Characterization of a murine cDNA encoding a member of the carboxylesterase multigene family

Cloning and molecular modeling of a thermostable carboxylesterase from the chicken uropygial glands

Starting from total uropygial glands mRNAs, chicken uropygial carboxylesterase (cuCES) cDNA was synthesized by RT-PCR and cloned into the PGEM-T vector. Amino acid sequence of the cuCES is compared to that of human liver carboxylesterase 1 (hCES1). Given the high amino acid sequence homology between the two enzymes, a 3-D structure model of the chicken carboxylesterase was built using the structure of hCES1 as template. By following this model and utilizing molecular dynamics (MD) simulations, the resistance of the chicken carboxylesterase at high temperatures could be explained. The docking of substrate analogs into the cuCES active site was used to explain the fact that the chicken carboxylesterase cannot hydrolyze efficiently large substrate molecules.

Production of ES1 plasma carboxylesterase knockout mice for toxicity studies

The LD(50) for soman is 10-20-fold higher for a mouse than a human. The difference in susceptibility is attributed to the presence of carboxylesterase in mouse but not in human plasma. Our goal was to make a mouse lacking plasma carboxylesterase. We used homologous recombination to inactivate the carboxylesterase ES1 gene on mouse chromosome 8 by deleting exon 5 and by introducing a frame shift for amino acids translated from exons 6 to 13. ES1-/- mice have no detectable carboxylesterase activity in plasma but have normal carboxylesterase activity in tissues. Homozygous ES1-/- mice and wild-type littermates were tested for response to a nerve agent model compound (soman coumarin) at 3 mg/kg sc. This dose intoxicated both genotypes but was lethal only to ES1-/- mice. This demonstrated that plasma carboxylesterase protects against a relatively high toxicity organophosphorus compound. The ES1-/- mouse should be an appropriate model for testing highly toxic nerve agents and for evaluating protection strategies against the toxicity of nerve agents.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Characterization of gene expression profiles for different types of mast cells pooled from mouse stomach subregions by an RNA amplification method

Background

Mast cells (MCs) play pivotal roles in allergy and innate immunity and consist of heterogenous subclasses. However, the molecular basis determining the different characteristics of these multiple MC subclasses remains unclear.

Results

To approach this, we developed a method of RNA extraction/amplification for intact in vivo MCs pooled from frozen tissue sections, which enabled us to obtain the global gene expression pattern of pooled MCs belonging to the same subclass. MCs were isolated from the submucosa (sMCs) and mucosa (mMCs) of mouse stomach sections, respectively, 15 cells were pooled, and their RNA was extracted, amplified and subjected to microarray analysis. Known marker genes specific for mMCs and sMCs showed expected expression trends, indicating accuracy of the analysis.

We identified 1,272 genes showing significantly different expression levels between sMCs and mMCs, and classified them into clusters on the basis of similarity of their expression profiles compared with bone marrow-derived MCs, which are the cultured MCs with so-called 'immature' properties. Among them, we found that several key genes such as Notch4 had sMC-biased expression and Ptgr1 had mMC-biased expression. Furthermore, there is a difference in the expression of several genes including extracellular matrix protein components, adhesion molecules, and cytoskeletal proteins between the two MC subclasses, which may reflect functional adaptation of each MC to the mucosal or submucosal environment in the stomach.

Conclusion

By using the method of RNA amplification from pooled intact MCs, we characterized the distinct gene expression profiles of sMCs and mMCs in the mouse stomach. Our findings offer insight into possible unidentified properties specific for each MC subclass.

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.

Structure and catalytic properties of carboxylesterase isozymes involved in metabolic activation of prodrugs

Mammalian carboxylesterases (CESs) comprise a multigene family whose gene products play important roles in biotransformation of ester- or amide-type prodrugs. They are members of an α,β-hydrolase-fold family and are found in various mammals. It has been suggested that CESs can be classified into five major groups denominated CES1-CES5, according to the homology of the amino acid sequence, and the majority of CESs that have been identified belong to the CES1 or CES2 family. The substrate specificities of CES1 and CES2 are significantly different. The CES1 isozyme mainly hydrolyzes a substrate with a small alcohol group and large acyl group, but its wide active pocket sometimes allows it to act on structurally distinct compounds of either a large or small alcohol moiety. In contrast, the CES2 isozyme recognizes a substrate with a large alcohol group and small acyl group, and its substrate specificity may be restricted by the capability of acyl-enzyme conjugate formation due to the presence of conformational interference in the active pocket. Since pharmacokinetic and pharmacological data for prodrugs obtained from preclinical experiments using various animals are generally used as references for human studies, it is important to clarify the biochemical properties of CES isozymes. Further experimentation for an understanding of detailed substrate specificity of prodrugs for CES isozymes and its hydrolysates will help us to design the ideal prodrugs.

Genomic structure and transcriptional regulation of the rat, mouse, and human carboxylesterase genes

The mammalian carboxylesterases (CESs) comprise a multigene family which gene products play important roles in biotransformation of ester- or amide-type prodrugs. Since expression level of CESs may affect the pharmacokinetic behavior of prodrugs in vivo, it is important to understand the transcriptional regulation mechanism of the CES genes. However, little is known about the gene structure and transcriptional regulation of the mammalian CES genes. In the present study, to investigate the transcriptional regulation of the promoter region of the CES1 and CES2 genes were isolated from mouse, rat and human genomic DNA by PCR amplification. A TATA box was not found the transcriptional start site of all CES promoter. These CES promoters share several common binding sites for transcription factors among the same CES families, suggesting that the orthologous CES genes have evolutionally conserved transcriptional regulatory mechanisms. The result of present study suggested that the mammalian CES promoters were at least partly conserved among the same CES families, and some of the transcription factors may play similar roles in transcriptional regulation of the human and murine CES genes.

Liver carboxylesterase cleaves surfactant protein (SP-) B and promotes surfactant subtype conversion

Conversion of the biophysically active large surfactant aggregate subtype of alveolar surfactant into the less surface active small surfactant aggregates occurs in vitro and in vivo, possibly in dependency of a carboxylesterase, entitled surfactant convertase. The substrate has yet not been safely identified. Utilizing the in vitro cycling assay we investigated conversion of an organic rabbit lavage extract reconstituted with SP-A. Porcine liver carboxylesterase, which is closely related to surfactant convertase, induced subtype conversion to a similar degree as compared with native lavage fluid containing endogenous convertase. In addition, we asked for cleavage products of SP-B and identified a approximately 12 kDa band upon cycling with liver carboxylesterase, having the same N-terminus as mature SP-B. A band of same molecular weight was found in native lavage fluid after in vitro conversion mediated by the endogenous convertase. We conclude that SP-B plays a pivotal role during subtype conversion and represents the substrate for surfactant convertase.

Structure, function and regulation of carboxylesterases

This review covers current developments in molecular-based studies of the structure and function of carboxylesterases. To allay the confusion of the classic classification of carboxylesterase isozymes, we have proposed a novel nomenclature and classification of mammalian carboxylesterases on the basis of molecular properties. In addition, mechanisms of regulation of gene expression of carboxylesterases by xenobiotics and involvement of carboxylesterase in drug metabolism and enzyme induction are also described.

Carboxylesterase 3 (EC 3.1.1.1) Is a Major Adipocyte Lipase

Hydrolysis of triglycerides is central to energy homeostasis in white adipose tissue (WAT). Hormone-sensitive lipase (HSL) was previously felt to mediate all lipolysis in WAT. Surprisingly, HSL-deficient mice show active HSL-independent lipolysis, suggesting that other lipase(s) also mediate triglyceride hydrolysis. To clarify this, we used functional proteomics to detect non-HSL lipase(s) in mouse WAT. After cell fractionation of intraabdominal WAT, most non-HSL neutral lipase activity is localized in the 100,000 x g infranatant and fat cake fractions. By oleic acid-linked agarose chromatography of infranatant followed by elution in a 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid gradient, we identified two peaks of esterase activity using p-nitrophenyl butyrate as a substrate. One of the peaks contained most of the lipase activity. In the corresponding fractions, gel permeation chromatography and SDS-PAGE, followed by tandem mass spectrometric analysis of excised Coomassie Blue-stained peptides, revealed carboxylesterase 3 (triacylglycerol hydrolase (TGH); EC 3.1.1.1). TGH is also the principle lipase of WAT fat cake extracts. Partially purified WAT TGH had lipase activity as well as lesser but detectable neutral cholesteryl ester hydrolase activity. Western blotting of subcellular fractions of WAT and confocal microscopy of fibroblasts following in vitro adipocytic differentiation are consistent with a distribution of TGH to endoplasmic reticulum, cytosol, and the lipid droplet. TGH is responsible for a major part of non-HSL lipase activity in WAT in vitro and may mediate some or all HSL-independent lipolysis in adipocytes.

IDENTIFICATION OF DI-(2-ETHYLHEXYL) PHTHALATE-INDUCED CARBOXYLESTERASE 1 IN C57BL/6 MOUSE LIVER MICROSOMES: PURIFICATION, CDNA CLONING, AND BACULOVIRUS-MEDIATED EXPRESSION

Several mouse carboxylesterase (CES) isozymes have been identified, but information about their roles in drug metabolism is limited. In this study, we purified and characterized a mouse CES1 isozyme that was induced by di-(2-ethylhexyl) phthalate. Purified mouse CES1 shared some biological characteristics with other CES isozymes, such as molecular weight of a subunit and isoelectronic point. In addition, purified mouse CES1 behaved as a trimer, a specific characteristic of CES1A subfamily isozymes. The purified enzyme possessed temocapril hydrolase activity, and it was found to contribute significantly to temocapril hydrolase activity in mouse liver microsomes. To identify the nucleotide sequences coding mouse CES1, antibody screening of a cDNA library was performed. The deduced amino acid sequence of the obtained cDNA, mCES1, exhibited striking similarity to those of CES1A isozymes. When expressed in Sf9 cells, recombinant mCES1 showed hydrolytic activity toward temocapril, as did purified mouse CES1. Based on these results, together with the findings that recombinant mouse CES1 had the same molecular weight of a subunit, the same isoelectronic point, and the same native protein mass as those of purified mouse CES1, it was concluded that mCES1 encoded mouse CES1. Furthermore, tissue expression profiles of mCES1 were found to be very similar to those of the human CES1 isozyme. This finding, together with our other results, suggests that mCES1 shares many biological properties with the human CES1 isozyme. The present study has provided useful information for study of metabolism and disposition of ester-prodrugs as well as ester-drugs.

Molecular cloning and characterization of a novel carboxylesterase-like protein that is physiologically present at high concentrations in the urine of domestic cats (Felis catus)

Normal mammals generally excrete only small amounts of protein in the urine, thus avoiding major leakage of proteins from the body. Proteinuria is the most commonly recognized abnormality in renal disease. However, healthy domestic cats ( Felis catus ) excrete proteins at high concentrations (about 0.5 mg/ml) in their urine. We investigated the possible cause of proteinuria in healthy cats, and discovered a 70 kDa glycoprotein, which was excreted as a major urinary protein in cat urine, irrespective of gender. To elucidate the biochemical functions and the excretion mechanism of this protein, we cloned the cDNA for this protein from a cat kidney cDNA library. The deduced amino acid sequence shared 47% identity with the rat liver carboxylesterase (EC 3.1.1.1), and both the serine hydrolase active site and the carboxylesterase-specific sequence were conserved. Therefore we named this protein cauxin (carboxylesterase-like urinary excreted protein). In contrast to the mammalian carboxylesterases, most of which are localized within the cells of various organs, cauxin was expressed specifically in the epithelial cells of the distal tubules, and was secreted efficiently into the urine, probably because it lacked the endoplasmic reticulum retention sequence (HDEL). Based on our finding that cauxin is not expressed in the immature cat kidney, we conclude that cauxin is involved in physiological functions that are specific for mature cats. Recently, cauxin-like cDNAs were found from human brain and teratocarcinoma cells. These data suggest that cauxin and cauxin-like human proteins are categorized as a novel group of carboxylesterase multigene family.

Mouse liver and kidney carboxylesterase (M-LK) rapidly hydrolyzes antitumor prodrug irinotecan and the N-terminal three quarter sequence determines substrate selectivity

Antitumor prodrug irinotecan is used for a variety of malignancies such as colorectal cancer. It is hydrolyzed to the metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), which exerts its antineoplastic effect. Several human and rodent carboxylesterases are shown to hydrolyze irinotecan, but the overall activity varies from enzyme to enzyme. This report describes a novel mouse liver and kidney carboxylesterase (M-LK) that is highly active toward this prodrug. Northern analyses demonstrated that M-LK was abundantly expressed in the liver and kidney and slightly in the intestine and lung. Lysates from M-LK transfected cells exhibited a markedly higher activity on irinotecan hydrolysis than lysates from the cells transfected with mouse triacylglycerol hydrolase (TGH) (6.9 versus 1.3 pmol/mg/min). Based on the immunostaining intensity with purified rat hydrolase A, M-LK had a specific activity of 173 pmol/mg/min, which ranked it as one of the most efficient esterases known to hydrolyze irinotecan. A chimeric carboxylesterase and its wild-type enzyme (e.g., M-LKn and M-LK), sharing three quarters of the entire sequence from the N-terminus, exhibited the same substrate preference toward irinotecan and two other substrates, suggesting that the N-terminal sequence determines substrate selectivity. M-LK transfected cells manifested more severe cytotoxicity than TGH transfected cells upon being exposed to irinotecan. Topoisomerase I inhibitors such as irinotecan represent a promising class of anticancer drugs. Identification of M-LK as an efficient carboxylesterase to activate irinotecan provides additional sequence information to locate residues involved in irinotecan hydrolysis and thus facilitates the design of new analogs.

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.

The cloning and expression of a murine triacylglycerol hydrolase cDNA and the structure of its corresponding gene

A novel murine cDNA for triacylglycerol hydrolase (TGH), an enzyme that is involved in mobilization of triacylglycerol from storage pools in hepatocytes, has been cloned and expressed. The cDNA consists of 1962 bp with an open reading frame of 1695 bp that encodes a protein of 565 amino acids. Murine TGH is a member of the CES1A class of carboxylesterases and shows a significant degree of identity to other carboxylesterases from rat, monkey and human. Expression of the cDNA in McArdle RH7777 hepatoma cells showed a 3-fold increase in the hydrolysis of p-nitrophenyl laurate compared to vector-transfected cells. The highest expression of TGH was observed in the livers of mice, with lower expression in kidney, heart, adipose and intestinal (duodenum/jejunum) tissues. The murine gene that encodes TGH was cloned and exon-intron boundaries were determined. The gene spans approx. 35 kb and contains 14 exons. The results will permit future studies on the function of this gene via gene-targeting experiments and analysis of transcriptional regulation of the TGH gene.

Identification of phenobarbitone-modulated genes in mouse liver by differential display

The molecular basis of how rodent nongenotoxic hepatocarcinogens such as phenobarbitone cause liver-tumor formation is poorly understood. An early effect of phenobarbitone exposure is to induce hepatocyte proliferation transiently, and there is evidence that this may be important for subsequent tumor development. In this investigation, we have used the differential display reverse transcriptase polymerase chain reaction technique to analyze differential gene expression in male C57B1/10J mouse liver during the mitogenic phase of the phenobarbitone response. Seventy-seven putative differentially expressed cDNAs were isolated by differential display, and 13 of them were subsequently confirmed as being differentially expressed (both increased and decreased by phenobarbitone). Seven of the cDNAs were homologous to known mouse or human genes (carboxylesterase, coagulation factor X, amine N-sulphotransferase, human protein disulphide isomerase-related protein, cytochrome c oxidase subunit IV, golgin-245, thioredoxin reductase, betaine-homocysteine methyl transferase) and the remainder were novel. The expression pattern of the sulphotransferase was further characterized, and in mouse liver it was found to be significantly induced by phenobarbitone and not by five other rodent nongenotoxic hepatocarcinogens. In summary, the technique has enabled the identification of previously uncharacterized genes whose expression patterns are differentially altered by phenobarbitone in the mouse liver.

Is dipalmitoylphosphatidylcholine a substrate for convertase?

Convertase has homology with carboxylesterases, but its substrate(s) is not known. Accordingly, we determined whether dipalmitoylphosphatidylcholine (DPPC), the major phospholipid in surfactant, was a substrate for convertase. We measured [(3)H]choline release during cycling of the heavy subtype containing [(3)H]choline-labeled DPPC with convertase, phospholipases A(2), B, C, and D, liver esterase, and elastase. Cycling with liver esterase or peanut or cabbage phospholipase D produced the characteristic profile of heavy and light peaks observed on cycling with convertase. In contrast, phospholipases A(2), B, and C and yeast phospholipase D produced a broad band of radioactivity across the gradient without distinct peaks. [(3)H]choline was released when natural surfactant containing [(3)H]choline-labeled DPPC was cycled with yeast phospholipase D but not with convertase or peanut and cabbage phospholipases D. Similarly, yeast phospholipase D hydrolyzed [(3)H]choline from [(3)H]choline-labeled DPPC after incubation in vitro, whereas convertase, liver esterase, or peanut and cabbage phospholipases D did not. Thus convertase, liver esterase, and plant phospholipases D did not hydrolyze choline from DPPC either on cycling or during incubation with enzyme in vitro. In conclusion, conversion of heavy to light subtype of surfactant by convertase may require a phospholipase D type hydrolysis of phospholipids, but the substrate in this reaction is not DPPC.

Toxicological significance in the cleavage of esterase-beta-glucuronidase complex in liver microsomes by organophosphorus compounds

Egasyn is an accessory protein of beta-glucuronidase (beta-G) in the liver microsomes. Liver microsomal beta-G is stabilized within the luminal site of the microsomal vesicles by complexation with egasyn which is one of the carboxylesterase isozymes. We investigated the effects of organophosphorus compounds (OPs) such as insecticides on the dissociation of egasyn-beta-glucuronidase (EG) complex. The EG complex was easily dissociated by administration of OPs, i.e. fenitrothion, EPN, phenthionate, and bis-beta-nitrophenyl phosphate (BNPP), and resulting beta-G dissociated was released into blood, leading to the rapid and transient increase of plasma beta-G level with a concomitant decrease of liver microsomal beta-G level. In a case of phenthionate treatment, less increase in plasma beta-G level was observed, as compared with those of other OPs. This may be explained by the fact that phenthionate was easily hydrolyzed by carboxylesterase. Similarly, carbamate insecticides such as carbaryl caused rapid increase of plasma beta-G level. In contrast, no significant increase of plasma beta-G level was observed when pyrethroid insecticides were administered to rats. This is due to the fact that pyrethroids such as phenthrin and allethrin were easily hydrolyzed by A-esterase as well as carboxylesterase. On the other hand, addition of OPs to the incubation mixture containing liver microsomes caused the release of beta-G from microsomes to the medium. From these in vivo and in vitro data, it is concluded that increase of the plasma beta-G level after OP administration is much more sensitive biomarker than cholinesterase inhibition to acute intoxication of OPs and carbamates.

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.

Cloning and sequencing of a novel murine liver carboxylesterase cDNA

Carboxylesterases (EC 3.1.1.1) comprise a group of serine hydrolases with at least 20 genetically distinct loci in mice. Here, we describe differential display PCR-based cloning of a cDNA, encoding a novel murine carboxylesterase termed ES-x, which was expressed predominantly in liver but also in kidney and lung. The cDNA of ES-x spanned a 2249-bp sequence with an open reading frame encoding 565 amino acids, including an N-terminal hydrophobic signal peptide which directs the synthesis into microsomal lumen and a C-terminal HVEL consensus sequence for retaining the protein in the lumen of the endoplasmic reticulum. The predicted amino acid sequence of ES-x exhibited 75% identity with rat liver pI 6.1 esterase. We further demonstrate that feeding mice with diets containing cholestyramine or sodium cholate increases mRNA-expression of ES-x in liver 2.5- to 3-fold.

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.

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).

The gene for albicidin detoxification from Pantoea dispersa encodes an esterase and attenuates pathogenicity of Xanthomonas albilineans to sugarcane

Albicidin phytotoxins are pathogenicity factors in a devastating disease of sugarcane known as leaf scald, caused by Xanthomonas albilineans. A gene (albD) from Pantoea dispersa has been cloned and sequenced and been shown to code for a peptide of 235 amino acids that detoxifies albicidin. The gene shows no significant homology at the DNA or protein level to any known sequence, but the gene product contains a GxSxG motif that is conserved in serine hydrolases. The AlbD protein, purified to homogeneity by means of a glutathione S-transferase gene fusion system, showed strong esterase activity on p-nitrophenyl butyrate and released hydrophilic products during detoxification of albicidins. AlbD hydrolysis of p-nitrophenyl butyrate and detoxification of albicidins required no complex cofactors. Both processes were strongly inhibited by phenylmethylsulfonyl fluoride, a serine enzyme inhibitor. These data strongly suggest that AlbD is an albicidin hydrolase. The enzyme detoxifies albicidins efficiently over a pH range from 5.8 to 8.0, with a broad temperature optimum from 15 to 35 degrees C. Expression of albD in transformed X. albilineans strains abolished the capacity to release albicidin toxins and to incite disease symptoms in sugarcane. The gene is a promising candidate for transfer into sugarcane to confer a form of disease resistance.

Lung "surfactant convertase" is a member of the carboxylesterase family

The extracellular conversion of lung surfactant from tubular myelin to the small vesicular form has previously been shown to require a serine-active enzyme called "surfactant convertase." In the present study, a 72kD serine-active enzyme previously identified in mouse lung alveolar lavage and having convertase activity was partially sequenced. Sixty-eight residues obtained from amino acid sequencing of this protein show that it is a new member of the mouse carboxylesterase family (EC 3.1.1.1). The 72kD lung protein also has esterase activity. A commercial esterase of the same family was able to reproduce surfactant convertase bioactivity in vitro, unlike several serine proteinases previously tested. We conclude that surfactant convertase is a carboxylesterase which mediates a biochemical step in the extracellular metabolism of surfactant.

Purification and cloning of a broad substrate specificity human liver carboxylesterase that catalyzes the hydrolysis of cocaine and heroin

A human liver carboxylesterase (hCE-2) that catalyzes the hydrolysis of the benzoyl group of cocaine and the acetyl groups of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine was purified from human liver. The purified enzyme exhibited a single band on SDS-polyacrylamide gel electrophoresis with a subunit mass of approximately 60 kDa. The native enzyme was monomeric. The isoelectric point of hCE-2 was approximately 4.9. Treatment with endoglycosidase H caused an increase in electrophoretic mobility indicating that the liver carboxylesterase was a glycoprotein of the high mannose type. The complete cDNA nucleotide sequence was determined. The authenticity of the cDNA was confirmed by a perfect sequence match of 78 amino acids derived from the hCE-2 purified from human liver. The mature 533-amino acid enzyme encoded by this cDNA shared highest sequence identity with the rabbit liver carboxylesterase form 2 (73%) and the hamster liver carboxylesterase AT51p (67%). Carboxylesterases with high sequence identity to hCE-2 have not been reported in mouse and rat liver. hCE-2 exhibited different drug ester substrate specificity from the human liver carboxylesterase called hCE-1, which hydrolyzes the methyl ester of cocaine. hCE-2 had higher catalytic efficiencies for hydrolysis of 4-methylumbelliferyl acetate, heroin, and 6-monoacetylmorphine and greater inhibition by eserine than hCE-1. hCE-2 may play an important role in the degradation of cocaine and heroin in human tissues.

High-resolution mapping of a linkage group on mouse chromosome 8 conserved on human chromosome 16Q

We have performed a high-resolution linkage analysis for the conserved segment on distal mouse Chromosome (Chr) 8 that is homologous to human Chr 16q. The interspecific backcross used involved M. m. molossinus and an M. m. domesticus line congenic for an M. spretus segment from Chr 8 flanked by phenotypic markers Os (oligosyndactyly) and e, a coat colormarker. From a total of 682 N2 progeny, the 191 animals revealing a recombination event between these phenotypic markers were typed for 23 internal loci. The following locus order with distances in cM was obtained: (centromere)-Os-4.1-Mmp2-0.2-Ces1,Es1, Es22-1.2-Mt1,D8Mit15-2.2-Got2, D8Mit11-3.7-Es30-0.3-Es2, Es7-0.9-Ctra1,Lcat-0.3-Cdh1, Cadp, Nmor1, D8Mit12-0.2-Mov34-2.5-Hp,Tat-0.2-Zfp4-1.6-Zfp1,+ ++Ctrb-10.9-e. In a separate interspecific cross involving 62 meioses, Dpep1 was mapped together with Aprt and Cdh3 at 12.9 cM distal to Hp, Tat, to the vicinity of e. Our data give locus order for markers not previously resolved, add Mmp2 and Dpep1 as new markers on mouse Chr 8, and indicate that Ctra1 is the mouse homolog for human CTRL. Comparison of the order of 17 mouse loci with that of their human homologs reveals that locus order is well conserved and that the conserved segment in the human apparently spans the whole long arm of Chr 16.

Ovarian teratomas associated with the insertion of an imprinted transgene

Ovarian teratomas are tumors that arise from female germ cells and are often a mixture of immature embryonal carcinoma cells and mature embryonic cells. Tissues derived from all three primary embryonic lineages (ectoderm, mesoderm, and endoderm) are typically found in the mature elements of a teratoma. In the case of the transgenic mouse line TG.KD, created with an imprinted transgene construct, malignant ovarian teratomas of a mixed germ cell tumor morphology occur in 15-20% of hemizygous female carriers of the transgene. The tumors frequently metastasize and can result in death of the mouse. Genetic analysis indicates that the tumors are associated with the transgenes integration site. Inbred FVB/N and female mice of other transgenic lines, also created in the inbred FVB/N strain with the same DNA construct as TG.KD, do not develop teratomas. In addition to teratomas, the integration of the transgene on Chromosome (Chr) 8 is associated with a perinatal lethality in homozygous transgenic carriers. The hemizygous genotypes of the teratomas suggest that they arise from early germ cells, prior to the completion of meiosis I.

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.

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.

Cloning and analysis of the esterase genes conferring insecticide resistance in the peach-potato aphid, Myzus persicae (Sulzer)

Full-length cDNA clones encoding the esterases (E4 and FE4) that confer insecticide resistance in the peach-potato aphid [Myzus persicae (Sulzer)] were isolated and characterized. The E4 cDNA contained an open reading frame of 1656 nucleotides, coding for a protein of 552 amino acids. The FE4 cDNA shared 99% identity with E4 over this region, the most important difference being a single nucleotide substitution resulting in the FE4 mRNA having an extra 36 nucleotides at the 3' end. The derived amino acid sequences for the N-terminus of E4 and FE4 were identical, with the first 23 residues being characteristic of a signal peptide and the next 40 residues being an exact match to the N-terminal sequence determined by Edman degradation of both purified proteins. The predicted molecular masses of 58.8 and 60.2 kDa for the E4 and FE4 polypeptides were consistent with those previously observed by in vitro translation of mRNA. Five potential N-linked glycosylation sites were present in both polypeptides, in accordance with earlier evidence that the native esterases are glycoproteins. Comparison of the aphid esterase protein sequences with other serine hydrolases provided evidence that their activity involves a charge-relay system with a catalytic triad the same as that found in acetylcholinesterase. Restriction mapping and sequencing of cloned genomic DNA showed that the E4 gene is spread over 4.3 kb with six introns and that the previously reported differences between the 3' ends of the E4 and FE4 genes result from single nucleotide substitutions and not gross differences in the DNA sequences.

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.

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.

Linkage of Agt and Actsk-1 to distal mouse chromosome 8 loci: a new conserved linkage

Angiotensinogen is an alpha 2-globulin involved in the maintenance of blood pressure and electrolyte balance. We have refined the position of the mouse angiotensinogen locus (Agt) on Chromosome (Chr) 8 and have also confirmed the assignment of the human angiotensinogen locus (AGT) to Chr 1. The segregation of several restriction fragment length variants (RFLVs) was followed in two interspecific backcross sets and in four recombinant inbred (RI) mouse sets. Analysis of the segregation patterns closely linked Agt to Aprt and Emv-2, which places the angiotensinogen locus on the distal end of mouse Chr 8. Additionally, a literature search has revealed that the strain distribution pattern (SDP) for the mouse skeletal alpha-actin locus 1 (Actsk-1, previously Acta1, Acta, or Acts) is nearly identical to the SDP for Agt in two RI sets. On the basis of this information we were able to reassign Actsk-1 to mouse Chr 8. By screening a panel of human-mouse somatic cell hybrids, we confirmed that the human angiotensinogen locus lies on Chr 1. This information describes a new region of conserved linkage homology between mouse Chr 8 and human Chr 1. It also defines the end of a large region of conserved linkage homology between mouse Chr 8 and human Chr 16.

Esterase-1: developmental expression in the mouse and distribution of related proteins in other species

Esterase 1 (Es-1) is a sexually dimorphic 65-kDa glycoprotein present in plasma and other murine tissues able to hydrolyze a variety of esters including fatty acid esters of estradiol. Like most other carboxylesterases, its function is unknown. To gain insight into the function of Es-1 and by analogy other carboxylesterases, we have examined the developmental regulation of Es-1 in the mouse and have looked for the presence of related proteins in the plasma of other species. Northern blot analysis of total RNA from the livers of mice of various ages using a 32P-labeled 470-bp Es-1 cDNA probe showed clear postpartum induction with no detectable Es-1 mRNA in fetal liver. Similarly, immunoblotting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an affinity-purified rabbit antibody to Es-1 showed no cross-reacting proteins in the plasma until after birth. Northern blot analysis of total RNA from a variety of adult mouse tissues showed the presence of substantial levels of Es-1 mRNA only in liver with lower levels in kidney, testes, and ovaries. Liver mRNA and plasma protein levels rose in parallel attaining full adult levels between 15 and 20 days of age. When plasma proteins were electrophoresed on 7% polyacrylamide gels under nondenaturing conditions, the antibody to Es-1 recognized a low mobility protein in mouse, rat, human, baboon, guinea pig, bovine, horse, and canine but not in chicken plasma. Consistent with the immunoblotting results, the Es-1 cDNA probe hybridized to restriction fragments from human, monkey, rat, and rabbit as well as mouse genomic DNA but not from chicken DNA indicating conservation of the esterase (or esterase-like) gene in mammalian species. The low mobility antigens in mouse and human plasma appeared also to cross-react with antibodies to human thyroglobulin, although antibodies to human thyroglobulin did not appear to recognize Es-1 under these conditions.

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.

Assignment of rat linkage group V to chromosome 19 by single-strand conformation polymorphism analysis of somatic cell hybrids

The rat provides a number of important models of human genetic disease; however, the rat genetic map has not been extensively developed. Although most rat chromosomes carry several gene assignments, some major linkage groups (LG) remain to be mapped. To determine the chromosome location of the largest unmapped linkage group in the rat (LG V containing multiple carboxylesterase loci), we used single-strand conformation polymorphism analysis to identify the rat esterase-10 gene in a panel of rat x mouse somatic cell hybrids. We found that the carboxylesterase gene family and hence LG V are located on rat chromosome 19. We have also confirmed the assignment of the angiotensinogen gene to rat chromosome 19 and have used a large set of recombinant inbred strains to map two anonymous variable number of tandem repeat (VNTR) markers to this chromosome. The current findings bring the total number of genes assigned to rat chromosome 19 from 3 to 19 and provide further evidence of substantial homology between this chromosome and chromosome 8 in the mouse.

Characterization and functional expression of a cDNA encoding egasyn (esterase-22): the endoplasmic reticulum-targeting protein of beta-glucuronidase

Egasyn (esterase-22), a member of the nonspecific carboxylesterase multigene family (E.C. 3.1.1.1), is the endoplasmic reticulum (ER)-targeting protein of beta-glucuronidase. We utilized the polymerase chain reaction (PCR) in the eventual isolation of murine egasyn cDNAs. PCR primers were based upon: (1) partial amino acid sequences derived from egasyn peptides and (2) a conserved active site region shared by carboxylesterases. The amino acid sequence deduced from the PCR product matched that obtained from egasyn protein. This product was utilized as a probe to screen a cDNA library. Two cDNAs whose composite sequence encoded an open reading frame of 562 amino acids were isolated. A message size of 1700-2000 bp was revealed by RNA blot hybridization analysis. S1 nuclease protection analyses detected mRNA in liver, kidney, lung, and submandibular gland, but not in spleen, brain, and testes. Genetic mapping confirmed the location of an egasyn cDNA fragment in cluster 1 of the esterase region on chromosome 8. Transfection of COS cells with the 2022-bp cDNA resulted in the expression of esterase activity, which comigrated on native gels with liver esterase-22. The features of the deduced amino acid sequence of the egasyn cDNA are compared with previously characterized carboxylesterases and with other lumenal ER proteins.