Long-chain acyl-CoA hydrolase from rat brain cytosol: purification, characterization, and immunohistochemical localization

Thioesterase enzyme families: Functions, structures, and mechanisms

Thioesterases are enzymes that hydrolyze thioester bonds in numerous biochemical pathways, for example in fatty acid synthesis. This work reports known functions, structures, and mechanisms of updated thioesterase enzyme families, which are classified into 35 families based on sequence similarity. Each thioesterase family is based on at least one experimentally characterized enzyme, and most families have enzymes that have been crystallized and their tertiary structure resolved. Classifying thioesterases into families allows to predict tertiary structures and infer catalytic residues and mechanisms of all sequences in a family, which is particularly useful because the majority of known protein sequence have no experimental characterization. Phylogenetic analysis of experimentally characterized thioesterases that have structures with the two main structural folds reveal convergent and divergent evolution. Based on tertiary structure superimposition, catalytic residues are predicted.

Structural basis for disulphide-CoA inhibition of a butyryl-CoA hexameric thioesterase

Acyl-coenzyme A thioesterases (ACTs) catalyse the hydrolysis of thioester bonds between fatty-acyl chains and coenzyme A (CoA), producing a free fatty-acyl chain and CoA. These enzymes are expressed ubiquitously across prokaryotes and eukaryotes, and play important roles in lipid metabolism. There are 25 thioesterase families, subdivided based on their active site configuration, protein oligomerization, and substrate specificity. Understanding the mechanism of regulation within these families is important due to their roles in controlling the cell concentration of a range of fatty acids and CoA-bound compounds. Here we report a structural basis for a novel mode of inhibition of an ACT from Staphylococcus aureus. The enzyme displays a hotdog fold composed of five β-strands wrapping around a central α-helix, and an additional 30 residue α-helix located at its C-terminus. We show that the enzyme is a hexamer and has specificity towards butyryl-CoA. Structural analysis revealed putative catalytic residues, and we show through site directed mutagenesis that Asn28, Asp43, and Thr60 are critical for activity. Additionally, we show that the Asn28Ala destabilises the enzyme oligomeric state into two distinct populations. Co-crystallization of the enzyme with the substrate butyryl-CoA produced a crystal with three CoA ligands bound in the enzyme active sites: CoA, butyryl-CoA, and disulphide-CoA, the latter of which inhibits enzyme activity. Our study provides new insights into the structure and specificity of hexameric thioesterases, inhibitory feedback mechanisms, and possible biotechnological applications in short-chain fatty acid production such as biofuels, pharmaceuticals, and industrial compounds.

Structural insights into GDP-mediated regulation of a bacterial acyl-CoA thioesterase

Thioesterases catalyze the cleavage of thioester bonds within many activated fatty acids and acyl-CoA substrates. They are expressed ubiquitously in both prokaryotes and eukaryotes and are subdivided into 25 thioesterase families according to their catalytic active site, protein oligomerization, and substrate specificity. Although many of these enzyme families are well-characterized in terms of function and substrate specificity, regulation across most thioesterase families is poorly understood. Here, we characterized a TE6 thioesterase from the bacterium Neisseria meningitidis Structural analysis with X-ray crystallographic diffraction data to 2.0-Å revealed that each protein subunit harbors a hot dog-fold and that the TE6 enzyme forms a hexamer with D3 symmetry. An assessment of thioesterase activity against a range of acyl-CoA substrates revealed the greatest activity against acetyl-CoA, and structure-guided mutagenesis of putative active site residues identified Asn²⁴ and Asp³⁹ as being essential for activity. Our structural analysis revealed that six GDP nucleotides bound the enzyme in close proximity to an intersubunit disulfide bond interactions that covalently link thioesterase domains in a double hot dog dimer. Structure-guided mutagenesis of residues within the GDP-binding pocket identified Arg⁹³ as playing a key role in the nucleotide interaction and revealed that GDP is required for activity. All mutations were confirmed to be specific and not to have resulted from structural perturbations by X-ray crystallography. This is the first report of a bacterial GDP-regulated thioesterase and of covalent linkage of thioesterase domains through a disulfide bond, revealing structural similarities with ADP regulation in the human ACOT12 thioesterase.

Acyl-CoA thioesterase 7 is involved in cell cycle progression via regulation of PKCζ-p53-p21 signaling pathway

Acyl-CoA thioesterase 7 (ACOT7) is a major isoform of the ACOT family that catalyzes hydrolysis of fatty acyl-CoAs to free fatty acids and CoA-SH. However, canonical and non-canonical functions of ACOT7 remain to be discovered. In this study, for the first time, ACOT7 was shown to be responsive to genotoxic stresses such as ionizing radiation (IR) and the anti-cancer drug doxorubicin in time- and dose-dependent manners. ACOT7 knockdown induced cytostasis via activation of the p53-p21 signaling pathway without a DNA damage response. PKCζ was specifically involved in ACOT7 depletion-mediated cell cycle arrest as an upstream molecule of the p53-p21 signaling pathway in MCF7 human breast carcinoma and A549 human lung carcinoma cells. Of the other members of the ACOT family, including ACOT1, 4, 8, 9, 11, 12, and 13 that were expressed in human, ACOT4, 8, and 12 were responsive to genotoxic stresses. However, none of those had a role in cytostasis via activation of the PKCζ-p53-p21 signaling pathway. Analysis of the ACOT7 prognostic value revealed that low ACOT7 levels prolonged overall survival periods in breast and lung cancer patients. Furthermore, ACOT7 mRNA levels were higher in lung cancer patient tissues compared to normal tissues. We also observed a synergistic effect of ACOT7 depletion in combination with either IR or doxorubicin on cell proliferation in breast and lung cancer cells. Together, our data suggest that a low level of ACOT7 may be involved, at least in part, in the prevention of human breast and lung cancer development via regulation of cell cycle progression.

Structural and Functional Characterization of the PaaI Thioesterase from Streptococcus pneumoniae Reveals a Dual Specificity for Phenylacetyl-CoA and Medium-chain Fatty Acyl-CoAs and a Novel CoA-induced Fit Mechanism

PaaI thioesterases are members of the TE13 thioesterase family that catalyze the hydrolysis of thioester bonds between coenzyme A and phenylacetyl-CoA. In this study we characterize the PaaI thioesterase from Streptococcus pneumoniae (SpPaaI), including structural analysis based on crystal diffraction data to 1.8-Å resolution, to reveal two double hotdog domains arranged in a back to back configuration. Consistent with the crystallography data, both size exclusion chromatography and small angle x-ray scattering data support a tetrameric arrangement of thioesterase domains in solution. Assessment of SpPaaI activity against a range of acyl-CoA substrates showed activity for both phenylacetyl-CoA and medium-chain fatty-acyl CoA substrates. Mutagenesis of putative active site residues reveals Asn(37), Asp(52), and Thr(68) are important for catalysis, and size exclusion chromatography analysis and x-ray crystallography confirm that these mutants retain the same tertiary and quaternary structures, establishing that the reduced activity is not a result of structural perturbations. Interestingly, the structure of SpPaaI in the presence of CoA provides a structural basis for the observed substrate specificity, accommodating a 10-carbon fatty acid chain, and a large conformational change of up to 38 Å in the N terminus, and a loop region involving Tyr(38)-Tyr(39). This is the first time PaaI thioesterases have displayed a dual specificity for medium-chain acyl-CoAs substrates and phenylacetyl-CoA substrates, and we provide a structural basis for this specificity, highlighting a novel induced fit mechanism that is likely to be conserved within members of this enzyme family.

Structural and functional characterization of TesB from Yersinia pestis reveals a unique octameric arrangement of hotdog domains

Acyl-CoA thioesterases catalyse the hydrolysis of the thioester bonds present within a wide range of acyl-CoA substrates, releasing free CoASH and the corresponding fatty-acyl conjugate. The TesB-type thioesterases are members of the TE4 thioesterase family, one of 25 thioesterase enzyme families characterized to date, and contain two fused hotdog domains in both prokaryote and eukaryote homologues. Only two structures have been elucidated within this enzyme family, and much of the current understanding of the TesB thioesterases has been based on the Escherichia coli structure. Yersinia pestis, a highly virulent bacterium, encodes only one TesB-type thioesterase in its genome; here, the structural and functional characterization of this enzyme are reported, revealing unique elements both within the protomer and quaternary arrangements of the hotdog domains which have not been reported previously in any thioesterase family. The quaternary structure, confirmed using a range of structural and biophysical techniques including crystallography, small-angle X-ray scattering, analytical ultracentrifugation and size-exclusion chromatography, exhibits a unique octameric arrangement of hotdog domains. Interestingly, the same biological unit appears to be present in both TesB structures solved to date, and is likely to be a conserved and distinguishing feature of TesB-type thioesterases. Analysis of the Y. pestis TesB thioesterase activity revealed a strong preference for octanoyl-CoA and this is supported by structural analysis of the active site. Overall, the results provide novel insights into the structure of TesB thioesterases which are likely to be conserved and distinguishing features of the TE4 thioesterase family.

Structural basis for regulation of the human acetyl-CoA thioesterase 12 and interactions with the steroidogenic acute regulatory protein-related lipid transfer (START) domain

Acetyl-CoA plays a fundamental role in cell signaling and metabolic pathways, with its cellular levels tightly controlled through reciprocal regulation of enzymes that mediate its synthesis and catabolism. ACOT12, the primary acetyl-CoA thioesterase in the liver of human, mouse, and rat, is responsible for cleavage of the thioester bond within acetyl-CoA, producing acetate and coenzyme A for a range of cellular processes. The enzyme is regulated by ADP and ATP, which is believed to be mediated through the ligand-induced oligomerization of the thioesterase domains, whereby ATP induces active dimers and tetramers, whereas apo- and ADP-bound ACOT12 are monomeric and inactive. Here, using a range of structural and biophysical techniques, it is demonstrated that ACOT12 is a trimer rather than a tetramer and that neither ADP nor ATP exert their regulatory effects by altering the oligomeric status of the enzyme. Rather, the binding site and mechanism of ADP regulation have been determined to occur through two novel regulatory regions, one involving a large loop that links the thioesterase domains (Phe(154)-Thr(178)), defined here as RegLoop1, and a second region involving the C terminus of thioesterase domain 2 (Gln(304)-Gly(326)), designated RegLoop2. Mutagenesis confirmed that Arg(312) and Arg(313) are crucial for this mode of regulation, and novel interactions with the START domain are presented together with insights into domain swapping within eukaryotic thioesterases for substrate recognition. In summary, these experiments provide the first structural insights into the regulation of this enzyme family, revealing an alternate hypothesis likely to be conserved throughout evolution.

Acyl coenzyme A thioesterase 7 regulates neuronal fatty acid metabolism to prevent neurotoxicity

Numerous neurological diseases are associated with dysregulated lipid metabolism; however, the basic metabolic control of fatty acid metabolism in neurons remains enigmatic. Here we have shown that neurons have abundant expression and activity of the long-chain cytoplasmic acyl coenzyme A (acyl-CoA) thioesterase 7 (ACOT7) to regulate lipid retention and metabolism. Unbiased and targeted metabolomic analysis of fasted mice with a conditional knockout of ACOT7 in the nervous system, Acot7(N-/-), revealed increased fatty acid flux into multiple long-chain acyl-CoA-dependent pathways. The alterations in brain fatty acid metabolism were concomitant with a loss of lean mass, hypermetabolism, hepatic steatosis, dyslipidemia, and behavioral hyperexcitability in Acot7(N-/-) mice. These failures in adaptive energy metabolism are common in neurodegenerative diseases. In agreement, Acot7(N-/-) mice exhibit neurological dysfunction and neurodegeneration. These data show that ACOT7 counterregulates fatty acid metabolism in neurons and protects against neurotoxicity.

Upregulation of fatty acyl-CoA thioesterases in the heart and skeletal muscle of rats fed a high-fat diet

In rodent models of diet-induced obesity, prolonged high-fat feeding increases the cellular uptake of fatty acids and causes lipotoxicity in the heart and skeletal muscle, where substrate overload to beta-oxidation generates mitochondrial stress. We examined the hypothesis that, because of its catalytic properties, acyl-CoA thioesterase (ACOT) would counteract these detrimental situations by modulating intracellular acyl-CoA levels. Rats were fed a low- or high-fat diet for up to 20 weeks, and the expressions of ACOT isoforms and fatty acid beta-oxidation enzymes were analyzed by western blotting. The expressions of ACOT1, ACOT2 and ACOT7 proteins in the heart and soleus muscle were significantly increased, by 2.0-7.6-fold, in rats fed the high-fat diet as compared with the low-fat diet group. These effects were accompanied by increases in carnitine palmitoyltransferase and acyl-CoA oxidase expression. However, ACOT was not induced in the extensor digitorum longus muscle or the liver. Subcellular fractionation of heart and soleus muscle homogenates confirmed expression of both the cytosolic and mitochondrial ACOT isoforms. These results underscore the functional relationship between ACOT and fatty acid oxidation, and suggest adaptive upregulation of ACOT to protect against fatty acid oversupply in the heart and skeletal muscle.

Thioesterases: a new perspective based on their primary and tertiary structures

Thioesterases (TEs) are classified into EC 3.1.2.1 through EC 3.1.2.27 based on their activities on different substrates, with many remaining unclassified (EC 3.1.2.-). Analysis of primary and tertiary structures of known TEs casts a new light on this enzyme group. We used strong primary sequence conservation based on experimentally proved proteins as the main criterion, followed by verification with tertiary structure superpositions, mechanisms, and catalytic residue positions, to accurately define TE families. At present, TEs fall into 23 families almost completely unrelated to each other by primary structure. It is assumed that all members of the same family have essentially the same tertiary structure; however, TEs in different families can have markedly different folds and mechanisms. Conversely, the latter sometimes have very similar tertiary structures and catalytic mechanisms despite being only slightly or not at all related by primary structure, indicating that they have common distant ancestors and can be grouped into clans. At present, four clans encompass 12 TE families. The new constantly updated ThYme (Thioester-active enzYmes) database contains TE primary and tertiary structures, classified into families and clans that are different from those currently found in the literature or in other databases. We review all types of TEs, including those cleaving CoA, ACP, glutathione, and other protein molecules, and we discuss their structures, functions, and mechanisms.

Functional and structural properties of mammalian acyl-coenzyme A thioesterases

Acyl-coenzyme A thioesterases (Acots) play important cellular roles in mammalian fatty acid metabolism through modulation of cellular concentrations of activated fatty acyl-CoAs. Acots catalyse the hydrolysis of the thioester bond present within acyl-CoA ester molecules to yield coenzyme A (CoASH) and the corresponding non-esterified fatty acid. Acyl-CoA thioesterases are expressed ubiquitously in both prokaryotes and eukaryotes and, in higher order organisms, the enzymes are expressed and localised in a tissue-dependent manner within the cytosol, mitochondria, peroxisomes and endoplasmic reticulum. Recent studies have led to advances in the functional and structural characterization of many mammalian Acot family members. These include the structure determination of both type-I and type-II Acot family members, structural elucidation of the START domain of ACOT11, identification of roles in arachidonic acid and inflammatory prostaglandin production by Acot7, and inclusion of a 13th Acot family member. Here, we review and analyse the current literature on mammalian Acots with respect to their characterization and summarize the current knowledge on the structure, function and regulation of this enzyme family.

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.

Sterol Regulatory Element-Binding Protein-2 modulates human brain acyl-CoA hydrolase gene transcription

The brain shows high catalyzing activity during hydrolysis of long-chain acyl-CoAs into fatty acids and CoA-SH. Brain acyl-CoA hydrolase (BACH) is responsible for most of the long-chain acyl-CoA hydrolyzing activity in the brain and is localized exclusively in neurons. We analyzed the human BACH gene promoter, focusing on transcriptional regulation by Sterol Regulatory Element-Binding Protein-2 (SREBP-2), which is a transcription factor that activates genes involved in cholesterol biosynthesis and uptake. When the nuclear form of SREBP-2 gene was transfected into human neuroblastoma cells, transcription of a BACH gene promoter-luciferase reporter gene was activated through a sterol regulatory element (SRE) motif. Moreover, a gel shift assay demonstrated that SREBP-2 specifically bound to the SRE motif. These results suggest that transcription of the BACH gene is activated by SREBP-2. This study also provides insights into BACH function in the interaction between the metabolism of acyl-CoAs and cholesterol in neurons.

Localization of a long-chain acyl-CoA hydrolase in spermatogenic cells in mice

Brain acyl-CoA hydrolase (BACH) hydrolyzes long-chain acyl-CoAs to free fatty acids and CoA-SH. BACH is highly distributed in brain and is localized in neurons, but not glial cells. This suggests that BACH plays a specific role in neurons. BACH is also detected in testis, although the expression profile of BACH is unknown in testis. In this study, developmental changes and cellular distribution of BACH were examined in mouse testis. Before postnatal day (P) 10, BACH was detected at very low levels by Western blotting. Then, BACH content rapidly increased from P14 and reached maximum levels at P21, remaining high until at least P70. The increase in BACH content corresponded to the appearance of pachytene spermatocytes, which was confirmed by immunohistochemistry. BACH was also detectable in spermatids, but not in spermatogonia, mature spermatozoa. These results suggest that BACH is expressed in a cell-specific manner and plays a role in spermatogenesis.

Each lipase has a unique sensitivity profile for organophosphorus inhibitors

Lipases sensitive to organophosphorus (OP) inhibitors play critical roles in cell regulation, nutrition, and disease, but little is known on the toxicological aspects in mammals. To help fill this gap, six lipases or lipase-like proteins are assayed for OP sensitivity in vitro under standard conditions (25 degrees C, 15 min incubation). Postheparin serum lipase, lipoprotein lipase (LPL) (two sources), pancreatic lipase, monoacylglycerol (MAG) lipase, cholesterol esterase, and KIAA1363 are considered with 32 OP pesticides and related compounds. Postheparin lipolytic activity in rat serum is inhibited by 14 OPs, including chlorpyrifos oxon (IC50 50-97 nM). LPL (bovine milk and Pseudomonas) generally is less inhibited by the insecticides or activated oxons, but the milk enzyme is very sensitive to six fluorophosphonates and benzodioxaphosphorin oxides (IC50 7-20 nM). Porcine pancreatic lipase is very sensitive to dioctyl 4-nitrophenyl phosphate (IC50 8 nM), MAG lipase of mouse brain to O-4-nitrophenyl methyldodecylphosphinate (IC50 0.6 nM), and cholesterol esterase (bovine pancreas) to all of the classes of OPs tested (IC50 < 10 nM for 17 compounds). KIAA1363 is sensitive to numerous OPs, including two O-4-nitrophenyl compounds (IC50 3-4 nM). In an overview, inhibition of 28 serine hydrolases (including lipases) by eight OPs (chlorpyrifos oxon, diazoxon, paraoxon, dichlorvos, and four nonpesticides) showed that brain acetylcholinesterase is usually less sensitive than butyrylcholinesterase, liver esterase, cholesterol esterase, and KIAA1363. In general, each lipase (like each serine hydrolase) has a different spectrum of OP sensitivity, and individual OPs have unique ranking of potency for inhibition of serine hydrolases.

Crystallization of the C-terminal domain of the mouse brain cytosolic long-chain acyl-CoA thioesterase

The mammalian long-chain acyl-CoA thioesterase, the enzyme that catalyses the hydrolysis of acyl-CoAs to free fatty acids, contains two fused 4HBT (4-hydroxybenzoyl-CoA thioesterase) motifs. The C-terminal domain of the mouse long-chain acyl-CoA thioesterase (Acot7) has been expressed in bacteria and crystallized. The crystals were obtained by vapour diffusion using PEG 2000 MME as precipitant at pH 7.0 and 290 K. The crystals have the symmetry of space group R32 (unit-cell parameters a = b = 136.83, c = 99.82 A, gamma = 120 degrees). Two molecules are expected in the asymmetric unit. The crystals diffract to 2.4 A resolution using the laboratory X-ray source and are suitable for crystal structure determination.

A revised nomenclature for mammalian acyl-CoA thioesterases/hydrolases

Acyl-CoA thioesterases, also known as acyl-CoA hydrolases, are a group of enzymes that hydrolyze CoA esters such as acyl-CoAs (saturated, unsaturated, branched-chain), bile acid-CoAs, CoA esters of prostaglandins, etc., to the corresponding free acid and CoA. However, there is significant confusion regarding the nomenclature of these genes. In agreement with the HUGO Gene Nomenclature Committee and the Mouse Genomic Nomenclature Committee, a revised nomenclature for mammalian acyl-CoA thioesterases/hydrolases has been suggested for the 12 member family. The family root symbol is ACOT, with human genes named ACOT1-ACOT12, and rat and mouse genes named Acot1-Acot12. Several of the ACOT genes are the result of splicing events, and these splice variants are cataloged.

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.

A 50-kDa isoform of mouse brain acyl-CoA hydrolase: expression and molecular properties

Brain acyl-CoA hydrolase (BACH) is responsible for most of the long-chain acyl-CoA hydrolyzing activity in the brain and is localized exclusively in neurons. There are two BACH isoforms: the major isoform, a 43-kDa BACH, and a lesser isoform, a 50-kDa BACH. In our previous work [Brain Res. Mol. Brain Res. 98 (2002) 81], a possibility was raised that these BACH isoforms might be generated from a single mRNA species via a mechanism of alternative use of translation start sites. However, the results obtained in the current study indicated that the 43-kDa BACH and 50-kDa BACH are not generated from a single mRNA species, but from distinct mRNA species transcribed by alternative use of transcription start sites. The molecular properties of the 50-kDa BACH were compared to those of the 43-kDa BACH. Palmitoyl-CoA hydrolase activity and protein stability were almost the same between both BACH isoforms. In addition, both 43-kDa BACH and 50-kDa BACH that were fused to green fluorescent protein showed cytosolic distribution. These results suggest that the 50-kDa BACH plays a similar role as the 43-kDa BACH. Therefore, since the 43-kDa BACH is expressed at higher levels than 50-kDa BACH, the 43-kDa BACH should largely contribute to understanding the physiological functions of the BACH gene in neurons.

Inducible expression of long-chain acyl-CoA hydrolase gene in cell cultures

A long-chain acyl-CoA hydrolase, BACH, is markedly distributed in the brain and localized in neurons. However, the physiological significance of BACH is unclear. To study the gene function, we expressed the mouse BACH gene in C3H 10T1/2 fibroblastic cells using a mifepristone (RU486)-inducible gene expression system. A cell clone, 10T-S6/44, was generated by stable transfection of two plasmids encoding a mifepristone-dependent transactivator and an inducible transgene product, BACH with a C-terminal MYC-tag (BACH-MYC). The transgene expression in the 10T-S6/44 cells was tightly regulated by mifepristone. Induction of BACH-MYC and an increase in palmitoyl-CoA hydrolase activity were observed in the cells treated with 3 x 10(-11) M mifepristone and reached maximal levels at a concentration of 1 x 10(-9) M for 48 h. The growth rate of cells showing the maximal induction of BACH-MYC was reduced, whereas phospholipid synthesis was unchanged. These results suggested that BACH affects specific cellular systems and functions, but not all acyl-CoA-utilizing processes.

Hepatic enzymatic synthesis and hydrolysis of CoA esters of solvent-derived oxa acids

Many ethylene glycol-derived solvents are oxidized to xenobiotic alkoxyacetic acids (3-oxa acids) by hepatic enzymes. The toxicity of these ubiquitous solvents has been associated with their oxa acid metabolites. For many xenobiotic carboxylic acids, the toxicity is associated with the CoA ester of the acid. In this study, related alkoxyacetic acids were evaluated as potential substrates for acyl-CoA synthetases found in mitochondrial, peroxisomal, and microsomal fractions isolated from rat liver. Likewise, chemically synthesized oxa acyl-CoAs were used as substrates for acyl-CoA hydrolases associated with the same rat liver fractions. Activities of the xenobiotic oxygen-substituted substrates were compared with analogous physiologic aliphatic substrates by UV-vis spectrophotometric methods. All of the solvent-derived oxa acids were reasonable substrates for the acyl-CoA synthetases, although their activity was usually less than the corresponding physiologic acid. Acyl-CoA hydrolase activities were decreased compared with acyl-CoA synthetase activities for all substrates, especially for the oxa acyl-CoAs. These studies suggest that these xenobiotic carboxylic acids may be converted to reactive acyl-CoA moieties which will persist in areas of the cell proximal to lipid synthesis, beta-oxidation, protein acylation, and amino acid conjugation. The interaction of these xenobiotic acyl-CoAs with those processes may be important to their toxicity and/or detoxification.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

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.

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.

Evidence for the involvement of polyunsaturated fatty acids in the regulation of long-chain acyl CoA thioesterases and peroxisome proliferation in rat carcinosarcoma

The feeding of high-fat diets rich in polyunsaturated fatty acids (PUFAs) caused a marked increase in the acyl CoA thioesterase activity of the Walker 256 tumour. Diets containing lower levels of PUFAs did not alter the activity of acyl CoA thioesterase and the exposure of LLC-WRC256 tumour cells, in culture, to PUFAs (150 microM) also was ineffective in altering activity. The tumours from n-3 PUFA-rich and control diets were analysed by transmission electron microscopy in order to compare peroxisomal content. The presence of PUFAs led to an almost 10-fold increase in the number of peroxisomes present in the tumour tissue. A common feature of the PUFA-treated tumour was the presence of many cells containing highly condensed heterochromatin at the periphery of the nucleus, indicative of apoptosis. The sparsity of endoplasmic reticulum and the lack of detection of mitochondrial acyl CoA thioesterase, MTE-I, led to the conclusion that the increase in tumour acyl CoA thioesterase activity may be due to an increase in the activity of the peroxisomal enzyme.

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.

In vitro study of fenoprofen chiral inversion in rat: comparison of brain versus liver

The extent and the overall stereoselectivity of the combined steps involved in the chiral inversion of fenoprofen, a non-steroidal anti-inflammatory drug, was investigated in rat brain microsomes and cytosol. Results were compared with those obtained with the same liver subcellular compartments. Brain microsomes catalysed the stereoselective activation of the R(-)-enantiomer to its coenzyme A thioester with a specific activity approximately 10-fold less than that obtained with liver microsomes. Rat brain microsomes and cytosol mediated the racemization and hydrolysis of both R(-)- and S( + )-fenoprofenoyl-CoA. In brain fractions the epimerase activity was lower than in liver, whereas the hydrolysis process appeared more efficient. Thus, the data indicated that the three-step mechanism occurred in brain subcellular compartments leading to a minor chiral inversion of fenoprofen compared with that in liver.

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].

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.

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.

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.

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.