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

Acyl-CoA thioesterase activity of peroxisomal ABC protein ABCD1 is required for the transport of very long-chain acyl-CoA into peroxisomes

The ABCD1 protein, one of the four ATP-binding cassette (ABC) proteins in subfamily D, is located on the peroxisomal membrane and is involved in the transport of very long chain fatty acid (VLCFA)-CoA into peroxisomes. Its mutation causes X-linked adrenoleukodystophy (X-ALD): an inborn error of peroxisomal β-oxidation of VLCFA. Whether ABCD1 transports VLCFA-CoA as a CoA ester or free fatty acid is controversial. Recently, Comatose (CTS), a plant homologue of human ABCD1, has been shown to possess acyl-CoA thioesterase (ACOT) activity, and it is suggested that this activity is required for transport of acyl-CoA into peroxisomes. However, the precise transport mechanism is unknown. Here, we expressed human His-tagged ABCD1 in methylotrophic yeast, and characterized its ACOT activity and transport mechanism. The expressed ABCD1 possessed both ATPase and ACOT activities. The ACOT activity of ABCD1 was inhibited by p-chloromercuribenzoic acid (pCMB), a cysteine-reactive compound. Furthermore, we performed a transport assay with ABCD1-containing liposomes using 7-nitro-2–1,3-benzoxadiazol-4-yl (NBD)-labeled acyl-CoA as the substrate. The results showed that the fatty acid produced from VLCFA-CoA by ABCD1 is transported into liposomes and that ACOT activity is essential during this transport process. We propose a detailed mechanism of VLCFA-CoA transport by ABCD1.

Biogenesis and Function of Peroxisomes in Human Disease with a Focus on the ABC Transporter

Peroxisomes are indispensable organelles in mammals including humans. They are involved in the β-oxidation of very long chain fatty acids, and the synthesis of ether phospholipids and bile acids. Pre-peroxisomes bud from endoplasmic reticulum and peroxisomal membrane and matrix proteins are imported to the pre-peroxisomes. Then, matured peroxisomes grow by division. Impairment of the biogenesis and function of peroxisomes results in severe diseases. Since I first undertook peroxisome research in Prof. de Duve's laboratory at Rockefeller University in 1985, I have continuously studied peroxisomes for more than 30 years, with a particular focus on the ATP-binding cassette (ABC) transporters. Here, I review the history of peroxisome research, the biogenesis and function of peroxisomes, and peroxisome disease including X-linked adrenoleukodystrophy. The review includes the targeting and function of the ABC transporter subfamily D.

Molecular Basis for Vitamin A Uptake and Storage in Vertebrates

The ability to store and distribute vitamin A inside the body is the main evolutionary adaptation that allows vertebrates to maintain retinoid functions during nutritional deficiencies and to acquire new metabolic pathways enabling light-independent production of 11-cis retinoids. These processes greatly depend on enzymes that esterify vitamin A as well as associated retinoid binding proteins. Although the significance of retinyl esters for vitamin A homeostasis is well established, until recently, the molecular basis for the retinol esterification enzymatic activity was unknown. In this review, we will look at retinoid absorption through the prism of current biochemical and structural studies on vitamin A esterifying enzymes. We describe molecular adaptations that enable retinoid storage and delineate mechanisms in which mutations found in selective proteins might influence vitamin A homeostasis in affected patients.

Distinct populations of hepatic stellate cells in the mouse liver have different capacities for retinoid and lipid storage

Hepatic stellate cell (HSC) lipid droplets are specialized organelles for the storage of retinoid, accounting for 50–60% of all retinoid present in the body. When HSCs activate, retinyl ester levels progressively decrease and the lipid droplets are lost. The objective of this study was to determine if the HSC population in a healthy, uninjured liver demonstrates heterogeneity in its capacity for retinoid and lipid storage in lipid droplets. To this end, we utilized two methods of HSC isolation, which leverage distinct properties of these cells, including their vitamin A content and collagen expression. HSCs were isolated either from wild type (WT) mice in the C57BL/6 genetic background by flotation in a Nycodenz density gradient, followed by fluorescence activated cell sorting (FACS) based on vitamin A autofluorescence, or from collagen-green fluorescent protein (GFP) mice by FACS based on GFP expression from a GFP transgene driven by the collagen I promoter. We show that GFP-HSCs have: (i) increased expression of typical markers of HSC activation; (ii) decreased retinyl ester levels, accompanied by reduced expression of the enzyme needed for hepatic retinyl ester synthesis (LRAT); (iii) decreased triglyceride levels; (iv) increased expression of genes associated with lipid catabolism; and (v) an increase in expression of the retinoid-catabolizing cytochrome, CYP2S1. Conclusion: Our observations suggest that the HSC population in a healthy, uninjured liver is heterogeneous. One subset of the total HSC population, which expresses early markers of HSC activation, may be “primed” and ready for rapid response to acute liver injury.

Vitamin A metabolism: an update

Retinoids are required for maintaining many essential physiological processes in the body, including normal growth and development, normal vision, a healthy immune system, normal reproduction, and healthy skin and barrier functions. In excess of 500 genes are thought to be regulated by retinoic acid. 11-cis-retinal serves as the visual chromophore in vision. The body must acquire retinoid from the diet in order to maintain these essential physiological processes. Retinoid metabolism is complex and involves many different retinoid forms, including retinyl esters, retinol, retinal, retinoic acid and oxidized and conjugated metabolites of both retinol and retinoic acid. In addition, retinoid metabolism involves many carrier proteins and enzymes that are specific to retinoid metabolism, as well as other proteins which may be involved in mediating also triglyceride and/or cholesterol metabolism. This review will focus on recent advances for understanding retinoid metabolism that have taken place in the last ten to fifteen years.

Acidic residues emulate a phosphorylation switch to enhance the activity of rat hepatic neutral cytosolic cholesterol esterase

Site-directed mutagenesis of rat hepatic neutral cytosolic cholesteryl ester hydrolase (rhncCEH) was used to substitute acidic, basic or neutral amino acid residues for Ser506, required for activation by protein kinase A. The substitution of acidic Asp506 resulted in esterase activities with cholesteryl oleate, p-nitrophenylcaprylate (PNPC) and p-nitrophenylacetate (PNPA) equivalent to those of native rhncCEH with Ser506. The substitution of 2 acidic residues (Asp505/506), emulating the 2 negative charges of phosphoserine, resulted in a 10-fold greater cholesterol esterase activity than that of native rhncCEH, similar to the activity of rhncCEH treated with protein kinase A. In contrast to mutants with Ser506, protein kinase A did not increase the specific activities of mutants with Asp505/506. The substitution of basic (Lys506) or neutral (Asn506) residues abolished activity with cholesteryl oleate but not PNPC or PNPA. The substitution of neutral Gln for basic residues Lys496/Arg503 also abolished cholesterol esterase activity but not PNPC- and PNPA-esterase activities. These structure-activity relationships are modeled by homology with a recently reported crystal structure for the homologous human triacylglycerol hydrolase. The results suggest that the cholesterol esterase activity of carboxylesterases is enhanced by interactions between one or more basic residues on helix alpha16 (residues 485-503) and acidic groups at residues 505-506 in the adjacent surface loop.

Biochemical and Molecular Characterization of ACH2, an Acyl-CoA Thioesterase from Arabidopsis thaliana

By using computer-based homology searches of the Arabidopsis genome, we identified the gene for ACH2, a putative acyl-CoA thioesterase. With the exception of a unique 129-amino acid N-terminal extension, the ACH2 protein is 17-36% identical to members of a family of acyl-CoA thioesterases that are found in both prokaryotes and eukaryotes. The eukaryotic homologs of ACH2 are peroxisomal acyl-CoA thioesterases that are up-regulated during times of increased fatty acid oxidation, suggesting potential roles in peroxisomal beta-oxidation. We investigated ACH2 to determine whether it has a similar role in the plant cell. Like its eukaryotic homologs, ACH2 carries a putative type 1 peroxisomal targeting sequence (-SKL(COOH)), and maintains all the catalytic residues typical of this family of acyl-CoA thioesterases. Analytical ultracentrifugation of recombinant ACH2-6His shows that it associates as a 196-kDa homotetramer in vitro, a result that is significant in light of the cooperative kinetics demonstrated by ACH2-6His in vitro. The cooperative effects are most pronounced with medium chain acyl-CoAs, where the Hill coefficient is 3.8 for lauroyl-CoA, but decrease for long chain acyl-CoAs, where the Hill coefficient is only 1.9 for oleoyl-CoA. ACH2-6His hydrolyzes both medium and long chain fatty acyl-CoAs but has highest activity toward the long chain unsaturated fatty acyl-CoAs. Maximum rates were found with palmitoleoyl-CoA, which is hydrolyzed at 21 micromol/min/mg protein. Additionally, ACH2-6His is insensitive to feedback inhibition by free CoASH levels as high as 100 microm. ACH2 is most highly expressed in mature tissues such as young leaves and flowers rather than in germinating seedlings where beta-oxidation is rapidly proceeding. Taken together, these results suggest that ACH2 activity is not linked to fatty acid oxidation as has been suggested for its eukaryotic homologs, but rather has a unique role in the plant cell.

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.

Involvement of liver carboxylesterases in the in vitro metabolism of lidocaine

Although lidocaine has been used clinically for more than half a century, the metabolism has still not been fully elucidated. In the present study we have addressed the involvement of hydroxylations, deethylations, and ester hydrolysis in the metabolism of lidocaine to 2,6-xylidine. Using microsomes isolated from male rat liver, we found that lidocaine is mainly metabolized by deethylation to N-(N-ethylglycyl)-2,6-xylidine, and N-(N-ethylglycyl)-2,6-xylidine is mainly metabolized to N-glycyl-2,6-xylidine, also by deethylation. However, 2,6-xylidine can be formed both from lidocaine and N-(N-ethylglycyl)-2,6-xylidine, but not from N-glycyl-2,6-xylidine, in an NADPH-independent reaction, suggesting that the amido bond in these compounds can be directly hydrolyzed by esterases. To test this hypothesis, we incubated lidocaine, N-(N-ethylglycyl)-2,6-xylidine, and N-glycyl-2,6-xylidine with purified liver carboxylesterases. Rat liver microsomal carboxylesterase ES-10, but not carboxylesterase ES-4, hydrolyzed lidocaine and N-(N-ethylglycyl)-2,6-xylidine to 2,6-xylidine, identifying this esterase as a candidate enzyme in the metabolism of lidocaine.

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.

Mutation of residues 423 (Met/Ile), 444 (Thr/Met), and 506 (Asn/Ser) confer cholesteryl esterase activity on rat lung carboxylesterase. Ser-506 is required for activation by cAMP-dependent protein kinase

Site-directed mutagenesis is used to identify amino acid residues that dictate reported differences in substrate specificity between rat hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) and rat lung carboxylesterase (LCE), proteins differing by only 4 residues in their primary sequences. Beginning with LCE, the substitution Met(423) --> Ile(423) alone or in combination with other mutations increased activity with p-nitrophenylcaprylate (PNPC) relative to more hydrophilic p-nitrophenylacetate (PNPA), typical of hncCEH. The substitution Thr(444) --> Met(444) was necessary but not sufficient for expression of cholesteryl esterase activity in COS-7 cells. The substitution Asn(506) --> Ser(506), creating a potential phosphorylation site, uniformly increased activity with both PNPA and PNPC, was necessary but not sufficient for expression of cholesteryl esterase activity and conferred susceptibility to activation by cAMP-dependent protein kinase, a property of hncCEH. The 3 mutations in combination were necessary and sufficient for expression of cholesteryl esterase activity by the mutated LCE. The substitution Gln(186) --> Arg(186) selectively reduced esterase activity with PNPA and PNPC but was not required for cholesteryl esterase activity. Homology modeling from x-ray structures of acetylcholinesterases is used to propose three-dimensional models for hncCEH and LCE that provide insight into the effects of these mutations on substrate specificity.

Altered expression of the carboxylesterases ES-4 and ES-10 by peroxisome proliferator chemicals

The nonspecific carboxylesterases (EC.3.1.1.1) are a large group of enzymes that play important roles in the metabolism of foreign xenobiotics and endogenous lipids, including activators of the peroxisome proliferator-activated receptor alpha, a nuclear receptor that is the central mediator of peroxisome proliferator (PP) effects in the rodent liver. A number of reports have demonstrated that PP exposure leads to alterations in levels of carboxylesterases in the liver. In this study, we determined by Western blot analysis whether exposure to diverse PP results in alteration of expression of two highly expressed microsomal carboxylesterases. Chronic exposure to the PP WY-14,643 (WY) and gemfibrozil (GEM), but not di-n-butyl phthalate (DBP), led to decreases in ES-4 in male rat livers. ES-4 was increased in female rat livers treated with GEM. WY exposure led to decreases in ES-10 in male and female rat livers. ES-10 was increased in female rats treated with DBP. Compared with other end points that are altered within days after PP exposure, the downregulation of ES-4 and ES-10 by WY was considerably slower, occurring between 1 and 5 weeks of exposure. Decreased expression of ES-4 was observed at doses of WY or GEM as low as 10 or 8000 ppm, respectively, whereas decreased expression of ES-10 was more resistant to changes by any PP occurring only with WY at doses as low as 50 ppm. After chronic exposure to WY or diethylhexyl phthalate in wild-type mice, kidney, but not liver, expression of ES-4 and ES-10 was downregulated. These decreases in kidney ES expression were not observed in PPARalpha-null mice lacking a functional PPARalpha gene, demonstrating the importance of this transcription factor in these changes. These studies demonstrate that ES protein expression is under complex control by PP that is sex- and compound-dependent. These results lend support to the hypothesis that PP exposure leads to a reprogramming of expression of enzymes important in the metabolism of PPARalpha activators.

Purification, molecular cloning, and functional expression of dog liver microsomal acyl-CoA hydrolase: a member of the carboxylesterase multigene family

To clarify the reason for the high acyl-CoA hydrolase (ACH) activity found in dog liver microsomes, the ACH was purified to homogeneity using column chromatography. The purified enzyme, named ACH D1, exhibited a subunit molecular weight of 60 KDa. The amino terminal amino acid sequence showed a striking homology with rat liver carboxylesterase (CES) isozymes. ACH D1 possessed hydrolytic activities toward esters containing xenobiotics in addition to acyl-CoA thioesters, and these activities were inhibited by a specific inhibitor of CES or by CES RH1 antibodies. These findings suggest that this protein is a member of the CES multigene family. Since ACH D1 appears to be a protein belonging to the CES family, we cloned the cDNA from a dog liver lambdagt10 library with a CES-specific probe. The clone obtained, designated CES D1, possessed several motifs characterizing CES isozymes, and the deduced amino acid sequences were 100% identical with those of ACH D1 in the first 18 amino acid residues. When it was expressed in V79 cells, it showed high catalytic activities toward acyl-CoA thioesters. In addition, the characteristics of the expressed protein were identical with those of ACH D1 in many cases, suggesting that CES D1 encodes liver microsomal ACH D1.

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.

Lipases and carboxylesterases: possible roles in the hepatic utilization of vitamin A

The formation and hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Although retinyl esters are found in a variety of tissues and cell types, most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This paper summarizes the current state of our knowledge about the identity, function and regulation of the hepatic enzymes that are potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases.

Microsomal long-chain acyl-CoA thioesterase (carboxylesterase ES-4) is regulated by thyroxine

Long chain acyl-CoA thioesterase activity is mainly located in microsomes after subcellular fractionation of liver from untreated rats. The physiological function and regulation of expression of this activity is not known. In the present study we have investigated the effect of thyroxine on expression of carboxylesterase ES-4, the major acyl-CoA thioesterase of liver microsomes. Thyroidectomy of rats decreased the palmitoyl-CoA thioesterase activity to about 25% of normal activity. This decrease was accompanied by similar decreases at the protein and mRNA levels (31% and 57%, respectively, of controls). Treatment with thyroxine completely reversed the effect of thyroidectomy and resulted in elevated levels in both thyroidectomized and control rats. For reasons of comparison we also studied the possibility that ES-10 and ES-2, two other members of the same gene family, are affected by thyroxine. ES-10 was not changed at the protein or mRNA level by any of the treatments, while ES-2 expression in liver was decreased by thyroxine treatment. The data shows that changes in activity and expression of ES-4 correlate to thyroxine status in the rat suggesting a physiological regulatory role by this hormone. Since thyroxine regulates the expression of lipogenic enzymes, these results are consistent with a function for this microsomal acyl-CoA thioesterase in fatty acid synthesis and/or secretion, rather than in oxidative degradation of fatty acids.

Formation of fatty acid ethyl esters in rat liver microsomes. Evidence for a key role for acyl-CoA: ethanol O-acyltransferase

Fatty acid ethyl esters have been detected in high concentrations in organs commonly damaged by alcohol abuse and are regarded as being important non-oxidative metabolites of ethanol. The formation of fatty acid ethyl esters (FAEEs) has been ascribed to two enzymic activities, acyl-CoA : ethanol O-acyltransferase (AEAT) and FAEE synthase. In the present study we determined AEAT and FAEE synthase activities in isolated rat liver microsomes and further characterized the microsomal AEAT activity in more detail. The determined AEAT and FAEE synthase activities were found to be similar (about 1.7 nmol.min-1.mg-1). However, the AEAT activity was increased about sixfold by the addition of 250 microm bis-(4-nitrophenyl) phosphate (a serine esterase inhibitor) to the incubation whereas FAEE synthase activity was completely inhibited. p-Hydroxymercuribenzoic acid (a cysteine-reacting compound) also stimulated AEAT activity (about fourfold) but had no effect on FAEE synthase activity. The effects of the inhibitors suggest that the formation of FAEEs by AEAT was severely counteracted by enzymic hydrolysis of the substrate (acyl-CoA) and to a lesser extent the product by serine esterases. dl-Melinamide, a hypocholesterolaemic drug, was found to be a very potent inhibitor of AEAT activity with an IC50 value of about 2.5 microm. Furthermore, we compared the activities of two purified microsomal carboxylesterases, ES-4 and ES-10, and identified ES-4 as the enzyme responsible for hydrolysis of FAEEs. The two carboxyesterases were also tested for FAEE synthase activity, but neither had any detectable activity. Esterase ES-4 was found to have some AEAT activity, but it was low. When measured under optimal conditions without competing hydrolysis the capacity of AEAT is thus considerably higher than FAEE synthase and the results are consistent with an important role for AEAT in the formation of ethyl esters. As the ratio acyl-CoA/non-esterified fatty acids is high under normal conditions, AEAT is probably the most important enzyme in fatty acid ethyl ester formation.

Immunoreactivity of neural crest-derived cells in thymic tissue developing under the rat kidney capsule

In order to study the functional development of a thymus in an experimental model, small pieces of adult rat thymic tissue were cultured for 9 days and implanted under the kidney capsule of littermates. The tissues were examined with a panel of antibodies raised against thymic and neural factors and neural crest cells at intervals from 5 to 13 days. At 5 days post-implantation, there were groups of L1+ cells within the implants that reacted with antibodies raised against neural and neural crest cell markers. L1+ cells were highly mitotic, rounded cells measuring 8.7 +/- 0.6 micrometer in diameter. Double immunostaining with different combinations of antibodies showed that 94% of the L1+ cells were also TH+, and many were HNK-1/NCAM+, PGP 9.5+, NGF+, chromogranin A+, VIP+, S100+, CGRP+, GAD+, and A2B5+. A few were also pan-cytokeratin+. These results indicate that these cells are derived from neural crest derived cells and belong to the neuroepithelial line of development. The L1+ cells were most numerous before nerves appeared (about Day 9) and reduced in number and extent as the thymus differentiated. The neural crest cells occasionally had long cytoplasmic extensions, but it was not possible to decide if they formed the nerves that appeared in the implants. Adult thymuses also contained a population of L1+ and HNK-1/NCAM+ cells, mainly in the subcapsular cortex, the septa, and the medulla. These cells could be a source of neural crest cells able to repopulate the implant. The adult thymus may always contain a reservoir of cells potentially capable of producing neuropeptides and transmitter factors required for thymic growth and regeneration.

Lipases and carboxylesterases: possible roles in the hepatic metabolism of retinol

The formation of hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Retinyl esters are found in a variety of tissues and cell types, but most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This review summarizes current knowledge about the identity, function, and regulation of the hepatic enzymes potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases. Although there is accumulating evidence that these enzymes function as retinyl ester hydrolases in vitro, it is not clear which play important physiological roles in hepatic retinyl ester metabolism.

The mammalian carboxylesterases: from molecules to functions

Multiple carboxylesterases (EC 3.1.1.1) play an important role in the hydrolytic biotransformation of a vast number of structurally diverse drugs. These enzymes are major determinants of the pharmacokinetic behavior of most therapeutic agents containing ester or amide bonds. Carboxylesterase activity can be influenced by interactions of a variety of compounds either directly or at the level of enzyme regulation. Since a significant number of drugs are metabolized by carboxylesterase, altering the activity of this enzyme class has important clinical implications. Drug elimination decreases and the incidence of drug-drug interactions increases when two or more drugs compete for hydrolysis by the same carboxylesterase isozyme. Exposure to environmental pollutants or to lipophilic drugs can result in induction of carboxylesterase activity. Therefore, the use of drugs known to increase the microsomal expression of a particular carboxylesterase, and thus to increase associated drug hydrolysis capacity in humans, requires caution. Mammalian carboxylesterases represent a multigene family, the products of which are localized in the endoplasmic reticulum of many tissues. A comparison of the nucleotide and amino acid sequence of the mammalian carboxylesterases shows that all forms expressed in the rat can be assigned to one of three gene subfamilies with structural identities of more than 70% within each subfamily. Considerable confusion exists in the scientific community in regards to a systematic nomenclature and classification of mammalian carboxylesterase. Until recently, adequate sequence information has not been available such that valid links among the mammalian carboxylesterase gene family or evolutionary relationships could be established. However, sufficient basic data are now available to support such a novel classification system.

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.

Proline-specific dipeptidyl peptidase from the blue blowfly Calliphora vicina hydrolyzes in vitro the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF)

To elucidate the mechanisms of inactivation of the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF) in the blue blowfly Calliphora vicina, we investigated its proteolytic degradation. In homogenates and membrane and soluble fractions, this hexapeptide (sequence: NPTNLH) was hydrolyzed into two fragments, NP and TNLH, suggesting the involvement of a proline-specific dipeptidyl peptidase. The dipeptidyl peptidase activity was highest in the late larval stage. It was purified 240-fold from soluble fractions of pupae of mixed age and classified on the basis of several catalytic properties as an invertebrate homologue of mammalian dipeptidyl peptidase IV (EC 3.4.14.5). Fly dipeptidyl peptidase IV has a molecular mass of 200 kDa, showed a pH optimum of 7.5-8.0 with the chromogenic substrate Gly-Pro-4-nitroanilide, and cleaved other chromogenic substrates with penultimate Pro or, with lower activity, Ala. It liberated Xaa-Pro dipeptides from the N-terminus of several bioactive peptides including substance P, neuropeptide Y, and peptide YY but not from bradykinin, indicating that the peptide bond between the two proline residues was resistant to cleavage. Fly dipeptidyl peptidase belongs to the serine class of proteases as the mammalian enzyme does; the fly enzyme, however, is not inhibited by several selective or nonselective inhibitors of its mammalian counterpart. It is suggested that dipeptidyl peptidases exert a regulatory role for the clearance not only of TMOF in files but for other bioactive peptides in various invertebrates.

Isolation of carboxylester lipase (CEL) isoenzymes from Candida rugosa and identification of the corresponding genes

The yeast Candida rugosa produces extracellular lipases which are widely used for industrial purposes. A commercial lipase preparation from this yeast can be separated into several isoenzymes which differ in carbohydrate content, isolelectric point, substrate specificity, and primary sequence. We have here purified and characterized three lipases, which also hydrolyze p-nitrophenyl esters, from a commercial preparation of this yeast. These three carboxylester lipases (CELs) elute differently on hydrophobic interaction chromatography, and have different carbohydrate contents and substrate specificities. Sequence analysis of their amino termini and peptides generated by LysC treatment showed that CEL-1 and CEL-3 probably have identical primary structure while CEL-2 was proven to be a different enzyme. Sequence comparison showed that both CEL-1 and CEL-3 are products of the LIP1 gene and that CEL-2 is the gene product of LIP2, cloned by Longhi et al. (Biochim. Biophys. Acta 1131, 227-232, 1992).

Purification and characterization of a neutral, bile salt-independent retinyl ester hydrolase from rat liver microsomes. Relationship To rat carboxylesterase ES-2

A neutral, bile salt-independent retinyl ester hydrolase (NREH) has been purified from a rat liver microsomal fraction. The purification procedure involved detergent extraction, DEAE-Sepharose ion exchange, Phenyl-Sepharose hydrophobic interaction, Sephadex G-100 and Sephacryl S-200 gel filtration chromatographies, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The isolated enzyme has an apparent molecular mass of approximately 66 kDa under denaturing conditions on SDS-PAGE. Analysis of the amino acid sequences of four peptides isolated after proteolytic digestion revealed that the enzyme is highly homologous with other rat liver carboxylesterases. In particular, the sequences of the four peptides of the NREH (60 amino acids total) were identical to those of a rat carboxylesterase expressed in the liver (Alexson, S. E. H., Finlay, T. H., Hellman, U., Svensson, L. T., Diczfalusy, U., and Eggertsen, G. (1994) J. Biol. Chem. 269, 17118-17124). Antibodies against this enzyme also react with the purified NREH. Purified NREH shows a substrate preference for retinyl palmitate over triolein and did not catalyze the hydrolysis of cholesteryl oleate. With retinyl palmitate as substrate, the enzyme had a pH optimum of 7 and showed apparent saturation kinetics, with half-maximal activity achieved at substrate concentrations (Km) of approximately 70 microM.

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.

Peroxisome proliferators alter the expression of estrogen-metabolizing enzymes

Exposure to some peroxisome proliferator chemicals (PPC) leads to toxic effects on sex organ function possibly by alterations of steroid hormone metabolism. A systematic search for genes whose mRNA levels are modulated by the PPC WY-14643 (WY) was carried out in rat liver, a site of steroid hormone metabolism. The sequence of one up-regulated cDNA (2480 bp) was predicted to encode a protein of 735 amino acids with 82% identity to the porcine 17 beta-hydroxysteroid dehydrogenase type IV (HSD IV) originally isolated as a 17 beta-estradiol dehydrogenase. The rat HSD IV was localized to peroxisomes and was regulated by diverse PPC by two distinct mechanisms. Induction of HSD IV and acyl-CoA oxidase (ACO) proteins in rat liver at different treatment times and concentrations of gemfibrozil (GEM) and di-n-butyl phthalate (DBP) were almost identical, suggesting that HSD IV mRNA induction involves the peroxisome proliferator-activated receptor alpha, a regulator of ACO. In contrast, HSD IV protein levels were only weakly induced by WY, a strong inducer of ACO protein, even though the levels of both HSD IV and ACO mRNA were strongly stimulated by WY. Thus HSD IV protein levels were uniquely regulated pretranslationally by WY. In addition to HSD IV we also identified the male-specific alpha 2u-globulin as a PPC down-regulated gene. This prompted us to examine the expression of another male-specific gene, CYP2C11, that catalyzes the hydroxylations of estradiol at the 2 and 16 alpha positions. Cyp2C11 protein expression in rat liver was either decreased or completely abolished after a 3-week treatment by GEM or WY, respectively. Decreased expression of enzymes which inactivate estradiol including Cyp2C11, and the reported increased expression of aromatase may explain why male rats exposed to diverse PPC have higher serum estradiol levels. These higher estradiol levels in male rats have been thought to be mechanistically linked to Leydig cell hyperplasia and adenomas. Increased conversion of estradiol to the less active estrone by HSD IV induction may explain how exposure to the phthalate di-(2-ethylhexyl) phthalate leads to decreases in serum estradiol levels and suppression of ovulation in female rats.

Acyl-coenzyme A:cholesterol O-acyltransferase is not identical to liver microsomal carboxylesterase

Acyl-coenzyme A (CoA):cholesterol O-acyltransferase (ACAT) is responsible for esterification of cholesterol in the cell. The enzyme has never been purified, but two cDNA sequences coding for this enzyme were recently reported. One of the sequences was identical to human liver carboxylesterase. We have used inhibitors to elucidate the relation between microsomal carboxylesterase, acyl-CoA hydrolase (ACH), and ACAT activities in rat liver. Low concentrations of serine esterase inhibitors strongly inhibited carboxylesterase and acyl-CoA hydrolase activities but stimulated ACAT activity. At higher concentrations, ACAT activity was also inhibited. A sulfhydryl-modifying agent was found to be a potent inhibitor of ACAT without affecting carboxylesterase activity. Similarly, two specific ACAT inhibitors, DL-melinamide and PD 138142-15, inhibited ACAT activity but did not affect carboxylesterase or ACH activities. Our data thus exclude ACAT as a liver microsomal carboxylesterase. The complex inhibition patterns observed with serine esterase inhibitors indicate that carboxylesterases and ACHs may interfere with ACAT activity by competing for the substrate. It is obvious that final identification of ACAT requires demonstration of an active homogenous protein.

Purification of a 60-kDa protein from chicken liver associated with the internal nuclear matrix and closely related to carboxylesterases

A 60-kDa protein was purified from chicken liver internal nuclear matrix and its nuclear localization was confirmed by immunofluorescence analysis. Structural information acquired from sequence analysis of the intact protein and of fragments obtained from enzymatic and chemical cleavages strongly suggests that it belongs to the carboxylesterases family, even if with some very peculiar features. The N-terminal sequence of the 60-kDa protein is completely different from the other carboxylesterases, but is similar to a region that is normally internal to all mammalian esterase sequences and localized after the serine residue at the active site. This suggests that the protein may be derived from a gene duplication and/or rearrangement. Since the 60-kDa protein shows a low esterase activity of about 0.2 micromol x min(-1) x mg(-1) using either p-nitrophenyl acetate or p-nitrophenyl butyrate as substrates, it is not possible to rule out that the protein shares only a sequence similarity with carboxylesterases and is not a true esterase. Otherwise it could be an esterase which has developed different properties, i.e. a special substrate specificity, the requirement of additional factors or a different stability in solution. In the latter case, this protein could be related to the physiological control of hydrolysis of exogenous and endogenous esters which can act on nuclear functions and/or metabolism.

Cloning and sequencing of rat liver carboxylesterase ES-4 (microsomal palmitoyl-CoA hydrolase)

A cDNA which encodes a carboxylesterase of 561 amino acid residues including a cleavable signal peptide is described. The enzyme expressed in COS cells migrates during PAGE (SDS-, and non-denaturing) as a single prominent band in the region of liver ES-4. It ends in the C-terminal cell-retention signal -HNEL, which, in COS cells overexpressing the enzyme, appears to be slightly less efficient than the signals -HTEL and -HVEL of ES-3 and ES-10 respectively. Glycosylation is not essential for intracellular retention, but leads to a higher activity. As do many carboxylesterases, the enzyme expressed in COS cells hydrolyses omicron-nitrophenyl acetate and alpha-naphthyl acetate. It also hydrolyses acetanilide, although less efficiently than ES-3, and, distinctively, palmitoyl-CoA. In addition to the four canonical Cys residues of the carboxylesterases, it contains a fifth, unpaired Cys336, which apparently is not essential for the catalytic properties. Indeed, treatment with iodoacetamide or substitution of Cys336 by Phe does not markedly alter the activity of the enzyme on the various substrates. The predicted structure of ES-4 is highly homologous to that of two other recently cloned esterases which also end in -HNEL [Yan, Yang, Brady and Parkinson (1994) J. Biol. Chem. 269, 29688-29696; Yan, Yang, and Parkinson (1995) Arch. Biochem. Biophys. 317, 222-234]. Together, these isoenzymes probably account for the closely spaced bands observed in the region of ES-4 in non-denaturing PAGE.

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.

Monomeric and dimeric forms of cholesterol esterase from Candida cylindracea. Primary structure, identity in peptide patterns, and additional microheterogeneity

Cholesterol esterase from Candida cylindracea was separated into two fractions, corresponding to a dimeric and a monomeric form. Fingerprint analysis after lysine cleavages shows identical patterns, suggesting lack of primary differences. Crystals obtained from the two proteins differ and suggest the possibility of an equilibrium between the two forms, influenced by the substrate cholesterol linoleate, which appears to stabilize the more active, dimeric form. All crystals have dimers as the asymmetric unit. The primary structure of the enzyme was determined at the peptide level and shows only one difference, Leu-350 instead of Ile, from a DNA-deduced amino acid sequence, and conservation of features typical for cholesterol esterases characterized.