain fatty acids. In fed animals, the portion of glycolysis from fructose 1,6-bisphosphate (FBP) to PEP was elevated 2 to 4 fold. In 48 hour starved animals, where gluconeogenesis is active, the portion of the gluconeogenetic pathway from FBP to glucose was elevated 1.5 to 3.5 fold with the exception of the butyrate treated animals where blood glucose was not elevated. The metabolites of the hexose-monophosphate pathway that were measured, namely 6-phosphogluconate, ribulose 5-phosphate and xylose 5-phosphate were increased in both fed and starved animals. The free cytoplasmic [NAD+]/[NADH], [NADP+]/[NADPH], and [epsilon ATP]/[epsilon ADP] X [epsilon Pi] ratios were all decreased in both fed and starved animals after short chain fatty acid administration. The liver content of calcium increased 1.2 to 2 fold in fed animals and 2 to 3 fold in starved animals while total liver magnesium was either unchanged or increased only 1.2 times. The liver pyrophosphate (PPi) content increased a minimum of 10 fold in fed animals and over 100 fold in starved animals. In all cases no PPi could be detected in vivo by 31P NMR even though in the starved rats the PPi levels approached those of ATP. The liver content of inorganic Pi increased 1.3 to 1.5 fold in fed animals and 1.5 to 2 fold in starved animals. The total "rapidly metabolizing" Pi pool, that includes adenine and guanine nucleotides, glycolytic and shunt intermediates, Pi and PPi increased 1.3 times in fed animals (from 13.8 mumole/g fresh weight) and 1.5 to 1.7 fold in starved animals (from 15.7 mumol/g fresh weight). The total phosphate taken up from blood and entering the rapidly turning over pool of liver phosphate ranged between 4 and 12 mumols/g of liver.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetate metabolism by tissues of the rabbit

Acetyl CoA synthetase (E.C.6.2.1.1) and acetyl CoA hydrolase (E.C.3.1.2.1) activities were assayed in sub-cellular fractions of rabbit liver, heart and kidney homogenates. The intracellular location of acetyl CoA hydrolase was predominantly mitochondrial in all tissues, whereas that for acetyl CoA synthetase varied between the tissues studied. The relationship between location of enzyme activity and metabolism of acetate in different tissues is discussed.

Flux of palmitate through the peroxisomal and mitochondrial beta-oxidation systems in isolated rat hepatocytes

Peroxisomes catalyze the beta-oxidation of fatty acids but their quantitative role in fatty acid catabolism in the intact hepatocyte is not yet clarified. In the present study peroxisomal beta-oxidation of [1-14C]palmitate was quantitated in hepatocytes without the use of metabolic inhibitors. It was assumed that acetyl-CoA formed by peroxisomal beta-oxidation enters the cytosolic pool of acetyl-CoA, whereas that from mitochondrial beta-oxidation enters the mitochondrial pool. The labeling of the two acetyl-CoA pools was assessed by measuring the incorporation of radioactivity into cholesterol (from cytosolic acetyl-CoA) and CO2 (from mitochondrial acetyl-CoA). The system was calibrated with [1-14C]acetate and [1-14C]butyrate because butyrate undergoes beta-oxidation only in mitochondria, whereas acetate forms acetyl-CoA primarily in the cytosol. The labeling ratio, [( 14C]cholesterol X 100)/[( 14C]cholesterol + [14C]CO2), reflects the site of formation of acetyl-CoA. This ratio was 0.51 for butyrate, 1.39 for acetate and 0.79 for palmitate. The difference between palmitate and butyrate was statistically significant (P less than 0.02). This indicates that not all of the palmitate was oxidized in mitochondria. By linear interpolation it was estimated that approximately 32% of the [1-14C]palmitate oxidation began in peroxisomes.

Partial purification of rat liver cytoplasmic acetyl-CoA synthetase; characterization of some properties

Rat liver cytoplasmic acetyl-CoA synthetase was partially purified (purification factor = 23, yield = 30%). The apparent Kms for acetate, coenzyme A, ATP and MgCl2 were determined and found to be 52.5 microM, 50.5 microM, 570 microM and 1.5 mM, respectively. The partially-purified enzyme showed a low affinity for short-chain carbon substrates other than acetate. The properties of the partially-purified enzyme were compared with those of enzymes from other sources.

Inhibition of lipogenesis in isolated hepatocytes by 3-amino-1,2,4-triazole

3-Amino-1,2,4-triazole has been found to be an inhibitor of fatty acid synthesis by isolated rat hepatocytes. Half-maximal inhibition of fatty acid synthesis occurs at approximately 20mM. Acetyl-CoA carboxylase activity in homogenates of hepatocytes is not affected by previous exposure of the intact cells to 3-amino-1,2,4-triazole. The drug opposes the activation of partially purified acetyl-CoA carboxylase by citrate, but does not influence enzyme activity in the absence of citrate. As compared to fatty acid synthesis, cholesterol synthesis by the hepatocytes is more drastically depressed by incubation of the cells with 3-amino-1,2,4-triazole. Half-maximal inhibition of cholesterogenesis occurs at approximately 5 mM 8-amino-1,2,4-triazole.

Inhibition by 5-(tetradecyloxy)-2-furoic acid of fatty acid and cholesterol synthesis in isolated rat hepatocytes

Fatty acid and cholesterol synthesis in isolated rat hepatocytes were strongly inhibited by 5-(tetradecyloxy)-2-furoic acid. With either 3H2O or [2(-14)C] acetate as the labeled precursor, the concentrations of inhibitor causing 50% decrease in fatty acid and cholesterol synthesis were, respectively, less than 0.005 mM and 0.020 mM. At 0.1 mM inhibitor, citrate concentration in cells from fed rats was increased by 75%; lactate and pyruvate concentrations were decreased by 30%; ethanol oxidation was decreased by 20%; with cells from starved rats, the mitochondrial [NAD+]/[NADH] was decreased. Other parameters were unaffected. Both its potency and its specificity indicate that 5-(tetradecyloxy)-2-furoic acid will be useful in studies on the regulation of lipid biosynthesis.

Studies on the response of cholesterol biogenesis in feeding in rats: evidence against the existence of diurnal rhythms

1. The biosynthesis of cholesterol was studied, by using various precursors, in rats subjected to several dietary regimes. 2. The use of 3H2O as a substrate to demonstrate differences in cholesterogenesis under various conditions was validated by using rats fed on cholesterol or cholestyramine. Cholesterol feeding resulted in decreased cholesterogenesis, whereas cholestyramine caused an increase. 3. With acetate as precursor, the biosynthesis of both digitonin-precipitable sterol and fatty acids was increased in vitro in response to a meal. 4. In rats fed ad libitum, hepatic cholesterogenesis was increased at midnight relative to mid-morning as measured by using acetate precursor in vitro. However, no such difference was found by using 3H2O in vivo. 5. The lipogenic response was measured in meal-fed rats by using 3H2O or octanoate in vivo. In contrast with findings with acetate in vitro, no postprandial stimulation of cholesterogenesis was seen with either 3H2O or octanoate as precursor, whereas fatty acid biosynthesis from either substrate was increased. 6. These findings are discussed with respect to current theories about the circadian rhythm of cholesterogenesis. Such theories are based on experiments using isolated enzyme measurements or non-physiological precursors such as acetate. 7. It is considered that results obtained with 3H2O give an accurate representation of cholesterogenesis under various conditions, and it is therefore suggested that hepatic cholesterogenesis in rats is not subjected to the same degree of diurnal rhythm as has previously been believed.

Mechanism of the stimulatory effect of fructose on ethanol oxidation in perfused rat liver

The stimulatory effect of fructose on ethanol oxidation was studied in livers from fasted rats perfused with Krebs-Henseleit-bicarbonate buffer in a non-recirculating system. Two series of experiments were performed: (A) ethanol was infused with stepwise increasing concentrations (0.1-20 mM) in the presence of 4 mM fructose; (B) fructose was infused with stepwise increasing concentrations (0.5-10 mM) in the presence of 2 mM ethanol. From measured metabolic rates the following parameters were calculated: energy-rich phosphates consumed for fructose metabolism which were provided from oxidative phosphorylation (delta approximately P); reducing equivalents derived from stimulated ethanol utilization which were disposed by mitochondrial oxidation (delta2H). Under the various conditions studied a linear relationship between these parameters was observed. The ratio delta approximately P/delta2H was about 2.0. It is suggested that fructose stimulates ethanol oxidation indirectly by increasing the energy consumption of the liver due to the production of glucose from fructose. Consequetnly, the rate of oxidative phosphorylation is increased and, therefore, the capacity of the respiratory chain for oxidizing reducing equivalents derived from ethanol is enhanced. The data support the more general hypothesis that the rate of ethanol oxidation depend upon the rate of hepatic energy consumption in a given metabolic state.

Organ and intracellular localization of short-chain acyl-CoA synthetases in rat and guinea-pig

1. Homogenates of rat epididymal fat pad, heart, kidney, lactating mammary gland, liver, skeletal muscle and small intestinal mucosa have been partitioned into a particulate and supernatant fraction. With reliable marker enzymes for the mitochondrial matrix and the cytosol: propionyl-CoA carboxylase and pyruvate kinase, the distributions of the acyl-CoA synthetase activities measured at 1 and 10 mM C2, C3 and C4 over mitochondria and cytosol have been calculated. From these values an estimate was made of the K0.5 of the fatty acids. 2. A distinct fatty acid-activating enzyme was assumed to be present in one of the compartments when that fatty acid was activated with a K0.5 less than or equal to 1.5 mM in an amount of greater than 13% of the total cellular activity. Adipose tissue, gut, liver and mammary gland, all organs of a high lipogenetic capacity, contained a cytosolic acetyl-CoA synthetase. At 1 mM acetate 60, 31, 77 and 83% of the total cellular activities in these organs were cytosolic in nature, with activities of 0.021, 0.32, 0.37 and 1.16 mumol C2 activated per min per g wet weight, respectively. 3. Mitochondrial acetyl-CoA and butyryl-CoA synthetases were found in adipose tissue, gut, heart, kidney, mammary gland and muscle. They were absent in liver. Adipose tissue and liver contained a mitochondrial propionyl-CoA synthetase with activities at 1 mM C3 of 0.014 and 1.50 mumol C3 activated per min per g wet weight, respectively. 4. At 1 mM, C2 was activated with decreasing rates by kidney, heart, mammary gland and gut (7.6-1.0 mumol C2 activated per min per g wet weight). C3 (1 mM) activation was about equal (1.6-1.9 mumol C3 activated per min per g wet weight) in liver, kidney and heart. C4 (1 mM) was activated with decreasing rates by heart, liver, kidney and gut (4.0-0.5 mumol C4 activated per min per g wet weight) in the order given. 5. The influence of the isolation method and the diet on fatty acid activation in small intestinal mucosal scrapings have been studied. To demonstrate the existence of cytosolic acetyl-CoA synthetase in fed animals a pre-treatment of everted intestine by low amplitude vibration has been found essential. Also C16 activation was highly (95%) decreased in a non-pre-vibrated preparation. 24 h starvation lowered cytosolic C2 and total C16 activation by 90 and 80%, respectively. Refeeding of starved rats with a balanced fat-free diet, and not with sucrose only, gave the same cytosolic C2 and total C16 activation as normally fed rats. 6. In guienea-pig heart, kidney, liver and muscle about the same partitions have been found as in the respective rat organs. The acetate activation in liver was factor 6 lower. Acetate and butyrate activation in guinea-pig muscle was much higher (6 and 37 times, respectively).

Rate-limiting steps in ethanol metabolism and approaches to changing these rates biochemically

Ethanol is oxidized to acetate primarily by a system involving liver alcohol and aldehyde dehydrogenases coupled with reoxidation of NADH by the mitochondria. All of these steps are at least partially rate-limiting in ethanol metabolism, with alcohol dehydrogenase and oxidative phosphorylation probably slower than the others. More research is required to assess the quantitative roles of various steps. Many agents are ineffective in changing the rate of metabolism of ethanol, but fructose and dinitrophenol may increase the rate by up to 1.5-fold in vivo. The failure of single agents to increase the rate substantially may indicate that when one step is accelerated, another step becomes rate-limited. Therefore, combinations of agents that affect several steps simultaneously may be required for acceleration. Effective experimental methods for inhibiting alcohol dehydrogenase in vivo are available.

Production and utilization of acetate in mammals

1. In an attempt to define the importance of acetate as a metabolic precursor, the activities of acetyl-CoA synthetase (EC 6.2.1.1) and acetyl-CoA hydrolase (Ec 3.1.2.1) were assayed in tissues from rats and sheep. In addition, the concentrations of acetate in blood and liver were measured, as well as the rates of acetate production by tissue slices and mitochondrial fractions of these tissues. 2. Acetyl-CoA synthetase occurs at high activities in heart and kidney cortex of both species as well as in rat liver and the sheep masseter muscle. The enzyme is mostly in the cytosol fraction of liver, whereas it is associated with the mitochondrial fraction in heart tissue. Both mitochondrial and cytosol activities have a K(m) for acetate of 0.3mm. Acetyl-CoA synthetase activity in liver was not altered by changes in diet, age or alloxan-diabetes. 3. Acetyl-CoA hydrolase is widely distributed in rat and sheep tissues, the highest activity being found in liver. Essentially all of the activity in liver and heart is localized in the mitochondrial fraction. Hepatic acetyl-CoA hydrolase activity is increased by starvation in rats and sheep and during the suckling period in young rats. 4. The concentrations of acetate in blood are decreased by starvation and increased by alloxan-diabetes in both species. The uptake of acetate by the sheep hind limb is proportional to the arterial concentration of acetate, except in alloxan-treated animals, where uptake is impaired. 5. Acetate is produced by liver and heart slices and also by heart mitochondrial fractions that are incubated with either pyruvate or palmitoyl-(-)-carnitine. Liver mitochondrial fractions do not form acetate from either substrate but instead convert acetate into acetoacetate. 6. We propose that acetate in the blood of rats or starved sheep is derived from the hydrolysis of acetyl-CoA. Release of acetate from tissues would occur under conditions when the function of the tricarboxylic acid cycle is restricted, so that the circulating acetate serves to redistribute oxidizable substrate throughout the body. This function is analogous to that served by ketone bodies.

Ethanol administration and the relationship of malonyl-coenzyme A concentrations to the rate of fatty acid synthesis in rat liver

1. The effect of ethanol on liver fatty acid synthesis was studied in vivo in 24h-starved and ;meal-fed' rats (i.e. fed for 3h per day and not ad libitum). 2. In the fed animal (3)H(2)O was incorporated into fat at a rate of 0.46mumol of C(2) units/min per g wet wt. of liver. Administration of either ethanol (3.2g/kg) or equicaloric amounts of glucose had no effect on the rate of (3)H(2)O incorporation into lipid. 3. In the 24h-starved animal, administration of the same dose of ethanol produced an increase in the rate of (3)H(2)O incorporation from 0.06 to 0.12mumol of C(2) units/min per g fresh wt. after 3h whereas [malonyl-CoA] increased from 0.006 to 0.009mumol/g. Glucose given in amounts equicaloric to ethanol was significantly more lipogenic, increasing both the (3)H(2)O incorporation from 0.06 to 0.20mumol of C(2) units/min per g and the malonyl-CoA content from 0.006 to 0.013 mumol/g wet wt. at 3h. 4. The decrease in the redox state of free cytoplasm NAD or NADP couples or the changes in content of citrate, glucose 6-phosphate and pyruvate of liver after ethanol administration had no measurable effect on the rate of fatty acid synthesis in vivo. 5. Under the conditions of the experiments there was no significant difference, among any of the groups, in the activity of liver fatty acid synthetase measured in vitro. A double-reciprocal plot of the rate of (3)H(2)O incorporation and the total tissue malonyl-CoA concentrations showed a striking relationship. It has been concluded that the rate of fatty acid synthesis in vivo is determined principally by the V(max.) of fatty acid synthetase and the concentration of free malonyl-CoA. 6. It has also been concluded that under the conditions of the present study, the synthesis of fatty acids de novo is unlikely to be an important factor in the increased liver lipid content associated with ethanol administration.