Induction of a novel long-chain acyl-CoA hydrolase in rat liver by administration of peroxisome proliferators

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

Cholesterol-lowering effect of bezafibrate is independent of peroxisome proliferator-activated receptor activation in mice

The hypocholesterolemic potential of peroxisome proliferator-activated receptor (PPAR) pan-activator bezafibrate has been documented. However, in addition to uncertainty about the contribution of PPAR alpha to its effect, there is a marked discrepancy in bezafibrate dosages used in previous rodent experiments (> or = 50 mg/kg/day) and those in clinical use (< or = 10 mg/kg/day). To investigate the association between bezafibrate-induced cholesterol reduction and PPAR alpha activation, wild-type and Ppar a-null mice were treated with bezafibrate at high (100 mg/kg/day) or low (10 mg/kg/day) doses and analyzed. High-dose treatment decreased hepatic cholesterol content in wild-type mice, but increased serum cholesterol concentration. In liver samples, simultaneous increases in the expression of numerous proteins involved in cholesterol biosynthesis and catabolism, as well as cholesterol influx and efflux, were observed, which made interpretation of phenotype changes subtle. These complicated responses were believed to be associated with intensive PPAR activation and accompanying up-regulation of liver X receptor alpha, farnesoid X receptor, and sterol regulatory element-binding protein 2 (SREBP2). In contrast, low-dose bezafibrate treatment decreased serum and hepatic cholesterol concentrations in a PPAR alpha-independent manner, probably from suppression of SREBP2-regulated cholesterogenesis and enhancement of cholesterol catabolism due to elevated 7alpha-hydroxylase levels. Interestingly, the low-dose treatment did not affect the expression of PPAR target genes or number of peroxisomes, suggesting the absence of PPAR activation. These results demonstrate that the action of bezafibrate on cholesterol metabolism may vary with dosage, and that the cholesterol-reducing effect found in mice at dosages similar to those administered to humans is independent of significant PPAR activation.

Structural basis for recruitment of tandem hotdog domains in acyl-CoA thioesterase 7 and its role in inflammation

Acyl-CoA thioesterases (Acots) catalyze the hydrolysis of fatty acyl-CoA to free fatty acid and CoA and thereby regulate lipid metabolism and cellular signaling. We present a comprehensive structural and functional characterization of mouse acyl-CoA thioesterase 7 (Acot7). Whereas prokaryotic homologues possess a single thioesterase domain, mammalian Acot7 contains a pair of domains in tandem. We determined the crystal structures of both the N- and C-terminal domains of the mouse enzyme, and inferred the structure of the full-length enzyme using a combination of chemical cross-linking, mass spectrometry, and molecular modeling. The quaternary arrangement in Acot7 features a trimer of hotdog fold dimers. Both domains of Acot7 are required for activity, but only one of two possible active sites in the dimer is functional. Asn-24 and Asp-213 (from N- and C-domains, respectively) were identified as the catalytic residues through site-directed mutagenesis. An enzyme with higher activity than wild-type Acot7 was obtained by mutating the residues in the nonfunctional active site. Recombinant Acot7 was shown to have the highest activity toward arachidonoyl-CoA, suggesting a function in eicosanoid metabolism. In line with the proposal, Acot7 was shown to be highly expressed in macrophages and up-regulated by lipopolysaccharide. Overexpression of Acot7 in a macrophage cell line modified the production of prostaglandins D2 and E2. Together, the results link the molecular and cellular functions of Acot7 and identify the enzyme as a candidate drug target in inflammatory disease.

Identification of fatty acid oxidation disorder patients with lowered acyl-CoA thioesterase activity in human skin fibroblasts

BACKGROUND

Acyl-CoA thioesterases are enzymes that hydrolyze acyl-CoAs to the free fatty acid and coenzyme A (CoASH). These enzymes have been identified in several cellular compartments and are thought to regulate intracellular levels of acyl-CoAs, free fatty acids and CoASH. However, to date no patients deficient in acyl-CoA thioesterases have been identified.

DESIGN

Acyl-CoA thioesterase activity was measured in human skin fibroblasts. Western-blot analysis was used to determine Type-II acyl-CoA thioesterase protein levels in patients.

RESULTS

Acyl-CoA thioesterase activity was found in human fibroblasts with all saturated acyl-CoAs from C4-CoA to C18-CoA, with highest activity detected with lauroyl-CoA and myristoyl-CoA (C12-CoA and C14-CoA). An antibody that recognizes the major isoforms of Type-II acyl-CoA thioesterases precipitated the majority of acyl-CoA thioesterase activity in fibroblasts, showing that the main thioesterase activity detected in fibroblasts is catalyzed by Type-II thioesterases. Measurement of acyl-CoA thioesterase activity from fibroblasts of 34 patients with putative fatty acid oxidation disorders resulted in the identification of three patients with lowered Type-II acyl-CoA thioesterase activity in fibroblasts. These patients also had lowered expression of Type-II acyl-CoA thioesterase protein in fibroblasts as judged by Western-blot analysis. However, mutation analysis failed to identify any mutation in the coding sequences for the mitochondrial acyl-CoA thioesterase II (MTE-II) or the cytosolic acyl-CoA thioesterase II (CTE-II).

CONCLUSIONS

We have described three patients with lowered Type-II acyl-CoA thioesterase protein and activity in human skin fibroblasts, which is the first description of patients with a putative defect in acyl-CoA thioesterases.

The expression of cytosolic and mitochondrial type II acyl-CoA thioesterases is upregulated in the porcine corpus luteum during pregnancy

Acyl-CoA thioesterases hydrolyze acyl-CoAs to free fatty acids and CoASH, thereby regulating fatty acid metabolism. This activity is catalyzed by numerous structurally related and unrelated enzymes, of which several acyl-CoA thioesterases have been shown to be regulated via the peroxisome proliferator-activated receptor alpha, strongly linking them to fatty acid metabolism. Two protein families have recently been characterized, the type I acyl-CoA thioesterase gene family and the type II protein family, which are expressed in cytosol, mitochondria and peroxisomes. Still, only little is known about regulation of their expression and precise functions in vivo. In the present study, we have investigated the activity and expression of acyl-CoA thioesterase in the porcine ovary during different phases of the estrus cycle. The activity was low in homogenates obtained during the immature and follicular phases, increasing nearly 4-fold during the luteal phase, with the highest activity being found in the pregnant corpus luteum (about 7-fold higher than in immature follicles). The increase in homogenate activity in corpus luteum from pregnant pigs was due to a moderate increase in the cytosolic activity, and an approximately 20-25-fold increase in the mitochondrial fraction. Western blot analysis showed no detectable expression of the type I acyl-CoA thioesterases (CTE-I and MTE-I) and revealed that the increased activity in cytosol and mitochondria is due to increased expression of the type II acyl-CoA thioesterases (CTE-II and MTE-II). This apparent hormonal regulation of expression of the type II acyl-CoA thioesterase may provide new insights into the functions of these enzymes in the mammalian ovary.

Profiling of hepatic gene expression in rats treated with fibric acid analogs

Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors whose ligands include fatty acids, eicosanoids and the fibrate class of drugs. In humans, fibrates are used to treat dyslipidemias. In rodents, fibrates cause peroxisome proliferation, a change that might explain the observed hepatomegaly. In this study, rats were treated with multiple dose levels of six fibric acid analogs (including fenofibrate) for up to two weeks. Pathological analysis identified hepatocellular hypertrophy as the only sign of hepatotoxicity, and only one compound at the highest dose caused any significant increase in serum ALT or AST activity. RNA profiling revealed that the expression of 1288 genes was related to dose or length of treatment and correlated with hepatocellular hypertrophy. This gene list included expression changes that were consistent with increased mitochondrial and peroxisomal beta-oxidation, increased fatty acid transport, increased hepatic uptake of LDL-cholesterol, decreased hepatic uptake of glucose, decreased gluconeogenesis and decreased glycolysis. These changes are likely linked to many of the clinical benefits of fibrate drugs, including decreased serum triglycerides, decreased serum LDL-cholesterol and increased serum HDL-cholesterol. In light of the fact that all six compounds stimulated similar or identical changes in the expression of this set of 1288 genes, these results indicate that hepatomegaly is due to PPARalpha activation, although signaling through other receptors (e.g. PPARgamma, RXR) or through non-receptor pathways cannot be excluded.

Human brain acyl-CoA hydrolase isoforms encoded by a single gene

Acyl-CoA hydrolases are a group of enzymes that catalyze the hydrolysis of acyl-CoA thioesters to free fatty acids and CoA-SH. The human brain acyl-CoA hydrolase (BACH) gene comprises 13 exons, generating several isoforms through the alternative use of exons. Four first exons (1a-1d) can be used, and three patterns of splicing occur at exon X located between exons 7 and 8 that contains an internal 3(')-splice acceptor site and creates premature stop codons. When examined with green fluorescent protein-fusion constructs expressed in Neuro-2a cells, the nuclear localization signal encoded by exon 9 was functional by itself, whereas the whole structure was cytosolic, suggesting nuclear translocation of the enzyme. This was consistent with dual staining of the cytosol and nucleus in certain neurons by immunohistochemistry using anti-BACH antibody. The mitochondrial targeting signals encoded by exons 1b and 1c were also functional and directed mitochondrial localization of BACH isoforms with the signals. Although BACH mRNA containing the sequence derived from exon 1a, but not exon X, was exclusively expressed in human brain, these results suggest that the human BACH gene can express long-chain acyl-CoA hydrolase activity in multiple intracellular compartments by generating BACH isoforms with differential localization signals to affect various cellular functions that involve acyl-CoAs.

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.

The peroxisome proliferator-induced cytosolic type I acyl-CoA thioesterase (CTE-I) is a serine-histidine-aspartic acid alpha /beta hydrolase

Long-chain acyl-CoA thioesterases hydrolyze long-chain acyl-CoAs to the corresponding free fatty acid and CoASH and may therefore play important roles in regulation of lipid metabolism. We have recently cloned four members of a highly conserved acyl-CoA thioesterase multigene family expressed in cytosol (CTE-I), mitochondria (MTE-I), and peroxisomes (PTE-Ia and -Ib), all of which are regulated via the peroxisome proliferator-activated receptor alpha (Hunt, M. C., Nousiainen, S. E. B., Huttunen, M. K., Orii, K. E., Svensson, L. T., and Alexson, S. E. H. (1999) J. Biol. Chem. 274, 34317-34326). Sequence comparison revealed the presence of putative active-site serine motifs (GXSXG) in all four acyl-CoA thioesterases. In the present study we have expressed CTE-I in Escherichia coli and characterized the recombinant protein with respect to sensitivity to various amino acid reactive compounds. The recombinant CTE-I was inhibited by phenylmethylsulfonyl fluoride and diethyl pyrocarbonate, suggesting the involvement of serine and histidine residues for the activity. Extensive sequence analysis pinpointed Ser(232), Asp(324), and His(358) as the likely components of a catalytic triad, and site-directed mutagenesis verified the importance of these residues for the catalytic activity. A S232C mutant retained about 2% of the wild type activity and incubation with (14)C-palmitoyl-CoA strongly labeled this mutant protein, in contrast to wild-type enzyme, indicating that deacylation of the acyl-enzyme intermediate becomes rate-limiting in this mutant protein. These data are discussed in relation to the structure/function of acyl-CoA thioesterases versus acyltransferases. Furthermore, kinetic characterization of recombinant CTE-I showed that this enzyme appears to be a true acyl-CoA thioesterase being highly specific for C(12)-C(20) acyl-CoAs.

Characterization of mouse homolog of brain acyl-CoA hydrolase: molecular cloning and neuronal localization

Acyl-CoA hydrolase could provide a mechanism via its potency to modulate cellular concentrations of acyl-CoAs for the regulation of various cellular events including fatty acid metabolism and gene expression. However, only limited evidence of this is available. To better understand the physiological role of this enzyme, we characterized a mouse brain acyl-CoA hydrolase, mBACH. The cloned cDNA for mBACH encoded a 338-amino-acid polypeptide with >95% identity to the human and rat homologs, indicating that the BACH gene is highly conserved among species. This was supported by the similarity in genomic organization of the BACH gene between humans and mice. Bacterially expressed mBACH was highly active against long-chain acyl-CoAs with a relatively broad specificity for chain length. While palmitoyl-CoA hydrolase activity was widely distributed in mouse tissues, it was marked in the brain, consistent with mBACH being almost exclusively distributed in this tissue, where >80% of the enzyme activity was explained by mBACH present in the cytosol. Immunohistochemistry demonstrated a neuronal localization of mBACH in both the central and peripheral nervous systems. In neurons, mBACH was distributed throughout the cell body and neurites. Although four isoforms except mBACH itself, that may be generated by the alternative use of exons of a single mBACH gene, were cloned, their mRNA levels in the brain were estimated to be negligible. However, a 50-kDa polypeptide besides the major one of 43-kDa seemed to be translated from the mBACH mRNA with differential in-frame ATG triplets used as the initiation codon. These findings will contribute to the functional analysis of the BACH gene using mice including genetic studies.

Cloning and regulation of peroxisome proliferator-induced acyl-CoA thioesterases from mouse liver

1.1. Acyl-CoA thioesterases hydrolyze acyl-CoAs to the corresponding free fatty acid plus CoASH. The activity is strongly induced in rat and mouse liver after feeding the animals peroxisome proliferators. To elucidate the role of these enzymes in lipid metabolism, we have cloned the cDNAs corresponding to the inducible cytosolic and mitochondrial type I enzymes (CTE-I and MTE-I) and studied tissue expression and nutritional regulation of expression of the mRNAs in mice. The constitutive expression of both mRNAs was low in liver, with CTE-I being expressed mainly in kidney and brown adipose tissue and MTE-I expressed in brown adipose tissue and heart. As expected, the expression in liver of both the CTE-I and MTE-I mRNAs was strongly induced (> 50-fold) by treatment with clofibrate. A similar level of induction was observed by fasting and a time-course study showed that both mRNAs were increased already at 6 hours after removal of the diet. Refeeding normal chow diet to mice fasted for 24 hours normalized the mRNA levels with a T1/2 of about 3-4 hours. Feeding mice a fat-free diet further decreased the expression, possibly indicating repression of expression. The strong expression of MTE-I and CTE-I in the heart was increased about 10-fold by fasting. To further characterize these highly regulated enzymes, we have cloned the corresponding genes and promoter regions. The structures of the two genes were found to be very similar, consisting of three exons and two introns. Exon-intron borders conform to general consensus sequences and especially the first exon appears to be highly conserved. The promoter regions of both the CTE-I and MTE-I genes contain putative peroxisome proliferator response elements (PPREs), suggesting an involvement of peroxisome proliferator-activated receptors in the regulation of these genes.

Sex-dependent regulation of hepatic peroxisome proliferation in mice by trichloroethylene via peroxisome proliferator-activated receptor alpha (PPARalpha)

The mechanism of trichloroethylene-induced liver peroxisome proliferation and the sex difference in response was investigated using wild-type Sv/129 and peroxisome proliferator-activated receptor alpha (PPARalpha)-null mice. Trichloroethylene treatment (0.75 g/kg for 2 weeks by gavage) resulted in liver peroxisome proliferation in wild-type mice, but not in PPARalpha-null mice, suggesting that trichloroethylene-induced peroxisome proliferation is primarily mediated by PPARalpha. No remarkable sex difference was observed in induction of peroxisome proliferation, as measured morphologically, but a markedly higher induction of several enzymes and PPARalpha protein and mRNA was found in males. On the other hand, trichloroethylene induced liver cytochrome P450 2E1, the principal enzyme responsible for metabolizing trichloroethylene to chloral hydrate, only in males, which resulted in similar expression levels in both sexes after the treatment. Trichloroethylene influenced neither the level of catalase, an enzyme involved in the reduction of oxidative stress, nor aldehyde dehydrogenase, the main enzyme catalyzing the conversion to trichloroacetic acid. These results suggest that trichloroethylene treatment causes a male-specific PPARalpha-dependent increase in cellular oxidative stress.

Peroxisome proliferator-induced long chain acyl-CoA thioesterases comprise a highly conserved novel multi-gene family involved in lipid metabolism

Long chain acyl-CoA esters are important intermediates in degradation and synthesis of fatty acids, as well as having important functions in regulation of intermediary metabolism and gene expression. Although the physiological functions for most acyl-CoA thioesterases have not yet been elucidated, previous data suggest that these enzymes may be involved in lipid metabolism by modulation of cellular concentrations of acyl-CoAs and fatty acids. In line with this, we have cloned four highly homologous acyl-CoA thioesterase genes from mouse, showing multiple compartmental localizations. The nomenclature for these genes has tentatively been assigned as CTE-I (cytosolic), MTE-I (mitochondrial), and PTE-Ia and Ib (peroxisomal), based on the identification of putative targeting signals. Although the various isoenzymes show between 67% and 94% identity at amino acid level, each individual enzyme shows a specific tissue expression. Our data suggest that all four genes are located within a very narrow cluster on chromosome 12 in mouse, similar to a sequence cluster on human chromosome 14, which identified four genes homologous to the mouse thioesterase genes. Four related genes were also identified in Caenorhabditis elegans, all containing putative PTS1 targeting signals, suggesting that the ancestral type I thioesterase gene(s) is/are of peroxisomal origin. All four thioesterases are differentially expressed in tissues examined, but all are inducible at mRNA level by treatment with the peroxisome proliferator clofibrate, or during the physiological condition of fasting, both of which conditions cause a perturbation in overall lipid homeostasis. These results strongly support the existence of a novel multi-gene family cluster of mouse acyl-CoA thioesterases, each with a distinct function in lipid metabolism.

Evidence for the involvement of the fatty acid and peroxisomal beta-oxidation pathways in the inhibition by dehydroepiandrosterone (DHEA) and induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benz(a)anthracene (BA) of cytochrome P4501B1 (CYP1B1) in mouse embryo fibroblasts (C3H10T1/2 cells)

Treatment of intact C3H10T1/2 cells or microsomes therefrom with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzanthracene (BA) enhanced CYP1B1 activity and CYP1B1 expression as revealed by elevations of CYP1B1-catalyzed DMBA metabolism, CYP1B1 apoprotein level and CYP1B1 gene expression. One hundred microM DHEA caused an 80-90% inhibition of cellular DMBA metabolism without inflicting cell death. Cytosolic glucose-6-phosphate dehydrogenase (G6PDH) was also inhibited in DHEA-treated cells, presumably due to the inhibition of NADP reduction. In contrast, neither DMBA metabolism nor CYP1B1 apoprotein was inhibited by DHEA in the microsomes isolated from these cells. DHEA (100 microM), TCDD (10 nM) and BA (10 microM) stimulated the activities and increased the apoprotein levels of two peroxisomal enzymes, namely, acyl CoA oxidase (ACOX) and acyl CoA hydrolase (ACH2) and also induced the expression of CYP1B1 and ACOX genes. Cytosolic fatty acyl-CoA beta-oxidation was also stimulated by DHEA, TCDD and BA. In corroboratory experiments, it was found that concomitant with the stimulation of the activity of a key enzyme regulator of fatty acid homeostasis, namely, glycerol-3-phosphate dehydrogenase (G3PDH), these agents enhanced arachidonic acid (AA) metabolism as judged by the release of [3H] from AA into the culture medium. Collectively, these data suggest that DHEA mediates the regulation of CYP1B1 and inhibits BA and TCDD-induced CYP1B1-catalyzed carcinogen (DMBA) activation in 10T1/2 cells through metabolic interactions that involve the activation of the peroxisomal and fatty acid beta-oxidation signaling pathways. These results also present evidence for the first time, for the possible peroxisomal effects of TCDD and BA which are similar to those of DHEA in this mouse embryo fibroblast cell line.

Existence of acyl-CoA hydrolase-mediated pathway supplying arachidonic acid for prostaglandin synthesis in microsomes from rabbit kidney medulla

We have previously shown that acyl-coenzyme A (CoA) hydrolase that hydrolyzes arachidonoyl-CoA (AA-CoA) to arachidonic acid (AA) and CoA is present in the cytosol of rabbit kidney medulla and that this enzyme can supply AA for prostaglandin (PG) synthesis in this region. In the present study, the existence of the acyl-CoA hydrolase-mediated pathway that supplies AA available for PG synthesis in microsomes from the kidney medulla was examined. AA-CoA (20 microM) was preincubated with the 105,000 g pellet (microsomes, 0.5 mg of protein) from the medulla for 5 min at 37 degrees C followed by incubation with the medulla microsomes (0.5 mg of protein) (the source of PG synthesizing enzymes) in the presence of hydroquinone and reduced glutathione for 5 min at 37 degrees C. The PGs formed were measured by high-pressure liquid chromatography using 9-anthryldiazomethane for derivatization. The addition of the microsomal fraction from the medulla in the preincubation mixture increased total PG formation from 3.86 to 8.70 nmol, and this stimulatory effect was somewhat weaker than that of the cytosolic fraction. On the other hand, the microsomal fraction in the kidney cortex has an extremely lower capacity to supply AA for PG synthesis than do medulla microsomes. These results suggest that, in kidney medulla, the microsomes as well as the cytosol have the potential route that supplies AA from AA-CoA for PG synthesis and that this pathway is mediated by acyl-CoA hydrolase.

cDNA cloning and genomic organization of peroxisome proliferator-inducible long-chain acyl-CoA hydrolase from rat liver cytosol

The cDNA for a peroxisome proliferator-inducible long-chain acyl-CoA hydrolase from rat liver cytosol, referred to as rLACH2, was isolated and its genomic structure was determined. The cDNA encoded a 419-amino-acid polypeptide with a calculated molecular weight of 46,011. Sequence analysis identified an active-site serine motif (Gly-x-Ser-x-Gly) common to carboxylesterases and lipases. When expressed in Escherichia coli, the cDNA directed expression of a protein immunoreactive to an anti-rLACH2 antibody with a molecular mass of 47 kDa, identical to that of purified rLACH2. Northern blot analysis showed marked induction of rLACH2 mRNA in the liver after feeding rats with di(2-ethylhexyl)phthalate, a peroxisome proliferator. The rLACH2 gene spanned about 19 kb and comprised 3 exons, the intron/exon boundaries of which were consistent with the donor/acceptor splice rule. A putative peroxisome proliferator response element (AGGTCATGGTTCA) was identified in the 5'-flanking region, suggesting the involvement of peroxisome proliferator-activated receptors in the regulation of rLACH2 gene expression.

Altered constitutive expression of fatty acid-metabolizing enzymes in mice lacking the peroxisome proliferator-activated receptor alpha (PPARalpha)

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the steroid/nuclear receptor superfamily and mediates the biological and toxicological effects of peroxisome proliferators. To determine the physiological role of PPARalpha in fatty acid metabolism, levels of peroxisomal and mitochondrial fatty acid metabolizing enzymes were determined in the PPARalpha null mouse. Constitutive liver beta-oxidation of the long chain fatty acid, palmitic acid, was lower in the PPARalpha null mice as compared with wild type mice, indicating defective mitochondrial fatty acid catabolism. In contrast, constitutive oxidation of the very long chain fatty acid, lignoceric acid, was not different between wild type and PPARalpha null mice, suggesting that constitutive expression of enzymes involved in peroxisomal beta-oxidation is independent of PPARalpha. Indeed, the PPARalpha null mice had normal levels of the peroxisomal acyl-CoA oxidase, bifunctional protein (hydratase + 3-hydroxyacyl-CoA dehydrogenase), and thiolase but lower constitutive expression of the D-type bifunctional protein (hydratase + 3-hydroxyacyl-CoA dehydrogenase). Several mitochondrial fatty acid metabolizing enzymes including very long chain acyl-CoA dehydrogenase, long chain acyl-CoA dehydrogenase, short chain-specific 3-ketoacyl-CoA thiolase, and long chain acyl-CoA synthetase are also expressed at lower levels in the untreated PPARalpha null mice, whereas other fatty acid metabolizing enzymes were not different between the untreated null mice and wild type mice. A lower constitutive expression of mRNAs encoding these enzymes was also found, suggesting that the effect was due to altered gene expression. In wild type mice, both peroxisomal and mitochondrial enzymes were induced by the peroxisome proliferator Wy-14,643; induction was not observed in the PPARalpha null animals. These data indicate that PPARalpha modulates constitutive expression of genes encoding several mitochondrial fatty acid-catabolizing enzymes in addition to mediating inducible mitochondrial and peroxisomal fatty acid beta-oxidation, thus establishing a role for the receptor in fatty acid homeostasis.

Molecular cloning and characterization of a mitochondrial peroxisome proliferator-induced acyl-CoA thioesterase from rat liver

We have previously reported the purification and characterization of the peroxisome proliferator-induced very-long-chain acyl-CoA thioesterase (MTE-I) from rat liver mitochondria [L.T. Svensson, S.E. H. Alexson and J.K. Hiltunen (1995) J. Biol. Chem. 270, 12177-12183]. Here we describe the cloning of the corresponding cDNA. One full-length clone was isolated that contained an open reading frame of 1359 bp encoding a polypeptide with a calculated molecular mass of 49707 Da. The deduced amino acid sequence contains a putative mitochondrial leader peptide of 42 residues. Expression of the cDNA in Chinese hamster ovary cells, followed by immunofluorescence, immunoelectron microscopy and Western blot analyses, showed that the product was targeted to mitochondria and processed to a mature protein of 45 kDa, which is similar to the molecular mass of the protein isolated from rat liver mitochondria. The recombinant enzyme showed the same acyl-CoA chain-length specificity as the isolated rat liver enzyme. Sequence analysis showed no similarity to known esterases, but a high degree (approx. 40%) of identity with bile acid-CoA:amino acid N-acyltransferase cloned from human and rat liver. A putative active-site serine motif (Gly-Xaa-Ser-Xaa-Gly) of several carboxylesterases and lipases was identified. Western and Northern blot analyses showed that MTE-I is constitutively expressed in heart and is strongly induced in liver by feeding rats with di(2-ethylhexyl)phthalate, a peroxisome proliferator, suggesting a role for the enzyme in lipid metabolism.

Peroxisome proliferator-induced acyl-CoA thioesterase from rat liver cytosol: molecular cloning and functional expression in Chinese hamster ovary cells

We have isolated and cloned a cDNA that codes for one of the peroxisome proliferator-induced acyl-CoA thioesterases of rat liver. The deduced amino acid sequence corresponds to the major induced isoform in cytosol. Analysis and comparison of the deduced amino acid sequence with the established consensus sequences suggested that this enzyme represents a novel kind of esterase with an incomplete lipase serine active site motif. Analyses of mRNA and its expression indicated that the enzyme is significantly expressed in liver only after peroxisome proliferator treatment, but isoenzymes are constitutively expressed at high levels in testis and brain. The reported cDNA sequence is highly homologous to the recently cloned brain acyl-CoA thioesterase [Broustas, Larkins, Uhler and Hajra (1996) J. Biol. Chem. 271, 10470-10476], but subtle differences throughout the sequence, and distinct differences close to the resulting C-termini, suggest that they are different enzymes, regulated in different manners. A full-length cDNA clone was expressed in Chinese hamster ovary cells and the expressed enzyme was characterized. The palmitoyl-CoA hydrolysing activity (Vmax) was induced approx. 9-fold to 1 micromol/min per mg of cell protein, which was estimated to correspond to a specific activity of 250 micromol/min per mg of cDNA-expressed enzyme. Both the specific activity and the acyl-CoA chain length specificity were very similar to those of the purified rat liver enzyme.

Molecular cloning and expression of cDNAs encoding rat brain and liver cytosolic long-chain acyl-CoA hydrolases

cDNAs encoding the long-chain acyl-CoA hydrolases (ACHs) from rat brain and liver, referred to as rBACH and rLACH1, respectively, were isolated and sequenced. The rBACH cDNA contained an open reading frame encoding a 338-amino acid polypeptide with a calculated molecular weight of 37,559, of which the deduced amino acid sequence matched partial amino acid sequences directly determined for peptides generated by tryptic digestion or CNBr cleavage of purified rBACH. The rLACH1 cDNA contained an open reading frame encoding a 343-amino acid polypeptide with a molecular weight of 38,240. When expressed in Escherichia coli, these cDNAs produced palmitoyl-CoA hydrolase activity and 44-kDa proteins with molecular masses similar to those of purified rBACH and rLACH1 (43 kDa). These expressed proteins and enzyme activity were immunoblotted and neutralized, respectively, by anti-rBACH or anti-rLACH1 antibodies. rLACH1 cDNA had 84 and 94% identity with rBACH cDNA at the nucleotide and amino acid levels, respectively. However, the 5'-end of the former cDNA which contained the N-terminal coding region of rLACH1 was entirely different from the corresponding region of rBACH cDNA, suggesting that these enzymes may be generated by alternative use of exons of the same gene. Northern blot analysis showed that ACH mRNA was expressed constitutively in the rat brain and testis, whereas its expression in the liver was inducible by treatment with the peroxisome proliferator. This study demonstrated the molecular diversity of ACH and suggested the presence of tissue-specific mechanisms to regulate the ACH gene expression.

Species differences in the intracellular distribution of ciprofibroyl-CoA hydrolase. Implications for peroxisome proliferation

Peroxisomal proliferators (HPP), such as ciprofibrate and clofibric acid, are species-specific drugs. Since HPP-coenzyme A derivatives might be involved in their action, we studied the subcellular distribution of liver ciprofibroyl-CoA hydrolase in rat and in two HPP-unresponsive species, humans and guinea pig. Total activity was similar in the three species and was not induced by clofibric acid treatment. In guinea pig, as in humans, the enzyme is localized in the mitochondrial and soluble fractions and no changes are observed after drug treatment. In the rat, the enzyme has a microsomal localization, but upon clofibric acid treatment it changes to a mitochondrial and soluble distribution, as in unresponsive species. These results raise the possibility that drug-induced hydrolases in rats might be normally expressed in humans and guinea pigs.

Molecular cloning and expression of cDNA encoding rat brain cytosolic acyl-coenzyme A thioester hydrolase

The cDNA encoding rat brain cytosolic acyl-CoA thioester hydrolase (ACT) has been cloned and sequenced, and the primary structure of the enzyme has been deduced. A partial amino acid sequence (38 amino acids) of the enzyme was determined using the peptides generated after CNBr digestion of the purified enzyme. Primers synthesized on the basis of this information were used to isolate two cDNA clones, each encoding the full length of the enzyme. The nucleotide sequences of these clones contained an open reading frame encoding a 358-amino acid polypeptide with a calculated molecular mass of 39.7 kDa, similar to that determined for the purified enzyme (40.9 kDa). The deduced ACT sequence showed no homology to the known sequences of any other thioesterases nor to any other known protein sequence. However, there was a strong homology to a number of expressed sequence tag human brain cDNA clones. The identity of the ACT cDNA was confirmed by the expression of ACT activity in Escherichia coli. There was a 10-15-fold increase in ACT-specific activity in the bacterial extracts after induction with isopropyl thiogalactoside, and the properties of the expressed enzyme (fusion protein) were the same as those of the purified rat brain ACT. Northern blot analysis showed that a 1.65-kilobase ACT transcript was present in rat brain and testis but not in any other rat tissues examined. However, the ACT mRNA was induced in the liver of rats that were fed Wy-14,643, a peroxisome proliferator and inducer of rodent liver cytosolic acyl-CoA thioesterase. These results indicate that the induced rat liver ACT is homologous to the constitutive rat brain ACT.

Peroxisome proliferators differentially regulate long-chain acyl-CoA thioesterases in rat liver

We have investigated the effects of peroxisome proliferators on rat liver long-chain acyl-CoA thioesterase activities. Subcellular fractionations of liver homogenates from control, clofibrate- and di(2-ethylhexyl)phthalate-treated rats confirmed earlier studies which demonstrated that peroxisome-proliferating drugs induce long-chain acyl-CoA thioesterase activity mainly in the mitochondrial and cytosolic fractions. The aim of the present study was to investigate whether the induced activities were due to increases in normally expressed enzymes, or due to induction of novel enzymes. To investigate whether structurally different peroxisome proliferators differentially induced thioesterase activities, we tested the effects of di(2-ethylhexyl)phthalate (a plastisizer) and the hypolipidemic drug clofibrate. For this purpose, we established an analytical size exclusion chromatography method. Chromatography of solubilised mitochondrial matrix proteins showed that the activity in control mitochondria was mainly due to enzymes with molecular masses of about 50 kDa and 35 kDa. The activity in samples prepared from clofibrate- and di(2-ethylhexyl)phthalate-treated rats eluted as proteins of about 40 kDa and 110 kDa. Highly purified peroxisomes contained two peaks of activity, which were not induced, that corresponded to molecular masses of 40 kDa and 80 kDa. The 80-kDa peak was shown to be due to dimerization by addition of glycerol. Chromatography of cytosolic fractions from control rat livers indicated the presence of long-chain acyl-CoA thioesterases with molecular masses of approximately 35 kDa and 125 kDa and a broad peak corresponding to a high-molecular-mass protein. The activity in cytosolic fractions from peroxisome-proliferator-treated rats eluted mainly as peaks corresponding to 40, 110 and 150 kDa. In addition, in the 110-kDa peak, a different degree of induction and different chain-length specificities were caused by clofibrate and di(2-ethylhexyl)phthalate, suggesting that these peroxisome proliferators differentially regulate the cytosolic acyl-CoA thioesterase activities. Western blot analysis showed that enzymes in the 40-kDa peak of the peroxisomal and cytosolic fractions were structurally related, but not identical, to a 40-kDa mitochondrial very-long-chain acyl-CoA thioesterase. Our data show that the increased acyl-CoA thioesterase activities in mitochondria and cytosol were mainly due to induction of acyl-CoA thioesterases which are not, or only weakly, expressed under normal conditions.

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.

Characterization of acyl-CoA thioesterase activity in isolated rat liver peroxisomes. Partial purification and characterization of a long-chain acyl-CoA thioesterase

A common function of peroxisomes in eukaryotic cells is beta-oxidation of fatty acids. In animal cells, beta-oxidation is compartmentalized to peroxisomes and mitochondria. Although regulation of beta-oxidation in mitochondria has been extensively studied, knowledge on its regulation in peroxisomes is still limited. We have considered the possibility that peroxisomes may contain acyl-CoA thioesterases with different substrate specificities that possibly regulate metabolism of different lipids by regulation of substrate availability. In the present study, we have investigated the presence of short-chain and long-chain acyl-CoA thioesterase activities in rat liver peroxisomes. Light-mitochondrial fractions, enriched in peroxisomes, were fractionated by Nycodenz density gradient centrifugation and gradient fractions were analyzed for acyl-CoA thioesterase and marker enzyme distributions. Fractionation of livers from normal rats showed that most of the long-chain acyl-CoA thioesterase activity was localized in microsomes and mitochondria, and only low activity was found in fractions containing peroxisomes. The gradient distribution of propionyl-CoA thioesterase activity showed this activity to be localized mainly in mitochondria and in fractions possibly representing lysosomes, with a small peak of activity in peroxisomal fractions. Di(2-ethylhexyl)phthalate treatment induced the specific propionyl-CoA thioesterase activity approximately threefold in the peak mitochondrial fractions and about onefold in peroxisomal fractions; the activity appeared to be almost exclusively localized to these organelles. The specific activity of myristoyl-CoA thioesterase was induced 1-2-fold in peroxisomal peak fractions and more than 10-fold in the mitochondrial peak fraction, whereas it was unchanged in microsomes. The chain-length specificity of acyl-CoA thioesterase activity in isolated peroxisomes suggests that peroxisomes contain an inducible short-chain thioesterase active on C2-C4 acyl-CoA species (possibly a 'propionyl-CoA' thioesterase). In addition, peroxisomes contain medium-chain to long-chain thioesterase activity, probably due to separate enzymes based on the different chain-length specificities observed in peroxisomes from normal and di(2-ethylhexyl)phthalate-treated rats. A long-chain acyl-CoA thioesterase was partially purified from isolated peroxisomes and found to be active only on fatty-acyl-CoA species longer than octanoyl-CoA. The protein is apparently a monomer of about 40 kDa and clearly different from microsomal long-chain acyl-CoA thioesterase. An induction of this long-chain thioesterase may explain the observed change in chain-length specificity in peroxisomes isolated from normal and di(2-ethylhexyl)phthalate-treated rats. Possible physiological functions of these thioesterases are discussed.

Characterization of microsomal long-chain acyl-CoA hydrolase activity in the rat submandibular gland

1. Long-chain fatty acyl-CoA hydrolase in submandibular gland microsomes was characterized and compared to that in liver ones. 2. In rat submandibular gland, the microsomal long-chain acyl-CoA hydrolase showed a higher relative activity for polyunsaturated fatty acyl-CoAs than that of liver microsomes. 3. It was suggested that the hydrolase in rat submandibular gland microsomes may play a role in modulation in the acyl-CoA pool size.

Activation of a peroxisome-proliferating catabolite of cholic acid to its CoA ester

We have shown that a microbial cholic acid catabolite (4R)-4-(2,3,4,6,6a beta,7,8,9,9a alpha,9b beta-decahydro-6a beta-methyl-3-oxo- 1H-cyclopenta[f]quinolin-7 beta-yl)valeric acid (DCQVA), is a potent peroxisome proliferator. In this paper a possible key stage in DCQVA metabolism, the activation of DCQVA to its CoA ester, has been investigated in rat liver microsomes and particulate fractions. The microsomal reaction was dependent on CoA, ATP, DCQVA (0.2-1 mM) and protein content. The reaction was decreased by storage at 4 degrees C, preincubation of microsomes at 37 degrees C for 5 min, or inclusion of Triton X-100 in the reaction mixture. Such treatments also enhanced generation of long-chain fatty acyl-CoAs, as determined by h.p.l.c. analysis. The same effect was caused by exposing the microsomes to phospholipase A2, suggesting that endogenous fatty acids may compete with DCQVA for esterification with CoA. Subcellular fractionation of rat liver demonstrated that the activity of DCQVA-CoA synthesis was localized predominantly in the microsomal fraction, in contrast to long-chain fatty acyl-CoA synthetase, which was distributed among all particulate fractions. Administration of clofibrate of rats did not affect the distribution of DCQVA-CoA synthesis activity. In contrast to a 2-fold induction of long-chain fatty acyl-CoA synthetase by clofibrate treatment, the activity of DCQVA-CoA synthesis in the microsomal fraction decreased by 80%. These results suggest that DCQVA is activated by an enzyme distinct from long-chain fatty acyl-CoA synthetase. The resulting perturbation of fatty acid metabolism may be involved in the mechanism whereby DCQVA causes peroxisome proliferation.

Isolation and characterization of microsomal acyl-CoA thioesterase. A member of the rat liver microsomal carboxylesterase multi-gene family

We have isolated and characterized an acyl-CoA thioesterase from rat liver microsomes. The enzyme consists mainly of a monomer of 59 kDa. However, the final preparation was found to contain minor amounts of a trimeric form of the protein. The enzyme was purified more than 85-fold from isolated microsomes and used for NH2-terminal sequence analysis and for analysis of peptides isolated after proteolytic digestion. The NH2-terminal sequence was unique but highly conserved compared to those of other carboxylesterases. Internal sequence data, covering almost 20% of the protein, showed high similarity to the deduced amino acid sequences from a cDNA encoding a carboxylesterase synthesized in the liver and subsequently secreted to the blood [Alexson, S. E. H., Finlay, T. H., Hellman, U., Diczfalusy, U. & Eggertsen, G., unpublished results] and nonspecific rat liver microsomal carboxylesterase with isoelectric point of 6.1 [Robbi, M., Beaufay, H. & Octave, J.-N. (1990) Biochem. J. 269, 451-458], thus confirming earlier suggestions that this enzyme is a member of the microsomal carboxylesterase multigene family. The peptide sequences contained two of the four conserved cysteic acid residues found in other carboxylesterases. Amino acid analysis indicated that the protein contains five cysteine residues in contrast to most other described carboxylesterases which contain four highly conserved cysteins. The purified protein was used for immunization and the antiserum was used to detect the protein as well as its trimeric form, which is a minor component, in isolated rat liver microsomes. The antiserum recognized proteins of similar sizes in microsomes and 100,000 x g supernatant prepared from hamster brown adipose tissue, a tissue known to contain very high activity of carboxylesterase, and to recognize carboxylesterases isolated from porcine and rabbit liver.

Cytosolic long-chain acyl-CoA hydrolase, a suitable parameter to measure hepatic response to peroxisome proliferators

The possibility of using cytosolic long-chain acyl-CoA as a parameter to measure the response of liver to peroxisome proliferators was studied. A subcutaneous (s.c.) injection of perfluorooctanoic acid (PFOA) to male Wistar rats caused an increase in activity of cytosolic long-chain acyl-CoA hydrolase. This increase in activity seems to be due to enzyme induction, since it was prevented by simultaneous administration of cycloheximide or actinomycin D with PFOA. The activity of cytosolic long-chain acyl-CoA hydrolase was increased in a dose-dependent manner by the administration of three peroxisome proliferators with diverse chemical structures: alpha-(p-chlorophenoxy)isobutyric acid (clofibric acid), 2,2'-(decamethylenedithio)diethanol (tiadenol) and PFOA. The increased activity produced by clofibric acid lasted throughout a 22-week treatment. A good correlation was found between the activities of cytosolic long-chain acyl-CoA hydrolase and peroxisomal beta-oxidation induced by the administration of the peroxisome proliferators. These results indicate that cytosolic long-chain acyl-CoA hydrolase is a suitable parameter for measuring the response of rat liver to challenges by peroxisome proliferators.

The presence of acyl-CoA hydrolase in rat brown-adipose-tissue peroxisomes

The subcellular distribution of acyl-CoA hydrolase was studied in rat brown adipose tissue, with special emphasis on possible peroxisomal localization. Subcellular fractionation by sucrose-density-gradient centrifugation, followed by measurement of short-chain (propionyl-CoA) acyl-CoA hydrolase in the presence of NADH, resulted in two peaks of activity in the gradient: one peak corresponded to the distribution of cytochrome oxidase (mitochondrial marker enzyme), and another peak of activity coincided with the peroxisomal marker enzyme catalase. The distribution of the NADH-inhibited short-chain hydrolase activity fully resembled that of cytochrome oxidase. The substrate-specificity curve of the peroxisomal acyl-CoA hydrolase activity indicated the presence of a single enzyme exhibiting a broad substrate specificity, with maximal activity towards fatty acids with chain lengths of 3-12 carbon atoms. The mitochondrial acyl-CoA hydrolase substrate specificity, in contrast, indicated the presence of at least two acyl-CoA hydrolases (of short- and medium-chain-length specificity). The peroxisomal acyl-CoA hydrolase activity was inhibited by CoA at low (microM) concentrations and by ATP at high concentrations (greater than 0.8 mM). In contrast with the mitochondrial short-chain hydrolase, the peroxisomal acyl-CoA hydrolase activity was not inhibited by NADH.

Subcellular localization of non-specific carboxylesterases, acylcarnitine hydrolase, monoacylglycerol lipase and palmitoyl-CoA hydrolase in rat liver

The subcellular distribution and sidedness on the membranes of four chemically and genetically distinct esterases (esterases ES-3, ES-4, ES-8, ES-15) in rat liver was investigated using selective substrates. (1) Rat liver homogenate was divided into nine subcellular fractions by differential centrifugation techniques. The cell fractions were assayed for the enzymatic hydrolysis of acetanilide (ES-3), propanidid, palmitoyl-CoA and monopalmitoylglycerol (ES-4), methyl butyrate and octanoylglycerol (ES-8), and decanoylcarnitine (ES-15). With all substrates, the highest specific activities were found in the rough and smooth endoplasmic reticulum fractions. This localization of the esterases was confirmed by labelling the cell fractions with the specific, covalently binding inhibitor bis(4-nitro[14C]phenyl) phosphate. The enzymatic hydrolysis of the palmitoyl esters in differing cell fractions did not completely parallel that of propanidid. This confirms the well-known existence of palmitoyl-CoA hydrolases other than esterase ES-4. (2) Density gradient fractionations with crude mitochondria indicated that a low amount of at least one of these carboxylesterases was an integral part of these organelles too. (3) Proteinase treatment reduced the non-specific esterase activities as well as lipase activities versus dioctanoylglycerol, acylcarnitines and palmitoyl-CoA only in detergent-disrupted microsomal vesicles. This might indicate a lumenal orientation of these enzymes. However, of the charged substrates palmitoylcarnitine and palmitoyl-CoA only the latter one showed the typical latency to be expected for a hydrolysis in the lumen of the endoplasmic reticulum.

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.

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.

Cardiac and hepatic acyl-CoA and acylcarnitine hydrolase activities in clofibrate-fed rabbits

Catalase activity in the heart of male rabbits was 21% of that found in the liver; clofibrate feeding (0.3% w/w for 10 days) resulted in an 80% increase in both cardiac and hepatic catalase activities. Fatty acyl-CoA oxidase activity in control heart was 11% of that found in control liver; this peroxisomal activity did not increase subsequent to clofibrate feeding. Only acyl-CoA hydrolase activity in the cardiac supernatant was elevated by clofibrate feeding. Acylcarnitine hydrolase activity was increased significantly in the homogenate, extract and supernatant of both heart and liver from the clofibrate-fed rabbit. Clofibrate feeding increased CoASH and carnitine tissue levels in heart and liver.

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.

Induction of rat hepatic long-chain acyl-CoA hydrolases by various peroxisome proliferators

Induction of cytosolic long-chain acyl-CoA hydrolases was investigated in rat liver after administration of various peroxisome proliferators and related compounds. Treatment of rats with di-(2-ethylhexyl)-phthalate, di-(2-ethylhexyl)-adipate or tiadenol induced hydrolases I and II, while acetylsalicylic acid induced only hydrolase II. Among the various phenoxyacetic acid derivatives and related compounds, 2,4,5-trichlorophenoxyacetic acid, 2-(4-chlorophenoxy)-2-methylacetic acid, 2-(2-chlorophenoxy)-2-methylpropionic acid and clofibric acid induced both hydrolases I and II, whereas 2, 4-dichlorophenoxyacetic acid induced only hydrolase II. All nine of the above-mentioned inducers also markedly increased the activity of peroxisomal beta-oxidation. Other compounds tested (2-chlorophenoxyacetic acid, 4-chlorophenoxyacetic acid, 4-chlorophenol, phenoxyacetic acid and phenoxy-2-methylacetic acid) were ineffective as inducers. These results suggest that inducers of acyl-CoA hydrolase II also enhance peroxisomal beta-oxidation activity, but do not necessarily induce acyl-CoA hydrolase I. The structure-inducing activity relationships of these compounds are discussed.

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.

Dicarboxylic aciduria: deficient [1-14C]octanoate oxidation and medium-chain acyl-CoA dehydrogenase in fibroblasts

Dicarboxylic aciduria, an inborn error of metabolism in man, is thought to be caused by defective beta-oxidation of six-carbon to ten-carbon fatty acids. Oxidation of [1-14C]octanoate was impaired in intact fibroblasts from three unrelated patients with dicarboxylic aciduria (19 percent of control), as was the activity of medium-chain (octanoyl-)acyl-CoA dehydrogenase in the supernatants of sonicated fibroblast mitochondria (5 percent of control). These data confirm that dicarboxylic aciduria is caused by an enzyme defect in the beta-oxidation cycle.

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

1. The activities of acyl-CoA hydrolase, catalase, urate oxidase and peroxisomal palmitoyl-CoA oxidation as well as the protein content and the level of CoASH and long-chain acyl-CoA were measured in subcellular fractions of liver from rats fed diets containing phenobarbital (0.1% w/w) or clofibrate (0.3% w/w). 2. Whereas phenobarbital administration resulted in increased microsomal protein, the clofibrate-induced increase was almost entirely attributed to the mitochondrial fraction with minor contribution from the light mitochondrial fraction. 3. The specific activity of palmitoyl-CoA hydrolase in the microsomal fraction was only slightly affected while the mitochondrial enzyme was increased to a marked extent (3-4-fold) by clofibrate. 4. Phenobarbital administration mainly enhanced the microsomal palmitoyl-CoA hydrolase. 5. The increased long-chain acyl-CoA and CoASH level observed after clofibrate treatment was mainly associated with the mitochondrial, light mitochondrial and cytosolic fractions, while the slight increase in the levels of these compounds found after phenobarbital feeding was largely of microsomal origin. 6. The findings suggest that there is an intraperoxisomal CoASH and long-chain acyl-CoA pool. 7. The specific activity of palmitoyl-CoA hydrolase, catalase and peroxisomal palmitoyl-CoA oxidation was increased in the lipid-rich floating layer of the cytosol-fraction. 8. The changes distribution of the peroxisomal marker enzymes and microsomal palmitoyl-CoA hydrolase after treatment with hypolipidemic drugs may be related to the origin of peroxisomes.