Molecular cloning and nucleotide sequence of cDNA of microsomal carboxyesterase E1 of rat liver

Insights into the physiological role of pig liver esterase: isoenzymes show differences in the demethylation of prenylated proteins

The possible physiological role of PLE (E.C. 3.1.1.1) located in the endoplasmic reticulum (ER) of pig liver cells in the conversion of endogenous compounds was investigated as it was reported, that PLE acts as prenylated methylated protein methyl esterase (PMPMEase) hydrolysing methylesters of prenylated proteins. Using the specific PMPMEase substrate benzoyl-glycyl-farnesyl-cysteine methyl ester (BzGFCM), six different PLE isoenzymes expressed recombinantly in the yeast Pichia pastoris were found active. Activities ranged from 1.6-15.6mU per mg protein and it is suggested that Pro285 has a major influence on high activity. In addition, the role of the C-terminal HAEL retention signal for translocation of pig liver esterase (PLE) in the endoplasmic reticulum (ER) of eukaryotic cells was studied using the gamma-isoenzyme of PLE expressed in Pichia pastoris. Using truncated versions (HAE, HA, H and without retention signal) of the enzyme it was found that in contrast to earlier reports no influence of the signal peptide on the expression rate of PLE was found. However, higher enzyme activities were obtained in the periplasmatic fraction compared to the supernatant irrespective of the presence or absence of HAEL and the trimeric formation seems to occur in the supernatant of P. pastoris X33 enabling an easier transition of monomeric forms through cell membranes.

Opossum carboxylesterases: sequences, phylogeny and evidence for CES gene duplication events predating the marsupial-eutherian common ancestor

Background

Carboxylesterases (CES) perform diverse metabolic roles in mammalian organisms in the detoxification of a broad range of drugs and xenobiotics and may also serve in specific roles in lipid, cholesterol, pheromone and lung surfactant metabolism. Five CES families have been reported in mammals with human CES1 and CES2 the most extensively studied. Here we describe the genetics, expression and phylogeny of CES isozymes in the opossum and report on the sequences and locations of CES1, CES2 and CES6 'like' genes within two gene clusters on chromosome one. We also discuss the likely sequence of gene duplication events generating multiple CES genes during vertebrate evolution.

Results

We report a cDNA sequence for an opossum CES and present evidence for CES1 and CES2 like genes expressed in opossum liver and intestine and for distinct gene locations of five opossum CES genes,CES1, CES2.1, CES2.2, CES2.3 and CES6, on chromosome 1. Phylogenetic and sequence alignment studies compared the predicted amino acid sequences for opossum CES with those for human, mouse, chicken, frog, salmon and Drosophila CES gene products. Phylogenetic analyses produced congruent phylogenetic trees depicting a rapid early diversification into at least five distinct CES gene family clusters: CES2, CES1, CES7, CES3, and CES6. Molecular divergence estimates based on a Bayesian relaxed clock approach revealed an origin for the five mammalian CES gene families between 328–378 MYA.

Conclusion

The deduced amino acid sequence for an opossum cDNA was consistent with its identity as a mammalian CES2 gene product (designated CES2.1). Distinct gene locations for opossum CES1 (1: 446,222,550–446,274,850), three CES2 genes (1: 677,773,395–677,927,030) and a CES6 gene (1: 677,585,520–677,730,419) were observed on chromosome 1. Opossum CES1 and multiple CES2 genes were expressed in liver and intestine. Amino acid sequences for opossum CES1 and three CES2 gene products revealed conserved residues previously reported for human CES1 involved in catalysis, ligand binding, tertiary structure and organelle localization. Phylogenetic studies indicated the gene duplication events which generated ancestral mammalian CES genes predated the common ancestor for marsupial and eutherian mammals, and appear to coincide with the early diversification of tetrapods.

Molecular cloning and analysis of a novel teratocyte-specific carboxylesterase from the parasitic wasp, Dinocampus coccinellae

Teratocytes derived from the embryonic membrane (serosa) of parasitoids are released into the host hemocoel when the parasitoid eggs hatch, where they perform several functions during the post-embryonic stage. A full-length cDNA encoding a putative carboxylesterase was isolated from the teratocytes of Dinocampus coccinellae and was designated as teratocyte-specific carboxylesterase (TSC). It contained an open reading frame of 2571 bp coding for a protein of 857 amino acids with a calculated molecular mass of 89 kDa. The deduced amino acid sequence had many structural features that are highly conserved among serine hydrolases including Ser, Glu and His as a catalytic triad, carboxylesterase type-B (FGGNPNSVTLLGYSAG)/ lipase-serine (VTLLGYSAGA) active sites, and six N-glycosylation sites. Interestingly, the mRNA encoding the TSC gene was expressed exclusively in teratocytes but not in the parasitoid larva or in the non-parasitized host. Most notably, the TSC protein was distinguished by an insertion of 294 amino acids towards the N-terminal region and was flanked by carboxylesterase domains. Furthermore, sequence alignment and homology search revealed these additional amino acids to be unique to TSC and the insertion contributed significantly to its molecular mass resulting in a larger protein than other esterases. In addition to sequence analysis, the possible role of TSC in relation to the host (Coccinella septempunctata) and parasitoid (D. coccinellae) system is discussed.

Effect of flavonoids from various Mediterranean plants on enzymatic activity of intestinal carboxylesterase

Flavonol compounds of three Mediterranean plants from the Algerian Atlas used traditionally in Arab folk medicine, Arenaria serpyllifolia, Rhamnus alaternus and Thapsia garganica, were found to inhibit the enzymatic activities of both rat intestine and purified porcine liver carboxylesterase in a concentration-dependent manner. Results indicate that the flavonol compounds from the aerial part of these plants lead to the inactivation of the CE pI = 5.1 with Ki of micromolar range. These results encourage us to perform further biological investigation.

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.

Identification, expression, and purification of a pyrethroid-hydrolyzing carboxylesterase from mouse liver microsomes

Carboxylesterases are enzymes that catalyze the hydrolysis of a wide range of ester-containing endogenous and xenobiotic compounds. Although the use of pyrethroids is increasing, the specific enzymes involved in the hydrolysis of these insecticides have yet to be identified. A pyrethroid-hydrolyzing enzyme was partially purified from mouse liver microsomes using a fluorescent reporter similar in structure to cypermethrin (Shan, G., and Hammock, B. D. (2001) Anal. Biochem. 299, 54-62 and Wheelock, C. E., Wheelock, A. M., Zhang, R., Stok, J. E., Morisseau, C., Le Valley, S. E., Green, C. E., and Hammock, B. D. (2003) Anal. Biochem. 315, 208-222) and subsequently identified as a carboxylesterase (NCBI accession number BAC36707). The expressed sequence tag was then cloned, expressed in baculovirus, and purified to homogeneity. Kinetic constants for a large number of both type I and type II pyrethroid or pyrethroid-like substrates were determined. This esterase possesses similar kinetic constants for cypermethrin and its fluorescent-surrogate (k(cat) = 0.12 +/- 0.03 versus 0.11 +/- 0.01 s(-1)). Compared with their cis- counterparts, trans-permethrin and cypermethrin were hydrolyzed 22- and 4-fold faster, respectively. Of the four fenvalerate isomers the (2R)(alphaR)-isomer was hydrolyzed at least 1 order of magnitude faster than any other isomer. However, it is unlikely that this enzyme accounts for the total pyrethroid hydrolysis in the microsomes because both isoelectrofocusing and native PAGE indicate the presence of a second region of cypermethrin-metabolizing enzymes. A second carboxylesterase gene (NCBI accession number NM_133960), isolated during a cDNA mouse liver library screening, was also found to hydrolyze pyrethroids. Both these enzymes could be used as preliminary tools in establishing the relative toxicity of new pyrethroids.

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.

Structure-function relationships in the carboxylic-ester-hydrolase superfamily. Disulfide bridge arrangement in porcine intestinal glycerol-ester hydrolase

CNBr fragments from porcine intestinal glycerol-ester hydrolase were separated by SDS/PAGE under reducing and nonreducing conditions, and their amino-acid sequences were analysed. Two intra-chain disulfide bridges were identified, namely Cys70-Cys99 (loop A) and Cys256-Cys267 (loop B). As the Cys71 sulfhydryl group could not be alkylated with iodoacetamide, it is suggested that the residue is blocked rather than being present in the free form. The two disulfide bridges of intestinal glycerol-ester hydrolase are present in the cholinesterase family, although the enzyme showed only about 35% identity with these proteins. Furthermore, the finding that glycerol-ester hydrolase was partly inactivated under reducing conditions suggests that one or both disulfide bridges are important for the enzyme conformation. Lastly, glycerol-ester hydrolase was also found to hydrolyse cholinergic substrates, although residues Trp86 and Asp74 which are considered to be the main constituents of the 'anionic' subsite responsible for substrate binding in cholinesterases were absent from loop A. Other amino-acid residues in the glycerol-ester hydrolase may therefore be responsible for the binding of cholinergic substrates to the enzyme.

Localization of a candidate surfactant convertase to type II cells, macrophages, and surfactant subfractions

Pulmonary surfactant exists in the alveolus in several distinct subtypes that differ in their morphology, composition, and surface activity. Experiments by others have implicated a serine hydrolase in the production of the inactive small vesicular subtype of surfactant (N. J. Gross and R. M. Schultz. Biochim. Biophys. Acta 1044: 222-230, 1990). Our laboratory recently identified this enzyme in the rat as the serine carboxylesterase ES-2 [F. Barr, H. Clark, and S. Hawgood. Am. J. Physiol. 274 (Lung Cell. Mol. Physiol. 18): L404-L410, 1998]. In the present study, we determined the cellular sites of expression of ES-2 in rat lung using a digoxygenin-labeled ES-2 riboprobe. ES-2 mRNA was localized to type II cells and alveolar macrophages but not to Clara cells. Using a specific ES-2 antibody, we determined the protein distribution of ES-2 in the lung by immunohistochemistry, and it was found to be consistent with the sites of mRNA expression. Most of the ES-2 in rat bronchoalveolar lavage is in the surfactant-depleted supernatant, but ES-2 was also consistently localized to the small vesicular surfactant subfraction presumed to form as a consequence of conversion activity. These results are consistent with a role for endogenous lung ES-2 in surfactant metabolism.

Molecular cloning, characterization, and differential expression pattern of mouse lung surfactant convertase

We recently reported the purification and partial amino acid sequence of "surfactant convertase," a 72-kDa glycoprotein involved in the extracellular metabolism of lung surfactant (S. Krishnasamy, N. J. Gross, A. L. Teng, R. M. Schultz, and R. Dhand. Biochem. Biophys. Res. Commun. 235: 180-184, 1997). We report here the isolation of a cDNA clone encoding putative convertase from a mouse lung cDNA library. The cDNA spans a 1,836-bp sequence, with an open reading frame encoding 536 amino acid residues in the mature protein and an 18-amino acid signal peptide at the NH2 terminus. The deduced amino acid sequence matches the four partial amino acid sequences (68 residues) that were previously obtained from the purified protein. The deduced amino acid sequence contains an 18-amino acid residue signal peptide, a serine active site consensus sequence, a histidine consensus sequence, five potential N-linked glycosylation sites, and a COOH-terminal secretory-type sequence His-Thr-Glu-His-Lys. Primer-extension analysis revealed that transcription starts 29 nucleotides upstream from the start codon. Northern blot analysis of RNA isolated from various mouse organs showed that convertase is expressed in lung, kidney, and liver as a 1,800-nucleotide-long transcript. The nucleotide and amino acid sequences of putative convertase are 98% homologous with mouse liver carboxylesterase. It thus may be the first member of the carboxylesterase family (EC 3.1.1.1) to be expressed in lung parenchyma and the first with a known physiological function.

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.

Identification of a putative surfactant convertase in rat lung as a secreted serine carboxylesterase

In the alveolar lumen, pulmonary surfactant converts from the contents of secreted lamellar bodies to tubular myelin to apoprotein-depleted vesicles during respiration. Using an in vitro system, researchers have reported that the conversion of tubular myelin to vesicles is blocked by inhibitors of serine hydrolase activity and have tentatively ascribed "convertase" activity to a diisopropyl fluorophosphate (DFP)-binding protein in mouse bronchoalveolar lavage (BAL). We purified and sequenced the homologous enzyme from rat BAL fluid. Amino acid sequence from the amino terminus and an internal cyanogen bromide peptide of the purified rat DFP-binding protein perfectly match the sequence of the carboxylesterase ES-2. Although ES-2 was initially cloned from liver, we found a 1.8-kilobase mRNA for ES-2 in decreasing relative abundance in rat liver, kidney, and lung but not in heart or spleen. Although further studies are required to establish the identity between "convertase" and ES-2 or a homologous member of the carboxylesterase family, our results raise the possibility that a protein with esterase/lipase activity plays a role in extracellular surfactant metabolism.

Molecular cloning and characterization of a novel putative carboxylesterase, present in human intestine and liver

A full-length cDNA coding for a putative intestinal carboxylesterase (iCE) was isolated from a human small intestine cDNA library. The cDNA has an open reading frame of 559 amino acids with up to 65% homology to other carboxylesterases of different mammalian species. The deduced amino-acid sequence contains many structural features, that are highly conserved among all carboxylesterase isoenzymes, like the serine esterase active site, an ER-retention signal and one Asn-Xxx-Thr site for N-linked carbohydrate addition. Northern blot analysis revealed that the corresponding mRNA is 3.4-3.6 kb in size and is preferentially expressed in human intestine with a weak signal also in liver. Analysis of cells from the gastrointestinal tract unveiled site-specific, transcriptional regulation of iCE, with higher expression in small intestine and lower expression in colon and rectum. The high expression in small intestine is attributable to a higher expression in jejunum compared to duodenum and ileum.

A novel fusidic acid resistance gene from Streptomyces lividans 66 encodes a highly specific esterase

Resistance to fusidic acid in Streptomyces lividans is due to secretion of an extracellular enzyme (FusH) that converts the steroid antibiotic into an inactive derivative. NH2-terminal and several internal amino acid sequences were prepared from the purified enzyme. Using one of the deduced oligonucleotides to probe a subgenomic DNA library, the fusH gene was cloned and sequenced. Sequence analysis located an ORF which, owing to the presence of two putative start codons, indicates a predicted protein with 520 or 509 amino acids. A signal peptide was identified by aligning the deduced amino acids with the N-terminal sequence determined for the mature enzyme. The C-terminal part of the deduced FusH contains a region of three tandemly repeated stretches of 50 amino acids, which is preceded and followed by amino acids showing high homology with the repeats. FusH was found to share a GDS motif with some deduced esterases. S. lividans transformants carrying fusH on a multicopy vector synthesized high levels of FusH. Purified FusH cleaved equally well an acetyl, a thioacetyl or a formyl group from the 16 beta-position of fusidic acid and its derivatives. However, a propionyl group at the 16 beta-position was attacked with difficulty and a 16 alpha-acetyl group was not hydrolysed at all. These data indicate that FusH is a highly specific esterase. The fusH gene is widely distributed among streptomycetes that modify fusidic acid to its inactive lactone derivative.

Purification and characterization of a porcine liver microsomal triacylglycerol hydrolase

We have purified an enzyme from porcine liver microsomes which catalyzes hydrolysis of triacylglycerols. The enzyme was solubilized from the membranes by the zwitterionic detergent 3-[(3- cholamidopropyl)dimethylammonio]-l-propansulfonate (CHAPS) and was purified to apparent homogeneity by sequential chromatography on Q-Sepharose, hydroxyapatite, Affi-Gel heparin, and Mono-Q. The purified hydrolase migrated in SDS-polyacrylamide gel electrophoresis (PAGE) as a single polypeptide band of an apparent molecular mass of 60 kDa. The enzyme hydrolyzed long-, medium-, and short-chain triacylglycerols, as well as a chromogenic lipase substrate, 1,2-O-dilauryl-rac-glycero-3-glutaric acid resorufin ester. The highest specific activity was obtained with tributyroylglycerol (240 mumol.min-1.mg-1). The reaction rate was maximal at pH 8.5. Sulfhydryl-directed reagents, such as N-ethylmaleimide (NEM), 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), and dodecyldithio-5-(2-nitrobenzoic acid) (C12-TNB) had no effect on the hydrolase activity; however, the enzyme was sensitive to HgCl2. Serine reagents, such as diethyl-p-nitrophenyl phosphate (E600) and diisopropyl fluorophosphate (DFP), used in 100-fold molar excess completely inhibited the activity, suggesting that it is a serine esterase. These results suggest that the enzyme may participate in the intracellular neutral lipid metabolism since the enzyme is located in the endoplasmic reticulum, an organelle where de novo triacylglycerol synthesis and assembly of lipoproteins take place.

Purification and characterization of rat liver microsomal fatty acid ethyl and 2-chloroethyl ester synthase and their relationship with carboxylesterase (pI 6.1)

Previous studies have shown that fatty acid ethyl ester synthase (FAEES) which catalyzes the formation of ethyl or 2-chloroethyl esters of long-chain fatty acids is localized in the microsomal fraction of rat liver. A recent study suggests that rat adipose tissue FAEES is similar to rat liver microsomal carboxylesterase (CE) [Tsujita and Okuda (1992) J. Biol. Chem. 267, 23489-23494]. Since the interrelationships among FAEES, 2-chloroethyl ester synthase (FACEES), and cholesterol esterase (ChE) are also not clear at present, we purified and characterized FAEES and FACEES from rat hepatic microsomes and studied their functional and structural relationships with CE and ChE. The results of these studies showed that CE, FAEES, and FACEES activities copurified during each step of purification. Although gel-filtration column chromatography of DEAE-Sephacel purified microsomal protein resolved into two peaks with an estimated molecular weight of 180 (major) and 60 kDa (minor, this paper describes characterization of only the 180 kDa protein. CE, FAEES, and FACEES activities associated with homogeneous 180 kDa protein could be inhibited by a beta-esterase inhibitor (diisopropyl fluorophosphate) in an identical manner. This protein, however, showed only the hydrolytic activity, but not the synthetic activity for cholesterol oleate, indicating that it is distinct from ChE. The purified protein could be immunoprecipitated with the antibodies raised against rat adipose tissue FAEES, but not with antibodies against rat pancreatic ChE, demonstrating again that the purified protein is distinct from ChE. A single band corresponding to 60 kDa upon SDS-PAGE, under reduced denaturing conditions, indicates that the purified protein is a trimer. N-terminal amino acid sequence of the first 27 residues were identical to that of rat hepatic microsomal CE [Robbi et al. (1990) Biochem. J., 451-458] which suggests structural similarity of the purified protein with rat hepatic microsomal CE. Therefore, the functional and structural properties of the purified protein demonstrate that FAEES, FACEES, as well as CE activities are expressed by the same protein, purified in this study, which exists as a trimer (180 kDa) and is involved in biosynthesis of long-chain fatty acid esters of xenobiotic alcohols. Further studies on purification and characterization of the enzymes responsible for the esterification of xenobiotic alcohols with endogenous fatty acids from various target organs need to be conducted to determine their functional and structural interrelationships. Inhibition and induction studies of these enzyme(s) and the extent of observed toxicity could be important in understanding their role in etiology of chronic diseases induced by alcohol abuse.

Purification and some properties of a carboxylesterase from ovine liver

Carboxylesterase ESB3 was extracted from ovine liver and purified to homogeneity by ammonium sulphate fractionation, hydrophobic interaction chromatography on Phenyl Sepharose, ion exchange chromatography on Mono-Q Sepharose and size exclusion chromatography on Superose 6. The enzyme is free of carboxylesterase ESB2 activity. The molecular mass of the enzyme is estimated 182 kDa as judged by size exclusion chromatography. Isoelectric focusing indicates the presence of six isoforms of pI 5.50-5.77 with three main isoforms of pI 5.55-5.65. The enzyme is active towards the substrates p-nitrophenyl acetate and the aliphatic substrates ethyl acetate, ethyl propionate, ethyl butyrate, and ethyl valerate. Of the ethyl esters the affinity is lowest towards acetate and highest towards ethyl butyrate. The enzyme is totally inhibited by phenylmethylsulphonyl fluoride (PMSF) and mercuric chloride but not affected by eserine or cupric chloride. The pH optimum of the enzyme is 7.5 and it is stable at 55 degrees C for 20 min.

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.

Molecular aspects of carboxylesterase isoforms in comparison with other esterases

The involvement of carboxylesterase, acetylcholinesterase, butyrylcholinesterase and cholesterol esterase in pharmacology and toxicology are well recognized. However, there are few papers concerning the comparative studies of these serine hydrolases in terms of molecular level. Recently, we have studied various aspects of carboxylesterases using cDNAs of carboxylesterase isozymes purified from 9 animal species and human liver microsomes, and found that there is high homology of the N-terminal amino acid sequences of the isozymes tested. On the other hand, we compared the amino acid sequences at the active site of the individual esterases and found that the sequences of all esterases tested are strictly conserved. These results strongly suggest that the esterases involved are classified into the serine hydrolase super family.

Purification and partial characterization of a novel thermophilic carboxylesterase with high mesophilic specific activity

An esterase activity obtained from a strain of Bacillus stearothermophilus was purified 5,133-fold to electrophoretic homogeneity with 26% recovery. The purified esterase had a specific activity of 2,032 mumol min-1 mg-1 based on the hydrolysis of p-nitrophenyl caproate at pH 7.0 and 30 degrees C. The apparent molecular mass was 50,000 +/- 2,000 daltons from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 45,000 +/- 3,000 daltons from gel filtration. Native polyacrylamide gels stained for esterase activity showed three bands. The isoelectric points were estimated to be 5.7, 5.8, and 6.0. Forty amino acid residues were sequenced at the N-terminus. The sequence showed no degeneracy, suggesting that the three esterases are functionally identical carboxylesterases differing by a limited number of amino acids. The enzyme showed maximum activity at pH 7.0 and was very stable at pH 6.0-8.9 with optimum stability at pH 6.0. At this pH and 60 degrees C the half-life was 170 h. Esterase activity was totally inhibited by phenylmethanesulfonyl fluoride, parahydroxymercuribenzoate, eserine, and tosyl-L-phenylalanine, but not by ethylendiaminetetra acetic acid. The esterase obeyed Michaelis-Menten kinetics in the hydrolysis of p-nitrophenyl esters, but both Vmax and KM were protein concentration-dependent. The esterase was able to hydrolyse a number of p-nitrophenyl derivatives (amino acid derivatives and aliphatic acids with different chain lengths).

Rat serum carboxylesterase. Cloning, expression, regulation, and evidence of secretion from liver

Multiple forms of carboxylesterase have been identified in rat liver, and five carboxylesterases (designated hydrolases A, B, C, S, and egasyn) have been cloned. Hydrolases A, B, C, and egasyn all have a C-terminal consensus sequence (HXEL) for retaining proteins in the endoplasmic reticulum, and these carboxylesterases are found in rat liver microsomes. In contrast, hydrolase S lacks this C-terminal consensus sequence and is presumed to be secreted. In order to test this hypothesis, a polyclonal antibody was raised against recombinant hydrolase S from cDNA-directed expression in Escherichia coli. In addition to hydrolases A, B, and C (57-59 kDa), this antibody recognized a 67-kDa protein in rat liver microsomes and a 71-kDa protein in rat serum. The 71-kDa protein detected in rat serum was also detected in the extracellular medium from primary cultures of rat hepatocytes. Non-denaturing gel electrophoresis with staining for esterase activity showed that a serum carboxylesterase comigrated with the 71-kDa protein. Immunoprecipitation of the 71-kDa enzyme from rat serum decreased esterase activity toward 1-naphthylacetate and para-nitrophenylacetate. The 71-kDa protein immunoprecipitated from rat serum had an N-terminal amino acid sequence identical to that predicted from the cDNA encoding hydrolase S, providing further evidence that hydrolase S is synthesized in and secreted by the liver. The levels of the 67-kDa protein in rat liver microsomes and the levels of the 71-kDa protein in rat serum were co-regulated. Deglycosylation of microsomes and serum converted the 67- and 71-kDa proteins to a 58-kDa peptide, which matches the molecular mass calculated from the cDNA for hydrolase S. These results suggest that the 67-kDa protein in liver microsomes is a precursor form of hydrolase S that undergoes further glycosylation before being secreted into serum. In rats, liver appears to be the only source of hydrolase S because no mRNA encoding hydrolase S could be detected in several extrahepatic tissues. Serum carboxylesterases have been found to play an important role in lipid metabolism and detoxication of organophosphates, therefore, the secretion of hydrolase S and the modulation of its expression by xenobiotics may have physiological as well as toxicological significance.

Co-localization of hydrolytic enzymes with widely disparate pH optima: implications for the regulation of lysosomal pH

Lysosomes are traditionally defined by their acidic interior, their content of degradative ‘acid hydrolases’, and the presence of distinctive membrane proteins. Terminal degradation of the N-linked oligosaccharides of glycoproteins takes place in lysosomes, and involves several hydrolases, many of which are known to have acidic pH optima. However, a sialic acid-specific 9-O-acetyl-esterase and a glycosyl-N-asparaginase, which degrade the outer- and inner-most linkages of N-linked oligosaccharides, respectively, both have pH optima in the neutral to alkaline range. By immunoelectron microscopy, these enzymes co-localize in lysosomes with several conventional acid hydrolases and with lysosomal membrane glycoproteins. Factors modifying the pH/activity profiles of these enzymes could not be found in lysosomal extracts. Thus, the function of the enzymes with neutral pH optima must depend either upon their minimal residual activity at acidic pH, or upon the possibility that lysosomes are not always strongly acidic. Indeed, when lysosomes are marked in living cells by uptake of fluorescently labeled mannose 6-phosphorylated proteins, the labeled organelles do not all rapidly accumulate Acridine Orange, a vital stain that is specific for acidic compartments. One plausible explanation is that lysosomal pH fluctuates, allowing hydrolytic enzymes with a wide range of pH optima to efficiently degrade macromolecules.

Structure, expression and gene sequence of a juvenile hormone esterase-related protein from metamorphosing larvae of Trichoplusia ni

A carboxylesterase with an encoded molecular size of 61 kDa and a high sequence similarity to juvenile hormone esterase (JHE) has been cloned from cDNA prepared from final instar larvae of Trichoplusia ni. The absence of a recognizable encoded signal peptide suggests that the enzyme, JHER (for JHE-related) may not be secreted, in contrast to JHE. When the amino acid sequence of JHE, JHER and other esterases were mapped onto the secondary and tertiary structure determined crystallographically for acetylcholinesterase, certain structural features for the substrate binding/catalytic site were identified as common only to JHE and JHER. However, several differences between JHE and JHER were identified in residues at the binding/catalytic site, suggesting that although the two enzymes prefer similar natural substrates, these substrates are not identical. JHER is present as a single-copy gene, transcribed during the feeding stage of the final stage of the final larval stadium, but not after metamorphic commitment to the pupal developmental programme. The gene transcribes a single-size message of 2.0 kb. The genes for JHER and JHE appear to be physically juxtaposed in the T. ni genome. The 5' flanking sequence to the JHER gene possesses some sequences in common with the JHE gene, but is also missing some regulatory elements previously identified in the JHE gene. Sequences conserved between the promoters for the two genes were identified that were different from previously reported regulatory elements of eukaryotic transcription factors.

Cloning and sequence analysis of a hamster liver cDNA encoding a novel putative carboxylesterase

A full-length cDNA encoding for a putative carboxylesterase was isolated from a hamster liver cDNA library. The cDNA consisting of 1911 base pairs contained an open reading frame of 1683 base pairs encoding for a polypeptide of 561 amino-acid residues, including 27 N-terminal amino-acid residues for signal peptide. The deduced amino-acid sequence of the cDNA is in 67% homology with the amino-acid sequence of rabbit form 2 carboxylesterase, which has not yet been cloned. It also had many structural features highly conserved among carboxylesterase isozymes.

Hormone-sensitive lipase is closely related to several bacterial proteins, and distantly related to acetylcholinesterase and lipoprotein lipase: identification of a superfamily of esterases and lipases

We have sequenced a gene from Bacillus acidocaldarius which encodes an open reading frame (ORF3) of 310 amino acids. The ORF3 was found to be related to the mammalian hormone-sensitive lipase (HSL). Searching the protein data base revealed five other bacterial proteins related to the HSL. Upon further sequence comparisons this HSL-group was found to be related to the family of carboxylesterases, and to a family of lipases (lipoprotein, hepatic and pancreatic lipases). The evolutionary relationship of these serine-dependent hydrolytic enzymes has not been studied previously, and it has not been known that these proteins belong to the same superfamily. Finally, the alignment of the HSL with the bacterial proteins allowed us to infer the location of the hormone-sensitive regulatory domain of the HSL-protein.

Glycosylation-dependent activity of baculovirus-expressed human liver carboxylesterases: cDNA cloning and characterization of two highly similar enzyme forms

A cDNA, designated hCE, encoding the entire sequence of a carboxylesterase, was isolated from a human liver lambda gt11 library. The hCE-deduced protein sequence contained 568 amino acids, including an 18 amino acid signal peptide sequence, and had a calculated molecular mass of the mature protein of 60,609 Da. A second cDNA, designated hCEv, was isolated from the same lambda gt11 library and contained a 3-bp deletion resulting in the loss of the final amino acid in the signal peptide sequence (Ala-1) and a second 3-bp deletion leading to an in-frame loss of Gln345. Expression of mRNA corresponding to both hCE and hCEv was detected in eight adult human liver samples, with individual levels varying 5-fold (hCE) and 12-fold (hCEv). A single immunoreactive protein was detected in 13 adult human liver samples when probed with antibody directed against a rat carboxylesterase. Based on allele-specific oligonucleotide hybridizations, we believe that the hCE and hCEv cDNAs represent two distinct members of the carboxylesterase family. The carboxylesterase genes were localized to human chromosome 16 using a somatic cell hybrid mapping strategy. Baculovirus expression of hCE in Sf9 cells produced a protein with an estimated molecular mass of 59,000 Da. This enzyme was able to hydrolyze aromatic and aliphatic esters but possessed no catalytic activity toward amides or a fatty acyl CoA ester. Baculovirus-mediated expression of the hCEv cDNA yielded a second protein of 56,000 Da resulting from inefficient N-glycosylation of the hCEv protein. Although the substrate specificity for the hCEv protein was identical to that of expressed hCE for any given substrate, the specific activity for the hCE protein was always higher than that for the hCEv protein. Tunicamycin inhibition studies provided the first evidence that N-glycosylation of these luminal enzymes is essential for maximal catalytic activity.

Purification, characterization and modulation of a microsomal carboxylesterase in rat liver for the hydrolysis of acyl-CoA

A carboxylesterase containing long-chain acyl-CoA hydrolase activity was purified to apparent homogeneity from rat liver microsomes. Palmitoyl-CoA was the most preferred substrate, followed by stearoyl-CoA and oleoyl-CoA. Arachidonoyl-CoA, linoleoyl-CoA and acetyl-CoA were not hydrolysed by the enzyme. The purified enzyme had no activity on the hydrolysis of phospholipids and neutral lipids. The molecular mass of the enzyme was found to be 56 kDa by SDS/PAGE and 64 kDa by gel-filtration chromatography. On isoelectric focusing, the purified enzyme behaved like the ES-4 type, with a pI of 6.15. Determination of the amino acid sequence revealed that its N-terminal sequence is 100% homologous with the only other known N-terminal sequence for a rat carboxylesterase isoenzyme (ES-10). Enzyme activity was inhibited by lysophosphatidic acid and activated by lysophosphatidylcholine. The modulation of enzyme activity by these lysophospholipids might represent a plausible mechanism for the physiological control of acyl-CoA concentrations.

Cloning and nucleotide sequence of a novel, male-predominant carboxylesterase in mouse liver

As a family of serine-dependent enzymes, the carboxylesterases (EC 3.1.1.1) demonstrate a broad substrate specificity. Mouse carboxylesterases comprise at least 20 genetically distinct loci. We cloned a full-length cDNA for a novel mouse carboxylesterase, Es-male which was expressed predominantly in male livers. This carboxylesterase consisted of 554 amino acid residues, and exhibited 43% and 42% similarities to the known mouse esterases Es-22 and pEs-N, respectively. Es-male contained a C-terminal ER-retention signal PEEL, indicating that it may be a microsomal carboxylesterase.

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

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

Relationship between sequence conservation and three-dimensional structure in a large family of esterases, lipases, and related proteins

Based on the recently determined X-ray structures of Torpedo californica acetylcholinesterase and Geotrichum candidum lipase and on their three-dimensional superposition, an improved alignment of a collection of 32 related amino acid sequences of other esterases, lipases, and related proteins was obtained. On the basis of this alignment, 24 residues are found to be invariant in 29 sequences of hydrolytic enzymes, and an additional 49 are well conserved. The conservation in the three remaining sequences is somewhat lower. The conserved residues include the active site, disulfide bridges, salt bridges, and residues in the core of the proteins. Most invariant residues are located at the edges of secondary structural elements. A clear structural basis for the preservation of many of these residues can be determined from comparison of the two X-ray structures.

The carboxylesterase family exhibits C-terminal sequence diversity reflecting the presence or absence of endoplasmic-reticulum-retention sequences

Resident proteins of the endoplasmic reticulum lumen are continuously retrieved from an early Golgi compartment by a receptor-mediated mechanism. The sorting or retention sequence on the endoplasmic reticulum proteins is located at the C-terminus and was initially shown to be the tetrapeptide KDEL in mammalian cells and HDEL in Saccharomyces cerevisiae. The carboxylesterases are a large family of enzymes primarily localized to the lumen of the endoplasmic reticulum. Retention sequences in these proteins have been difficult to identify due to atypical and heterogeneous C-terminal sequences. Utilizing the polymerase chain reaction with degenerate primers, we have identified and characterized the C-termini of four members of the carboxylesterase family from rat liver. Three of the carboxylesterases sequences contained C-terminal sequences (HVEL, HNEL or HTEL) resembling the yeast sorting signal which were reported to be non-functional in mammalian cells. A fourth carboxylesterase contained a distinct C-terminal sequence, TEHT. A full-length esterase cDNA clone, terminating in the sequence HVEL, was isolated and was used to assess the retention capabilities of the various esterase C-terminal sequences. This esterase was retained in COS-1 cells, but was secreted when its C-terminal tetrapeptide, HVEL, was deleted. Addition of C-terminal sequences containing HNEL and HTEL resulted in efficient retention. However, the C-terminal sequence containing TEHT was not a functional retention signal. Both HDEL, the authentic yeast retention signal, and KDEL were efficient retention sequences for the esterase. These studies show that some members of the rat liver carboxylesterase family contain novel C-terminal retention sequences that resemble the yeast signal. At least one member of the family does not contain a C-terminal retention signal and probably represents a secretory form.

Topogenesis of carboxylesterases: a rat liver isoenzyme ending in -HTEHT-COOH is a secreted protein

We have cloned a rat liver cDNA that encodes a carboxylesterase isoenzyme, as revealed by immunoprecipitation, cytochemical staining and inhibition by bis-p-nitrophenylphosphate of the product expressed in transfected COS cells. The predicted polypeptide ends in -HTEHT-COOH. The product is secreted by COS cells with a half-time of about 1 hour, after maturation of oligosaccharide chains in the Golgi complex. A variant ending in -HTEL-COOH is stable in the cells. This strengthens the existing evidence that the HXEL-COOH end signals proteins for retrieval from the secretory traffic in animal cells. The encoded enzyme still remains to be identified. It shows 98% homology to an esterase sequenced earlier (Takagi et al. 1988, J. Biochem. 104, 801-806; Long et al. 1988, Biochem. Biophys. Res. Commun. 156, 866-873); however it must be an enzyme from the serum, not from the microsomes.

Cloning and analysis of a cDNA encoding a human liver carboxylesterase

A human liver carboxylesterase (CE)-encoding cDNA has been cloned using synthetic oligodeoxyribonucleotides (oligos) based on the known amino acid (aa) sequences of rabbit and rat liver CEs. The oligos hybridize specifically to DNA encoding liver CEs. The longest cDNA obtained from screening several cDNA libraries encodes about 80% of the protein and translates into an aa sequence which has a high degree of similarity with the sequences of liver CEs from other species. On hybridization to mRNA isolated from human liver, the cDNA gave a single band of about 2.0 kb consistent with its encoding a protein of less than 68 kDa. DNA obtained from a number of human livers and probed with the CE cDNA gave identical hybridization patterns. These patterns were moderately complex by comparison with published data.

Purification and characterization of a human liver arylacetamide deacetylase

Arylacetamide deacetylation is an important enzyme activity in the metabolic activation of arylamine substrates to ultimate carcinogens, best described as a carboxylesterase/amidase type of reaction. A 7-fold variation in the Vmax of 2-acetylaminofluorene deacetylation in 24 human livers was observed. An acetylaminofluorene deacetylase was purified 90 fold from human liver microsomes by PEG-fractionation, anion exchange and hydrophobic interaction chromatography. The purified 45kD protein showed no amino acid sequence homology to other carboxylesterases, neither in its N-terminus nor in tryptic peptides. Antibodies raised against the deacetylase recognized the protein with high specificity. This report thus describes the first arylacetamide deacetylase in human liver.

Evolutionary origin of thyroglobulin by duplication of esterase gene

Rat liver microsomal carboxyesterase E1 was found to have homology with five esterases and with the COOH-terminal parts of two thyroglobulins. A phylogenetic tree constructed for these proteins shows that this new superfamily has evolved from a common ancestral gene that encoded a carboxyesterase. The tree also shows that the ancestral gene already existed before the divergence of vertebrates and invertebrates and later its duplicated genes gained various kinds of esterase activity. According to the tree, one of the duplicated genes evolved into the COOH-terminal half of thyroglobulin by a gene fusion with a DNA sequence whose evolutionary origin is unknown.

Cloning and sequencing of a human liver carboxylesterase isoenzyme

A human liver lambda gt11 library was screened with antibodies raised to a purified rat liver carboxylesterase, and several clones were isolated and sequenced. The longest cDNA contained an open reading frame of 507 amino acids that represented 92% of the sequence of a mature carboxylesterase protein. This sequence possessed many structural features that are highly conserved among rabbit and rat liver carboxylesterase proteins, including Ser, His, and Asp residues that comprise the active site, two pairs of Cys residues that may participate in disulfide bond formation, and one Asn-Xxx-Thr site for N-linked carbohydrate addition. When the clone was used to probe human liver genomic DNA that had been digested with various restriction enzymes, many hybridizing bands of differing intensities were observed. The results suggest that the carboxylesterases exist as several isoenzymes in humans, and that they are encoded by multiple genes.

A different cytochrome P450 form is induced in primary cultures of rat hepatocytes

A 49-kDa protein (P49) was discovered in the primary cultures of rat hepatocytes. P49 cross-reacted with the antibodies against purified P450IIC11 [formerly P-450(M-1)]. P49 was located in microsomes and highly induced after plating of isolated hepatocytes on collagen-coated culture dishes. To characterize P49, cDNA clones were screened from a rat liver lambda gt11 expression library. From sequence analysis of the cloned cDNAs, the amino acid sequence of P49 was deduced, and the protein was identified as a previously uncharacterized form of cytochrome P450. P49 consists of 489 amino acids and shows approximately 60% similarity with the members of class IIC subfamily of rat cytochrome P450, such as P450IIC11 and P450IIC12 [formerly P-450(F-1)]. RNA blot analysis indicates that the mRNA translating P49 was induced approximately 20- to 30-fold at 70 hr in the primary cultures compared with the liver of adult rats. Induction of P49 was not affected by density of the plated cells and the presence or absence of several hormones, serum, or antibiotics in the culture medium. On the other hand, lower induction of P49 was seen when the hepatocytes were cultured on Matrigel-coated plates. Expression of P49 mRNA was low in the liver of adult rats and was not detectable in the livers of 1- and 2-week-old male and female rats. P49 is an additional form of cytochrome P450, which is induced in the primary culture of rat hepatocytes.

Mutagenesis of essential functional residues in acetylcholinesterase

The cholinesterases are serine hydrolases that show no global similarities in sequence with either the trypsin or the subtilisin family of serine proteases. The cholinesterase superfamily includes several esterases with distinct functions and other proteins devoid of the catalytic serine and known esterase activity. To identify the residues involved in catalysis and conferring specificity on the enzyme, we have expressed wild-type Torpedo acetylcholinesterase (EC 3.1.1.7) and several site-directed mutants in a heterologous system. Mutation of serine-200 to cysteine results in diminished activity, while its mutation to valine abolishes detectable activity. Two conserved histidines can be identified at positions 425 and 440 in the cholinesterase family; glutamine replacement at position 440 eliminates activity whereas the mutation at 425 reduces activity only slightly. The assignment of the catalytic histidine to position 440 defines a rank ordering of catalytic residues in cholinesterases distinct from trypsin and subtilisin and suggests a convergence of a catalytic triad to form a third, distinct family of serine hydrolases. Mutation of glutamate-199 to glutamine yields an enzyme with a higher Km and without the substrate-inhibition behavior characteristic of acetylcholinesterase. Hence, modification of the acidic amino acid adjacent to the serine influences substrate association and the capacity of a second substrate molecule to affect catalysis.

Nucleotide sequence of cDNA coding for rat liver pI 6.1 esterase (ES-10), a carboxylesterase located in the lumen of the endoplasmic reticulum

A commercial rat liver cDNA library in lambda gt11 was screened with a rabbit antiserum to native pI 6.1 esterase (ES-10). The inserts of the immunoreactive clones were short (0.9-1.1 kbp). One of these was used as a probe to rescreen the library, yielding 30 clones, two of which contained relatively long (approx. 1.9 kbp) and widely overlapping cDNA inserts. They did not contain the first two nucleotide residues of the initiator codon, nor the 5'-end untranslated portion of the mRNA. These were derived from a home-made rat liver cDNA library in lambda gt11, screened with an oligonucleotide corresponding to the 5'-end of the already known cDNA sequence. The nucleotide sequence consists of 48 bp of 5'-end non-coding region, 1695 bp of coding region and 212 bp of 3'-end non-coding region including a 20 bp poly(A) tail. The signal peptide and the mature protein subunit are 18 and 547 residues long respectively. Tyr is confirmed as N-terminal residue. The predicted amino acid sequence is highly similar to those of rabbit liver esterase forms 1 (77% identity) and 2 (56% identity), determined by protein sequencing [Korza & Ozols (1988) J. Biol. Chem. 263, 3486-3495; Ozols (1989) J. Biol. Chem. 264, 12533-12545]. The three enzymes share the Ser and His residues presumed to be part of the active site, four Cys residues and a high proportion of charged side chains at their C-terminus. The C-terminal tetrapeptides of the three esterases (-HVEL, -HIEL and -HTEL for pI 6.1 and forms 1 and 2 esterases respectively) are reminiscent of, but not identical with, the localization signal identified in other proteins of the endoplasmic-reticulum lumen (-KDEL in animal cells [Munro & Pelham (1987) Cell 48, 899-907]; -HDEL in yeast [Pelham, Hardwick & Lewis (1988) EMBO J. 7, 1757-1762]). We still lack direct evidence to decide whether or not these C-terminal tetrapeptides commit esterases to reside in the endoplasmic reticulum. In that case the antepenultimate residue (D, V, I or T) would be only weakly stringent, and some sequences primed by H instead of K would be recognized in animal as well as in yeast cells.

Characterization of molecular species of liver microsomal carboxylesterases of several animal species and humans

Seven carboxylesterase isozymes were purified to electrophoretic homogeneity from liver microsomes of mouse, hamster, guinea pig, rabbit, and monkey by the same procedure used previously to obtain three isozymes from the rat, and their physical, enzymological, and immunological properties were compared with those of the rat isozymes. The substrate specificity and immunological reactivity of liver microsomal carboxylesterases from pig, cow, beagle dog, and human were also examined for comparison, though these enzymes were not purified. The ten purified preparations have similar subunit weight (57,000-64,000), but their isoelectric points differ widely (4.7-6.5). The purification procedure of all isozymes included concanavalin A-Sepharose column chromatography. The isozymes were not eluted from the column with a high concentration of sodium chloride, but were efficiently eluted with alpha-methylmannoside. This observation suggested that the carboxylesterases studied are glycoproteins. All the isozymes except rat RL1 and RL2 possess a high hydrolytic activity toward all the substrates examined. Long-chain monoglyceride was hydrolyzed by the purified carboxylesterase isozymes. Anti-rat RH1 immunoglobulin G was found to possess high cross-reactivity with all isozymes tested, except monkey MK2, by immunoblotting analysis. The amino acid compositions of carboxylesterase isozymes showed considerable similarities, except for monkey MK2. The amino-terminal amino acid sequences showed a striking homology, except for monkey MK2, though the amino-terminal amino acid itself was different in every isozyme. Hepatic microsomal carboxylesterases in mammals play an important role in drug and lipid metabolism in the endoplasmic reticulum, and it is noteworthy that the isozymes from various species examined here showed considerable similarities in physical, enzymatic, and immunochemical properties.