Entry of [(1,2-13C2)acetyl]-L-carnitine in liver tricarboxylic acid cycle and lipogenesis: a study by 13C NMR spectroscopy in conscious, freely moving rats.

The biochemical pathways involved in acetyl-L-carnitine utilization were investigated in conscious, freely moving rats by 13C NMR spectroscopy. Following 4-h [(1,2-13C2)acetyl]-L-carnitine infusion in fasted animals, the free carnitine levels in serum were increased, and an efflux of unlabelled acetyl-L-carnitine from tissues was observed. [(1,2-13C2)Acetyl]-L-carnitine was found to enter biosynthetic pathways in liver, and the acetyl moiety was incorporated into both cholesterol and 3-hydroxybutyrate carbon skeleton. In accord with the entry of [(1,2-13C2)acetyl]-L-carnitine in the mitochondrial acetylCoA pool associated with tricarboxylic acid cycle, the 13C label was also found in liver glutamate, glutamine, and glutathione. The analysis of the 13C-labelling pattern in 3-hydroxybutyrate and cholesterol carbon skeleton provided evidence that the acetyl-L-carnitine-derived acetylCoA pool used for ketone bodies synthesis in mitochondria was homogeneous, whereas cholesterol was synthesized from two different acetylCoA pools located in the extra- and intramitochondrial compartment, respectively. Furthermore, cholesterol molecules were shown to be preferentially synthesized by the metabolic route involving the direct channelling of CoA-activated mitochondria-derived ketone bodies into 3-hydroxy-3-methylglutarylCoA pathway, prior to equilibration of their acyl groups with extramitochondrial acetylCoA pool via acetoacetylCoA thiolase.     

Critical analysis of the use of 14C-acetate for measuring in vivo rat cholesterol synthesis

The bulk of cholesterol produced by the liver and the gut enters the mobile pool of body cholesterol. This process is called internal secretion in contrast with the fraction of biosynthesized cholesterol directly eliminated in the feces (fecal external secretion). In rats, under various conditions, a linear relationship was found between the rates of internal secretion measured by the isotope equilibrium method (range: 10-60 mg/day) and the sum of sterol radioactivities measured in liver and intestine 70 min after a [14C]-acetate pulse. In fact, a better correlation was found between the radioactivities of liver sterols and the values for internal secretion. In this new relationship, the ordinate at the origin corresponds to a minimal internal secretion of about 10 mg/day, which implies an important extrahepatic cholesterol production, probably from the gut. Indeed, in adult male rats, fed a semi-purified sucrose-rich diet, the relative contribution of this organ to the internal secretion was higher than in adult rats fed a commercial diet and higher than in young animals, whatever the circadian period. It can be concluded that some of the discrepancies observed in the literature about the relative participation of the intestine and the liver in the internal secretion of cholesterol are probably due to differences in experimental and nutritional conditions (age and sex of the animals, diet composition, time of the circadian cycle) rather than to the cholesterol precursor used (3H2O or [14C] acetate) to assess the activity of cholesterol synthesis. Indeed, a comparative study of 3H2O and [14C]acetate incorporation into sterols of enterocytes indicated the same crypt-villus radioactive gradient, regardless of the intestinal site studied (duodenum, jejunum or ileum) and thus validated the use of [14C]acetate. Other experiments however, showed evidence of some local differences in the cytosolic dilution of labeled acetyl CoA by the endogenous cholesterol precursor in rats under various conditions (control or cholestyramine-enriched diet, parenteral nutrition). After intravenous infusion of 1,2[13C]acetate, mass fragmentography of free cholesterol isolated from liver and intestine indicated different 13C-labeling patterns of newly synthesized molecules. They indicate that cholesterol is generally synthesized from acetyl CoA with a lower 13C-content in the liver than in the intestine. The local endogenous flow of acetyl CoA used for cholesterol synthesis was about 2-fold higher in the hepatocytes than in the enterocytes. This conclusion was confirmed by the results obtained with several experimental groups exhibiting a large range of both internal secretion of cholesterol and sterol radioactivities in liver and intestine after [14C]acetate injection.(ABSTRACT TRUNCATED AT 400 WORDS)     

Subcellular localization of fatty acid acetyl coa and malonyl CoA elongation systems in rat adipocytes

The distribution of fatty acid elongation systems in the subcellular fractions of isolated fat cells was studied. These systems were found to be located essentially in the particulate fractions, Acetyl CoA elongation was found to occur predominantly in the mitochondria (2.8 nmoles of acetyl CoA incorporated into fatty acids/mg of proteins per 30 min) while malonyl CoA elongation was predominant in the purified plasma membranes (10 nmoles of malonyl CoA incorporated into fatty acids/mg of proteins per 30 min). The acetyl CoA elongation activity in the mitochondria and the malonyl CoA elongation activity in the purified plasma membranes were found to be 6.4 and 2.4 times higher, respectively, than in the purified microsomes, the next most active fraction. In liver, on the other hand, greater malonyl CoA elongation activity was found in microsomes than in plasma membranes.     

Absolute rates of cholesterol synthesis in extrahepatic tissues measured with 3H-labeled water and 14C-labeled substrates.

This study was undertaken to develop techniques for measuring absolute rates of sterol synthesis in extrahepatic tissues in vitro and to estimate the magnitude of the errors inherent in the use of various 14C-labeled substrates for such measurements. Initial studies showed that significant errors were introduced when rates of synthesis were estimated using [3H]water since about 20 nmol of water were bound to each mg of tissue cholesterol isolated as the digitonide. This source of error could be eliminated by subtracting apparent incorporation rates obtained at 0 degrees C from those obtained at 37 degrees C or by regenerating and drying the free sterol. In a second set of experiments, the H/C incorporation ratio in cholesterol was determined in the liver by measuring the absolute rates of hydrogen and acetyl CoA flux into sterols. The ratio of 0.69 +/- 0.03 was found to be independent of the rate of hepatic cholesterol synthesis, the rate of hepatic acetyl CoA generation, or the source of the acetyl CoA. In a third set of studies, rates of incorporation of [3H]water or 14C-labeled acetate, octanoate, and glucose into digitonin-precipitable sterols were simultaneously measured in nine different extrahepatic tissues. Assuming that the H/C ratio measured in the liver also applied to these tissues, the [3H]water incorporation rates were multipled by the reciprocal of the H/C ratio to give the absolute rates of sterol synthesis in each tissue. When these were compared to the incorporation rates determined with the 14C-labeled substrates the magnitude of the errors in the rates of sterol synthesis obtained with these substrates in each tissue could be assessed. Only [14C]octanoate gave synthesis rates approaching 100% of those obtained with [3H]water and this occurred only in the intestine and kidney; in the other extrahepatic tissues this substrate gave rates of 6--66+ of the absolute rates. Rates of [14C]acetate incorporation in sterols varied from 4 to 62% of the [3H]water incorporation rates while those obtained with [14C]glucose were only 2--88% of the true rates. These studies document the large and highly variable errors inherent in estimating rates of sterol synthesis in extrahepatic tissues using 14C-labeled substrates under in vitro conditions.     

On the estimation of alternative pathways of fatty acid oxidation in the liverIn vivo

The relative contributions of mitochondrial β-oxidation and peroxisomal β-oxidation and peroxisomal ω-oxidation to the oxidation of a given fatty acidin vivo can be quantitated by an isotopic method. The approach requires infusion of a fatty acid labelled on two specific carbon atoms (e.g. [1-¹⁴C] and [11-¹⁴C] palmitate) to an isotopic steady state, with subsequent isolation and degradation of an acetylated conjugate as a product of the liver cytosolic acetyl CoA pool and of ketone bodies as a product of the liver mitochondrial acetyl CoA pool.     

The synthesis of bile acids in perfused rat liver subjected to chronic biliary drainage

1. Isolated rat liver was perfused with heparinized whole blood under physiological pressure resulting in the secretion of bile at about the rate observed in vivo. 2. The preparation remained metabolically active for 4h and was apparently normal in function and microscopic appearance. 3. When the perfusate plasma and liver cholesterol pool was labelled by the introduction of [2-(14)C]mevalonic acid the specific radioactivity of the perfusate cholesterol increased. The biliary acids (cholic acid and chenodeoxycholic acid) were labelled and had the same specific radioactivity. 4. Livers removed from rats immediately after, and 40h after, the start of total biliary drainage, were perfused; increased excretion rates of both cholic acid and chenodeoxycholic acid were found when the liver donors had been subjected to biliary drainage. 5. The incorporation of [2-(14)C]mevalonic acid or rat lipoprotein labelled with [(14)C]cholesterol into bile acids was studied. 6. A dissociation between the mass of bile acid excreted and the rate of incorporation of (14)C was found. This was attributed to the changing specific radioactivity of the cholesterol pool acting as the immediate bile acid precursor.     

Spectrophotometric studies of acyl-coenzyme A synthetases of rat liver mitochondria

1. Deca-2,4,6,8-tetraenoic acid is a substrate for both ATP-specific (EC 6.2.1.2 or 3) and GTP-specific (EC 6.2.1.-) acyl-CoA synthetases of rat liver mitochondria. The enzymic synthesis of decatetraenoyl-CoA results in new spectral characteristics. The difference spectrum for the acyl-CoA minus free acid has a maximum at 376nm with epsilon(mM) 34. Isosbestic points are at 345nm and 440nm. 2. The acylation of CoA by decatetraenoate in mitochondrial suspensions can be continuously measured with a dual-wavelength spectrophotometer. 3. By using this technique, three distinct types of acyl-CoA synthetase activity were demonstrated in rat liver mitochondria. One of these utilized added CoA and ATP, required added Mg(2+) and corresponded to a previously described ;external' acyl-CoA synthetase. The other two acyl-CoA synthetase activities utilized intramitochondrial CoA and did not require added Mg(2+). Of these two ;internal' acyl-CoA synthetases, one was insensitive to uncoupling agents, was inhibited by phosphate or arsenate, and corresponded to the GTP-specific enzyme. The other corresponded to the ATP-specific enzyme. 4. Atractylate inhibited the activity of the two internal acyl-CoA synthetases only when the energy source was added ATP. 5. The amount of intramitochondrial CoA acylated by decatetraenoate was independent of whether the internal ATP-specific or GTP-specific acyl-CoA synthetase was active. It is concluded that these two internal acyl-CoA synthetases have access to the same intramitochondrial pool of CoA. 6. The amount of intramitochondrial CoA that could be acylated with decatetraenoate was decreased by the addition of palmitoyl-dl-carnitine, 2-oxoglutarate, or pyruvate. These observations indicated that pyruvate dehydrogenase (EC 1.2.4.1), oxoglutarate dehydrogenase (EC 1.2.4.2), carnitine palmitoyltransferase (EC 2.3.1.-), citrate synthase (EC 4.1.3.7), and succinyl-CoA synthetase (EC 6.2.1.4) all have access to the same intramitochondrial pool of CoA as do the two internal acyl-CoA synthetases.     

New experiments of biotin enzymes.

The objects of structural studies on biotin-enzymes were acetyl CoA-carboxylase and pyruvate carboxylase of Saccharomyces cerevisiae and beta-methylcrotonyl CoA-carboxylase and acetyl CoA-carboxylase of Achromobacter IV S. It was found that these enzymes can be arranged in three groups. In the first group, as represented by acetyl CoA-carboxylase of Achromobacter, the active enzyme could be resolved in three types of functional components: (1) the biotin-carboxyl carrier protein, (2) the biotin carboxylase, and (3) the carboxyl transferase. In the second group, as represented by beta-methylcrotonyl CoA-carboxylase from Achromobacter only two types of polypeptides are present. The one carries the biotin carboxylase activity together with the biotin-carboxyl-carrier protein, the other one carries the carboxyl transferase activity. In this third group, as represented by the two enzymes of yeast, all three catalytic functions are incorporated in one multifunctional polypeptide chain. The evolution of the different enzymes is discussed. The animal tissues acetyl CoA-carboxylase is under metabolic control, as known from previous studies. It thus has to be expected that the levels of malonyl CoA in livers of rats in all states of depressed fatty acid synthesis are much lower than under normal conditions because the carboxylation of acetyl CoA is strongly reduced and cannot keep pace with the consumption of malonyl CoA by fatty acid synthetase. A new highly sensitive assay method for malonyl CoA was developed which uses tritiated NADPH and measures the incorporation of radioactivity into the fatty acids formed from malonyl CoA in the presence of purified fatty acid synthetase. The application of this method to liver extracts showed that the level of malonyl CoA which amounts to about 7 nmoles per gram of wet liver drops to less than 10% within a starvation period of 24 hr and even further if the starvation period is extended to 48 hr. A low malonyl CoA concentration is also found in the alloxan diabetic animals and in animals being fed a fatty diet after starvation. On the other hand, feeding a carbohydrate rich diet leads to malonyl CoA levels surpassing the levels found after feeding a balanced diet. These observations reconfirm the concept that fatty acid synthesis is principally regulated by the carboxylation of acetyl CoA.     

Metabolism of octanoate and its effect on glucose and palmitate utilization by isolated fat cells.

Octanoate is avidly incorporated into triglycerides by isolated rat adipocytes in the presence of glucose via direct esterification without prior beta-oxidation to acetyl CoA. This was shown by separation of the products formed from (1-14C) octanoate into lipid classes using Florisil columns, and after alkaline hydrolysis of the triglyceride fraction, by cochromatogrpahy with authentic fatty acids on reverse-phase Celite columns. The relative contribution of (U-14C) glucose and (1-14C) octanoate to triglyceride synthesis and CO2 formation were studied under a variety of conditions. Concentrations of octanoate below 0.5 mM have a stimulatory effect on the conversion of (U-14C) glucose to CO2, triglycerides and esterified fatty acids. However, a marked depression of fatty acid synthesis from (U-14C) glucose was observed in the presence of millimolar concentrations of octanoate. Octanoate had no effect on the esterification of palmitate, but palmitate strongly depressed the ability of rat adipocytes to esterify octanoate.     

Rapid activation and inactivation of fatty acid synthesis from glucose in vivo

The flux of glucose carbon to total body fatty acids was measured in unanesthetized mice either after fasting or 50-80 min after they nibbled a small test meal containing 120 mg of glucose (fasted-refed). Flux was calculated from plasma [(14)C]glucose specific activity curves and from total body (14)C-labeled fatty acid 30 min after intravenous injection of tracer [(14)C]glucose. Mobilization of liver glycogen, changes in the body glucose pool size, and total flux of carbon through the glucose pool during periods of fasting and refeeding were defined. Liver glycogen was almost completely depleted 8 hr after food removal. Body glucose pool size fell during fasting and increased after refeeding the test meal. Irreversible disposal rate of glucose C varied directly with body glucose pool size; but flux of glucose C into fatty acids increased exponentially as body glucose concentration increased. Within an hour after nibbling a small test meal, the flux of glucose C into total body fatty acids increased 700% in mice previously starved for 24 hr. However, flux of glucose C into fatty acids in postabsorptive mice (food removed for 2 hr; livers rich in glycogen) was only about 2% of the value calculated from published studies in which the incorporation of an intubated [(14)C]glucose load into total body fatty acid was measured in mice. A possible explanation for this phenomenon is presented.     

The transport of lipoprotein cholesterol into bile: a reassessment of kinetic studies in the experimental animal

Studies were undertaken to assess the contribution of lipoprotein cholesterol to bile and to determine whether already-existent hepatic free cholesterol and the free cholesterol which is newly generated from the hydrolysis of hepatic cholesteryl esters are equally available for secretion into bile or constitute metabolically separate pools. Rats with a bile fistula were injected with an intravenous bolus of high-density lipoprotein recombinants containing free [14C]cholesterol and [3H]cholesteryl esters. Results showed (1) that bile free [14C]cholesterol secretion was a constant and linear proportion of the whole liver free [14C]cholesterol pool, (2) that secretion into bile of free [3H]cholesterol was in direct proportion to the rate of hydrolysis of hepatic [3H]cholesteryl esters, and (3) that rates of biliary cholesterol secretion were very similar when secretion was calculated using the specific activity of free [14C]cholesterol and free [3H]cholesterol in the entire liver to 'label' the precursor free cholesterol pool. Furthermore, rates of secretion that were calculated using either isotope closely approximated the mass of free cholesterol that was directly measured in bile. Results thus indicate that because of equilibration and extensive dilution by the large pool of already-existent free cholesterol, the transport of isotopic cholesterol from lipoproteins cannot be used to directly assess the contribution of lipoprotein cholesterol to the cholesterol that is secreted in bile. These studies further suggest that the totality of preformed free cholesterol in the liver is in metabolic equilibrium in one single kinetic pool and that all hepatic free cholesterol is potentially available for secretion into bile.     

Cyclic AMP-sensitive activation of hepatic sterol synthesis and 3-hydroxy-3-methylglutaryl coenzyme A reductase.

We previously showed that preincubation of a 10,000 g supernatant (S(10)) from rat liver for 20 min at 37 degrees C dramatically increased the subsequent incorporation of [(14)C]acetate into sterols. No activation was seen with [(14)C]mevalonate as substrate. In the present studies we have examined the effect of preincubation on HMG CoA reductase. When microsomes were isolated from S(10) by calcium precipitation, preincubation of S(10) increased the specific activity of HMG CoA reductase threefold. No activation of HMG CoA reductase was observed in microsomes isolated by ultracentrifugation. Activation was cyclic AMP-sensitive. When cyclic AMP (0.001-1.0 mM) and MgATP (1 mM) were present during the preincubation period, there was little or no activation of HMG CoA reductase activity or of sterol synthesis from acetate. MgATP alone did not prevent activation. Neither cyclic AMP nor MgATP was inhibitory when present only during the assay of sterol synthesis. We propose that the in vitro activation represents the reversal of a physiologic cyclic AMP-mediated mechanism for the control of hepatic HMG CoA reductase. That a phosphoprotein phosphatase may catalyze the activation was supported by the observation that sodium fluoride, an inhibitor of phosphoprotein phosphatases, inhibited the activation. These results suggest that hormone-induced changes in the cellular level of cyclic AMP may regulate the activity of HMG CoA reductase and the rate of hepatic cholesterol synthesis.     

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.     

The effects of alpha-adrenergic stimulation on the regulation of the pyruvate dehydrogenase complex in the perfused rat liver

The regulation of the pyruvate dehydrogenase multienzyme complex was investigated during alpha-adrenergic stimulation with phenylephrine in the isolated perfused rat liver. The metabolic flux through the pyruvate dehydrogenase reaction was monitored by measuring the production of 14CO2 from infused [1-14C] pyruvate. In livers from fed animals perfused with a low concentration of pyruvate (0.05 mM), phenylephrine infusion significantly inhibited the rate of pyruvate decarboxylation without affecting the amount of pyruvate dehydrogenase in its active form. Also, phenylephrine caused no significant effect on tissue NADH/NAD+ and acetyl-CoA/CoASH ratios or on the kinetics of pyruvate decarboxylation in 14CO2 washout experiments. Phenylephrine inhibition of [1-14C]pyruvate decarboxylation was, however, closely associated with a decrease in the specific radioactivity of perfusate lactate, suggesting that the pyruvate decarboxylation response simply reflected dilution of the labeled pyruvate pool due to phenylephrine-stimulated glycogenolysis. This suggestion was confirmed in additional experiments which showed that the alpha-adrenergic-mediated inhibitory effect on pyruvate decarboxylation was reduced in livers perfused with a high concentration of pyruvate (1 mM) and was absent in livers from starved rats. Thus, alpha-adrenergic agonists do not exert short term regulatory effects on pyruvate dehydrogenase in the liver. Furthermore, the results suggest either that the rat liver pyruvate dehydrogenase complex is insensitive to changes in mitochondrial calcium or that changes in intramitochondrial calcium levels as a result of alpha-adrenergic stimulation are considerably less than suggested by others.     

On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

Octanoate applied to rat liver mitochondria respiring with glutamate plus malate or succinate (plus rotenone) under resting-state (State 4) conditions stimulates oxygen uptake and decreases the membrane potential, both effects being sensitive to oligomycin but not to carboxyatractyloside. Octanoate also decreases the rate of pyruvate carboxylation under the same conditions, this effect being correlated with the decrease of intramitochondrial content of ATP and increase of AMP. The decrease of pyruvate carboxylation and the change of mitochondrial adenine nucleotides are both reversed by 2-oxoglutarate. Fatty acids of shorter chain length have similar effects, though at higher concentrations. Addition of octanoate in the presence of fluoride (inhibitor of pyrophosphatase) produces intramitochondrial accumulation of pyrophosphate, even under conditions when oxidation of octanoate is prevented by rotenone. In isolated hepatocytes incubated with lactate plus pyruvate, octanoate also increases oxygen uptake and produces a shift in the profile of adenine nucleotides similar to that observed in isolated mitochondria. It decreases the ‘efficiency’ of gluconeogenesis, as expressed by the ratio between an increase of glucose production and an increase of oxygen uptake upon addition of gluconeogenic substrates (lactate plus pyruvate), and increases the reduction state of mitochondrial NAD. These effects taken together are not compatible with uncoupling, but point to intramitochondrial hydrolysis of octanoyl-CoA and probably also shorter chain-length acyl-CoAs. This mechanism probably functions as a ‘safety valve’ preventing a drastic decrease of intramitochondrial free CoA under a large supply of medium- and short-chain fatty acids.     

Gonadotropin modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in desensitized luteinized rat ovary

These studies were done to examine the effect of gonadotropin on rat luteal 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity (the rate-limiting step in cholesterol biosynthesis) in ovaries of pregnant mare's serum gonadotropin (PMSG)-human chorionic gonadotropin (hCG) primed rats. Administration of hCG stimulated HMG CoA reductase activity in a time- and dose-dependent manner: significant increases were noted within 4 h, with maximum effects (30-40-fold increases) seen 24 h after hCG (25 IU) administration. This effect was specific in that only LH, of several hormones tested, was as effective as hCG in stimulating HMG CoA reductase activity, and no change in the activity of either liver microsomal HMG CoA reductase or luteal microsomal NADPH-cytochrome c reductase was seen after hCG. The gonadotropin-induced increase in HMG CoA reductase activity seemed to be due to a net increase in enzyme activity, not to a change in the phosphorylated/dephosphorylated state of the enzyme. Pretreatment of animals with aminoglutethimide, an inhibitor of the conversion of cholesterol to steroid (pregnenolone), prevented the hCG-induced rise in HMG CoA reductase activity, whereas treatment with 4-aminopyrazolo[3,4-d]pyrimidine (4-APP), which depletes cellular cholesterol content, led to striking increases in enzyme activity. However, the combined effects of 4-APP and hCG were additive, suggesting that the stimulating effect of hCG on HMG CoA reductase activity is not entirely due to a depletion of cellular sterol content of luteinized ovaries. Similarly, cholesteryl ester and cholesterol syntheses as measured by [14C]acetate conversion were also increased by hCG and 4-APP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulatory effects of sterols and bile acids on hepatic 3-hydroxy-3-methylglutaryl CoA reductase and cholesterol 7alpha-hydroxylase in the rat

Specific activities of the hepatic microsomal enzymes 3-hydroxy-3-methylglutaryl CoA (HMG CoA) reductase and cholesterol 7alpha-hydroxylase were studied in rats fed sterols and bile acids. The administration of bile acids (taurocholate, taurodeoxycholate, taurochenodeoxycholate) at a level of 1% of the diet for 1 wk reduced the activity of HMG CoA reductase. Taurocholate and taurodeoxycholate, but not taurochenodeoxycholate, inhibited cholesterol 7alpha-hydroxylase. Dietary sitosterol produced increases in the specific activity of HMG CoA reductase (3.6-fold) and cholesterol 7alpha-hydroxylase (1.4-fold), and biliary cholesterol concentrations in this group more than doubled. Compared with controls fed the stock diet, the simultaneous administration of sitosterol and taurochenodeoxycholate resulted in a 60% decrease of HMG CoA reductase activity and no change in cholesterol 7alpha-hydroxylase activity or biliary cholesterol concentration. Rats fed sitosterol plus taurocholate had nearly normal HMG CoA reductase activity, but cholesterol 7alpha-hydroxylase was inhibited and biliary cholesterol remained high. Bile acid secretion rates and biliary bile acid composition were similar in controls and sterol-fed animals. In all groups receiving bile acids, biliary secretion of bile acids was nearly doubled and bile acid composition was shifted in the direction of the administered bile acid. It is concluded that the composition of the bile acid pool influences the hepatic concentrations of the rate-controlling enzymes of bile acid synthesis.     

Hepatic uptake and metabolism of benzoate: a multiple indicator dilution, perfused rat liver study

Multiple, noneliminated references ((51)Cr-labeled erythrocytes, (125)I-albumin, [(14)C]- or [(3)H]sucrose, and [(2)H](2)O), together with [(3)H]hippurate or [(14)C]benzoate, were injected simultaneously into the portal vein of the perfused rat liver during single-pass delivery of benzoate (5-1,000 microM) and hippurate (5 microM) to investigate hippurate formation kinetics and transport. The outflow dilution data best fit a space-distributed model comprising vascular and cellular pools for benzoate and hippurate; there was further need to segregate the cellular pool of benzoate into shallow (cytosolic) and deep (mitochondrial) pools. Fitted values of the membrane permeability-surface area products for sinusoidal entry of unbound benzoate were fast and concentration independent (0.89 +/- 0.17 ml. s(-1). g(-1)) and greatly exceeded the plasma flow rate (0.0169 +/- 0.0018 ml. s(-1). g(-1)), whereas both the influx of benzoate into the deep pool and the formation of hippurate occurring therein appeared to be saturable. Results of the fit to the dilution data suggest rapid uptake of benzoate, with glycination occurring within the deep and not the shallow pool as the rate-determining step.     

Changes in catalytic activity and association state of pyruvate carboxylase which are dependent on enzyme concentration

The specific activity of chicken liver pyruvate carboxylase has been shown to decrease with decreasing enzyme concentration, even at 100 microM, which is close to the estimated physiological concentration. The kinetics of the loss of enzyme specific activity following dilution were biphasic. Incubation of dilution-inactivated enzyme with ATP, acetyl CoA, Mg2+ + ATP or, to a lesser degree, with Mg2+ alone resulted in a high degree of reactivation, while no reactivation occurred in the presence of pyruvate. The association state of the enzyme before, during, and after dilution inactivation has been assessed by gel filtration chromatography. These studies indicate that on dilution, there is dissociation of the catalytically active tetrameric enzyme species into inactive dimers. Reactivation of the enzyme resulted in reassociation of enzymic dimers into tetramers. The enzyme was shown to form high molecular weight aggregates at high enzyme concentrations.     

Growth of Paracoccus denitrificans on [2,3-13C]succinate and [1,4-13C]succinate. I. The flux of carbon in energy metabolism and the operation of the TCA cycle

The metabolism of Paracoccus denitrificans, grown on either [2,3-13C]succinate or [1,4-13C]succinate, was investigated by using gas chromatography-mass spectrometry. The distribution of label in a group of metabolites closely related to the TCA-cycle intermediates showed that the flux of carbon from succinate in energy metabolism in vivo was via pyruvate (malic enzyme) and acetyl CoA. The labelling pattern of the carboxyl groups showed that one fifth of the succinate pool was formed by the regeneration of succinate via the TCA cycle, and four fifths was supplied externally as substrate from the medium.