The mammalian carboxylesterases: from molecules to functions

Twelve Novel Single Nucleotide Polymorphisms in the CES2 Gene Encoding Human Carboxylesterase 2 (hCE­2)

Twelve novel single nucleotide polymorphisms (SNPs) were found in the CES2 gene from 153 Japanese individuals, who were administered irinotecan or steroidal drugs. The detected SNPs were as follows:1) SNP, MPJ6_CS2001; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CTGGAACAACTCG/CCTCCCCTCGGAA-3'. 2) SNP, MPJ6_CS2002; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-AACCACCACCGCT/CGATCCTAGCAGG-3'. 3) SNP, MPJ6_CS2003; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-AAATGTTTGTCAA/GGTGGATAAATGA-3'. 4) SNP, MPJ6_CS2004; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCTCCTATCGATC/GCCCCAGCGCGCT-3'. 5) SNP, MPJ6_CS2005; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GCCAGTCCCATCC/TGGACCACACACA-3'. 6) SNP, MPJ6_CS2006; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GCTGGGCAACCCG/AGGCTGAGCGGGG-3'. 7) SNP, MPJ6_CS2007; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CAAGCACGCAACC/TGGCAACTGGGGC-3'. 8) SNP, MPJ6_CS2008; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CATGGAGAGTGGC/TGTGGCCCTCCTG-3'. 9) SNP, MPJ6_CS2009; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCTGTTCTTGGCC/TAGGGCCTTGGGC-3'. 10) SNP, MPJ6_CS2010; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-CCCATCCCCAGCT/AACAGACTCTCTC-3'. 11) SNP, MPJ6_CS2011; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-TCCACCTGGGGTA/GGATGTTGCCTCC-3'. 12) SNP, MPJ6_CS2013; GENE NAME, CES2; ACCESSION NUMBER, NT_010498.13; LENGTH, 25 bases; 5'-GGACTGGGGACCG/AAGGTCTCGGGGG-3'The frequencies were 0.029 for MPJ6_CS2004 and CS2013, 0.026 for MPJ6_CS2009, 0.013 for MPJ6_CS2001, 0.007 for MPJ6_CS2003, and 0.003 for the other 7 SNPs. Among these SNPs, MPJ6_CS2005 (100C>T) resulted in an amino acid alteration (R34W), and MPJ6_CS2007 (579C>T) and MPJ6_CS2008 (765C>T) were synonymous (T193T and G255G, respectively). Furthermore, MPJ6_CS2011 (IVS8-2A>G) resulted in variation at ther 3' splice acceptor site.

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.

Use of ab initio calculations to predict the biological potency of carboxylesterase inhibitors

Carboxylesterases are important enzymes responsible for the hydrolysis and metabolism of numerous pharmaceuticals and xenobiotics. These enzymes are potently inhibited by trifluoromethyl ketone containing (TFK) inhibitors. We demonstrated that the ketone hydration state was affected by the surrounding chemical moieties and was related to inhibitor potency, with inhibitors that favored the gem-diol conformation exhibiting greater potency. Ab initio calculations were performed to determine the energy of hydration of the ketone, and the values were correlated with esterase inhibition data for a series of carboxylesterase inhibitors. This system was examined in three different mammalian models (human liver microsomes, murine liver microsomes, and commercial porcine liver esterase) and in an insect enzyme preparation (juvenile hormone esterase). In all cases, the extent of ketone hydration was strongly correlated with biological potency. Our results showed a very strong correlation with the extent of hydration, accounting for 94% of activity for human liver microsome esterase inhibition (p < 0.01). The atomic charge on the carbon atom of the carbonyl group in the TFK also strongly correlated with inhibitor potency, accounting for 94% of inhibition activity in human liver microsomes (p < 0.01). In addition, we provide crystallographic evidence of intramolecular hydrogen bonding in sulfur-containing inhibitors and relate these data to gem-diol formation. This study provides insight into the mechanism of carboxylesterase inhibition and raises the possibility that inhibitors that too strongly favor the gem-diol configuration have decreased potency due to low rate of ketone formation.

Comprehensive evaluation of carboxylesterase-2 expression in normal human tissues using tissue array analysis

Carboxylesterases play an important role in the hydrolytic biotransformation of a number of structurally diverse endogenous compounds and medications. Several distinct carboxylesterase isoforms have been described in human liver, brain, and placenta. Carboxylesterase-2 has been identified as the key enzyme in the metabolic activation of the irinotecan, a topoisomerase I inhibitor commonly used in the treatment of many solid tumors. The tissue distribution and intensity of protein expression of carboxylesterase-2 have not been defined in any organ or tissue. This study used a carboxylesterase-2-specific antibody and tissue array analysis to detect carboxylesterase-2 expression in human normal tissues by immunohistochemistry. Carboxylesterase-2 is present in a wide variety of organs and tissues. The highest carboxylesterase-2 expression occurs in hepatocyte, small intestine mucosa, kidney proximal convoluted tubule, and adrenal cortex cells. The results suggest that liver and gastrointestinal tract with carboxylesterase-2 are likely the most important sites of conversion of irinotecan to the active metabolite SN-38, but carboxylesterase-2 within the other tissues may be contributive to this process. In the central nervous system, carboxylesterase-2 expression was confined to capillary endothelial cells, consistent with the enzyme having a role to protect the central nervous system from toxic esters and perhaps being a component of a blood-brain barrier system.

Purification and properties of a phospholipase A2/lipase preferring phosphatidic acid, bis(monoacylglycerol) phosphate, and monoacylglycerol from rat testis

Phospholipase A(2) (PLA(2)) was purified to homogeneity from the supernatant fraction of rat testis homogenate. The purified 63-kDa enzyme did not require Ca(2+) ions for activity and exhibited both phosphatidic acid-preferring PLA(2) and monoacylglycerol lipase activities with a modest specificity toward unsaturated acyl chains. Anionic detergents enhanced these activities. Serine-modifying irreversible inhibitors, (p-amidinophenyl) methanesulfonyl fluoride and methylarachidonyl fluorophosphonate, inhibited both activities to a similar extent, indicating a single active site is involved in PLA(2) and lipase activities. The sequence of NH(2)-terminal 12 amino acids of purified enzyme was identical to that of a carboxylesterase from rat liver. The optimal pH for PLA(2) activity (around 5.5) differed from that for lipase activity (around 8.0). At pH 5.5 the enzyme also hydrolyzed bis(monoacylglycerol) phosphate, or lysobisphosphatidic acid (LBPA), that has been hitherto known as a secretory PLA(2)-resistant phospholipid and a late endosome marker. LBPA-enriched fractions were prepared from liver lysosome fractions of chloroquine-treated rats, treated with excess of pancreatic PLA(2), and then used for assaying LBPA-hydrolyzing activity. LBPA and the reaction products were identified by microbore normal phase high performance liquid chromatography/electrospray ionization ion-trap mass spectrometry. These enzymatic properties suggest that the enzyme can metabolize phosphatidic and lysobisphosphatidic acids in cellular acidic compartments.

Secreted and tumour targeted human carboxylesterase for activation of irinotecan

Irinotecan (CPT-11) is an anticancer agent for the treatment of colon cancer. CPT-11 can be considered as a prodrug, since it needs to be activated into the toxic drug SN-38 by the enzyme carboxylesterase. An approach to achieve tumour specific activation of CPT-11 is to transduce the cDNA encoding carboxylesterase into tumour cells. A secreted form of carboxylesterase may diffuse through a tumour mass and may activate CPT-11 extracellularly. This could enhance the antitumour efficacy by exerting a bystander effect on untransduced cells. In addition a secreted tumour-targeted form of carboxylesterase should prevent leakage of the enzyme from the site of the tumour into the circulation. We have constructed a secreted form of human liver carboxylesterase-2 by deletion of the cellular retention signal and by cloning the cDNA downstream of an Ig kappa leader sequence. The protein was secreted by transfected cells and showed both enzyme activity and efficient CPT-11 activation. To obtain a secreted, tumour-targeted form of carboxylesterase-2 the cDNA encoding the human scFv antibody C28 directed against the epithelial cell adhesion molecule EpCAM, was inserted between the leader sequence and carboxylesterase-2. This fusion protein showed CPT-11 activation and specific binding to EpCAM expressing cells. Importantly, in combination with CPT-11 both recombinant carboxylesterase proteins exerted strong antiproliferative effects on human colon cancer cells. They are, therefore, promising new tools for gene directed enzyme prodrug therapy approaches for the treatment of colon carcinoma with CPT-11.

Identification of microsomal rat liver carboxylesterases and their activity with retinyl palmitate

Retinyl esters are a major endogenous storage source of vitamin A in vertebrates and their hydrolysis to retinol is a key step in the regulation of the supply of retinoids to all tissues. Some members of nonspecific carboxylesterase family (EC 3.1.1.1) have been shown to hydrolyze retinyl esters. However, the number of different isoenzymes that are expressed in the liver and their retinyl palmitate hydrolase activity is not known. Six different carboxylesterases were identified and purified from rat liver microsomal extracts. Each isoenzyme was identified by mass spectrometry of its tryptic peptides. In addition to previously characterized rat liver carboxylesterases ES10, ES4, ES3, the protein products for two cloned genes, AB010635 and D50580 (GenBank accession numbers), were also identified. The sixth isoenzyme was a novel carboxylesterase and its complete cDNA was cloned and sequenced (AY034877). Three isoenzymes, ES10, ES4 and ES3, account for more than 95% of rat liver microsomal carboxylesterase activity. They obey Michaelis-Menten kinetics for hydrolysis of retinyl palmitate with Km values of about 1 micro m and specific activities between 3 and 8 nmol.min-1.mg-1 protein. D50580 and AY034877 also hydrolyzed retinyl palmitate. Gene-specific oligonucleotide probing of multiple-tissue Northern blot indicates differential expression in various tissues. Multiple genes are highly expressed in liver and small intestine, important tissues for retinoid metabolism. The level of expression of any one of the six different carboxylesterase isoenzymes will regulate the metabolism of retinyl palmitate in specific rat cells and tissues.

Intestinal absorption enhancement of the ester prodrug tenofovir disoproxil fumarate through modulation of the biochemical barrier by defined ester mixtures

The effect of discrete esters and ester mixtures on the intestinal stability and absorption of tenofovir disoproxil fumarate (tenofovir DF, an esterase-sensitive prodrug of the antiviral tenofovir) was compared with the effect of strawberry extract, which has been shown to enhance the absorption of the prodrug across Caco-2 monolayers and in rat ileum. In addition, the mechanism of absorption enhancement was investigated. In rat intestinal homogenates, complete inhibition of the conversion of tenofovir DF (as obtained by strawberry extract) could only be obtained at relatively high concentrations of the discrete esters or by using mixtures of esters (e.g., propyl p-hydroxybenzoate 0.02%, octyl acetate 0.02%, ethyl caprylate 0.01%). Coincubation of tenofovir DF with this mixture also resulted in an enhancement of its absorption in the in vitro Caco-2 system as well as in rat ileum. As tenofovir DF is a substrate for P-glycoprotein (P-gp)-related efflux carriers in the Caco-2 model, the modulatory effect of the ester mixtures was studied on the functionality of P-gp using cyclosporin A (CsA) as a model substrate. Strawberry extract as well as the mixture of three esters interfered with the absorptive transport of CsA across Caco-2 monolayers, illustrating that both mixtures interfere with both esterase-activity and P-gp functionality. This concerted barrier was not observed in rat ileum, suggesting differential functional activities of the biochemical barrier toward tenofovir DF in different absorption systems. Overall, our results illustrate that modulation of the biochemical barrier (metabolism and efflux) of tenofovir DF by ester mixtures can be used to increase the intestinal absorption of tenofovir DF in an in vitro and an in situ absorption model; the mechanism of action appears to be a complex interplay of different systems; the differential expression of carriers and enzymes in different systems illustrates the difficulty of extrapolating observations between different systems/species.

Involvement of liver carboxylesterases in the in vitro metabolism of lidocaine

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

Current progress on esterases: from molecular structure to function

This article reports on a symposium sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the April 2001 Experimental Biology meeting. Current developments in molecular-based studies into the structure and function of cholinesterases, carboxylesterases, and paraoxonases are described. This article covers mechanisms of regulation of gene expression of the various esterases by developmental factors and xenobiotics, as well as the interplay between physiological and chemical regulation of enzyme activity.

Human and rodent carboxylesterases: immunorelatedness, overlapping substrate specificity, differential sensitivity to serine enzyme inhibitors, and tumor-related expression

Carboxylesterases hydrolyze numerous endogenous and foreign compounds with diverse structures. Humans and rodents express multiple forms of carboxylesterases, which share a high degree of sequence identity (approximately 70%). Alignment analyses locate in carboxylesterases several functional subsites such the catalytic triad as seen in acetylcholinesterase. The aim of this study was to determine among human and rodent carboxylesterases the immunorelatedness, overlapping substrate specificity, differential sensitivity to serine enzyme inhibitors, tissue distribution, and tumor-related expression. Six antibodies against whole carboxylesterases or synthetic peptides were tested for their reactivity toward 11 human or rodent recombinant carboxylesterases. The antibodies against whole proteins generally exhibited a broader cross-reactivity than the anti-peptide antibodies. All carboxylesterases hydrolyzed para-nitrophenylacetate and para-nitrophenylbutyrate. However, the relative activity varied markedly from enzyme to enzyme (>20-fold), and some carboxylesterases showed a clear substrate preference. Carboxylesterases with the same functional subsites had a similar profile on substrate specificity and sensitivity toward phenylmethylsulfonyl fluoride (PMSF) and paraoxon, suggesting that these subsites play determinant roles in the recognition of substrates and inhibitors. Among three human carboxylesterases, HCE-1 hydrolyzed both substrates to a similar extent, whereas HCE-2 and HCE-3 showed an opposite substrate preference. All three enzymes were inhibited by PMSF and paraoxon, but they showed a marked difference in relative sensitivities. Based on immunoblotting analyses, HCE-1 was present in all tissues examined, whereas HCE-2 and HCE-3 were expressed in a tissue-restricted pattern. Colon carcinomas expressed slightly higher levels of HCE-1 and HCE-2 than the adjacent normal tissues, whereas the opposite was true with HCE-3.

Pharmacodynamics and pharmacogenomics of diverse receptor-mediated effects of methylprednisolone in rats using microarray analysis

Corticosteroids such as methylprednisolone (MPL) produce many of their anti-inflammatory, immunosuppressive, and exaggerated physiological effects by receptor and gene-mediated mechanisms. The temporal pattern of change in four genes in rat tissues was measured by quantitative Northern hybridization and rtPCR after a single dose of MPL. Two profiles were observed: two genes with enhanced expression showed a slow onset and moderate rate of decline within a 24 hr time frame while two genes with reduced expression exhibited a rapid onset and prolonged suppression over a > or = 72 hr time span. These patterns are consistent with and rationalized by pharmacodynamic expectations based on earlier models. cDNA microarrays used to assess the expression levels of 5200 genes at one optimal time-point showed marked variation in baseline values. Of these, 20 genes showed statistically significant enhanced expression with increases ranging from 130 to 1690%, 31 genes exhibited reduced expression ranging from 31 to 72% of control. Many genes could be categorized as affecting acute phase/immune response, energy metabolism, microsomal metabolism, and hepatic function. These studies provide the first simultaneous assessment of the diversity in pharmacogenomic effects of corticosteroids. They also provide some insight into the advantages and limitations of microarray measurements in regard to the pharmacodynamics of drugs having complex, multi-faceted, and integrated mechanisms of action.

ERp29 is a ubiquitous resident of the endoplasmic reticulum with a distinct role in secretory protein production

ERp29 was recently characterized biochemically as a novel protein that resides in mammalian endoplasmic reticulum (ER). Here we applied immunochemical procedures at the cellular level to investigate the hypothesized role of ERp29 in secretory protein production. ERp29 was localized exclusively to the ER/nuclear envelope of MDCK cells using confocal immunocytochemistry and comparative markers of the ER lumen, ER/Golgi membrane, nuclei, and mitochondria. A predominant association with rough ER was revealed by sucrose-gradient analysis of rat liver microsomes. Immunohistochemistry showed ERp29 expression in 35 functionally distinct cell types of rat, establishing ERp29 as a general ER marker. The ERp29 expression profile largely paralleled that of protein disulfide isomerase (PDI), the closest relative of ERp29, consistent with a role in secretory protein production. However strikingly different ERp29/PDI ratios were observed in various cell types, suggesting independent regulation and functional roles. Together, these findings associate ERp29 primarily with the early stages of secretory protein production and implicate ERp29 in a distinct functional role that is utilized in most cells. Our identification of several ERp29-enriched cell types suggests a potential selectivity of ERp29 for non-collagenous substrates and provides a physiological foundation for future investigations.

The role of ketamine on plasma cocaine pharmacokinetics in rat

Ketamine has gained attention recently because of re-emergence of its abuse especially in combination with cocaine. When more than one drug is present simultaneously, the potential for drug--drug interaction exists, which can be pharmacokinetic, pharmacodynamic or both in nature. The objective of this study was to investigate the effect of ketamine on plasma cocaine pharmacokinetics to assess the role that the kinetic component may play in the interaction of these agents. Moreover, the effect of repetitive administration of ketamine pretreatment on the pharmacokinetics of cocaine was addressed. Male Sprague-Dawley rats were treated with cocaine alone (5 mg/kg i.v.), ketamine alone (100 mg/kg by gavage), or ketamine followed by cocaine (the same routes and doses). Blood samples were withdrawn at different time points post-injection and analyzed for determination of cocaine, its metabolites (benzoylecgonine and norcocaine) and ketamine. The results demonstrated that ketamine caused a significant decrease in cocaine's area under the curve (AUC) and elimination half-life while its total clearance was increased. The AUC of benzoylecgonine was increased by 1.5-fold after the combination compared with cocaine alone. However, cocaine did not affect ketamine's pharmacokinetic parameters. In the pretreatment study, ketamine was given orally for 3 days, followed 18 h later by a single i.v. of cocaine. Further enhancement of cocaine metabolism occurred with the appearance of norcocaine. This investigation revealed that ketamine enhances cocaine metabolism and may affect its toxicological profile.

Differential inhibition of hepatic, pancreatic, and plasma fatty acid ethyl ester synthase by tri-o-tolylphosphate in rats

Fatty acid conjugation of alcohols, catalyzed by fatty acid ethyl ester synthase (FAEES), results in the formation of lipophilic esters. Although the activity of FAEES is reported in almost all organs, including plasma, the interrelationship among various proteins expressing FAEES activity in different organs/tissues is not well understood. Earlier, we have reported an inhibition of FAEES activity in human hepatoma cells by tri-o-tolylphosphate (TOTP; serine esterase inhibitor). The present study was undertaken to further characterize the hepatic, plasma, and pancreatic FAEES in rats after ip injection of 10, 25, 50, or 100 mg/kg TOTP in corn oil or vehicle alone. After 18 h, animals were euthanized and FAEES activity in the plasma and postnuclear fractions of hepatic and pancreatic homogenates were assayed by measuring the ester formation following incubation with [1-(14)C]oleic acid and ethanol or methanol as substrates. Significant inhibition of FAEES activity was observed in hepatic postnuclear fraction. The esterase activity also showed a pattern similar to fatty acid ethyl and methyl ester synthesizing activity. A trend similar to hepatic synthesizing and hydrolyzing activities was also found in the plasma of TOTP-treated rats. However, no inhibition of synthetic activity toward formation of fatty acid ethyl or methyl esters or p-nitrophenyl acetate hydrolyzing activity was observed in the pancreas of rats after TOTP exposure. Our results also show that the protein expressing FAEES activity in the pancreas does not cross-react with antibodies to rat adipose tissue FAEES using Western blot analysis, which recognizes approximately 60- and approximately 84-kDa proteins in the liver and plasma, respectively. Furthermore, the inhibition in liver is at the functional level of enzyme as no change was observed between control and treated animals by immunohistochemistry. We conclude that fatty acid ethyl or methyl ester synthesizing enzyme(s) in the liver and plasma, which are inhibited by TOTP, are different from that present in the pancreas.

Phosphonate O-deethylation of [4-(4-bromo-2-cyano-phenylcarbamoyl) benzyl]-phosphonic acid diethyl ester, a lipoprotein lipase-promoting agent, catalyzed by cytochrome P450 2C8 and 3A4 in human liver microsomes

NO-1886 ([4-(4-bromo-2-cyano-phenylcarbamoyl) benzyl]-phosphonic acid diethyl ester) increases lipoprotein lipase activity, resulting in a reduction in plasma triglycerides and an increase in high-density lipoprotein cholesterol. The metabolism of NO-1886 in human liver was investigated in the present study. Ester cleavage of NO-1886 from diethyl phosphonate to monoethyl phosphonate was the major metabolic pathway catalyzed by cytochrome P450. In addition, the minor metabolic pathway in human liver was the hydrolysis of the amide bond of NO-1886 by a specific cytosolic esterase. Eadie-Hofstee plots of phosphonate O-deethylation of NO-1886 in human liver microsomes showed a biphasic curve, indicating low- and high-K(m) components. Inhibition experiments with chemical inhibitors and antibodies against various cytochrome P450 isoforms suggested the involvement of CYP2C8 and CYP3A in the phosphonate O-deethylation. Recombinant CYP3A4 and CYP2C8 expressed in baculovirus-infected insect cells and human lymphoblastoid cells exhibited a high activity for phosphonate O-deethylation of NO-1886. The recombinant cytochrome P450 enzymes indicated that CYP2C8 and CYP3A4 were responsible for the low- and high-K(m) components in human liver microsomes, respectively. The selectivity of CYP2C8 in catalyzing phosphonate O-deethylation indicates that coadministration of drugs that are metabolized by the same enzyme requires careful consideration.

Altered liver gene expression in CCl4-cirrhotic rats is partially normalized by insulin-like growth factor-I

We have previously shown that the administration of low doses of insulin-like growth factor-I (IGF-I) to CCl4-cirrhotic rats improves liver function and reduces fibrosis. To better understand the mechanisms behind the hepatoprotective effects of IGF-I, and to identify those genes whose expression is affected in cirrhosis and after IGF-1 treatment, we have performed differential display of mRNA analysis by means of polymerase chain reaction (PCR) in livers from control and CCl4-cirrhotic rats treated or not with IGF-I. We have identified 16 genes that were up- or down-regulated in the cirrhotic liver. IGF-I treatment partially normalized the expression of eight of these genes, including serine proteinase inhibitors such as serpin-2 and alpha-1-antichymotripsin, alpha-1-acid glycoprotein, and alpha-2u-globulin. Additionally, we show that IGF-I enhanced the regenerative activity in the cirrhotic liver, as determined by the increased expression of the proliferating cell nuclear antigen (PCNA). Finally, IGF-I treatment partially restored the expression of growth hormone receptor (GHR) and the levels of global genomic DNA methylation, which are reduced in human and experimental cirrhosis. Taken together, our observations confirm the hepatoprotective effects of IGF-I, and suggest that this action can be exerted in part through the normalization of liver gene expression, growth hormone (GH) responsiveness and global genomic DNA methylation.

Synthesis, hydrolytic activation and cytotoxicity of etoposide prodrugs

Two 4'-propylcarbonoxy derivatives (2,3) of etoposide (1), a topoisomerase II inhibitor, were synthesized and evaluated as potential prodrugs for anticancer therapy. Their activation via hydrolysis mechanisms was determined as a function of pH in buffer solutions, in human serum and in the presence of carboxyl ester hydrolase. Cytotoxicity was determined on various tumor cell lines and compared to the parent compound. On cell lines exhibiting resistance to etoposide we observed an enhanced cytotoxicity of the prodrugs of up to three orders of magnitude.

Appraisal of risks from nonoccupational exposure to chlorpyrifos

The toxicological database for chlorpyrifos indicates that humans are not more sensitive than laboratory animals to the toxic effects. Although an oral dose of 1 mg/kg-day resulted in measurable levels of chlorpyrifos in the blood, daily dosing at this level from 9 days to 2 years did not affect brain acetylcholinesterase activity (AChE) in laboratory animals. Developmental toxicity did not occur at doses below maternal toxicity. Most nonoccupational illnesses resulting from entry into areas treated with chlorpyrifos likely stem from odor, rather than the ability of the organophosphate to inhibit AChE. Based on biological monitoring studies, chronic aggregate nonoccupational exposures to chlorpyrifos ranged from 0.0002 mg/kg-day (adults) to 0.0005 mg/kg-day (infants and small children)-1 order of magnitude less than exposures estimated by standard procedures. Other biological monitoring data indicated that cumulative exposure to all organophosphate pesticides ranged from 0.0003 mg/kg-day (adults) to 0.003 mg/kg-day (children). Considering all these factors, the risks of aggregate, nonoccupational exposure to chlorpyrifos have been overstated by more than a 1000-fold.

Heterologous expression, purification, and characterization of human triacylglycerol hydrolase

Triacylglycerol hydrolase mobilizes stored triacylglycerol some of which is used for very-low-density lipoprotein assembly in the liver. A full-length cDNA coding for a human triacylglycerol hydrolase (hTGH) was isolated from a human liver cDNA library. The cDNA has an open reading frame of 576 amino acids with a cleavable 18-amino-acid signal sequence. The deduced amino acid sequence shows that the protein belongs to the carboxylesterase family. The hTGH was highly expressed in Escherichia coli as a 6xHis-tagged fusion protein, with the tag at the N-terminus in place of the signal peptide. However, the expressed protein was insoluble and inactive. Expression was confirmed by immunoblotting and N-terminal amino acid sequencing of the purified protein. Expression of hTGH with its native signal sequence and a C-terminal 6xHis-tag in Sf9 cells using the baculovirus expression system yielded active enzyme. N-terminal amino acid sequencing of the purified expressed protein showed correct processing of the signal peptide. The enzyme also undergoes glycosylation within the endoplasmic reticulum lumen. The results suggest that hTGH expressed in insect cells is properly folded. Therefore, baculovirus expression of hTGH and facile purification of the His-tagged enzyme will allow detailed characterization of the structure/activity relationship.

Synthesis of new carboxylesterase inhibitors and evaluation of potency and water solubility

Carboxylesterases are essential enzymes in the hydrolysis and detoxification of numerous pharmaceuticals and pesticides. They are vital in mediating organophosphate toxicity and in activating many prodrugs such as the chemotherapeutic agent CPT-11. It is therefore important to study the catalytic mechanism responsible for carboxylesterase-induced hydrolysis, which can be accomplished through the use of potent and selective inhibitors. Trifluoromethyl ketone (TFK)-containing compounds are the most potent esterase inhibitors described to date. The inclusion of a thioether moiety beta to the carbonyl further increased TFK inhibitor potency. In this study, we have synthesized the sulfone analogues of a series of aliphatic and aromatic substituted thioether TFKs to evaluate their potency and solubility properties. This structural change shifted the keto/hydrate equilibrium from <9% hydrate to >95% hydrate, forming almost exclusively the gem-diol. These new compounds were evaluated for their inhibition of carboxylesterase activity in three different systems, rat liver microsomes, commercial porcine esterase, and juvenile hormone esterase in cabbage looper (Trichoplusia ni) hemolymph. The most potent inhibitor of rat liver carboxylesterase activity was 1,1,1-trifluoro-3-(decane-1-sulfonyl)-propan-2,2-diol, which inhibited 50% of the enzyme activity (IC(50)) at 6.3 +/- 1.3 nM and was 18-fold more potent than its thioether analogue. However, the sulfone derivatives were consistently poorer inhibitors of porcine carboxylesterase activity and juvenile hormone esterase activity, with IC(50) values ranging from low micromolar to millimolar. The compound 1,1,1-trifluoro-3-(octane-1-sulfonyl)-propan-2,2-diol was shown to have a 10-fold greater water solubility than its thioether analogue, 1,1,1-trifluoro-3-octylsulfanyl-propan-2-one (OTFP). These novel compounds provide further evidence of the differences between esterase orthologs, suggesting that additional development of esterase inhibitors may ultimately provide a battery of ortholog and/or isoform selective inhibitors analogous to those available for other complex enzyme families with overlapping substrate specificity.

Regulation of rat carboxylesterase expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): a dose-dependent decrease in mRNA levels but a biphasic change in protein levels and activity

Carboxylesterases play an important role in the metabolism of endogenous lipids and foreign compounds; therefore, xenobiotic regulation of carboxylesterase gene expression has both physiological and pharmacological significance. We previously reported that beta-naphthoflavone and 3-methylcholanthrene, two potent inducers for cytochrome P4501A enzymes, had opposing effects on the expression of hydrolase S, a secretory carboxylesterase. Beta-naphthoflavone caused suppression, whereas 3-methylcholanthrene caused induction of the expression of this enzyme. The aim of the present study was to determine the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), another prototypical cytochrome P4501A inducer, on the expression of this and several other rat carboxylesterases (hydrolases A, B, and C) in liver and extrahepatic tissues. Rats received TCDD treatment at nonlethal (<or=10 microg/kg), sublethal (30 microg/kg), or lethal doses (90 microg/kg). The overall hydrolytic activity in the liver microsomes toward both 1-naphthyl- and para-nitrophenylacetate was markedly increased in the nonlethal dosage groups, but markedly decreased in the lethal dosage group. Consistent with such a biphasic dose-response relationship, the levels of enzyme proteins exhibited an initial increase, followed by a decrease in response to nonlethal and lethal doses, respectively. In contrast, treatment with TCDD caused a dose-dependent decrease on the levels of mRNA encoding these enzymes. All liver carboxylesterases showed a similar pattern of change on activity, protein, and mRNA levels, suggesting that TCDD coregulates the expression of these genes. In the extrahepatic tissues, a similar biphasic change was observed in activity and in protein and mRNA levels. In both liver and kidney, the expression of cytochrome P4501A1 (CYP1A1) was significantly induced in a dose-dependent manner. TCDD is known to upregulate the expression of CYP1A1 gene through the aryl hydrocarbon receptor (AhR). The differential effects on the expression of liver carboxylesterases and CYP1A1 suggest that TCDD regulates the expression of hydrolytic enzymes via a mechanism(s) other than the AhR-mediated transcription activation, as observed in the CYP1A1 regulation. The different patterns of change on protein and mRNA levels in the nonlethal dosage groups suggest that TCDD regulates the expression of hepatic carboxylesterases by acting on both transcription and translation.

Purification and characterization of rat hepatic microsomal low molecular weight fatty acid ethyl ester synthase and its relationship to carboxylesterases

We reported purification of a high molecular weight (HMW) (ca. 180 kD) and a low molecular weight (LMW) (ca. 60 kD) protein fractions from digitonized rat liver microsomes using ammonium sulfate precipitation followed by ion exchange and gel filtration column chromatography. Both fractions expressed fatty acid ethyl ester (FAEE) synthase as well as p-nitrophenyl acetate (PNPA)-hydrolyzing (esterase) activities. The HMW fraction was found to be a trimer with subunit molecular weight ca. 60 kD and structurally and functionally similar to rat hepatic microsomal carboxylesterase (CE, pI 6.1) and adipose tissue FAEE synthase. In this article, we report further purification and characterization of the LMW (minor) fraction expressing FAEE synthase activity and its structural and functional relationship to hepatic microsomal CEs. Using isoelectric focusing (IEF) followed by gel filtration-high-performance liquid chromatography (GF-HPLC), five proteins were purified, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. The isoelectric point values of 6.5, 5.8, 5.6, 5.3, and 5.0 were found for the purified LMW proteins by IEF and each showed a peak corresponding to ca. 60 kD molecular weight by GF-HPLC, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. Sodium dodecyl sulfate-polyacrylamide gel elecrophoresis (SDS-PAGE) analysis of the GF-HPLC purified LMW proteins revealed that these proteins are monomers (ca. 60 kD). All the purified LMW proteins cross-reacted with antibodies to rat adipose tissue FAEE synthase. Coelution of PNPA-hydrolyzing and FAEE synthase activity at each step of purification and cross-reactivity with rat adipose tissue FAEE synthase antibodies suggest that the purified proteins are related to various hepatic microsomal CEs. This conclusion is further supported by the homology of N-terminal amino acid sequence of the purified LMW proteins to various hepatic microsomal CEs and protease precursors. Therefore, LMW FAEE synthase activity most probably is expressed by various isozymes of hepatic microsomal CEs, which are also involved in the biotransformation of xenobiotic alcohols and amines.

Biotransformation of organophosphorus compounds

This study reviews current understanding of mechanisms of biotransformation of organophosphorus compounds (OPC). The first part of this article covers chemical aspects of biotransformation describing reactions that lead to activation or detoxication of OPC. The second part explains biochemical mechanisms of biotransformation describing the role of enzymes that are involved in this process. Among them are the enzymes that take part in metabolic activation of OPC such as cytochrome P450 system, NADPH-cytochrome P450 reductase and flavin-containing monooxygenases. Among enzymes participating in detoxication of OPC, the role of phosphoric triester hydrolases, carboxylesterases and glutathione redox system is explained. This article also deals with other aspects of detoxication of OPC such as protein binding and the role of tissue depots for these compounds.

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.

Structural constraints affect the metabolism of 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11) by carboxylesterases

7-Ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin [CPT-11 (irinotecan)] is a water-soluble camptothecin-derived prodrug that is activated by esterases to yield the potent topoisomerase I poison SN-38. We identified a rabbit liver carboxylesterase (CE) that was very efficient at CPT-11 metabolism; however, a human homolog that was more than 81% identical to this protein activated the drug poorly. Recently, two other human CEs have been isolated that are efficient in the conversion of CPT-11 to SN-38, yet both demonstrate little homology to the rabbit protein. To understand this phenomenon, we have characterized a series of esterases from human and rabbit, including several chimeric proteins, for their ability to metabolize CPT-11. Computer predictive modeling indicated that the ability of each enzyme to activate CPT-11 was dependent on the size of the entrance to the active site. Kinetic studies with a series of nitrophenyl and naphthyl esters confirmed these predictions, indicating that activation of CPT-11 by a CE is constrained by size-limited access of the drug to the active site catalytic amino acid residues.

Structure-activity relationships in the deacetylation of O-glucosides of N-hydroxy-N-arylacylamides by mammalian liver microsomes

Structure-activity relationships in the deacylation of O-glucosides of N-hydroxy-N-aryl-acylamides were investigated to provide insights into the metabolic activation of carcinogenic/mutagenic O-glycosides of N-hydroxy-N-arylacylamides. In the subcellular fractions obtained from porcine liver, the deacetylation activity toward O-glucoside of N-hydroxyacetanilide (GAc) was mainly localized in the microsomes. Both the 2-chloro (2ClGAc) and 2-methyl (2MeGAc) derivatives of GAc were not deacetylated by the microsomes. Other compounds having either 3- or 4-substituent (chloro or methyl), however, were deacetylated and showed higher V(max)/K(m) values than that of GAc. 4-Methyl derivative (4MeGAc) was shown to competitively inhibit the deacetylation activity toward GAc, and the K(i) value of 4MeGAc was comparable with its K(m) value obtained in the microsome-catalyzed deacetylation. These apparent K(m) values were shown to correspond to their lipophilicities estimated from retention times on high-performance liquid chromatography (HPLC). As for the effect of acyl groups, the order of V(max)/K(m) values was N-propionyl derivative (GPr)>GAc>N-butyryl derivative (GBu). From the optimized structures and energy levels of the frontier orbitals of these compounds, calculated by the semi-empirical AM1 method, the effects of 2-substituents and acyl groups on the deacetylation activity is thought to be due to a steric factor. From the energy levels of the frontier orbitals of GAc and its 3- or 4-substituted derivatives, the compound having a lower level of LUMO was shown to be deacetylated effectively. There were marked species differences in the microsomal deacetylation activity toward GAc, and the highest activity was found in the rabbit, followed by the porcine, hamster, rat and then bovine liver microsomes.

Design, synthesis, computational prediction, and biological evaluation of ester soft drugs as inhibitors of dihydrofolate reductase from Pneumocystis carinii

A series of lipophilic soft drugs structurally related to the nonclassical dihydrofolate reductase (DHFR) inhibitors trimetrexate and piritrexim have been designed, synthesized, and evaluated in DHFR assays, with special emphasis on the inhibition of P. carinii DHFR. The best inhibitors, encompassing an ester bond in the bridge connecting the two aromatic systems, were approximately 10 times less potent than trimetrexate and piritrexim. The metabolites were designed to be poor inhibitors. Furthermore, molecular dynamics simulations of three ligands in complex with DHFR from Pneumocystis carinii and from the human enzyme were conducted in order to better understand the factors determining the selectivity. A correct ranking of the relative inhibition of DHFR was achieved utilizing the linear interaction energy method. The soft drugs are intended for local administration. One representative ester was selected for a pharmacokinetic study in rats where it was found to undergo fast metabolic degradation to the predicted inactive metabolites.

A role for Sp1 in the transcriptional regulation of hepatic triacylglycerol hydrolase in the mouse

Microsomal triacylglycerol hydrolase (TGH) hydrolyzes stored triacylglycerol in cultured hepatoma cells (Lehner, R., and Vance, D. E. (1999) Biochem. J. 343, 1-10). We studied expression of TGH in murine liver and found both protein and mRNA increased dramatically at 27 days after birth. Nuclear run-on assays demonstrated that this was due to increased transcription. We cloned 542 base pairs upstream of the transcriptional start site of the murine TGH gene. Electrophoretic mobility shift assays demonstrated enhanced binding of hepatic nuclear proteins from 27-day-old mice to the murine TGH promoter, yielding three differentially migrating complexes. DNase I footprint analysis localized these complexes to two distinct regions: site A contains a putative Sp binding site, and site B contains a degenerate E box. We transfected primary murine hepatocytes with a series of 5'-deletion constructs upstream of the reporter luciferase cDNA. Positive control elements were identified in a segment containing site A. Competitive electrophoretic mobility shift assays and supershift assays demonstrated that site A binds Sp1 and Sp3. Transcriptional activation assays in Schneider SL-2 insect cells demonstrated that Sp1 is a potent activator of the TGH promoter. These experiments directly link increased TGH expression at the time of weaning to transcriptional regulation by Sp1.

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.

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

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

Kinetics study of the biotransformation of an oligonucleotide prodrug in cells extract by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry

The fate of a dodecathymidine prodrug in cell extract was monitored by MALDI-TOF MS. This technique allows a facile identification and a relative quantification of metabolites produced. We showed that the relative peak intensities were similar to the relative metabolite proportions that permitted the determination of their half-lives. We found a good fit between the calculated kinetics curves and the experimental points. The oligonucleotide prodrug was fully metabolized to yield the dodecathymidine phosphorothioate likely through a carboxyesterase mediated mechanism.

Biology of monocyte-specific esterase

The monocyte-specific (carboxyl)esterase (MSE) is a marker enzyme which is well-known to hematologists as its detection is part of the traditional cytochemical stainings of leukemia cells. There are a variety of synonyms for MSE among hematologists and biochemists. Biochemically, MSE is well-characterized, but should be discerned from other esterases with similar or identical substrate specificities and other features. Intensive analysis of normal and malignant hematopoietic cells and leukemia-lymphoma cell lines using isoelectric focusing established the specificity of this enzyme for monocytes and related cells, hence its designation as monocyte-specific esterase. Cloning of the gene led to its molecular characterization and provided new opportunities to examine MSE expression also at the RNA level which confirmed the monocyte/macrophage specificity. The availability of the gene sequences of various serine esterases and lipases which also hydrolyze ester bonds allowed for the identification of identical isolates from different tissues and the construction of an unrooted dendrogram based on sequence homologies of 22 enzymes. The detailed regulation of the gene and the functional role of MSE have remained largely unknown as of yet. However, DNA binding sites for various transcription factors have already been detected. Some evidence suggests involvement in physiological detoxification processes and in the immune defense against tumor cells. A more thorough understanding of the in vivo function of this truly unique enzyme should be helped by characterizing the signals and signal transduction mechanisms which lead to MSE expression.

Association of carboxyl esterase with facilitative glucose transporter isoform 4 (GLUT4) intracellular compartments in rat adipocytes and its possible role in insulin-induced GLUT4 recruitment

Facilitative glucose transporter isoform 4 (GLUT4) in rat adipocytes is largely sequestered in intracellular sites, and insulin recruits GLUT4 from these sites to the cell surface. The process is known to involve multiple intracellular compartments and associated proteins, many of which are yet to be identified. Recently, we purified three distinct insulin-sensitive intracellular GLUT4 compartments (G4T(L), G4H, and G4L) in rat adipocytes and unraveled several new resident proteins in these compartments. Here, we describe one of them, a 62-kDa protein, purified and identified as rat adipose tissue carboxyl esterase (p62/CE) by matrix-assisted laser desorption/ionization time of flight mass spectroscopy, reverse transcription-polymerase chain reaction, gene cloning, and immunological and enzymatic activity measurements. p62/CE in rat adipocytes was 80% cytosolic and 20% microsome-associated. It was found in all of the three insulin-sensitive intracellular GLUT4 compartments, and particularly enriched in G4T(L,) a compartment thought to represent GLUT4 endocytic vesicles. Significantly, an antibody against p62/CE introduced into rat adipocytes completely abolished the insulin-induced GLUT4 recruitment to the plasma membrane in host cells without affecting the basal GLUT4 distribution. Together, these findings suggest that p62/CE plays a key role in insulin-induced GLUT4 recruitment in rat adipocytes, probably by hydrolyzing acylglycerols or acyl-CoA esters to the respective free acids that are required for GLUT4 transport vesicle budding and/or fusion.

Cholesteryl ester hydrolase in human monocyte/macrophage: cloning, sequencing, and expression of full-length cDNA

The sensitive technique of RT-PCR was used to identify cholesteryl ester hydrolase (CEH) expressed in human macrophages. This enzyme is thought to regulate the availability of intracellular free cholesterol for efflux. The expected 667-bp product was obtained starting with RNA from human peripheral blood and THP-1 monocytes and macrophages. The cDNA for human macrophage CEH was then cloned by PCR-based screening of a lambda-gt11 cDNA library. The full-length cDNA was sequenced and found to exhibit 76% homology (at the nucleotide and conceptually translated protein level) to hepatic CEH, an enzyme shown to be involved in hepatic cholesterol homeostasis and regulated by cholesterol at the transcription level via sterol response elements in the proximal promoter. Identification of the conserved catalytic triad (Ser(221</SUP >, His(468), and Glu354)) and the SEDCLY motif places human macrophage CEH in the family of carboxylesterases. A greater than 20-fold increase in CEH activity was observed when COS-1 and COS-7 cells were transiently transfected with an eukaryotic expression vector, pcDNA3.1/V5/His-TOPO, containing the cDNA for human macrophage CEH. Using this full-length cDNA as a probe, a 2.2-kb transcript was identified by Northern blot analysis of total RNA from human peripheral blood and THP -1 macrophages. Overexpression of human macrophage CEH resulted in an impairment of upregulation of low-density lipoprotein (LDL) receptor mRNA in Chinese hamster ovary (CHO-K1) cells grown in cholesterol-deficient environment. These data identify the human macrophage CEH, demonstrate its expression in human peripheral blood macrophage and human macrophage cell line, THP-1, and suggest its role in the intracellular cholesteryl ester metabolism.

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.

Soft drug design: general principles and recent applications

Soft drug design represents a new approach aimed to design safer drugs with an increased therapeutic index by integrating metabolism considerations into the drug design process. Soft drugs are new therapeutic agents that undergo predictable metabolism to inactive metabolites after exerting their therapeutic effect. Hence, they are obtained by building into the molecule, in addition to the activity, the most desired way in which the molecule is to be deactivated and detoxified. In an attempt to systematize and summarize the related work done in a number of laboratories, including ours, the present review presents an overview of the general soft drug design principles and provides a variety of specific examples to illustrate the concepts. A number of already marketed drugs, such as esmolol, remifentanil, or loteprednol etabonate, resulted from the successful application of such design principles. Many other promising drug candidates are currently under investigation in a variety of fields including possible soft antimicrobials, anticholinergics, corticosteroids, beta-blockers, analgetics, ACE inhibitors, antiarrhythmics, and others. Whenever possible, pharmacokinetic and pharmacodynamic properties are briefly summarized and compared to those of other compounds used in the same field.

Quantitative structure-metabolism relationships: steric and nonsteric effects in the enzymatic hydrolysis of noncongener carboxylic esters

An attempt to quantitatively describe human blood in vitro hydrolysis data for more than 80 compounds belonging to seven different noncongener series of ester-containing drugs is presented. A parameter not yet explored in pharmaceutical studies, the inaccessible solid angle Omega(h), calculated around different atoms was used as a measure of steric hindrance, and the steric hindrance around the carbonyl sp(2) oxygen (Omega(h)(O=)) proved the most relevant parameter. The obtained final equation, log t(1/2) = -3.805 + 0.172Omega(h)(O=) - 10.146q(C=) + 0.112QLogP, also includes the AM1-calculated charge on the carbonyl carbon (q(C=)) and a calculated log octanol-water partition coefficient (QLogP) as parameters and accounts for 80% of the variability in the log half-lives of 67 compounds. A number of structures are still mispredicted, but the equation agrees very well with a recently proposed mechanism for hydrolysis by carboxylesterases. The model, with a predictive power tested here on three unrelated structures, should be useful in estimating approximate rates of hydrolysis for prodrug or soft drug candidates ahead of their synthesis.

Carboxyl/cholinesterases: a case study of the evolution of a successful multigene family

The evolution of organismal diversity among the Metazoa is dependent on the proliferation of genes and diversification of functions in multigene families. Here we analyse these processes for one highly successful family, the carboxyl/cholinesterases. One key to the expansion of the functional niche of this group of enzymes is associated with versatile substrate binding and catalytic machinery. Qualitatively new functions can be obtained by substitution of one or a very few amino acids. This crudely adapted new functionality is then refined rapidly by a pulse of change elsewhere in the molecule; in one case about 13% amino acid divergence occurred in 5-10 million years. Furthermore, we postulate that the versatility of the substrate binding motifs underpins the recruitment of several family members to additional noncatalytic signal transduction functions.

cDNA cloning, characterization and stable expression of novel human brain carboxylesterase

The DNA sequence encoding a novel human brain carboxylesterase (CES) has been determined. The protein is predicted to have 567 amino acids, including conserved motifs, such as GESAGG, GXXXXEFG, and GDHGD which comprise a catalytic triad, and the endoplasmic reticulum retention motif (HXEL-COOH) observed in CES families. Their gene products exhibited hydrolase activity towards temocapril, p-nitrophenyl-acetate and long-chain acyl-CoA. Since the molecular masses of these gene products are similar to those that exist in capillary endothelial cells of the human brain [Yamamda et al. (1994) Brain Res. 658, 163-167], these CES isozymes may function as a blood-brain barrier to protect the central nervous system from ester or amide compounds.

Human carboxylesterases in term placentae: enzymatic characterization, molecular cloning and evidence for the existence of multiple forms

The placenta is a temporary organ that is known to metabolize numerous endogenous and xenobiotic compounds. Carboxylesterases represent a family of enzymes which hydrolyse a variety of esters, amides and thioesters. Many studies have demonstrated that carboxylesterases are widely distributed among mammalian tissues, but little is known about these enzymes in the placenta. The present study was conducted to establish the kinetic parameters of placental carboxylesterases toward several p -nitrophenol and 1-naphthol esters, and to establish the molecular basis for these enzymes in the placenta. The enzymatic rate of the hydrolysis of 1-naphthylacetate and carboxylic esters of p -nitrophenol as a function of substrate concentration (0.01-1.00 m m) was examined with human placental microsomes pooled from six placentae. Data from these studies yielded a linear Lineweaver-Burk plot with each substrate examined. K(m)values for these substrates ranged from 92 to 370 microm, and V(max)values ranged from 85 to 170 nmol/mg/min. These results suggest that each substrate is hydrolysed by a single enzyme, or enzymes that are kinetically indistinguishable, or that one of them is dominant. Microsomes from all individual placentae contained esterase activity toward all four substrates, and exhibited a one- to three-fold variation. The activity toward p -nitrophenylacetate correlated well with the activity toward 1-naphthylacetate (r(2)=0.957). In contrast, the activity toward p -nitrophenylbutyrate correlated poorly with the activity toward 1-naphthylacetate (r(2)=0.121). These results suggest that placental microsomes have more than one carboxylesterase activity. Screening of a placental cDNA library with gene-trapping hybridization resulted in the isolation of three distinct cDNAs, designated PCE-1, PCE-2 and PCE-3. PCE-1 and PCE-2 have a significant sequence identity (approx 99 per cent) with liver carboxylesterases hCE and hCE-2, respectively. PCE-3 has a 96 per cent sequence identity with hCE but only at the first 874 nucleotide of the 5' end. The rest of the 1396 nucleotides of the 3' end exhibit no significant sequence identity with any known mammalian carboxylesterases. A probe derived from the 3' end of PCE-3 detected an approx 2.2 kb messenger transcript, the size of a regular carboxylesterase. However, the entire PCE-3 cDNA has multiple internal stop codons and encodes only 269 amino acids; half the size of a regular carboxylesterase. Northern blotting experiments detected the transcripts coding for PCE-1, PCE-2 or PCE-3 in all placentae, and the levels of these messengers showed an approx six-fold individual variation. Placenta 6 had the highest activity toward all four substrates, and highest levels of the messengers for PCE-1, PCE-2 and PCE-3. In contrast, placenta 1 had relatively high levels of messengers for PCE-1 and PCE-2, but the activity toward these four substrates was only moderate. These results suggest that a discrepancy between the messenger level and the enzyme protein exists or that there are other as yet unidentified carboxylesterase(s) in the placenta which contribute to the hydrolytic activity. Carboxylesterases are known to involve the detoxication and metabolic activation of various drugs, environmental toxicants and carcinogens. Therefore, placental carboxylesterases have both pharmacological and toxicological significance in the development of the fetus.

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.