Hepatic enzymes, CoASH and long-chain acyl-CoA in subcellular fractions as affected by drugs inducing peroxisomes and smooth endoplasmic reticulum

Coenzyme A biosynthetic machinery in mammalian cells

CoA (coenzyme A) is an essential cofactor in all living organisms. CoA and its thioester derivatives [acetyl-CoA, malonyl-CoA, HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) etc.] participate in diverse anabolic and catabolic pathways, allosteric regulatory interactions and the regulation of gene expression. The biosynthesis of CoA requires pantothenic acid, cysteine and ATP, and involves five enzymatic steps that are highly conserved from prokaryotes to eukaryotes. The intracellular levels of CoA and its derivatives change in response to extracellular stimuli, stresses and metabolites, and in human pathologies, such as cancer, metabolic disorders and neurodegeneration. In the present mini-review, we describe the current understanding of the CoA biosynthetic pathway, provide a detailed overview on expression and subcellular localization of enzymes implicated in CoA biosynthesis, their regulation and the potential to form multi-enzyme complexes for efficient and highly co-ordinated biosynthetic process.

Structural and enzymatic characterization of NanS (YjhS), a 9-O-Acetyl N-acetylneuraminic acid esterase from Escherichia coli O157:H7

There is a high prevalence of sialic acid in a number of different organisms, resulting in there being a myriad of different enzymes that can exploit it as a fermentable carbon source. One such enzyme is NanS, a carbohydrate esterase that we show here deacetylates the 9 position of 9-O-sialic acid so that it can be readily transported into the cell for catabolism. Through structural studies, we show that NanS adopts a SGNH hydrolase fold. Although the backbone of the structure is similar to previously characterized family members, sequence comparisons indicate that this family can be further subdivided into two subfamilies with somewhat different fingerprints. NanS is the founding member of group II. Its catalytic center contains Ser19 and His301 but no Asp/Glu is present to form the classical catalytic triad. The contribution of Ser19 and His301 to catalysis was confirmed by mutagenesis. In addition to structural characterization, we have mapped the specificity of NanS using a battery of substrates.

Simple and unique purification by size-exclusion chromatography for an oligomeric enzyme, rat liver cytosolic acetyl-coenzyme A hydrolase

An overview of the purification of an oligomeric enzyme, an extramitochondrial acetyl-coenzyme A hydrolase from rat liver, is presented. The enzyme has been purified to homogeneity using two successive size-exclusion chromatography runs, first for the monomeric and second for the oligomeric form of the enzyme. The sequential gel-filtration steps efficiently removed the contaminants of any molecular size, first of different size from that of the monomeric form of the enzyme (K(av)=0.47 on Superdex 200) and second of different size from that of the oligomeric form (K(av)=0.33), allowing us to purify the enzyme in high purity. This strategy provides an excellent model for purifying many other oligomeric proteins including key enzymes or allosteric enzymes regulating metabolism.

The role Acyl-CoA thioesterases play in mediating intracellular lipid metabolism

Acyl-CoA thioesterases are a group of enzymes that catalyze the hydrolysis of acyl-CoAs to the free fatty acid and coenzyme A (CoASH), providing the potential to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. These enzymes are localized in almost all cellular compartments such as endoplasmic reticulum, cytosol, mitochondria and peroxisomes. Acyl-CoA thioesterases are highly regulated by peroxisome proliferator-activated receptors (PPARs), and other nutritional factors, which has led to the conclusion that they are involved in lipid metabolism. Although the physiological functions for these enzymes are not yet fully understood, recent cloning and more in-depth characterization of acyl-CoA thioesterases has assisted in discussion of putative functions for specific enzymes. Here we review the acyl-CoA thioesterases characterized to date and also address the diverse putative functions for these enzymes, such as in ligand supply for nuclear receptors, and regulation and termination of fatty acid oxidation in mitochondria and peroxisomes.

Molecular cloning and functional expression of rat liver cytosolic acetyl-CoA hydrolase

A cytosolic acetyl-CoA hydrolase (CACH) was purified from rat liver to homogeneity by a new method using Triton X-100 as a stabilizer. We digested the purified enzyme with an endopeptidase and determined the N-terminal amino-acid sequences of the two proteolytic fragments. From the sequence data, we designed probes for RT-PCR, and amplified CACH cDNA from rat liver mRNA. The CACH cDNA contains a 1668-bp ORF encoding a protein of 556 amino-acid residues (62 017 Da). Recombinant expression of the cDNA in insect cells resulted in overproduction of functional acetyl-CoA hydrolase with comparable acyl-CoA chain-length specificity and Michaelis constant for acetyl-CoA to those of the native CACH. Database searching shows no homology to other known proteins, but reveals high similarities to two mouse expressed sequence tags (91% and 93% homology) and human mRNA for KIAA0707 hypothetical protein (50% homology) of unknown function.

Role of acyl-CoA binding protein in acyl-CoA metabolism and acyl-CoA-mediated cell signaling

Long-chain acyl-CoA esters (LCA) act both as substrates and intermediates in metabolism and as regulators of various intracellular functions. Acyl-CoA binding protein (ACBP) binds LCA with high affinity and is believed to play an important role in intracellular acyl-CoA transport and pool formation and therefore also for the function of LCA as metabolites and regulators of cellular functions . The free concentration of cytosolic LCA is efficiently buffered to low nanomole concentration by ACBP and fatty acid binding protein (FABP). An additional important factor is the activity of acyl-CoA hydrolases. The estimated cellular free LCA concentration is two to four orders of magnitude lower than the concentrations reported to be necessary to regulate most LCA-affected cellular functions. Preliminary evidence indicates that the regulatory effect of LCA might be mediated by the LCA/ACBP complex.

Activity of the phosphatidylcholine biosynthetic pathway modulates the distribution of fatty acids into glycerolipids in proliferating cells

PtdCho accumulation is a periodic, S phase-specific event that is modulated in part by cell cycle-dependent fluctuations in CTP:phosphocholine cytidylyltransferase (CCT) activity. A supply of fatty acids is essential to generate the diacylglycerol (DG) precursors for phosphatidylcholine (PtdCho) biosynthesis but it is not known whether the DG supply is also coupled to the cell cycle. Although the rate of fatty acid synthesis in a macrophage cell line was dramatically stimulated in response to the growth factor, CSF-1, it was not regulated by the cell cycle. Increased fatty acid synthesis correlated with elevated acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) steady-state mRNA levels. Cellular fatty acid synthesis was essential for membrane PL synthesis. Cerulenin inhibition of endogenous fatty acid synthesis also inhibited PtdCho synthesis, which was not relieved by exogenous fatty acids. Inhibition of CCT activity by the addition of lysophosphatidylcholine (lysoPtdCho) or temperature-shift of a conditionally defective CCT diverted newly synthesized DG to the TG pool where it accumulated. Enforced expression of CCT stimulated PtdCho biosynthesis and reduced TG synthesis. Thus, the cellular DG supply did not regulate PtdCho biosynthesis and CCT activity governs the partitioning of DG into either the PL or TG pools, thereby controlling both PtdCho and TG biosynthesis.

Role of acylCoA binding protein in acylCoA transport, metabolism and cell signaling

Long chain acylCoA esters (LCAs) act both as substrates and intermediates in intermediary metabolism and as regulators in various intracellular functions. AcylCoA binding protein (ACBP) binds LCAs with high affinity and is believed to play an important role in intracellular acylCoA transport and pool formation and therefore also for the function of LCAs as metabolites and regulators of cellular functions [1]. The major factors controlling the free concentration of cytosol long chain acylCoA ester (LCA) include ACBP [2], sterol carrier protein 2 (SCP2) [3] and fatty acid binding protein (FABP) [4]. Additional factors affecting the concentration of free LCA include feed back inhibition of the acylCoA synthetase [5], binding to acylCoA receptors (LCA-regulated molecules and enzymes), binding to membranes and the activity of acylCoA hydrolases [6].

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Fatty acyl-CoA oxidase activity is induced before long-chain acyl-CoA hydrolase activity and acyl-CoA binding protein in liver of rat treated with peroxisome proliferating 3-thia fatty acids

1. In this study we explored the relationship between specific acyl-CoA esters and induction of acyl-CoA binding protein (ACBP) and enzymes related to the proliferation of peroxisomes. Male Wistar rats were administered a single dose (150 mg/day/kg) of sulphur-substituted fatty acid analogues, and the effects of tetradecylthioacetic acid and 3-thiadicarboxylic acid, which both act as peroxisome proliferators, were compared with the effects of tetradecylthiopropionic acid and palmitic acid which do not induce peroxisome proliferation. 2. The hepatic level of total long-chain acyl-CoA was significantly increased within 12 h of feeding these fatty acids, except in rat fed tetradecylthioacetic acid. Hplc chromatograms of liver extracts prepared from rat fed tetradecylthioacetic acid showed that tetradecylthioacetyl-CoA ester accumulated in the liver 4 h after feeding and had disappeared after 24 h. In liver extracts of the tetradecylthiopropionic acid-treated rat tetradecylthiopropionyl-CoA was not observed, but the appearance of a new long-chain acyl-CoA ester, probably a metabolite of tetradecylthiopropionic acid, was detected. This new peak reached a maximum 4h after feeding. In rat fed tetradecylthioacetic acid and 3-thiadicarboxylic acid the hepatic level of fatty acyl-CoA oxidase mRNA increased 8 h after feeding, while the acyl-CoA oxidase activity had increased after 12 h. 3. The early accumulation of specific tetradecylthioacetyl-CoA suggests that this ester may be a possible mediator of the induction of fatty acyl-CoA oxidase. The level of hepatic acyl-CoA binding protein, long-chain acyl-CoA hydrolase activity and long-chain acyl-CoA synthetase activity did not change after a single dose of all four fatty acids. Prolonged administration of 3-thia fatty acids resulted, however, in a dose- and time-dependent increase in hepatic ACBP content and ACBP mRNA level. The amount of ACBP increased in parallel to the long-chain acyl-CoA hydrolase activity. The correlated induction of fatty acyl-CoA binding protein and long-chain acyl-CoA hydrolase seems to be dependent on a sustained accumulation of total long-chain acyl-CoA esters.

Very long chain and long chain acyl-CoA thioesterases in rat liver mitochondria. Identification, purification, characterization, and induction by peroxisome proliferators

We have previously reported that long chain acyl-CoA thioesterase activity was induced about 10-fold in rat liver mitochondria, when treating rats with the peroxisome proliferator di(2-ethylhexyl)phthalate (Wilcke M., and Alexson S. E. H (1994) Eur. J. Biochem. 222, 803-811). Here we have characterized two enzymes which are responsible for the majority of long chain acyl-CoA thioesterase activity in mitochondria from animals treated with peroxisome proliferators. A 40-kDa enzyme was purified and characterized as a very long chain acyl-CoA thioesterase (MTE-I). The second enzyme was partially purified and characterized as a long chain acyl-CoA thioesterase (MTE-II). MTE-I was inhibited by p-chloromercuribenzoic acid, which implicates the importance of a cysteine residue in, or close, to the active site. Antibodies against MTE-I demonstrated the presence of immunologically related acyl-CoA thioesterases in peroxisomes and cytosol. High expression of MTE-I was found in liver from peroxisome proliferator treated rats and in heart and brown fat from control and induced rats. Comparison of physical and catalytical characteristics of the enzymes studied here and previously purified acyl-CoA thioesterases suggest that MTE-I and MTE-II are novel enzymes.

Species variation in organellar location and activity of L-pipecolic acid oxidation in mammals

The oxidation of L-pipecolic acid to alpha-aminoadipic acid was studied in eight species of mammals using an assay system more sensitive than those previously employed. After percoll-gradient fractionation, activity was localized to the mitochondrial-enriched fractions in tissues from rabbit, guinea pig, pig, dog, and sheep, with guinea pig kidney cortex showing greatest specific activity. These results contrast with the peroxisomal oxidation of L-pipecolic acid observed in macaques and man (Mihalik and Rhead 1989; Mihalik et al. 1989). Rats and mice had undetectable levels of both peroxisomal and mitochondrial L-pipecolic acid oxidation. In the rat, peroxisomal oxidation activity was not induced by feeding with either clofibrate or clofibrate and L-pipecolic acid. Thus, among mammals, both the ability to oxidize L-pipecolic acid and the organellar location of this oxidation is species dependent.

Fatty acid metabolism in liver of rats treated with hypolipidemic sulphur-substituted fatty acid analogues

The purpose of this study was to investigate early biochemical changes and possible mechanisms via which alkyl(C12)thioacetic acid (CMTTD, blocked for beta-oxidation), alkyl(C12)thiopropionic acid (CETTD, undergo one cycle of beta-oxidation) and a 3-thiadicarboxylic acid (BCMTD, blocked for both omega- (and beta-oxidation) influence the peroxisomal beta-oxidation in liver of rats. Treatment of rats with CMTTD caused a stimulation of the palmitoyl-CoA synthetase activity accompanied with increased concentration of hepatic acid-insoluble CoA. This effect was already established during 12-24 h of feeding. From 2 days of feeding, the cellular level of acid-insoluble CoA began to decrease, whereas free CoASH content increased. Stimulation of [1-14C]palmitoyl-CoA oxidation in the presence of KCN, palmitoyl-CoA-dependent dehydrogenase (termed peroxisomal beta-oxidation) and palmitoyl-CoA hydrolase activities were revealed after 36-48 h of CMTTD-feeding. Administration of BCMTD affected the enzymatic activities and altered the distribution of CoA between acid-insoluble and free forms comparable to what was observed in CMTTD-treated rats. It is evident that treatment of peroxisome proliferators (BCMTD and CMTTD), the level of acyl-CoA esters and the enzyme activity involved in their formation precede the increase in peroxisomal and palmitoyl-CoA hydrolase activities. In CMTTD-fed animals the activity of cyanide-insensitive fatty acid oxidation remained unchanged when the mitochondrial beta-oxidation and carnitine palmitoyltransferase operated at maximum rates. The sequence and redistribution of CoA and enzyme changes were interpreted as support for the hypothesis that substrate supply is an important factor in the regulation of peroxisomal fatty acid metabolism, i.e., the fatty acyl-CoA species appear to be catabolized by peroxisomes at high rates only when uptake into mitochondria is saturated. Administration of CETTD led to an inhibition of mitochondrial fatty acid oxidation accompanied with a rise in the concentration of acyl-CoA esters in the liver. Consequently, fatty liver developed. The peroxisomal beta-oxidation was marginally affected. Whether inhibition of mitochondrial beta-oxidation may be involved in regulation of peroxisomal fatty acid metabolism and in development of fatty liver should be considered.

Inhibition of palmitoyl co-enzyme A hydrolase in mitochondria and microsomes by pharmaceutical organic anions

Rat microsomes and mitochondria were isolated and incubated with selected pharmaceutical organic anions at concentrations of 0, 0.2, 0.5, 1.0, and 2 mM. Activity of palmitoyl CoA hydrolase (PCAH) was shown to be reduced in a dose-dependent manner in microsomes by ibuprofen, valproate, acetyl salicylate, 2,4-dichlorophenoxyacetate (2,4-D), and 4-pentenoate, but not salicylate. Mitochondrial PCAH activity was inhibited by clofibrate, ibuprofen, valproate, and 2,4-D. Mitochondrial oxidative phosphorylation was impaired or uncoupled by each of the mitochondrial PCAH inhibitors. The inhibition of PCAH by some of these agents may lead to fatty acyl CoA accumulation. Very low concentrations of fatty acyl CoA are known to cause mitochondrial uncoupling and increase permeability. This action may play a role in the mitochondrial injury caused by some of these agents or related disease processes.

Effects of the tumor promoter 12-O-tetradecanoylphorbol 13-acetate on peroxisomal activities and enzyme activities involved in lipid metabolism in rat liver

The effects of 12-O-tetradecanoylphorbol 13-acetate (TPA) on hepatic lipids and key enzymes involved in esterification, hydrolysis and oxidation of long-chain fatty acids at increasing doses were investigated in rats. TPA administration tended to decrease the mitochondrial activities of palmitoyl-CoA synthetase and carnitine palmitoyltransferase. The microsomal palmitoyl-CoA synthetase activity was increased. TPA administration was also associated with a dose-dependent increase of glycerophosphate acyltransferase activity both in the mitochondrial and microsomal fractions in particular. The data are consistent with a decreased catabolism of long-chain fatty acids at the mitochondrial level, and an increased capacity for esterification of fatty acids in the microsomal fraction. Peroxisomal beta-oxidation was increased about 2-fold in the peroxisome-enriched fraction of TPA-treated rats while the catalase and urate oxidase activities were only marginally affected. TPA administration revealed elevated capacity for hydrolysis of palmitoyl-CoA and palmitoyl-L-carnitine in the microsomal fraction. Neither increased cytosolic palmitoyl-CoA hydrolase activity nor increased hydroxylation of lauric acid nor changes of the hepatic content of cytochrome P-450 isoenzymic forms were observed in the TPA-treated animals. There was no induction of the protein content of the bifunctional enoyl-CoA hydratase. Thus, TPA behaves more like choline-deficient diet and ethionine treatment than well-known peroxisome proliferators. It seems possible that TPA selectively stimulated the peroxisomal activities, i.e., peroxisomal beta-oxidation rather than evoking a peroxisome proliferation capacity.

Beta-oxidation of valproate. II. Effects of fasting, glucose, and clofibrate in rats

The effects of glucose infusion, fasting, and clofibrate pretreatment on valproate (VPA) disposition were investigated in rats to determine the role of endogenous fatty acid beta-oxidation in the metabolic formation of 2-en-VPA. Rats undergoing each treatment received a continuous steady-state infusion of VPA and a single intravenous (i.v.) bolus of 2-en-VPA. Elimination clearance of VPA was significantly higher (median 31%, p = 0.002) with glucose infusion as compared with fasting but was unchanged by clofibrate pretreatment as compared with control. Formation clearance of 2-en-VPA was significantly higher with glucose infusion as compared with fasting (median 147%, p = 0.001) and with clofibrate pretreatment as compared with control (median 73%, p = 0.041). Fractional metabolism of VPA by this route averaged 6% in fasted and control rats and 10% in glucose-infused and clofibrate-pretreated rats. Thus, VPA elimination clearance was not greatly influenced by effects on this route in rats. Elimination clearance of 2-en-VPA was also higher with glucose infusion as compared with fasting (median 149%, p = 0.002), and with clofibrate pretreatment as compared with control (median 167%, p less than 0.001). These observations are consistent with glucose-sparing release of endogenous fatty acids (FAs) to compete with VPA for beta-oxidation, and increased beta-oxidative activity after clofibrate treatment. The results of this study provide strong in vivo evidence for involvement of beta-oxidation in metabolism of VPA.

Alkylthioacetic acid (3-thia fatty acids)--a new group of non-beta-oxidizable, peroxisome-inducing fatty acid analogues. I. A study on the structural requirements for proliferation of peroxisomes and mitochondria in rat liver

The induction of peroxisome proliferation was examined in rat liver after administration of equal concentrations (1 mmol/kg body weight) of 1,10-bis(carboxymethylthiodecane) (BCMTD), 1-mono(carboxymethylthiotetradecane) (CMTTD), 1-mono(carboxymethylthiooctane) (CMTO), 1-mono(carboxyethylthiotetradecane) (CETTD), palmitic acid and hexadecanedioic acid (HDDA). BCMTD, a non-beta-oxidizable and non-omega-oxidizable sulphur-substituted fatty acid analogue was considerably more potent than CMTTD (only non-beta-oxidizable) in inducing enlargement of the liver and increasing peroxisomal activities (monitored by peroxisomal beta-oxidation, palmitoyl-CoA hydrolase and catalase activities). Morphometric analysis of randomly selected hepatocytes revealed that BCMTD and CMTTD treatment increased the number and size of peroxisomes and the relative volume fraction of the peroxisomes. All these cellular responses were more marked with BCMTD than compared with CMTTD. CMTO, a non-beta-oxidizable fatty acid analogue containing a lower hydrophobic alkyl-end than CMTTD and CETTD (a beta-oxidizable fatty acid analogue), showed a slight increase (1.4-1.8-fold) of peroxisomal beta-oxidation and caused marginally morphological changes of peroxisomes compared with CMTTD and BCMTD. The most striking effect of the alkylthiopropionic acid (CETTD) was an enhancement of the hepatic triacylglycerol level. Palmitic acid and hexadecanedioic acid only marginally affected the peroxisomal activities, but no morphological changes of peroxisomes and fat droplets were observed. The presented data strongly suggest that a minimal structural requirement for a peroxisome proliferator may be (1) a carboxylic acid group linked to (2) a hydrophobic backbone which (3) cannot be beta-oxidized i.e., the fatty acid analogues have a sulphur atom in the beta-position. It is also conceivable that blockage for omega-oxidation may potentiate the peroxisome-proliferating activities in as much as BCMTD was more potent than CMTTD. Two mitochondrial marker enzymes, carnitine palmitoyltransferase and succinate phenazine methosulphate oxidoreductase were differently affected after administration of the investigated compounds. Furthermore, BCMTD and CMTTD as well as HDDA treatments increased the number of mitochondria, but the mitochondria tended to be smaller. The overall results presented here indicate that the structural requirements for proliferation of mitochondria are not identical to those for proliferation of peroxisomes.

Ca2+-mediated action of long-chain acyl-CoA on liver mitochondria energy-linked processes

The decrease of steady-state transmembrane potential (delta psi) and loss of accumulated Ca2+ are magnified if palmitoyl-CoA is added to rat liver mitochondria exposed to Ca2+ and phosphate. The extent of this damage increases with increasing concentration of long-chain acyl-CoA. Addition of L-carnitine with or without the addition of palmitoyl-CoA considerably delays the deenergization. In the latter case, there is a substantial decrease in the assayed endogenous long-chain acyl-CoA content. This protective action of L-carnitine is abolished by L-aminocarnitine, a powerful inhibitor of carnitine palmitoyl transferase (palmitoyl-CoA: L-carnitine O-palmitoyltransferase, EC 2.3.1.21.). The removal of Ca2+ by EGTA, or the inhibition of its uptake by Ruthenium red or Mg2+ further enhances the degree of protection.

The induction of liver peroxisomal proliferation by beta,beta'-methyl-substituted hexadecanedioic acid (MEDICA 16)

Treatment of rats by beta,beta'-methyl-substituted hexadecanedioic acid (MEDICA 16) resulted in a dose- and time-dependent increase in liver peroxisomal enoyl-CoA hydratase and cyanide-insensitive palmitoyl-CoA oxidation with a concomitant increase in the volume density of peroxisomes as determined by morphometry. The induced peroxisomal proliferation was sustained as long as treatment was maintained and was accompanied by an increase in liver weight. Incubation of cultured rat hepatocytes in the presence of MEDICA 16 added to the culture medium resulted in a dose-dependent increase in peroxisomal beta-oxidation activities with a concomitant elevation of the volume density of peroxisomes. The induction of peroxisomal proliferation by MEDICA 16 in culture could be prevented in the presence of carnitine palmitoyltransferase inhibitors added to the culture medium, e.g. 2-bromopalmitate, 2-tetradecylglycidic acid or 2-[5-(4-chlorophenyl)-pentyl]oxirane-2-carboxylate. The induction of liver peroxisomes by MEDICA 16 conforms to the previously defined requirement for an amphipathic carboxylate in initiating peroxisomal proliferation. The prevention of peroxisomal proliferation by carnitine acyltransferase inhibitors may implicate the involvement of this acyltransferase in the induction of peroxisomal proliferation by xenobiotic or native amphipathic carboxylates.

Effect of short-term DHEA administration on liver metabolism of lean and obese rats

Lean and obese Zucker rats, 7-8 wk of age, were treated with dehydroepiandrosterone (DHEA) for either 3 or 7 days to determine the initial cellular event(s) that might be responsible for the antiobesity activity of DHEA. Epididymal, retroperitoneal, and brown adipose tissue weights were unaltered by either 3 or 7 days of DHEA treatment. Liver weight was not affected by 3 days of treatment but was 13 and 18% higher in 7-day DHEA-treated lean and obese rats, respectively, compared with their corresponding control group. Mitochondrial state 3 respiration rates with glutamate-malate and succinate as substrates were elevated by an average of 35% in 3- and 7-day DHEA-treated obese rats and by 15-20% in 7-day DHEA-treated lean rats compared with rates obtained in the corresponding control groups. State 3 respiration was not affected in 3-day DHEA-treated lean rats compared with control lean rats. The specific activities of long-chain fatty acyl-coenzyme A synthase and hydrolase and the levels of free CoA were increased by severalfold in cellular fractions of both DHEA-treated lean and obese rats compared with their respective control group. Hepatic malic enzyme activity, which was shown earlier to be elevated with long-term DHEA treatment, was unaltered by either 3 or 7 days of DHEA administration. The above results suggest the involvement of mitochondrial respiration and fatty acid deacylation/reacylation in the antiobesity mechanism of action of DHEA.

Prevention of peroxisomal proliferation by carnitine palmitoyltransferase inhibitors in cultured rat hepatocytes and in vivo

1. The induction of peroxisomal beta-oxidation activities by bezafibrate in cultured rat hepatocytes and in the rat in vivo was prevented by inhibitors of carnitine acyltransferase, e.g. 2-bromopalmitate, 2-[5-(4-chlorophenyl)pentyl]oxirane-2-carboxylate or 2-tetradecylglycidic acid. 2. The prevention of peroxisomal proliferation by carnitine palmitoyltransferase inhibitors could not be accounted for by inhibition of mitochondrial beta-oxidation, since 2-bromo-octanoate, acting as an inhibitor of beta-oxidation, did not prevent the induction of peroxisomal activities in cultured rat hepatocytes. 3. The putative role of the acylcarnitine derivative of bezafibrate was analysed by studying the formation of bezafibroylcarnitine with bezafibroyl-CoA as substrate. However, no bezafibroylcarnitine formation was demonstrated in the presence of rat liver preparations capable of catalysing transfer to carnitine of medium- or long-chain fatty acids. 4. The prevention of peroxisomal proliferation by carnitine acyltransferase inhibitors may help in dissecting the causal relationship between the multiple effects mediated by peroxisomal proliferators.

Induction of cytosolic clofibroyl-CoA hydrolase activity in liver of rats treated with clofibrate

Among subcellular fractions of liver homogenates of rats, the clofibroyl-CoA hydrolase activity is found mainly in the cytosolic fraction. It is here shown that the subcellular distribution of clofibroyl-CoA hydrolase appears to be different from the distribution of palmitoyl-CoA hydrolase activity. Thus, in contrast to the case with palmitoyl-CoA, no hydrolysis of clofibroyl-CoA was catalysed by the microsomal fraction. Furthermore, the hydrolysis of palmitoyl-CoA and clofibroyl-CoA in the cytosolic fraction seemed to be catalyzed by two different enzymes. Rats treated with clofibrate (0.3%, w/w) showed a significant increased clofibroyl-CoA hydrolase activity where the cytosolic hydrolase was increased 3.5-fold. Clofibrate administration also elevated the specific clofibroyl-CoA hydrolase activity by factors of 1.7 and 1.5 in the mitochondrial and the light-mitochondrial fractions, respectively. Thus, it is possible that clofibroyl-CoA hydrolase has also a multiorganelle localization.

Beta-oxidation of 2-ethyl-5-carboxypentyl phthalate in rodent liver

[7-14C]-2-Ethyl-5-carboxypentyl phthalate was isolated and purified from urine of rats given [7-14C]-di-(2-ethylhexyl) phthalate. This metabolite was shown to serve as a precursor for 2-ethyl-3-carboxypropyl phthalate in vivo. 2-Ethyl-5-carboxypentyl phthalate was oxidized to 2-ethyl-3-carboxypropyl phthalate in liver slices from control or, much more rapidly, from clofibrate-pretreated rats. Inhibition by KCN in liver slices from untreated rats, and strong inhibition by acrylate, suggested that formation of 2-ethyl-3-carboxypropyl phthalate involved mitochondrial beta-oxidation. The strong enhancement of the production of this compound by clofibrate (a very weak inducer for mitochondrial dehydrogenases), and strong inhibition by chlorpromazine suggested that peroxisomes may also be able to oxidize 2-ethyl-5-carboxypentyl phthalate. We were able to detect beta-oxidation of 2-ethyl-5-carboxypentyl phthalate to 2-ethyl-3-carboxypropyl phthalate using purified mitochondria, but strong phthalate monoester hydrolase activity observed during incubation of the former compound with purified peroxisomes made it impossible to determine whether 2-ethyl-3-carboxypropyl phthalate could be produced in the latter organelle or not. 2-Ethyl-5-carboxypentyl phthalate was such an inefficient substrate for beta-oxidation compared to palmitic acid that it is unlikely that it contributes significantly to the production of H2O2 in rats chronically exposed to di-(2-ethylhexyl) phthalate. Normal fatty acids are most likely to serve as the dominant substrates for peroxisomal beta-oxidase.

Effect of a high-fat diet with partially hydrogenated fish oil on long-chain fatty acid metabolizing enzymes in subcellular fractions of rat liver

Hepatic metabolism of long-chain fatty acids were studied in young male rats fed a semisynthetic diet containing 20% (w/w) partially hydrogenated fish oil (PHFO)2, with or without 2% (w/w) linoleic acid. The enzymic activities involved in the formation and breakdown of long-chain acyl-CoA were both increased in the animals fed the semisynthetic diet, compared to pellet-fed control animals. Thus, the specific palmitoyl-CoA synthetase activity increased slightly in both the mitochondrial (1.4-fold) and the microsomal (1.6-fold) fractions. In the peroxisome-enriched fraction the activity was increased (about 2.6-fold) only on addition of linoleic acid to the diet. The data are consistent with an increased catabolism of long-chain fatty acids by a peroxisomal and a mitochondrial pathway. Thus, the total carnitine palmitoyltransferase activity increased 2-fold in the mitochondrial fraction, and was partly prevented by added linoleic acid. Peroxisomal beta-oxidation activity was also increased (about 7-fold) in livers of PHFO-fed rats, but did not change when linoleic acid was added. The PHFO-fed rats also revealed elevated capacity for hydrolysis of palmitoyl-CoA in both the mitochondrial (2.4-fold) and the cytosolic (2.0-fold) fractions and the latter was almost completely and selectively prevented by added linoleic acid. The s values of mitochondria and peroxisomes varied with the dietary regime, and some of the observed changes in the specific activities of the fatty acid metabolizing enzymes with multiple subcellular localization can be explained as an effect of changes in the s values of the organelles. Thus, the s value of mitochondria increased 1.8-fold as a result of PHFO feeding, but was fully prevented by linoleic acid in the diet. On the other hand, the s values of peroxisomes decreased by about 50% on feeding a PHFO diet, and by about 25% with added linoleic acid.

Involvement of long-chain acyl CoA in the antagonistic effects of halothane and L-carnitine on mitochondrial energy-linked processes

Incubation of rat liver mitochondria in the presence of halothane induced a consistent impairment of mitochondrial oxidative phosphorylation without significantly affecting the steady-state of transmembrane electrical potential. These alterations of mitochondrial energy-linked processes were associated with a consistent accumulation of long-chain acyl CoA. Addition of L-carnitine partially prevented the effects of halothane on oxidative phosphorylation and completely abolished the halothane-induced long-chain acyl CoA accumulation. The possibility is discussed that the damaging action of halothane on mitochondrial functions might be partially ascribed to the noxious action of the excess of long-chain acyl CoA induced the anesthetic.

The tumour promoter 12-O-tetradecanoylphorbol-13-acetate increases the activities of some peroxisome-associated enzymes in in vitro cell culture

A study was conducted on the effects of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on peroxisomal enzyme activities in mouse embryo fibroblasts C3H/10T1/2 C18 cells and chemically transformed C3H/10T1/2 MCA16 cells. TPA is a potent tumour promoter and treatment with this compound of the two cell lines induced peroxisomal fatty acid beta-oxidation, carnitine acetyltransferase, palmitoyl-CoA hydrolase, and catalase activities after 240 h of treatment. Stimulation of the corresponding enzyme activities was dose-related and cycloheximide inhibited the TPA-induced enzyme activities, except that of carnitine acetyltransferase. The MCA16 cells appeared to be more sensitive than the C18 cells in inducing peroxisome-associated enzyme activities after TPA treatment. The activities of the microsomal marker, NADPH-cytochrome c reductase and the mitochondrial marker, glutamate dehydrogenase were not enhanced by TPA treatment. The results indicate that TPA has peroxisomal effects and may be classified as a peroxisome proliferator.

Identity of purified monoacylglycerol lipase, palmitoyl-CoA hydrolase and aspirin-metabolizing carboxylesterase from rat liver microsomal fractions. A comparative study with enzymes purified in different laboratories

Two purified carboxylesterases that were isolated from a rat liver microsomal fraction in a Norwegian and a German laboratory were compared. The Norwegian enzyme preparation was classified as palmitoyl-CoA hydrolase (EC 3.1.2.2) in many earlier papers, whereas the German preparation was termed monoacylglycerol lipase (EC 3.1.1.23) or esterase pI 6.2/6.4 (non-specific carboxylesterase, EC 3.1.1.1). Antisera against the two purified enzyme preparations were cross-reactive. The two proteins co-migrate in sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Both enzymes exhibit identical inhibition characteristics with Mg2+, Ca2+ and bis-(4-nitrophenyl) phosphate if assayed with the two substrates palmitoyl-CoA and phenyl butyrate. It is concluded that the two esterase preparations are identical. However, immunoprecipitation and inhibition experiments confirm that this microsomal lipase differs from the palmitoyl-CoA hydrolases of rat liver cytosol and mitochondria.

Correlation between the cellular level of long-chain acyl-CoA, peroxisomal beta-oxidation, and palmitoyl-CoA hydrolase activity in rat liver. Are the two enzyme systems regulated by a substrate-induced mechanism?

Data obtained in earlier studies with rats fed diets containing high doses of peroxisome proliferators (niadenate, tiadenol, clofibrate, or nitotinic acid) are used to look for a quantitative relationship between peroxisomal beta-oxidation, palmitoyl-CoA hydrolase, palmitoyl-CoA synthetase and carnitine palmitoyltransferase activities, and the cellular concentration of their substrate and reaction products. The order of the hyperlipidemic drugs with regard to their effect on CoA derivatives and enzyme activities was niadenate greater than tiadenol greater than clofibrate greater than nicotinic acid. Linear regression analysis of long-chain acyl-CoA content versus palmitoyl-CoA hydrolase and peroxisomal beta-oxidation activity showed highly significant linear correlations both in the total liver homogenate and in the peroxisome-enriched fractions. A dose-response curve of tiadenol showed that carnitine palmitoyltransferase and palmitoyl-CoA synthetase activities and the ratio of long-chain acyl-CoA to free CoASH in total homogenate rose at low doses before detectable changes occurred in the peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity. A plot of this ratio parallelled the palmitoyl-CoA synthetase activity. The specific activity of microsomally localized carnitine palmitoyl-transferase was low and unchanged up to a dose where no enhanced peroxisomal beta-oxidation was observed, but over this dose the activity increased considerably so that the specific of the enzyme in the mitochondrial and microsomal fractions became comparable. The mitochondrial palmitoyl-CoA synthetase activity decreased gradually. The correlations may be interpreted as reflecting a common regulation mechanism for palmitoyl-CoA hydrolase and peroxisomal beta-oxidation enzymes, i.e., the cellular level of long-chain acyl-CoA acting as the metabolic message for peroxisomal proliferation resulting in induction of peroxisomal beta-oxidation and palmitoyl-CoA hydrolase activity. The findings are discussed with regard to their possible consequences for mitochondrial fatty acid oxidation and the conversion of long-chain acyl-L-carnitine to acyl-CoA derivatives.

Purification and characterization of a fatty acyl-CoA hydrolase from the uropygial glands of Peking ducks (Anas domesticus)

In a previous communication the occurrence of a medium-chain acyl-CoA hydrolase designated thioesterase-B in the uropygial gland of mallard ducks was reported [L. Rogers, P. E. Kolattukudy, and M. J. de Renobales (1982) J. Biol. Chem. 257, 880-886]. In the present study, thioesterase-B was purified from the postmicrosomal supernatant of homogenized uropygial glands from Peking ducks (Anas domesticus). Most of the contaminating thioesterase activities were removed by ammonium sulfate fractionation. The 55% ammonium sulfate supernatant, containing thioesterase-B, was chromatographed on hydroxylapatite followed by gel filtration on Sephadex G-100. The remaining contaminants were removed by chromatofocusing followed by desalting on Sephadex G-75. This procedure gave a 26% yield with a nearly 200-fold purification. Gel filtration of the purified enzyme showed that the molecular weight of the native enzyme was 56,300, whereas sodium dodecyl sulfate-gel electrophoresis of components separated by chromatofocusing showed that the purified enzyme contained enzymatically active proteins of molecular weights 59,400, 58,300, 56,000, and 55,800. The four species differed slightly in pI (4.9, 4.7, 4.45, and 4.40) but they were kinetically and immunologically indistinguishable. All four had the same N-terminal sequence. The purified thioesterase preparation showed a pH optimum of 9.3 with C12-CoA but the pH optimum was dependent on the chain length of the acyl group. At pH 8.0, C10 was the preferred substrate with less activity on C12, C8, and C14. The enzymatic activity was stimulated by bovine serum albumin and was inhibited by p-hydroxymercuribenzoate. Involvement of active serine in catalysis was suggested by inhibition of the enzyme by diethylpyrocarbonate, diisopropylfluorophosphate and phenylmethylsulfonyl fluoride.

Effects of high fat diets on the activity of palmitoyl-CoA hydrolase in rat liver

Palmitoyl-CoA hydrolase [EC 3.1.2.2.] activity in rat liver was found to be enhanced by high fat diets. Partially hydrogenated marine oil and high-erucic acid rapeseed oil diets produced a greater increase than a diet containing soybean oil. With diets containing from 5 to 30% (w/w) of partially hydrogenated marine oil the increase in palmitoyl-CoA hydrolase activity was similar to the increase observed in peroxisomal beta-oxidation activity (correlation coefficient r = 0.94). A positive correlation (r = 0.86) also was observed between the activity of palmitoyl-CoA hydrolase and previously determined levels of long-chain acyl-CoA. The results presented may suggest a common "induction" mechanism for palmitoyl-CoA hydrolase and peroxisomal beta-oxidation enzymes, possibly exerted through an increased cellular level of long-chain acyl-CoA.

Induction of peroxisomal enzymes and palmitoyl-CoA hydrolase in rats treated with cholestyramine and nicotinic acid

Male Wistar rats were given 200 mg/kg/day nicotinic acid or 1000 mg/kg/day cholestyramine by stomach tube for ten days. Peroxisomal palmitoyl-CoA oxidation (cyanide-insensitive) and the activities of palmitoyl-CoA hydrolase and urate oxidase were significantly increased in the total liver homogenate. Subcellular fractionation showed enhanced enzyme activities after drug treatment mainly in the peroxisome-containing fractions. The increase in urate oxidase activity and its subcellular distribution suggest that the tested drugs induce core-containing peroxisomes. The findings are similar to those previously reported with low doses of peroxisome-proliferating hypolipidemic drugs and with acetylsalicylic acid, a drug which is structurally similar to nicotinic acid. Since cholestyramine is not absorbed, its influence on hepatic enzymes probably occurs indirectly as a consequence of enhanced catabolism of cholesterol.

Enhancement of long-chain acyl-CoA hydrolase activity in peroxisomes and mitochondria of rat liver by peroxisomal proliferators

The present study has confirmed previous findings of long-chain acyl-CoA hydrolase activities in the mitochondrial and microsomal fractions of the normal rat liver. In addition, experimental evidence is presented in support of a peroxisomal localization of long-chain acyl-CoA hydrolase activity. (a) Analytical differential centrifugation of homogenates from normal rat liver revealed that this activity (using palmitoyl-CoA as the substrate) was also present in a population of particles with an average sedimentation coefficient of 6740 S, characteristic of peroxisomal marker enzymes. (b) The subcellular distribution of the hydrolase activity was greatly affected by administration of the peroxisomal proliferators clofibrate and tiadenol. The specific activity was enhanced in the mitochondrial fraction and in a population of particles with an average sedimentation coefficient of 4400 S, characteristic of peroxisomal marker enzymes. Three populations of particles containing lysosomal marker enzymes were found by analytical differential centrifugation, both in normal and clofibrate-treated rats. Our data do not support the proposal that palmitoyl-CoA hydrolase and acid phosphatase belong to the same subcellular particles. In livers from rats treated with peroxisomal proliferators, the specific activity of palmitoyl-CoA hydrolase was also enhanced in the particle-free supernatant. Evidence is presented that this activity at least in part, is related to the peroxisomal proliferation.

Enzymatic changes in rat liver associated with low and high doses of a peroxisome proliferator

The activities of a number of lipid-metabolizing and subcellular marker enzymes were measured in total homogenates and subcellular fractions prepared from the livers of male rats fed diets containing 0.05, 0.1, 0.3, and 0.5% of the hypolipidemic drug tiadenol, resulting in mean drug intake of 45, 90, 330, and 530 mg/day/kg body wt, respectively. In the total homogenates, a massive induction of palmitoyl-CoA hydrolase and peroxisomal palmitoyl-CoA oxidation accompanied by increased free CoASH and long-chain acyl-CoA content was observed at the highest dose levels whereas little change occurred up to 90 mg/day/kg/body wt. The palmitoyl-CoA synthetase activity increased slightly up to 90 mg/day/kg body wt, but higher doses resulted in decreased enzyme activity. Catalase activity increased with the dose to be elevated by a factor of approximately 1.6 at 330 mg/day/kg, whereas the activities of urate oxidase decreased. The specific activities of palmitoyl-CoA hydrolase and peroxisomal palmitoyl-CoA oxidation increased in all fractions, but most markedly in the cytosol. The changes in the activities and the distribution of subcellular marker enzymes and the increase of the peroxisome-associated polypeptide (PPA-80) are in keeping with a peroxisome proliferating effect resulting in formation of premature organelles with altered properties. Since high doses of many hypolipidemic drugs produce hepatic tumors and peroxisomal proliferation in rodents and since no increase in peroxisomes is found in human liver after therapeutic use of lower doses, the dose-response relationship is of interest for the evaluation of the toxicology of this class of agents.

Long-chain Acyl-CoA levels in liver from rats fed high-fat diets: is it of significance for an increased peroxisomal beta-oxidation?

The levels of long-chain acyl-CoA in the livers of rats given diets containing various amounts of dietary oils were investigated. Increasing the amount of soybean oil in the diet from 5% to 25% (w/w) led to a 40% increase in long-chain acyl-CoA. With partially hydrogenated marine oil, a sigmoidal dose-response curve was obtained, giving a 60% increase when 20% or more of this oil was in the diet. All high-fat diets tested resulted in higher levels of long-chain acyl-CoA than the low-fat control containing soybean oil. The increase was most prominent with partially hydrogenated marine and rapeseed oils. With diets containing partially hydrogenated marine oil, the ratio of long-chain acyl-CoA to acid-soluble CoA was increased after 3 days, but decreased after 3 weeks, to a value similar to that observed in animals fed soybean oil because of an extensive increase in acid-soluble CoA. Increased levels of long-chain acyl-CoA were also observed after clofibrate was administered, but the increase was less prominent than observed with high-fat diets. When comparing the levels of long-chain acyl-CoA observed after 3 days on different diets with the peroxisomal beta-oxidation activity previously determined after 3 weeks on the corresponding diets, a straight line was obtained. These results are discussed in relation to the possibility that long-chain acyl-CoA induces peroxisomal beta-oxidation activity.

Influence of dietary status on liver palmitoyl-CoA hydrolase, peroxisomal enzymes, CoASH and long-chain acyl-CoA in rats

In the livers of fasted rats, the activity of mitochondrial palmitoyl-CoA hydrolase was increased whereas the microsomal palmitoyl-CoA hydrolase activity decreased. Refeeding with a high-carbohydrate diet (glucose), the corresponding enzyme activities were decreased while refeeding with a high-fat diet (sheep tallow) increased the enzyme activities over the control values. The increased content of long-chain acyl-CoA and free CoASH under fasting and fat-refeeding was mainly attributed to the mitochondrial fraction with the remainder in the light mitochondrial fraction which contains peroxisomes. The results suggest a correlation of the compartmentation of the palmitoyl-CoA hydrolase and the content and compartmentation of the CoA derivatives in the liver under different nutritional states. The peroxisomal palmitoyl-CoA oxidase activity was increased by fasting. Fat-refeeding increased the activity even more; 1.8-fold as compared to the fasting animals. On the other hand, the activities of other peroxisomal enzymes which are not directly involved in the fatty acid metabolism such as urate oxidase were decreased to approximately the same extent by fasting. Re-feeding with glucose and fat further decreased the corresponding enzyme activity, particularly seen in the glucose-refed group.

The existence of separate peroxisomal pools of free coenzyme a and long-chain acyl-CoA in rat liver, demonstrated by a specific high performance liquid chromatography method

1. Tiadenol administration of rats lead to an increased hepatic content of unesterified and esterified CoA. 2. Liver homogenates from normal tiadenol treated rats were fractionated by differential centrifugation and fractions enriched in peroxisomes were subfractionated by isopycnic density gradient centrifugation. 3. The analysis demonstrated that purified peroxisomes contained a separate pool of free CoASH and long-chain acyl-CoA. 4. The data also provides indications of the presence of palmitoyl-CoA synthetase in the peroxisomes.