Evidence that the production of acetate in rat hepatocytes is a predominantly cytoplasmic process

Complex regulatory effects of gut microbial short-chain fatty acids on immune tolerance and autoimmunity

Immune tolerance deletes or suppresses autoreactive lymphocytes and is established at multiple levels during the development, activation and effector phases of T and B cells. These mechanisms are cell-intrinsically programmed and critical in preventing autoimmune diseases. We have witnessed the existence of another type of immune tolerance mechanism that is shaped by lifestyle choices, such as diet, microbiome and microbial metabolites. Short-chain fatty acids (SCFAs) are the most abundant microbial metabolites in the colonic lumen and are mainly produced by the microbial fermentation of prebiotics, such as dietary fiber. This review focuses on the preventive and immunomodulatory effects of SCFAs on autoimmunity. The tissue- and disease-specific effects of dietary fiber, SCFAs and SCFA-producing microbes on major types of autoimmune diseases, including type I diabetes, multiple sclerosis, rheumatoid arthritis and lupus, are discussed. Additionally, their key regulatory mechanisms for lymphocyte development, tissue barrier function, host metabolism, immunity, autoantibody production, and inflammatory effector and regulatory lymphocytes are discussed. The shared and differential effects of SCFAs on different types and stages of autoimmune diseases are discussed.

Regulation of common neurological disorders by gut microbial metabolites

The gut is connected to the CNS by immunological mediators, lymphocytes, neurotransmitters, microbes and microbial metabolites. A mounting body of evidence indicates that the microbiome exerts significant effects on immune cells and CNS cells. These effects frequently result in the suppression or exacerbation of inflammatory responses, the latter of which can lead to severe tissue damage, altered synapse formation and disrupted maintenance of the CNS. Herein, we review recent progress in research on the microbial regulation of CNS diseases with a focus on major gut microbial metabolites, such as short-chain fatty acids, tryptophan metabolites, and secondary bile acids. Pathological changes in the CNS are associated with dysbiosis and altered levels of microbial metabolites, which can further exacerbate various neurological disorders. The cellular and molecular mechanisms by which these gut microbial metabolites regulate inflammatory diseases in the CNS are discussed. We highlight the similarities and differences in the impact on four major CNS diseases, i.e., multiple sclerosis, Parkinson's disease, Alzheimer's disease, and autism spectrum disorder, to identify common cellular and molecular networks governing the regulation of cellular constituents and pathogenesis in the CNS by microbial metabolites.

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.

Compartmentation of Metabolism of the C12-, C9-, and C5-n-dicarboxylates in Rat Liver, Investigated by Mass Isotopomer Analysis: ANAPLEROSIS FROM DODECANEDIOATE

We investigated the compartmentation of the catabolism of dodecanedioate (DODA), azelate, and glutarate in perfused rat livers, using a combination of metabolomics and mass isotopomer analyses. Livers were perfused with recirculating or nonrecirculating buffer containing one fully (13)C-labeled dicarboxylate. Information on the peroxisomal versus mitochondrial catabolism was gathered from the labeling patterns of acetyl-CoA proxies, i.e. total acetyl-CoA, the acetyl moiety of citrate, C-1 + 2 of β-hydroxybutyrate, malonyl-CoA, and acetylcarnitine. Additional information was obtained from the labeling patterns of citric acid cycle intermediates and related compounds. The data characterize the partial oxidation of DODA and azelate in peroxisomes, with terminal oxidation in mitochondria. We did not find evidence of peroxisomal oxidation of glutarate. Unexpectedly, DODA contributes a substantial fraction to anaplerosis of the citric acid cycle. This opens the possibility to use water-soluble DODA in nutritional or pharmacological anaplerotic therapy when other anaplerotic substrates are impractical or contraindicated, e.g. in propionic acidemia and methylmalonic acidemia.

Luminescent ruthenium probe for the determination of acetyl phosphate in complex biological matrices

The first probe for the fluorogenic determination of acetyl phosphate (AcP), (bpy)(2)Ru(1,10-phenanthroline-5,6-dione dioxime) (RuPDO), was prepared and its reaction with AcP was studied in detail. The emission of the weakly luminescent RuPDO is red shifted and strongly enhanced upon reaction with AcP in the presence of metal cations like Zn(2+) or Cu(2+). The reaction occurs within 60 min incubation time under highly biocompatible conditions (aqueous buffer of pH 7, 37 °C). A linear dynamic range from 10 to 200 µmol L(-1) is observed with an LOD of AcP of 3.4 µmol L(-1) (for RuPDO-Zn). Other bio-phosphates studied show only weak interference. Furthermore, the applicability of the probe in complex biological matrices was evaluated.

Acetate produced in the mitochondrion is the essential precursor for lipid biosynthesis in procyclic trypanosomes

Acetyl-CoA produced in mitochondria from carbohydrate or amino acid catabolism needs to reach the cytosol to initiate de novo synthesis of fatty acids. All eukaryotes analyzed so far use a citrate/malate shuttle to transfer acetyl group equivalents from the mitochondrial matrix to the cytosol. Here we investigate how this acetyl group transfer occurs in the procyclic life cycle stage of Trypanosoma brucei, a protozoan parasite responsible of human sleeping sickness and economically important livestock diseases. Deletion of the potential citrate lyase gene, a critical cytosolic enzyme of the citrate/malate shuttle, has no effect on de novo biosynthesis of fatty acids from (14)C-labeled glucose, indicating that another route is used for acetyl group transfer. Because acetate is produced from acetyl-CoA in the mitochondrion of this parasite, we considered genes encoding cytosolic enzymes producing acetyl-CoA from acetate. We identified an acetyl-CoA synthetase gene encoding a cytosolic enzyme (AceCS), which is essential for cell viability. Repression of AceCS by inducible RNAi results in a 20-fold reduction of (14)C-incorporation from radiolabeled glucose or acetate into de novo synthesized fatty acids. Thus, we demonstrate that the essential cytosolic enzyme AceCS of T. brucei is responsible for activation of acetate into acetyl-CoA to feed de novo biosynthesis of lipids. To date, Trypanosoma is the only known eukaryotic organism that uses acetate instead of citrate to transfer acetyl groups over the mitochondrial membrane for cytosolic lipid synthesis.

Mitochondria, ciglitazone and liver: a neglected interaction in biochemical pharmacology

Peroxisome proliferator activated receptors (PPARs) are a class of nuclear receptors now actively investigated for their involvement in lipid and glucidic metabolism, immune regulation and cell differentiation. Drugs binding and activating PPARs are therefore attracting attention for their potential therapeutic role in various diseases like type 2 diabetes, dyslipidemias, atherosclerosis, obesity (i.e., metabolic syndrome). Agonists of these receptors have been already used in therapeutic protocols: fibrates (PPAR-alpha ligands) are being used in hyperlipidemias, and thiazolidinediones (mainly PPAR-gamma ligands) are being employed as insulin sensitizers. The latter drugs introduction into therapy, however, showed very soon some unwanted effects (hepatotoxicity at first and myocardiotoxicity later on) which confirmed some contradictory data already suggested by pre-clinical trial-experiments. In this study we show that some PPAR ligands impair mitochondrial oxidative metabolism in human liver cell line mainly by deranging NADH oxidation. Intriguingly, the PPAR-gamma ligand ciglitazone caused a dose-dependent inhibition of NADH-cytochrome c reductase that resulted, at a drug concentration of 50 microM, of about 60% (P<0.001), while other PPAR ligands with different receptor affinity - positive controls like clofibrate (0.7 mM), gemfibrozil (0.23 mM) and bezafibrate (1 mM) - reduced the activity of mitochondrial Complex I by about 20% (P<0.01, P<0.01 and P<0.05, respectively). The induced mitochondrial dysfunction imposed a series of metabolic compensatory adaptations characterized by a significant shift to anaerobic glycolysis. These findings underline the undervalued non-genomic effects of PPAR ligands and can provide a better understanding of the pharmacotoxicological profiles of these drugs and of their roles in the therapy of diabetes mellitus.

The acetate switch

To succeed, many cells must alternate between life-styles that permit rapid growth in the presence of abundant nutrients and ones that enhance survival in the absence of those nutrients. One such change in life-style, the "acetate switch," occurs as cells deplete their environment of acetate-producing carbon sources and begin to rely on their ability to scavenge for acetate. This review explains why, when, and how cells excrete or dissimilate acetate. The central components of the "switch" (phosphotransacetylase [PTA], acetate kinase [ACK], and AMP-forming acetyl coenzyme A synthetase [AMP-ACS]) and the behavior of cells that lack these components are introduced. Acetyl phosphate (acetyl approximately P), the high-energy intermediate of acetate dissimilation, is discussed, and conditions that influence its intracellular concentration are described. Evidence is provided that acetyl approximately P influences cellular processes from organelle biogenesis to cell cycle regulation and from biofilm development to pathogenesis. The merits of each mechanism proposed to explain the interaction of acetyl approximately P with two-component signal transduction pathways are addressed. A short list of enzymes that generate acetyl approximately P by PTA-ACKA-independent mechanisms is introduced and discussed briefly. Attention is then directed to the mechanisms used by cells to "flip the switch," the induction and activation of the acetate-scavenging AMP-ACS. First, evidence is presented that nucleoid proteins orchestrate a progression of distinct nucleoprotein complexes to ensure proper transcription of its gene. Next, the way in which cells regulate AMP-ACS activity through reversible acetylation is described. Finally, the "acetate switch" as it exists in selected eubacteria, archaea, and eukaryotes, including humans, is described.

Bezafibrate induces a mitochondrial derangement in human cell lines: a PPAR-independent mechanism for a peroxisome proliferator

Bezafibrate is a hypolipidemic drug that belongs to the group of peroxisome proliferators because it binds to peroxisome proliferator-activated receptors type alpha (PPARs). Peroxisome proliferators produce a myriad of extraperoxisomal effects, which are not necessarily dependent on their interaction with PPARs. An investigation on the peculiar activities of bezafibrate could clarify some of the molecular events and the relationship with the biochemical and pharmacological properties of this class of compounds. In this view, the human acute promyelocytic leukemia HL-60 cell line and human rabdomiosarcoma TE-671 cell line were cultured in media containing bezafibrate and a number of observations such as spectrophotometric analysis of mitochondrial respiratory chain enzymes, NMR metabolite determinations, phosphofructokinase enzymatic analysis, and differentiation assays were carried on. Bezafibrate induced a derangement of NADH cytochrome c reductase activity accompanied by metabolic alterations, mainly a shift to anaerobic glycolysis and an increase of fatty acid oxidation, as shown by NMR analysis of culture supernatants where acetate, lactate, and alanine levels increased. On the whole, the present results suggest a biochemical profile and a therapeutic role of this class of PPARs ligands more complex than those previously proposed.

Mitochondrial respiratory chain dysfunction, a non-receptor-mediated effect of synthetic PPAR-ligands: biochemical and pharmacological implications

Peroxisome proliferator activated receptors (PPARs) are a class of nuclear receptors involved in lipid and glucidic metabolism, immune regulation, and cell differentiation. Many of their biological activities have been studied by using selective synthetic activators (mainly fibrates and thiazolidinediones) which have been already employed in therapeutic protocols. Both kinds of drugs, however, showed pharmacotoxicological profiles, which cannot be ascribed by any means to receptor activation. To better understand these non-receptorial or extrareceptorial aspects, the effect of different PPAR-ligands on the metabolic status of human HL-60 cell line has been investigated. At this regard, NMR analysis of cell culture supernatants was accomplished in order to monitor modifications at the level of cell metabolism. Cell growth and chemiluminescence assays were employed to verify cell differentiation. Results showed that all the considered PPAR-ligands, although with different potencies and independently from their PPAR binding specificity, induced a significant derangement of the mitochondrial respiratory chain consisting in a strong inhibition of NADH-cytochrome c reductase activity. This derangement has been shown to be strictly correlated to the adaptive metabolic modifications, as evidenced by the increased formation of lactate and acetate, due to the stimulation of anaerobic glycolysis and fatty acid beta-oxidation. It is worthy noting that the mitochondrial dysfunction appeared also linked to the capacity of any given PPAR-ligand to induce cell differentiation. These data could afford an explanation of biochemical and toxicological aspects related to the therapeutic use of synthetic PPAR-ligands and suggest a revision of PPAR pathophysiologic mechanisms.

Acetate and Butyrate Are the Major Substrates for De Novo Lipogenesis in Rat Colonic Epithelial Cells

The objective of these experiments was to investigate the source of substrates used for lipid synthesis and the pathways of substrate incorporation into lipids by epithelial cells of the colon. Within replicates, cells were exposed to all treatments evaluated in that experiment. By comparing the relative incorporation rates of several 14C-labeled substrates into lipids, acetate made a significantly larger carbon contribution to lipids than propionate, butyrate, glucose or glutamine under the in vitro conditions utilized in this study. Other major carbon contributors were butyrate and 3-hydroxybutyrate. Glucose, glutamine and propionate made only minor contributions. (-)-Hydroxycitrate did not affect the incorporation of acetate or butyrate carbon into lipids, even though it inhibited colonic ATP-citrate lyase. These data suggest that SCFA carbon used in the synthesis of lipids by colonocytes is not likely transported to the cytosol as citrate. Competition experiments suggest that ketone bodies and butyrate contribute to a single precursor pool for lipogenesis. Ketone bodies did not significantly suppress acetate incorporation into lipid, however. Incorporation of 3H2O and 14C-acetate was significantly greater into phospholipids than into free fatty acids and triacylglycerides, suggesting that the major role of lipogenesis is for membrane synthesis. In conclusion, colonocytes appear to synthesize lipids using a pathway distinct from the liver by incorporating mainly SCFA and ketone bodies into lipids, and by using citrate to a limited extent, if at all, to transport acetyl units from the mitochondria to the cytosol.

Chemistry and biochemistry of (-)-hydroxycitric acid from Garcinia

(-)-Hydroxycitric acid [(-)-HCA] is the principal acid of fruit rinds of Garcinia cambogia, Garcinia indica, and Garcinia atroviridis. (-)-HCA was shown to be a potent inhibitor of ATP citrate lyase (EC 4.1.3.8), which catalyzes the extramitochondrial cleavage of citrate to oxaloacetate and acetyl-CoA: citrate + ATP + CoA --> acetyl-CoA + ADP + P(i) + oxaloacetate. The inhibition of this reaction limits the availability of acetyl-CoA units required for fatty acid synthesis and lipogenesis during a lipogenic diet, that is, a diet high in carbohydrates. Extensive animal studies indicated that (-)-HCA suppresses the fatty acid synthesis, lipogenesis, food intake, and induced weight loss. In vitro studies revealed the inhibitions of fatty acid synthesis and lipogenesis from various precursors. However, a few clinical studies have shown controversial findings. This review explores the literature on a number of topics: the source of (-)-HCA; the discovery of (-)-HCA; the isolation, stereochemistry, properties, methods of estimation, and derivatives of (-)-HCA; and its biochemistry, which includes inhibition of the citrate cleavage enzyme, effects on fatty acid synthesis and lipogenesis, effects on ketogenesis, other biological effects, possible modes of action on the reduction of food intake, promotion of glycogenesis, gluconeogenesis, and lipid oxidation, (-)-HCA as weight-controlling agent, and some possible concerns about (-)-HCA, which provides a coherent presentation of scattered literature on (-)-HCA and its plausible mechanism of action and is provocative of further research.

Acetyl-CoA synthetase 2, a mitochondrial matrix enzyme involved in the oxidation of acetate

Using peptide sequences derived from bovine cardiac acetyl-CoA synthetase (AceCS), we isolated and characterized cDNAs for a bovine and murine cardiac enzyme designated AceCS2. We also isolated a murine cDNA encoding a hepatic type enzyme, designated AceCS1, identical to one reported recently (Luong, A., Hannah, V. C., Brown, M. S., and Goldstein, J. L. (2000) J. Biol. Chem. 275, 26458-26466). Murine AceCS1 and AceCS2 were purified to homogeneity and characterized. Among C2-C5 short and medium chain fatty acids, both enzymes preferentially utilize acetate with similar affinity. The AceCS2 transcripts are expressed in a wide range of tissues, with the highest levels in heart, and are apparently absent from the liver. The levels of AceCS2 mRNA in skeletal muscle were increased markedly under ketogenic conditions. Subcellular fractionation revealed that AceCS2 is a mitochondrial matrix enzyme. [(14)C]Acetate incorporation indicated that acetyl-CoAs produced by AceCS2 are utilized mainly for oxidation.

Molecular characterization of human acetyl-CoA synthetase, an enzyme regulated by sterol regulatory element-binding proteins

Through suppressive subtractive hybridization, we identified a new gene whose transcription is induced by sterol regulatory element-binding proteins (SREBPs). The gene encodes acetyl-CoA synthetase (ACS), the cytosolic enzyme that activates acetate so that it can be used for lipid synthesis or for energy generation. ACS genes were isolated previously from yeast, but not from animal cells. Recombinant human ACS was produced by expressing the cloned cDNA transiently in human cells. After purification by nickel chromatography, the 701-amino acid cytosolic enzyme was shown to function as a monomer. The recombinant enzyme produced acetyl-CoA from acetate in a reaction that required ATP. As expected for a gene controlled by SREBPs, ACS mRNA was induced when cultured cells were deprived of sterols and repressed by sterol addition. The pattern of regulation resembled the regulation of enzymes of fatty acid synthesis. ACS mRNA was also elevated in livers of transgenic mice that express dominant-positive versions of all three isoforms of SREBP. We conclude that ACS mRNA, and hence the ability of cells to activate acetate, is regulated by SREBPs in parallel with fatty acid synthesis in animal cells.

Acetogenesis does not replace ketogenesis in fasting piglets infused with hexanoate

The current studies were performed to better understand the physiological relevance of acetate in the poorly ketogenic piglet and to determine if endogenous acetogenesis rises with increased mitochondrial fatty acid beta-oxidation, analogous to ketogenesis. Plasma acetate concentration values in newborn, fasted, or suckled piglets (230-343 microM) were at least 10-fold higher than the ketone bodies, a pattern opposite to that in 24- to 48-h suckled rats (77-175 microM). Employing continuous infusion techniques with sodium [3H]acetate tracer in fasting approximately 40-h-old piglets, acetate rate of appearance (Ra) was found to be 34 +/- 4 micromol . min-1 . kg body wt-1. This basal Ra was double that observed in animals coinfused with sodium [1-14C]hexanoate (P < 0.001), despite active oxidation of the latter as determined by 14CO2 production. Active acetogenesis in vivo and relatively abundant acetate in piglet blood are consistent with the hypothesis that acetate plays an important physiological role in piglets. However, the negative impact of hexanoate oxidation upon acetate Ra and the lack of significant changes in circulating acetate in newborn, suckled, and fasted piglets draws into question the extent of analogy between acetogenesis and ketogenesis in vivo.

Effect of propionate on fatty acid and cholesterol synthesis and on acetate metabolism in isolated rat hepatocytes

In the present study the actual role of propionic acid in the control of fatty acid and cholesterol synthesis was investigated in isolated liver cells from fed rats maintained in the presence of near-physiological concentrations of glucose, glutamine and acetate. Using 3H2O for lipid labelling, propionate appears as an effective inhibitor of fatty acid synthesis and to a lesser extent of cholesterol synthesis, even at the lowest concentration used (0.6 mmol/l). Butyrate is a potent activator of both synthetic pathways, and the activating effect was not counteracted by propionate. Using 1-[14C]acetate, it was observed that propionate at a moderate concentration, or 1 mmol oleate/l, are both very effective inhibitors of 14C incorporation into fatty acid and cholesterol. This incorporation was drastically inhibited when propionate and oleate were present together in the incubation medium. The net utilization of acetate by rat hepatocytes was impaired by propionate, in contrast to oleate. 1-[14C]butyrate was utilized at a high rate for fatty acid synthesis, but to a lesser extent for cholesterol synthesis; both processes were unaffected by propionate. Intracellular citrate concentration was not markedly depressed by propionate, whereas it was strongly elevated by butyrate. In conclusion, propionate may represent an effective inhibitor of lipid synthesis when acetate is a major source of acetyl-CoA, a situation which is encountered with diets rich in readily-fermentable fibres. The present findings also suggest that propionate may be effective at concentrations close to values measured in vivo in the portal vein.

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

The relative contributions of mitochondrial beta-oxidation and peroxisomal beta-oxidation and peroxisomal omega-oxidation to the oxidation of a given fatty acid in 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.

Tracing hepatic gluconeogenesis relative to citric acid cycle activity in vitro and in vivo. Comparisons in the use of [3-13C]lactate, [2-13C]acetate, and alpha-keto[3-13C]isocaproate

The validity of the use of a carbon tracer for investigating liver intermediary metabolism in vivo requires that the labeling pattern of liver metabolites not be influenced by metabolism of the tracer in other tissues. To identify such specific tracer, livers from 48-h starved rats were perfused with recirculating buffer containing [3-13C]lactate, [2-13C]acetate, or alpha-keto[3-13C]isocaproate. Conscious 48-h starved rats were infused with the same tracers for 5 h. The labeling patterns of liver glutamate and extracellular glucose were assayed by gas chromatography-mass spectrometry. In vivo data were corrected for 13CO2 reincorporation into C-1 of glutamate and C-3 and C-4 of glucose, using data from control rats infused with NaH13CO3. With [3-13C]lactate the labeling pattern of liver glutamate was the same in perfused organs and in vivo. In contrast, with [2-13C]acetate and alpha-keto[3-13C]isocaproate the labeling pattern of liver glutamate in vivo was clearly influenced by the expected labeling pattern of citric acid cycle intermediates formed in non-gluconeogenic organs, presumably glutamine made in muscle. Indeed, the labeling pattern of plasma glutamine and liver glutamate were similar in experiments with [3-13C]lactate but different in experiments with [2-13C]acetate and alpha-keto[3-13C]isocaproate. Similar conclusions were drawn from the labeling patterns of glucose. Therefore, labeled lactate appears as the best tracer for studies of liver intermediary metabolism in vivo. Our data also show that a substantial fraction of alpha-ketoisocaproate metabolism occurs in peripheral tissues.

The effects of propionate and butyrate on acetate metabolism in rat hepatocytes

1. Two mM propionate or butyrate inhibited the mitochondrial uptake of acetate by rat hepatocytes. 2. With propionate the inhibition was so strong that the net formation of acetate in the cytoplasm, usually masked by the mitochondrial uptake, appeared directly as a net output of acetate into the medium; showing that this net formation of acetate, reported by [Crabtree B., Gordon M.-J. and Christie S. L. (1990) Biochem. J. 270, 219-225] is not an artefact arising from a misinterpretation of isotopic data. 3. The results also suggest that propionate and butyrate inhibit peroxisomal metabolism.

Biphasic effect of oleic acid on hepatic cholesterogenesis

Livers isolated from adult male rats were perfused in vitro with oleic acid (0.6 mM) as a complex with bovine serum albumin. Albumin alone was infused in control experiments. Oleic acid exerted a biphasic effect on incorporation of 3H2O into cholesterol, which was inhibitory during the first hour of perfusion, but exhibited a net stimulatory effect over a four hour period. No differences were observed in total activity or apparent phosphorylation state of HMG-CoA reductase after one hour of perfusion, with or without addition of oleic acid, implying that some other step limits the rate of cholesterol synthesis during this interval. After four hours of perfusion, HMG-CoA reductase activity was higher in livers perfused with oleic acid than in those perfused in its absence, in agreement with the observed differences in rates of cholesterol synthesis.

Systemic and regional acetate kinetics in dogs

Little is known about the sites of production and uptake of acetate in nonruminants. We placed blood sampling catheters in the femoral artery and in the femoral, portal, hepatic, and renal veins of mongrel dogs (n = 11). The animals were studied in the conscious state 2 wk later during a primed continuous infusion of [1-14C]acetate. Systemic acetate turnover, oxidation, and clearance were determined, as well as regional uptake and release, by measuring 14CO2 excretion as well as plasma concentration and specific activity at the five sampling sites. Results showed systemic acetate turnover was 8.8 +/- 1.9 mumol.kg-1.min-1, approximating 5% of energy expenditure in dogs. Simultaneous uptake and release of acetate was demonstrated in intestine, liver, kidney, and hindlimb. The intestine was the greatest contributor to acetate production, whereas the liver was the most important site of uptake. Plasma acetate oxidation was 77 +/- 4% of turnover. Both systemic clearance (129 +/- 22 ml.kg-1.min-1) and tissue fractional extraction (68-85%) were many times greater than values reported for glucose, free fatty acids, lactate, or amino acids. In conclusion, most tissues simultaneously take up and release acetate in dogs. This may represent a mechanism for interorgan transport of energy, especially under conditions of caloric deprivation.

Metabolic aspects of peroxisomal beta-oxidation

In the course of the last decade peroxisomal beta-oxidation has emerged as a metabolic process indispensable to normal physiology. Peroxisomes beta-oxidize fatty acids, dicarboxylic acids, prostaglandins and various fatty acid analogues. Other compounds possessing an alkyl-group of six to eight carbon atoms (many substituted fatty acids) are initially omega-oxidized in endoplasmic reticulum. The resulting carboxyalkyl-groups are subsequently chain-shortened by beta-oxidation in peroxisomes. Peroxisomal beta-oxidation is therefore, in contrast to mitochondrial beta-oxidation, characterized by a very broad substrate-specificity. Acyl-CoA oxidases initiate the cycle of beta-oxidation of acyl-CoA esters. The next steps involve the bi(tri)functional enzyme, which possesses active sites for enoyl-CoA hydratase-, beta-hydroxyacyl-CoA dehydrogenase- and for delta 2, delta 5 enoyl-CoA isomerase activity. The beta-oxidation sequence is completed by a beta-ketoacyl-CoA thiolase. The peroxisomes also contain a 2,4-dienoyl-CoA reductase, which is required for beta-oxidation of unsaturated fatty acids. The peroxisomal beta-hydroxyacyl-CoA epimerase activity is due to the combined action of two enoyl-CoA hydratases. (For a recent review of the enzymology of beta-oxidation enzymes see Ref. 225.) The broad specificity of peroxisomal beta-oxidation is in part due to the presence of at least two acyl-CoA oxidases, one of which, the trihydroxy-5 beta-cholestanoyl-CoA (THCA-CoA) oxidase, is responsible for the initial dehydrogenation of the omega-oxidized cholesterol side-chain, initially hydroxylated in mitochondria. Shortening of this side-chain results in formation of bile acids and of propionyl-CoA. In relation to its mitochondrial counterpart, peroxisomal beta-oxidation in rat liver is characterized by a high extent of induction following exposure of rats to a variety of amphipathic compounds possessing a carboxylic-, or sulphonic acid group. In rats some high fat diets cause induction of peroxisomal fatty acid beta-oxidation and of trihydroxy-5 beta-cholestanoyl-CoA oxidase. Induction involves increased rates of synthesis of the appropriate mRNA molecules. Increased half-lives of mRNA- and enzyme molecules may also be involved. Recent findings of the involvement of a member of the steroid hormone receptor superfamily during induction, suggest that induction of peroxisomal beta-oxidation represents another regulatory phenomenon controlled by nuclear receptor proteins. This will likely be an area of intense future research. Chain-shortening of fatty acids, rather than their complete beta-oxidation, is the prominent feature of peroxisomal beta-oxidation.(ABSTRACT TRUNCATED AT 400 WORDS)

Measurement of the rates of acetyl-CoA hydrolysis and synthesis from acetate in rat hepatocytes and the role of these fluxes in substrate cycling

1. Acetyl-CoA hydrolysis, acetyl-CoA synthesis from acetate and several related fluxes were measured in rat hepatocytes. 2. In contrast with acetyl-CoA hydrolysis, most of the acetyl-CoA synthesis from acetate occurred in the mitochondria. 3. Acetyl-CoA hydrolysis was not significantly affected by 24 h starvation or (-)-hydroxycitrate. 4. In the cytoplasm there was a net flux of acetyl-CoA to acetate, and substrate cycling between acetate and acetyl-CoA in this compartment was very low, accounting for less than 0.1% of the total heat production by the animal. 5. A larger cycle, involving mitochondrial and cytoplasmic acetate and acetyl-CoA, may operate in fed animals, but would account for only approx 1% of total heat production. 6. It is proposed that the opposing fluxes of mitochondrial acetate utilization and cytoplasmic net acetate production may provide sensitivity, feedback and buffering, even when these fluxes are not linked to form a conventional substrate cycle.