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

Purification, molecular cloning, and functional expression of inducible liver acylcarnitine hydrolase in C57BL/6 mouse, belonging to the carboxylesterase multigene family

To identify the peroxisome proliferator-inducible acylcarnitine hydrolase in C57BL/6 mice, acylcarnitine hydrolase was purified to homogeneity using column chromatography. The purified enzyme, named ACH M1, had a subunit molecular weight of 60kDa. ACH M1 could hydrolyze classical carboxylesterase (CES) substrates as well as palmitoyl-dl-carnitine and these activities were inhibited by anti-rat CES antibodies. The peptide fragments of ACH M1 were identical to those of the deduced amino acid sequence of mouse CES2 isozyme. These findings suggested that ACH M1 was a member of the CES2 family. The mouse CES2 cDNA, designated mCES2, was cloned from mouse liver. The recombinant mCES2 expressing in Sf9 cells showed high level of catalytic activity toward acylcarnitines. Furthermore, the biological characteristics of the expressed protein were identical with those of ACH M1 in many cases, suggesting that mCES2 encodes mouse liver ACH M1.

Expression and partial purification of a recombinant secretory form of human liver carboxylesterase

Serine-dependent carboxylesterases (EC 3.1.1.1) are found in a variety of tissues with high activity detected in the liver. Carboxylesterases (CaE) hydrolyze aliphatic and aromatic esters, and aromatic amides, and play an important role in the detoxification of xenobiotic chemicals that contain organophosphate (OP) compounds. The detoxifying ability of CaE is limited by its low concentration in serum where it encounters OP compounds. Studies in our laboratory have shown that a pRC/CMV-hCaE plasmid construct, stably integrated into 293T cells, expresses a human liver CaE in culture. However, the enzyme remained inside the cell and reached a low steady-state level of expression. The goals of this study were to overexpress a functional human liver CaE from a recombinant cDNA in a human cell line and to isolate and purify the recombinant protein. To accomplish these goals, a single amino acid change was made in the C-terminal retrieval signal, HIEL (His-Ile-Glu-Leu), of human liver CaE. The mutation produced a unique Eco47III restriction site, which aided in clone selection. The recombinant plasmid, pRc/CMV-mhCaE, was isolated and stably integrated into human 293T cells. Expression of the altered cDNA resulted in secretion of an active CaE up to levels of 500 enzyme units per liter of growth medium. Secretory CaE displayed isoelectric focusing patterns similar to those of the native enzyme with no observable changes in activity. The secreted enzyme was partially purified by hydrophobic interaction chromatography and Cibacron blue affinity chromatography. Partial enzyme purification was achieved, and CaE retained a high level of enzymatic activity.

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.

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.

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.

Mosquito carboxylesterase Est alpha 2(1) (A2). Cloning and sequence of the full-length cDNA for a major insecticide resistance gene worldwide in the mosquito Culex quinquefasciatus

Organophosphorus insecticide resistance in Culex mosquitoes is commonly caused by increased activity of one or more esterases. The commonest phenotype involves elevation of the esterases Est alpha 2 (A2) and Est beta 2 (B2). A cDNA encoding the Est alpha 2 esterase has now been isolated from a Sri Lankan insecticide-resistant mosquito (Culex quinquefasciatus, Say) expression library. In line with a recently suggested nomenclature system (Karunaratne, S. H. P. P. (1994) Characterization of Multiple Variants of Carboxylesterases Which Are Involved in Insecticide Resistance in the Mosquito Culex quinquefasciatus. Ph.D. thesis, University of London), as the first sequenced variant of this esterase, it is now referred to as Est alpha 2(1). The full-length cDNA of est alpha 2(1) codes for a 540-amino acid protein, which has high homology with other esterases and lipases and belongs to the serine or B-esterase enzyme family. The predicted secondary structure of Est alpha 2(1) is similar to the consensus secondary structure of proteins within the esterase/lipase family where the secondary and tertiary structures have been resolved. The level of identity (approximately 47% at the amino acid level) between the est alpha 2(1) and the various Culex est beta (B1 and B2) cDNA alleles that have been cloned and sequenced suggests that the two esterase loci are closely related and arose originally from duplication of a common ancestral gene. The lack of a distinct hydrophobic signal sequence for Est alpha 2(1) and two possible N-linked glycosylation sites, both situated close to the active site serine, suggest that it is a nonglycosylated protein that is not exported from the cell. Southern and dot blot analysis of genomic DNA from various insecticide-resistant and susceptible mosquito strains show that the est alpha 2(1) gene, like est beta 2(1), is amplified in resistant strains. The restriction fragment length polymorphism patterns, after probing Southern blots of EcoRI-digested genomic DNA with esta alpha 2(1) cDNA, show that the amplified and nonamplified est alpha alleles differ in the resistant and susceptible Sri Lankan mosquitoes.

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.

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.