Regulation of two rat liver microsomal carboxylesterase isozymes: species differences, tissue distribution, and the effects of age, sex, and xenobiotic treatment of rats

Hydrolytic biotransformation of the bumetanide ester prodrug DIMAEB to bumetanide by esterases in neonatal human and rat serum and neonatal rat brain-A new treatment strategy for neonatal seizures?

OBJECTIVES

The loop diuretic bumetanide has been proposed previously as an adjunct treatment for neonatal seizures because bumetanide is thought to potentiate the action of γ-aminobutyric acid (GABA)ergic drugs such as phenobarbital by preventing abnormal intracellular accumulation of chloride and the subsequent "GABA shift." However, a clinical trial in neonates failed to demonstrate such a synergistic effect of bumetanide, most likely because this drug only poorly penetrates into the brain. This prompted us to develop lipophilic prodrugs of bumetanide, such as the N,N-dimethylaminoethyl ester of bumetanide (DIMAEB), which rapidly enter the brain where they are hydrolyzed by esterases to the parent compound, as demonstrated previously by us in adult rodents. However, it is not known whether esterase activity in neonates is sufficient to hydrolyze ester prodrugs such as DIMAEB.

METHODS

In the present study, we examined whether esterases in neonatal serum of healthy term infants are capable of hydrolyzing DIMAEB to bumetanide and whether this activity is different from the serum of adults. Furthermore, to extrapolate the findings to brain tissue, we performed experiments with brain tissue and serum of neonatal and adult rats.

RESULTS

Serum from 1- to 2-day-old infants was capable of hydrolyzing DIMAEB to bumetanide at a rate similar to that of serum from adult individuals. Similarly, serum and brain tissue of neonatal rats rapidly hydrolyzed DIMAEB to bumetanide.

SIGNIFICANCE

These data provide a prerequisite for further evaluating the potential of bumetanide prodrugs as add-on therapy to phenobarbital and other antiseizure drugs as a new strategy for improving pharmacotherapy of neonatal seizures.

Toxicokinetics of cis- and trans-Permethrin: Influence of Isomer, Maturation, and Sex

Permethrin exposure of children and adults is widespread in many populations, but knowledge of its relative toxicokinetics (TK) and health risks in immature age groups is lacking. Studies were conducted in rats to determine the influence of immaturity and sex (on plasma and target organ dosimetry of each of the insecticide’s 2 isomers, cis- and trans-permethrin [CIS and TRANS]). Postnatal day 15, 21, and 90 (adult), Sprague Dawley rats were orally administered a graduated series of doses of CIS and TRANS in corn oil. Serial sacrifices were conducted over 24 h to obtain plasma, brain, liver, skeletal muscle, and fat profiles of CIS and TRANS. Levels of TRANS decreased relatively rapidly, despite administration of relatively high doses. Concentrations of each isomer in plasma, brain, and other tissues monitored were inversely proportional to the animals’ age. The youngest pups exhibited 4-fold higher plasma and brain area under the curves than did adults. Little difference was observed in the TK of CIS or TRANS between adult male and female rats, other than higher initial plasma and liver CIS levels in females. Elevated exposure of the immature brain appears to be instrumental in increased susceptibility to the acute neurotoxicity of high-dose permethrin (Cantalamessa [1993]), but it remains to be established whether age-dependent TK is relevant to long-term, low-level risks.

Role of geraniol against lead acetate-mediated hepatic damage and their interaction with liver carboxylesterase activity in rats

In this study, the effect of geraniol (50 mg/kg for 30 d), a natural antioxidant and repellent/antifeedant monoterpene, in a rat model of lead acetate-induced (500 ppm for 30 d) liver damage was evaluated. Hepatic malondialdehyde increased in the lead acetate group. Reduced glutathione unchanged, but glutathione S-transferase, glutathione reductase, as well as carboxylesterase activities decreased in geraniol, lead acetate and geraniol + lead acetate groups. 8-OhDG immunoreactivity, mononuclear cell infiltrations and hepatic lead concentration were lower in the geraniol + lead acetate group than the lead acetate group. Serum aspartate aminotransferase and alanine aminotransferase activities increased in the Pb acetate group. In conclusion, lead acetate causes oxidative and toxic damage in the liver and this effect can reduce with geraniol treatment. However, we first observed that lead acetate, as well as geraniol, can affect liver carboxylesterase activity.

Carboxylesterases in lipid metabolism: from mouse to human

Mammalian carboxylesterases hydrolyze a wide range of xenobiotic and endogenous compounds, including lipid esters. Physiological functions of carboxylesterases in lipid metabolism and energy homeostasis in vivo have been demonstrated by genetic manipulations and chemical inhibition in mice, and in vitro through (over)expression, knockdown of expression, and chemical inhibition in a variety of cells. Recent research advances have revealed the relevance of carboxylesterases to metabolic diseases such as obesity and fatty liver disease, suggesting these enzymes might be potential targets for treatment of metabolic disorders. In order to translate pre-clinical studies in cellular and mouse models to humans, differences and similarities of carboxylesterases between mice and human need to be elucidated. This review presents and discusses the research progress in structure and function of mouse and human carboxylesterases, and the role of these enzymes in lipid metabolism and metabolic disorders.

Isoform-Specific Regulation of Mouse Carboxylesterase Expression and Activity by Prototypical Transcriptional Activators

Nuclear receptors and transcription factors regulate the mRNA expression of many drug metabolizing enzymes, including the carboxylesterases (Ces). However, there are few data regarding whether these changes in mRNA expression result in alteration of protein levels or activity. In the present study, we sought to determine the isoform-specific regulation of hepatic Ces mRNA expression and activity following the administration of pharmacological activators of the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and nuclear factor E2-related protein (Nrf2) to mice. The CAR activator 1,4-bis-[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) and PXR ligand pregnenolone-16a-carbonitrile (PCN) increased Ces mRNA expression of various Ces2 isoforms, whereas the Nrf2 activator butylated hydroxyanisole primarily reduced Ces3a mRNA expression and induced Ces1g mRNA. TCPOBOP and PCN increased Ces2 hydrolytic activity in an isoform-specific manner. Taken together, these data demonstrate that activation of CAR, PXR, and Nrf2 regulates not only Ces mRNA expression, but also isoform-specific activity.

Ontogenic expression of human carboxylesterase-2 and cytochrome P450 3A4 in liver and duodenum: postnatal surge and organ-dependent regulation

Human carboxylesterase-2 (CES2) and cytochrome P450 3A4 (CYP3A4) are two major drug metabolizing enzymes that play critical roles in hydrolytic and oxidative biotransformation, respectively. They share substrates but may have opposite effect on therapeutic potential such as the metabolism of the anticancer prodrug irinotecan. Both CES2 and CYP3A4 are expressed in the liver and the gastrointestinal tract. This study was conducted to determine whether CES2 and CYP3A4 are expressed under developmental regulation and whether the regulation occurs differentially between the liver and duodenum. A large number of tissues (112) were collected with majority of them from donors at 1-198 days of age. In addition, multi-sampling (liver, duodenum and jejunum) was performed in some donors. The expression was determined at mRNA and protein levels. In the liver, CES2 and CYP3A4 mRNA exhibited a postnatal surge (1 versus 2 months of age) by 2.7 and 29 fold, respectively. CYP3A4 but not CES2 mRNA in certain pediatric groups reached or even exceeded the adult level. The duodenal samples, on the other hand, showed a gene-specific expression pattern at mRNA level. CES2 mRNA increased with age but the opposite was true with CYP3A4 mRNA. The levels of CES2 and CYP3A4 protein, on the other hand, increased with age in both liver and duodenum. The multi-sampling study demonstrated significant correlation of CES2 expression between the duodenum and jejunum. However, neither duodenal nor jejunal expression correlated with hepatic expression of CES2. These findings establish that developmental regulation occurs in a gene and organ-dependent manner.

Low level chlorpyrifos exposure increases anandamide accumulation in juvenile rat brain in the absence of brain cholinesterase inhibition

The prevailing dogma is that chlorpyrifos (CPF) mediates its toxicity through inhibition of cholinesterase (ChE). However, in recent years, the toxicological effects of developmental CPF exposure have been attributed to an unknown non-cholinergic mechanism of action. We hypothesize that the endocannabinoid system may be an important target because of its vital role in nervous system development. We have previously reported that repeated exposure to CPF results in greater inhibition of fatty acid amide hydrolase (FAAH), the enzyme that metabolizes the endocannabinoid anandamide (AEA), than inhibition of either forebrain ChE or monoacylglycerol lipase (MAGL), the enzyme that metabolizes the endocannabinoid 2-arachidonylglycerol (2-AG). This exposure resulted in the accumulation of 2-AG and AEA in the forebrain of juvenile rats; however, even at the lowest dosage level used (1.0mg/kg), forebrain ChE inhibition was still present. Thus, it is not clear if FAAH activity would be inhibited at dosage levels that do not inhibit ChE. To determine this, 10 day old rat pups were exposed daily for 7 days to either corn oil or 0.5mg/kg CPF by oral gavage. At 4 and 12h post-exposure on the last day of administration, the activities of serum ChE and carboxylesterase (CES) and forebrain ChE, MAGL, and FAAH were determined as well as the forebrain AEA and 2-AG levels. Significant inhibition of serum ChE and CES was present at both 4 and 12h. There was no significant inhibition of the activities of forebrain ChE or MAGL and no significant change in the amount of 2-AG at either time point. On the other hand, while no statistically significant effects were observed at 4h, FAAH activity was significantly inhibited at 12h resulting in a significant accumulation of AEA. Although it is not clear if this level of accumulation impacts brain maturation, this study demonstrates that developmental CPF exposure at a level that does not inhibit brain ChE can alter components of endocannabinoid signaling.

Dexamethasone regulates differential expression of carboxylesterase 1 and carboxylesterase 2 through activation of nuclear receptors

Carboxylesterases (CESs) play important roles in the metabolism of endogenous and foreign compounds in physiological and pharmacological responses. The aim of this study was to investigate the effect of dexamethasone at different doses on the expression of CES1 and CES2. Imidapril and irinotecan hydrochloride (CPT-11) were used as special substrates for CES1 and CES2, respectively. Rat hepatocytes were cultured and treated with different concentrations of dexamethasone. The hydrolytic activity of CES1 and CES2 was tested by incubation experiment and their expression was quantitated by real-time PCR. A pharmacokinetic study was conducted in SD rats to further evaluate the effect of dexamethasone on CESs activity in vivo. Western blotting was performed to investigate the regulatory mechanism related to pregnane X receptor (PXR) and glucocorticoid receptor (GR). The results showed that exposure of cultured rat hepatocytes to nanomolar dexamethasone inhibited the imidapril hydrolase activity, which was slightly elevated by micromolar dexamethasone. For CES2, CPT-11 hydrolase activity was induced only when dexamethasone reached micromolar levels. The real-time PCR demonstrated that CES1 mRNA was markedly decreased by nanomolar dexamethasone and increased by micromolar dexamethasone, whereas CES2 mRNA was significantly increased by micromolar dexamethasone. The results of a complementary animal study showed that the concurrent administration of dexamethasone significantly increased the plasma concentration of the metabolite of imidapril while the ratio of CPT-11 to its metabolite SN-38 was significantly decreased. PXR protein was gradually increased by serial concentrations of dexamethasone. However, only nanomolar dexamethasone elevated the level of GR protein. The different concentrations of dexamethasone required suggested that suppression of CES1 may be mediated by GR whereas the induction of CES2 may result from the role of PXR. It was concluded that dexamethasone at different concentrations can differentially regulate CES1 and CES2.

Identification of human cytochrome P450 isoforms and esterases involved in the metabolism of mirabegron, a potent and selective β3-adrenoceptor agonist

1. Human cytochrome P450 (CYP) enzymes and esterases involved in the metabolism of mirabegron, a potent and selective human β(3)-adrenoceptor agonist intended for the treatment of overactive bladder, were identified in in vitro studies. 2. Incubations of mirabegron with recombinant human CYP enzymes showed significant metabolism of mirabegron by CYP2D6 and CYP3A4 only. Correlation analyses showed a significant correlation between mirabegron metabolism and testosterone 6β-hydroxylation (CYP3A4/5 marker activity). In inhibition studies using antiserum against CYP3A4, a strong inhibition (at maximum 80% inhibition) of the metabolism of mirabegron was observed, whereas the inhibitory effects of monoclonal antibodies against CYP2D6 were small (at maximum 10% inhibition). These findings suggest that CYP3A4 is the primary CYP enzyme responsible for in vitro oxidative metabolism of mirabegron, with a minor role of CYP2D6. 3. Mirabegron hydrolysis was catalyzed in human blood, plasma and butyrylcholinesterase (BChE) solution, but not in human liver microsomes, intestinal microsomes, liver S9, intestinal S9 and recombinant acetylcholinesterase solution. K(m) values of mirabegron hydrolysis in human blood, plasma and BChE solution were all similar (13.4-15.2 μM). The inhibition profiles in human blood and plasma were also similar to those in BChE solution, suggesting that mirabegron hydrolysis is catalyzed by BChE.

Age-related inducibility of carboxylesterases by the antiepileptic agent phenobarbital and implications in drug metabolism and lipid accumulation

Carboxylesterases (CES) constitute a class of hydrolytic enzymes that play critical roles in drug metabolism and lipid mobilization. Previous studies with a large number of human liver samples have suggested that the inducibility of carboxylesterases is inversely related with age. To directly test this possibility, neonatal (10 days of age) and adult mice were treated with the antiepileptic agent phenobarbital. The expression and hydrolytic activity were determined on six major carboxylesterases including ces1d, the ortholog of human CES1. Without exception, all carboxylesterases tested were induced to a greater extent in neonatal than adult mice. The induction was detected at mRNA, protein and catalytic levels. Ces1d was greatly induced and found to rapidly hydrolyze the antiplatelet agent clopidogrel and support the accumulation of neutral lipids. Phenobarbital represents a large number of therapeutic agents that induce drug metabolizing enzymes and transporters in a species-conserved manner. The higher inducibility of carboxylesterases in the developmental age likely represents a general phenomenon cross species including human. Consequently, individuals in the developmental age may experience greater drug-drug interactions. The greater induction of ces1d also provides a molecular explanation to the clinical observation that children on antiepileptic drugs increase plasma lipids.

Cholinesterase inhibition and toxicokinetics in immature and adult rats after acute or repeated exposures to chlorpyrifos or chlorpyrifos-oxon

The effect of age or dose regimen on cholinesterase inhibition (ChEI) from chlorpyrifos (CPF) or CPF-oxon (CPFO) was studied in Crl:CD(SD) rats. Rats were exposed to CPF by gavage in corn oil, rat milk (pups), or in the diet (adults) or to CPFO by gavage in corn oil. Blood CPF/CPFO levels were measured. With acute exposure, ChEI NOELs were 2 mg/kg CPF for brain and 0.5 mg/kg CPF for red blood cells (RBCs) in both age groups. In pups, ChEI and blood CPF levels were similar using either milk or corn oil vehicles. Compared to gavage, adults given dietary CPF (12 h exposure) had greater RBC ChEI, but lower brain ChEI at corresponding CPF doses, indicating an effect of dose rate. With repeated CPF exposures, ChEI NOELs were the same across ages (0.5 and 0.1 mg/kg/day for brain and RBCs, respectively). With CPFO dosing, the ChEI NOELs were 0.1 mg/kg (acute) and 0.01 mg/kg/day (repeated doses) for RBCs with no ChEI in brain at CPFO doses up to 0.5 (pup) or 10 mg/kg (adult) for acute dosing or 0.5 mg/kg/day for both ages with repeat dosing. Thus, there were no age-dependent differences in CPF ChEI via acute or repeated exposures. Pups had less ChEI than adults at comparable blood CPF levels. Oral CPFO resulted in substantial RBC ChEI, but no brain ChEI, indicating no CPFO systemic bioavailability to peripheral tissues.

Transcription factor-mediated regulation of carboxylesterase enzymes in livers of mice

The induction of drug-metabolizing enzymes by chemicals is one of the major reasons for drug-drug interactions. In the present study, the regulation of mRNA expression of one arylacetamide deacetylase (Aadac) and 11 carboxylesterases (Cess) by 15 microsomal enzyme inducers (MEIs) was examined in livers of male C57BL/6 mice. The data demonstrated that Aadac mRNA expression was suppressed by three aryl hydrocarbon receptor (AhR) ligands, two constitutive androstane receptor (CAR) activators, two pregnane X receptor (PXR) ligands, and one nuclear factor erythroid 2-related factor 2 (Nrf2) activator. Ces1 subfamily mRNA expression was not altered by most of the MEIs, whereas Ces2 subfamily mRNA was readily induced by the activators of CAR, PXR, and Nrf2 but not by peroxisome proliferator-activated receptor α activators. Studies using null mice demonstrated that 1) AhR was required for the 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated suppression of Aadac and Ces3a; 2) CAR was involved in the 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene-mediated induction of Aadac, Ces2c, Ces2a, and Ces3a; 3) PXR was required for the pregnenolone-16α-carbonitrile-mediated induction of Aadac, Ces2c, and Ces2a; 4) Nrf2 was required for the oltipraz-mediated induction of Ces1g and Ces2c; and 5) PXR was not required for the DEX-mediated suppression of Cess in livers of mice. In conclusion, the present study systematically investigated the regulation of Cess by MEIs in livers of mice and demonstrated that MEIs modulated mRNA expression of mouse hepatic Cess through the activation of AhR, CAR, PXR, and/or Nrf2 transcriptional pathways.

Surge in expression of carboxylesterase 1 during the post-neonatal stage enables a rapid gain of the capacity to activate the anti-influenza prodrug oseltamivir

BACKGROUND

Oseltamivir, a widely used anti-influenza drug, is hydrolytically activated by carboxylesterase 1 (CES1). The expression of this carboxylesterase is developmentally regulated. This study was performed to determine when after birth infants acquire competence of activating this prodrug.

METHODS

Liver tissue samples were collected and divided into 5 age groups: group 1 (1-31 d old), group 2 (35-70 d old), group 3 (89-119 d old), group 4 (123-198 d old), and group 5 (>18 years of age). These samples were analyzed for oseltamivir hydrolysis and CES1 expression.

RESULTS

Liver samples in group 1 expressed the lowest level of CES1 with the lowest hydrolytic activity toward oseltamivir. A 4-7-fold increase between groups 1 and 2 (1-31 vs 35-70 d of age) was detected in the hydrolysis and expression analyses, respectively. Liver samples in the other 3 pediatric groups (35-198 d of age) exhibited similar expression and hydrolysis levels. Overall, liver samples in group 1 had CES1 expression and hydrolysis levels that were 10% of those of adults, whereas liver samples in the other 3 pediatric groups had levels that were ∼50% of adult levels.

CONCLUSIONS

The post-neonatal surge in CES1 expression ensures the hydrolytic capacity to be gained rapidly after birth in infants, but the larger variability during this period suggests that caution should be exercised on the extrapolated dosing regimens of ester drugs from other age groups.

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.

Mechanisms of lung tumorigenesis by ethyl carbamate and vinyl carbamate

Vinyl carbamate (VC) and ethyl carbamate (EC) induce the formation of lung tumors. The mechanism involves a two-step oxidation of EC to VC and VC to an epoxide, both of which are mediated mainly by CYP2E1. Interaction of the epoxide with DNA leads to the formation of DNA adducts, including 1,N(6)ethenodeoxyadenosine and 1,N(4)-ethenodeoxycytidine. The production of DNA adducts correlated with capacities for the bioactivation of VC, which are higher in the lungs of A/J than in C57BL/6 mice. Importantly, CYP2E1 is higher in the lungs of A/J than in C57BL/6 mice. Studies using F(1) (Big Blue x A/J) transgenic mice revealed the formation of mutations in the lambda cII gene after treatment with VC. Mutations induced by VC were mainly A:T-->G:C transitions and A:T-->T:A transversions, while mutations induced by EC were mainly G:C-->A:T transitions. An EC dose that was 17-fold higher than that for VC was required to produce a similar level of mutant frequency in the lung. Pretreatment of mice with the CYP2E1 inhibitor, diallyl sulfone, significantly inhibited the mutant frequencies induced by VC. Mutations in the endogeneous Kras2 gene were found in codon 61 of exon 2 and were identified as A:T transversions and A-->G transitions in the second base and A-->T transversions in the third base. These mutations were reduced by treatment of mice with diallyl sulfone before VC and coincided with a reduction in the number of lung tumors with Kras2 mutations. These findings affirmed that the metabolism of EC and VC is a prerequisite for, or at least substantially contributes to, initiation of the cascade of events leading to lung tumor formation.

Human carboxylesterases HCE1 and HCE2: ontogenic expression, inter-individual variability and differential hydrolysis of oseltamivir, aspirin, deltamethrin and permethrin

Carboxylesterases hydrolyze chemicals containing such functional groups as a carboxylic acid ester, amide and thioester. The liver contains the highest carboxylesterase activity and expresses two major carboxylesterases: HCE1 and HCE2. In this study, we analyzed 104 individual liver samples for the expression patterns of both carboxylesterases. These samples were divided into three age groups: adults (>or= 18 years of age), children (0 days-10 years) and fetuses (82-224 gestation days). In general, the adult group expressed significantly higher HCE1 and HCE2 than the child group, which expressed significantly higher than the fetal group. The age-related expression was confirmed by RT-qPCR and Western immunoblotting. To determine whether the expression patterns reflected the hydrolytic activity, liver microsomes were pooled from each group and tested for the hydrolysis of drugs such as oseltamivir and insecticides such as deltamethrin. Consistent with the expression patterns, adult microsomes were approximately 4 times as active as child microsomes and 10 times as active as fetal microsomes in hydrolyzing these chemicals. Within the same age group, particularly in the fetal and child groups, a large inter-individual variability was detected in mRNA (430-fold), protein (100-fold) and hydrolytic activity (127-fold). Carboxylesterases are recognized to play critical roles in drug metabolism and insecticide detoxication. The findings on the large variability among different age groups or even within the same age group have important pharmacological and toxicological implications, particularly in relation to pharmacokinetic alterations of ester drugs in children and vulnerability of fetuses and children to pyrethroid insecticides.

Dexamethasone suppresses the expression of multiple rat carboxylesterases through transcriptional repression: evidence for an involvement of the glucocorticoid receptor

Carboxylesterases play important roles in the metabolism of xenobiotics and detoxication of insecticides. Without exception, all mammalian species studied express multiple forms of carboxylesterases. Several rat carboxylesterases are well-characterized including hydrolase A, B and S, and the expression of these enzymes is significantly suppressed by glucocorticoid dexamethasone. In this study, we used multiple experimental systems and presented a molecular mechanism for the suppression. Rats receiving one or more daily injections of dexamethasone consistently expressed lower HA, HB and HS. The suppression occurred at the levels of mRNA, protein and hydrolytic activity. In hepatoma cell line H4-II-E-C3, nanomolar dexamethasone caused significant decreases in HA, HB and HS mRNA, and the decreases were abolished by antiglucocorticoid RU486. Additionally, dexamethasone at nanomolar concentrations repressed the promoters of carboxylesterases, and the repression was reduced by glucocorticoid receptor-beta, a dominant negative regulator of the glucocorticoid receptor (GR). In contrast, co-transfection of the pregnane X receptor (PXR) increased the reporter activities, but the increase occurred only at micromolar concentrations of dexamethasone. These findings establish that both GR and PXR are involved in the regulated expression of rat carboxylesterases by dexamethasone but their involvement depends on the concentrations.

Species differences in hydrolase activities toward OT-7100 responsible for different bioavailability in rats, dogs, monkeys and humans

OT-7100 (5-n-butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo[1,5-a] pyrimidine) is an amide moiety-bearing pyrazolopyrimidine derivative with a potential analgesic effect. To determine the factors responsible for observed species differences in the bioavailability of this drug, human and experimental animal samples were used to investigate in vitro microsomal and cytosolic hydrolase activities in the liver and small intestine vis-à-vis the pharmacokinetics of OT-7100. The AUC(0-t) values of OT-7100 after oral administration in rats, dogs and monkeys were 0.163, 0.0383 and 0.00147 microg h ml(-1) divided by mg kg(-1), respectively. The bioavailabilities of OT-7100 after oral administration in rats, dogs and monkeys were 36, 17 and 0.3%, respectively. The plasma concentration-time profiles of intravenously administrated OT-7100 in rats, dogs and monkeys were similar. The hydrolase activities toward OT-7100 in liver microsomes or cytosol were approximately similar in rats, dogs, monkeys and humans. In contrast, hydrolase activities of small intestinal microsomes from monkeys were higher (36.1 ng mg protein(-1) min(-1)) than those of rats, dogs and humans (5.4, 1.4 and 4.3 ng mg protein(-1) min(-1), respectively). These results suggest that the primary factor influencing first-pass metabolism for the OT-7100 is enzymatic hydrolysis in the small intestine. This information provides an important index for extrapolating the pharmacokinetics of drugs in humans using studies on monkeys.

Ontogeny of hepatic and plasma metabolism of deltamethrin in vitro: role in age-dependent acute neurotoxicity

Deltamethrin (DLM) is a relatively potent and widely used pyrethroid insecticide. Inefficient detoxification has been proposed to be the primary reason for the greater sensitivity of immature rats to the acute neurotoxicity of DLM. The objective of this study was to test this hypothesis by characterizing the age dependence of DLM metabolism in vitro, as well as toxic signs and blood levels of the neurotoxic parent compound following administration of 10 mg DLM/kg p.o. in glycerol formal. Metabolism was quantified in vitro by monitoring the disappearance of the parent compound from plasma [via carboxylesterases (CaEs)] and liver microsomes [via CaEs and cytochromes P450 (P450s)] obtained from 10-, 21-, and 40-day-old male Sprague-Dawley rats. Mean (+/-S.E.) intrinsic clearances (Vmax/Km) in these respective age groups by liver P450s (4.99+/-0.32, 16.99+/-1.85, and 38.45+/-7.03) and by liver CaEs (0.34+/-0.05, 1.77+/-0.38, and 2.53+/-0.19) and plasma CaEs (0.39+/-0.06, 0.80+/-0.09, and 2.28+/-0.56) increased significantly (p<or=0.05) with age, because of progressive increases in Vmax. Intrinsic clearance of DLM by plasma CaEs and liver P450s reached adult levels by 40 days, but clearance by liver CaEs did not. Hepatic P450s played the predominant role in DLM biotransformation in young and adult rats. The incidence and severity of neurotoxic effects varied inversely with age. Correspondingly, blood DLM areas under the concentration versus time curve (AUCs) and Cmax values progressively decreased with increasing age. Internal exposure to DLM (blood AUCs) was closely correlated with toxic signs (salivation and tremors). The present study provides evidence that the limited metabolic capacity of immature rats contributes to elevated systemic exposure and ensuing neurotoxic effects of DLM.

Mutation of F417 but not of L418 or L420 in the lipid binding domain decreases the activity of triacylglycerol hydrolase

Human triacylglycerol hydrolase (hTGH) has been shown to play a role in hepatic lipid metabolism. Triacylglycerol hydrolase (TGH) hydrolyzes insoluble carboxylic esters at lipid/water interfaces, although the mechanism by which the enzyme adsorbs to lipid droplets is unclear. Three-dimensional modeling of hTGH predicts that catalytic residues are adjacent to an alpha-helix that may mediate TGH/lipid interaction. The helix contains a putative neutral lipid binding domain consisting of the octapeptide FLDLIADV (amino acid residues 417-424) with the consensus sequence FLXLXXXn (where n is a nonpolar residue and X is any amino acid except proline) identified in several other proteins that bind or metabolize neutral lipids. Deletion of this alpha-helix abolished the lipolytic activity of hTGH. Replacement of F417 with alanine reduced activity by 40% toward both insoluble and soluble esters, whereas replacement of L418 and L420 with alanine did not. Another potential mechanism of increasing TGH affinity for lipid is via reversible acylation. Molecular modeling predicts that C390 is available for covalent acylation. However, neither chemical modification of C390 nor mutation to alanine affected activity. Our findings indicate that F417 but not L418, L420, or C390 participates in substrate hydrolysis by hTGH.

Comparative carboxylesterase activities in infant and adult liver and their in vitro sensitivity to chlorpyrifos oxon

Maturational expression of carboxylesterase activity in laboratory animals has been correlated with age-related differences in sensitivity to many organophosphorus insecticides including chlorpyrifos. Little information is available, however, on the maturational expression of liver carboxylesterases in humans. Human liver carboxylesterase activity was compared in tissues from infants (2-24 months) and adults (20-36 years). There was no significant difference between mean infant and adult carboxylesterase activities. The carboxylesterase activity rank order was: 2 months<3 months<20 years<24 months<4 months<36 years<21 years<8 months<34 years<35 years. Proteins (3 microg) were separated and blotted using antibodies against rat hydrolase S (HS), human carboxylesterase (HCE) types 1 and 2, and CYP3A4. Again, there were no significant differences in staining density between infant and adult tissues with any isozyme. Aliquots of each sample were pre-incubated (30 min, 37 degrees C) with chlorpyrifos oxon to evaluate in vitro sensitivity. Based on 95% confidence intervals, no significant differences in IC50 values were obtained in 3-month to 36-year samples (range: 1.42-2.12 nM), while the IC50 was significantly lower in the 2-month sample (0.45 nM). Carboxylesterase activity across samples was correlated with cytochrome b5 content and HS immunosignal but not with other microsomal activities (total cyt P450 content, testosterone hydroxylation, coumarin hydroxylation, and EROD). The results suggest that, in contrast to rodents, human liver carboxylesterase expression changes relatively little during postnatal maturation.

Dexamethasone-induced methylprednisolone hemisuccinate hydrolase: Its identification as a member of the rat carboxylesterase 2 family and its unique existence in plasma

Carboxylesterases (CESs) play important roles in the metabolism of many ester-drugs. In the present study, we identified and characterized dexamethasone-induced methylprednisolone hemisuccinate (MPHS) hydrolase in rat liver microsomes. Intraperitoneal injection of dexamethasone resulted in a significant increase in the level of MPHS hydrolase activity accompanied by induction of a specific CES isozyme. Since the biochemical characteristics of the induced CES isozyme were very similar to those of rat CES RL4, we hypothesized that these were the same enzymes. The results of nano-electrospray ionization tandem mass spectrometry analysis revealed that both dexamethasone-induced CES isozyme and CES RL4 possessed identical peptide fragments to those of , a rat CES2 isozyme, supporting our hypothesis. Furthermore, the results of reverse transcription-polymerase chain reaction showed that the amount of mRNA in dexamethasone-treated liver was greater than that in control liver. To confirm that encodes dexamethasone-induced CES isozyme, cDNA cloning was performed and the obtained cDNA was expressed in Sf9 cells by using a baculovirus-mediated expression system. The recombinant CES protein could hydrolyze MPHS and exhibited biochemical characteristics similar to those of CES RL4. Collectively, the results indicated that dexamethasone-induced MPHS hydrolase in liver microsomes is a rat CES2 isozyme. Interestingly, the results also showed that this rat CES2 isozyme exists in plasma and that the amount of this protein is increased by dexamethasone. These findings, together with the findings described above, provide important information for the study of phramacokinetics and pharmacodynamics of ester-drugs as well as for the study of CESs.

Ghrelin degradation by serum and tissue homogenates: identification of the cleavage sites

The endogenous ligand for the GH secretagogue receptor is ghrelin, a peptide recently purified from the stomach. Ghrelin is n-octanoylated on the Ser(3) residue, and this modification is essential for its interaction with the receptor. The degradation of ghrelin by rat and human serum, purified commercial enzymes, and tissues homogenates was analyzed by combining HPLC and mass spectrometry. In serum, ghrelin was desoctanoylated, without proteolysis. The desoctanoylation was significantly reduced by phenylmethylsulfonyl fluoride, a serine proteases and esterases inhibitor. In rat serum, the carboxylesterase inhibitor bis-p-nitrophenyl-phosphate totally inhibited ghrelin desoctanoylation, and a correlation was found between ghrelin desoctanoylation and carboxylesterase activity. Moreover, purified carboxylesterase degraded ghrelin. Thus, carboxylesterase could be responsible for ghrelin desoctanoylation in that species. In human serum, ghrelin desoctanoylation was partially inhibited by eserine salicylate and sodium fluoride, two butyrylcholinesterase inhibitors, but not by bis-p-nitrophenyl-phosphate and EDTA. Purified butyrylcholinesterase was able to degrade ghrelin, and there was a correlation between the butyrylcholinesterase and ghrelin desoctanoylation activities in human sera. This suggested that several esterases, including butyrylcholinesterase, contributed to ghrelin desoctanoylation in human serum. In contact with tissues homogenates, ghrelin was degraded by both desoctanoylation and N-terminal proteolysis. We identified five cleavage sites in ghrelin between residues -Ser(2)-(acyl)Ser(3)- (stomach and liver), -(acyl?)Ser(3)-Phe(4)- (stomach, liver, and kidney), -Phe(4)-Leu(5)- (stomach and kidney), -Leu(5)-Ser(6)- and -Pro(7)-Glu(8)- (kidney). In all cases, the resulting fragments were biologically inactive.

Further assessment of an in vitro screen that may help identify organophosphorus pesticides that are more acutely toxic to the young

Some, but not all, organophosphorus pesticides are more acutely toxic to the young as compared to adults. We have developed an in vitro assay that measures the detoxification potential (via carboxylesterase and A-esterases) of tissues. Previous results using this in vitro screen correlated with the marked in vivo sensitivity of the young to chlorpyrifos and also correlated with the equal sensitivity of the young and adult to methamidophos (Padilla et al., 2000). We have now extended these observations to two other pesticides that have already been shown in the literature to be more toxic to the young: parathion (paraoxon) and malathion (malaoxon). In our in vitro assay, liver or plasma from 7-d-old rats were much less efficacious than adult tissues at detoxification of the active metabolites of these two pesticides. Using our in vitro assay we also tested the active metabolite of diazinon, diazoxon, and again found that young liver or plasma possessed much less detoxification capability than adult tissues. From these results, we predicted that young animals would be more sensitive to diazinon, which, in fact, was the case: When postnatal day (PND) 17 or adult rats were given a dosage of 75 mg/kg diazinon, adult brain cholinesterase (ChE) was only inhibited 38%, while the brain ChE in the PND 17 animals showed much more inhibition (75%). We conclude that our in vitro screen may prove to be a useful, quick, convenient test for identifying which organophosphorus pesticides may be more acutely toxic to the young as compared to adults.

Regulation of the enzymes of hepatic microsomal triacylglycerol lipolysis and re-esterification by the glucocorticoid dexamethasone

Hepatic VLDL (very-low-density lipoprotein) assembly is a complex process that is largely regulated by the provision of lipid for apolipoprotein B assembly. Intracellular stored TAG (triacylglycerol) undergoes an initial lipolysis followed by re-esterification of the lipolytic products to form TAG prior to their incorporation into a VLDL particle. TGH (TAG hydrolase) is a lipase that hydrolyses intracellular TAG within the hepatocyte. We have utilized both dexamethasone-injected mouse and primary hepatocyte models to address whether stimulation of TAG biosynthesis by the synthetic glucocorticoid, dexamethasone, altered hepatic lipolysis and re-esterification and the provision of stored TAG for lipoprotein secretion. Dexamethasone treatment resulted in decreased TGH expression, primarily due to a dexamethasone-induced decrease in TGH mRNA stability. The expression and activities of diacylglycerol acyltransferases 1 and 2 were stimulated by dexamethasone. The combination of reduced intracellular TAG lipolysis and increased TAG biosynthesis contributed to the accumulation of TAG within the livers of dexamethasone-injected mice. The rate of hepatic TAG secretion in dexamethasonetreated mice was maintained at similar levels as in control mice. Our data demonstrate that stimulation of de novo TAG synthesis by dexamethasone increased the proportion of secreted TAG that was derived from de novo sources, while the utilization of stored TAG for secretion was reduced. The results show that, during markedly increased TAG synthesis, some TAGs are diverted from the cytosolic storage pool and are utilized directly for VLDL assembly within the endoplasmic reticulum lumen.

Age dependence of organophosphate and carbamate neurotoxicity in the postnatal rat: extrapolation to the human

One important aspect of risk assessment for the organophosphate and carbamate pesticides is to determine whether their neurotoxicity occurs at lower dose levels in human infants compared to adults. Because these compounds probably exert their neurotoxic effects through the inhibition of acetylcholinesterase (AChE), the above question can be narrowed to whether the cholinesterase inhibition and neurotoxicity they produce is age-dependent, both in terms of the effects produced and potency. The rat is the animal model system most commonly used to address these issues. This paper first discusses the adequacy of the postnatal rat to serve as a model for neurodevelopment in the postnatal human, concluding that the two species share numerous pathways of postnatal neurodevelopment, and that the rat in the third postnatal week is the neurodevelopmental equivalent of the newborn human. Then, studies are discussed in which young and adult rats were dosed by identical routes with organophosphates or carbamates. Four pesticides were tested in rat pups in their third postnatal week: aldicarb, chlorpyrifos, malathion, and methamidophos. The first three, but not methamidophos, caused neurotoxicity at dose levels that ranged from 1.8- to 5.1-fold lower (mean 2.6-fold lower) in the 2- to 3-week-old rat compared to the adult. This estimate in the rat, based on a limited data set of three organophosphates and a single carbamate, probably represents the minimum difference in the neurotoxicity of an untested cholinesterase-inhibiting pesticide that should be expected between the human neonate and adult. For the organophosphates, the greater sensitivity of postnatal rats, and, by analogy, that expected for human neonates, is correlated with generally lower levels of the enzymes involved in organophosphate deactivation.

Regulation of triacylglycerol hydrolase expression by dietary fatty acids and peroxisomal proliferator-activated receptors

Triacylglycerol hydrolase (TGH) is an enzyme that catalyzes the lipolysis of intracellular stored triacylglycerol (TG). Peroxisomal proliferator-activated receptors (PPAR) regulate a multitude of genes involved in lipid homeostasis. Polyunsaturated fatty acids (PUFA) are PPAR ligands and fatty acids are produced via TGH activity, so we studied whether dietary fats and PPAR agonists could regulate TGH expression. In 3T3-L1 adipocytes, TGH expression was increased 10-fold upon differentiation, compared to pre-adipocytes. 3T3-L1 cells incubated with a PPARgamma agonist during the differentiation process resulted in a 5-fold increase in TGH expression compared to control cells. Evidence for direct regulation of TGH expression by PPARgamma could not be demonstrated as TGH expression was not affected by a 24-h incubation of mature 3T3-L1 adipocytes with the PPARgamma agonist. Feeding mice diets enriched in fatty acids for 3 weeks did not affect hepatic TGH expression, though a 3-week diet enriched in fatty acids and cholesterol increased hepatic TGH expression 2-fold. Two weeks of clofibrate feeding did not significantly affect hepatic TGH expression or microsomal lipolytic activities in wild-type or PPARalpha-null mice, indicating that PPARalpha does not regulate hepatic TGH expression. Therefore, TGH expression does not appear to be directly regulated by PPARs or fatty acids in the liver or adipocytes.

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.

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.

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.

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.

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.

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.

Age-related effects of chlorpyrifos on acetylcholine release in rat brain

Chlorpyrifos (CPF) is an organophosphorus insecticide that elicits toxicity through inhibition of acetylcholinesterase (AChE). Young animals are markedly more sensitive than adults to the acute toxicity of CPF. We evaluated acetylcholine (ACh) release and its muscarinic receptor-mediated regulation (i.e. muscarinic autoreceptor function, MAF) during maturation as a possible contributing factor to age-related differences in sensitivity. Cortical and striatal slices were prelabeled with [3H]choline chloride, superfused in the presence or absence of the anticholinesterase physostigmine (PHY, 20 microM) and stimulated twice (S1 and S2) with a high concentration of potassium chloride (20 mM). Depolarization-stimulated ACh release (DSAR) was lowest in neonatal, intermediate in juvenile and markedly higher in adult tissues. MAF was not detectable in tissues from neonatal rats but was present in juvenile and adult tissues. ACh release and MAF were studied at 4, 24 and 96 h following oral exposure to CPF (0, 0.5 or 1 x LD10). In general, 40-60% and 80-90% maximal AChE inhibition followed exposure to the respective 0.5 and 1 x LD10 dosages. DSAR was decreased in neonatal cortex 1 day after LD10 exposure but increased in juvenile striatum 1 day after LD10 treatment. In adults, DSAR was reduced at 4 and 24 h after exposure, but increased 96 h after CPF exposure. In juveniles, MAF was reduced in both brain regions at 24 h after 0.5LD10 exposure and at 24 and 96 h after LD10 exposure in cortex. A later reduction in MAF was noted in adult tissues (i.e. only at 96 h after LD10 treatment). Together, the results suggest that ACh release dynamics in brain vary markedly during postnatal maturation and that acute CPF exposure can alter ACh release in an age-related manner. The functional status of presynaptic processes regulating neurotransmitter release may contribute to age-related neurotoxicity elicited by high-dose exposures to chlorpyrifos.

Characterization of esterases involved in the stereoselective hydrolysis of ester-type prodrugs of propranolol in rat liver and plasma

An inhibition study showed that the stereoselective hydrolysis of butyryl propranolol (butyryl PL) in rat liver microsomes and plasma involves carboxylesterase. The hydrolysis of (S)-butyryl PL in plasma was specifically inhibited by eserine and bis-nitrophenyl phosphate (BNPP), compared to the (R)-isomer, despite the non-stereoselective hydrolysis of butyryl PL in plasma. In addition, inhibition of hydroloysis by eserine and BNPP showed little stereoselectivity for butyryl PL in liver, although liver microsomes showed an (S)-preferential hydrolysis for butyryl PL (R/S ratio of Vmax/Km: 2.1 +/- 0.2). The hydrolysis of butyryl PL was not inhibited by a polyclonal antibody against a high affinity carboxylesterase (hydrolase A, RH1). Moreover, the high Km value and the high IC50 for phenylmethylsulfonyl fluoride (PMSF) against the hydrolysis of butyryl PL in rat liver microsomes suggest that a low affinity carboxylesterase (perhaps hydrolase B) might be involved in this hydrolysis in rat liver.

High-affinity nasal extraction of vinyl acetate vapor is carboxylesterase dependent

Vinyl acetate induces nasal tumors in rats, but not mice. Species differences in airflow patterns, physiology, and biochemistry complicate extrapolation of nasal dosimetry from rats to humans. Physiologically based pharmacokinetic modeling of vinyl acetate dosimetry in rats suggested the presence of a saturable metabolic removal pathway in rat nasal mucus. We explored the possibility that this pathway is either a cytochrome P-450 2E1 (CYP2E1) or high-affinity carboxylesterase. Nasal extraction of vinyl acetate vapor (150 ppm) was measured in the surgically isolated nasal cavity of anesthetized rats. Vinyl acetate (150 ppm) was extracted with 73% efficiency in controls. Pretreatment of rats with the CYP2E1 inhibitor diallyl sulfide (DAS) had no effect on extraction, despite significantly reducing CYP2E1 activity. Pretreatment with bis(p-nitrophenyl) phosphate (BNPP), a carboxylesterase inhibitor, reduced extraction to approximately 41%. Acetaldehyde production was similarly unaffected by DAS but was reduced to 55% of control by BNPP. Rat nasal mucus carboxylesterase activity had a K(m) value (32 microM) similar, within a factor of 2, to the value predicted by the physiologically based model, although V(max) was significantly lower than the model prediction. Histochemical observations support the inference that the high-affinity carboxylesterase is bound to the luminal plasma membrane of nasal tissue and is not readily released by nasal lavage, providing an explanation for the low V(max) of the lavage enzyme. This high-affinity isoenzyme could be important in the removal of odorants from the sensory cell-rich nasal olfactory epithelium.

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.

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

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

Effect of phenobarbital and beta-naphthoflavone on activities of different rat esterases after paraoxon exposure

1. The effects of two model inducers of the cytochrome P450 system, phenobarbital (PB) and beta-naphthoflavone (NF), on the toxicity of paraoxon were studied in rats. 2. Paraoxon toxicity was measured by inhibition of brain acetylcholinesterase (AChE) activity. 3. PB treatment did not affect the toxicity of paraoxon, whereas NF increased the inhibition of brain AChE. PB administration slightly increased the activities of some peripheral cholinesterases and carboxylesterases, as well as liver microsomal paraoxonase (Pxase). 4. NF administration, in contrast, decreased the activities of peripheral esterases. Serum Pxase activity was reduced by both inducers. 5. Hepatic CYP2B and CYP1A were markedly induced by PB and NF, respectively. 6. Cytochrome P450 isoenzymes induced by PB or NF seemed not to be critical in the detoxification of paraoxon in vivo. NF caused a general reduction of peripheral esterases, which led to an increase in paraoxon toxicity. 7. The results indicated the great importance of peripheral cholinesterases and carboxylesterases as a detoxifying mechanism of paraoxon. The role of serum paraoxonase was not critical.

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.

Species differences in the disposition of propranolol prodrugs derived from hydrolase activity in intestinal mucosa

The bioavailability of propranolol (PL) after oral administration of ester-type prodrug was compared in rat and dog, and the possible reason for species difference was investigated. In dog, the oral bioavailability of PL was enhanced by the use of prodrug due to saturation of metabolism of PL. In contrast, high (10 mg/kg) and low (2.5 mg/kg) doses of butyryl PL and isovaleryl PL failed to improve oral bioavailability of PL in rats. The hydrolase activities for prodrugs in rat liver were lower than those of dog (by 4-12-fold), but those of rat intestinal mucosa were significantly higher than those of dog (50-260-fold). Although it is clear from the in vitro hydrolysis using subcellular fractions that the rapid hydrolysis in intestinal mucosa was mainly due to cytosolic components, the brush-border membrane vesicle in rat intestine also showed hydrolase activity for both prodrugs. In situ absorption experiment in rat revealed an improvement in the apparent absorption rate of PL as the result of prodrug use (1.3-fold) and the nearly complete hydrolysis of isovaleryl PL during intestinal absorption, which is a slower hydrolyzed prodrug than butyryl PL in intestinal mucosa and liver. The defects for enhancing oral bioavailability in rats appears to be based on an unsaturation of metabolism for PL, which is derived from a decrease in PL concentration in hepatocytes, owing to rapid hydrolysis of the prodrug in intestinal mucosa and slow hydrolysis of the prodrug in liver. Furthermore, human intestinal mucosa showed a surprisingly high hydrolase activity in microsomes. Therefore, the oral bioavailability of PL after administration of prodrugs might be not significantly improved in human.

Species differences in stereoselective hydrolase activity in intestinal mucosa

PURPOSE

The aim of this study is to investigate species differences in the stereoselective hydrolysis for propranolol ester prodrugs in mammalian intestinal mucosa and Caco-2 cells.

METHODS

Hydrolase activities for propranolol prodrugs and p-nitrophenylacetate in man (age: 51-71 years), the beagle dog (age: 4 years) and Wistar rat (age: 8 weeks) intestinal mucosa, and also in Caco-2 cells (passage between 60-70) were estimated by determining the rate of production of proparanolol and p-nitrophenol, respectively.

RESULTS

The hydrolase activities for both propranolol prodrugs and p-nitrophenylacetate were in the order of man > rat >> Caco-2 cells > dog for intestinal microsomes, and rat > Caco-2 cells = man > dog for intestinal cytosol. Dog microsomes showed stereoselective hydrolysis for propranolol prodrugs, but not those from human or rat. Interestingly, both subcellular fractions of Caco-2 cells showed remarkable R-enantioselectivity except acetyl propranolol. Enzyme kinetic experiments for each enantiomer of butyryl propranolol in microsomes revealed that dog possesses both low and high affinity hydrolases. Both Km and Vmax values in rat were largest among examined microsomes, while Vmax/Km was largest in man. Finally, it was shown that the carboxylesterases might contribute to the hydrolysis of propranolol prodrug in all species by inhibition experiments.

CONCLUSIONS

The hydrolase activities for propranolol prodrugs and p-nitrophenylacetate in intestinal mucosa showed great species differences and those in human intestine were closer to those of rat intestine than dog intestine or Caco-2 cells.

Hepatic clearance of ONO-5046, a novel neutrophil elastase inhibitor, in rats and in the rat perfused liver

The hepatic clearance of ONO-5046 (N-[2-[4-(2,2-dimethylpropionyloxy)phenylsulphonylamino]benz oyl]aminoacetic acid), a low-molecular-weight neutrophil elastase inhibitor, has been investigated in rats and in the rat perfused liver. This ester was easily hydrolysed to its inactive metabolite EI-601 (N-[2-[(4-hydroxyphenyl)sulphonylamino]benzoyl]aminoacetic acid) in liver homogenate and in erythrocytes suspension in-vitro. On the other hand, it was stable in biological media such as plasma and whole blood, which contain plasma proteins. Scatchard plot analysis of ONO-5046 binding to bovine serum albumin (BSA) in-vitro indicated that the association constant (K) and number of binding sites (n) were 6.91 x 10(4) (M(-1)) and 4.33, respectively. Thus, ONO-5046 (100 microM) would bind to plasma proteins to an extent >99% at physiological plasma-protein concentrations. The total plasma clearance of ONO-5046 in rats was constant (approximately 9 mL min(-1) kg(-1)) under different steady-state plasma concentrations (5-50 microM) a value equivalent to the hepatic clearance. In the rat perfused liver, the hepatic extraction ratio of ONO-5046 was significantly reduced by adding BSA to the dosing solution. Thus, the relatively low hepatic clearance of ONO-5046, which has an ester linkage in its structure and is naturally susceptible to enzymatic hydrolysis, was found to be because of the extremely high protein-binding of the compound.

Age- and gender-related differences in the time course of behavioral and biochemical effects produced by oral chlorpyrifos in rats

It is well known that young animals are generally more sensitive to lethal effects of cholinesterase-inhibiting pesticides, but there are sparse data comparing less-than-lethal effects. We compared the behavioral and biochemical toxicity of chlorpyrifos in young (postnatal Day 17; PND17) and adult (about 70 days old) rats. First, we established that the magnitude of the age-related differences decreased as the rat matures. Next, we evaluated the time course of a single oral dose of chlorpyrifos in adult and PND17 male and female rats. Behavioral changes were assessed using a functional observational battery (with age-appropriate modifications for pre-weanling rats) and an evaluation of motor activity. Cholinesterase (ChE) activity was measured in brain and peripheral tissues and muscarinic receptor binding assays were conducted on selected tissues. Rats received either vehicle (corn oil) or chlorpyrifos (adult dose: 80 mg/kg; PND17 dose: 15 mg/kg); these doses were equally effective in inhibiting ChE. The rats were tested, and tissues were then taken at 1, 2, 3.5, 6.5, 24, 72, 168, or 336 h after dosing. In adult rats, peak behavioral changes and ChE inhibition occurred in males at 3.5 h after dosing, while in females the onset of functional changes was sooner, the time course was more protracted and recovery was slower. In PND17 rats, maximal behavioral effects and ChE inhibition occurred at 6.5 h after dosing, and there were no gender-related differences. Behavioral changes showed partial to full recovery at 24 to 72 h, whereas ChE inhibition recovered markedly slower. Blood and brain ChE activity in young rats had nearly recovered by 1 week after dosing, whereas brain ChE in adults had not recovered at 2 weeks. Muscarinic-receptor binding assays revealed apparent down-regulation in some brain areas, mostly at 24 and 72 h. PND17 rats generally showed more receptor down-regulation than adults, whereas only adult female rats showed receptor changes in striatal tissue that persisted for 2 weeks. Thus, compared to adults (1) PND17 rats show similar behavioral changes and ChE inhibition although at a five-fold lower dose; (2) the onset of maximal effects is somewhat delayed in the young rats; (3) ChE activity tended to recover more quickly in the young rats; (4) young rats appear to have more extensive muscarinic receptor down-regulation, and (5) young rats show no gender-related differences.