Cloning and expression of a rat liver phenobarbital-inducible UDP-glucuronosyltransferase (2B12) with specificity for monoterpenoid alcohols

Glucosylation of (±)-Menthol by Uridine-Diphosphate-Sugar Dependent Glucosyltransferases from Plants

Menthol is a cyclic monoterpene alcohol of the essential oils of plants of the genus Mentha, which is in demand by various industries due to its diverse sensorial and physiological properties. However, its poor water solubility and its toxic effect limit possible applications. Glycosylation offers a solution as the binding of a sugar residue to small molecules increases their water solubility and stability, renders aroma components odorless and modifies bioactivity. In order to identify plant enzymes that catalyze this reaction, a glycosyltransferase library containing 57 uridine diphosphate sugar-dependent enzymes (UGTs) was screened with (±)-menthol. The identity of the products was confirmed by mass spectrometry and nuclear magnetic resonance spectroscopy. Five enzymes were able to form (±)-menthyl-β-d-glucopyranoside in whole-cell biotransformations: UGT93Y1, UGT93Y2, UGT85K11, UGT72B27 and UGT73B24. In vitro enzyme activity assays revealed highest catalytic activity for UGT93Y1 (7.6 nkat/mg) from Camellia sinensis towards menthol and its isomeric forms. Although UGT93Y2 shares 70% sequence identity with UGT93Y1, it was less efficient. Of the five enzymes, UGT93Y1 stood out because of its high in vivo and in vitro biotransformation rate. The identification of novel menthol glycosyltransferases from the tea plant opens new perspectives for the biotechnological production of menthyl glucoside.

Effect of natural borneol on the pharmacokinetics and distribution of nimodipine in mice

The purpose of this study was to investigate the effect of natural borneol (NB) on the pharmacokinetics and distribution of nimodipine in mice. A single dose of nimodipine was administered intravenously (2 mg/kg) to mice pretreated with NB (250 mg/kg) or vehicle. Blood as well as brain, liver, and kidney tissue samples were collected at 5, 10, 20, 40, and 60 min post-dose nimodipine. The concentrations of nimodipine in plasma and tissues were determined by ultra performance liquid chromatography (UPLC) coupled with UV detection, and the pharmacokinetic parameters were calculated based on non-compartmental analysis. NB increased the plasma AUC5-60 min by 26 % compared to the vehicle. In addition, brain concentrations of nimodipine in NB-treated mice were significantly higher than those in control mice with the increased AUC5-60 min by 30 %. In liver and kidney, NB also caused 26 and 47 % increase in AUC5-60 min, respectively. These results implicated that NB may inhibit the metabolism or elimination of nimodipine and enhance its distribution in brain and kidney tissue.

Inhibition of morphine glucuronidation in the liver microsomes of rats and humans by monoterpenoid alcohols

Morphine is an important drug used to alleviate moderate to severe pain. This opiate is mainly metabolized by glucuronidation to a non-analgesic metabolite, morphine-3-glucuronide (M-3-G) and an active metabolite morphine-6-glucuronide (M-6-G). To understand the modulation of morphine glucuronidation activity by environmental factors, the effect of endogenous and food-derived compounds on morphine uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) in rat and human microsomes was evaluated examining the 50% inhibitory concentration (IC(50)). The liver microsomes from Sprague-Dawley rats (RLM) and humans (HLM, 150 donors, pooled microsomes) were used as enzyme sources. Of 27 compounds tested, monoterpenoid alcohols, such as borneol and iso-borneol, exhibited a strong inhibitory effect on morphine glucuronidation in rat liver microsomes (RLM), whereas we failed to detect any inhibitory effect of endogenous substances including amino acids and sugars. The substances which have the ability to inhibit the activity in RLM are also inhibitory toward morphine glucuronidation in HLM and UGT2B7 baculosomes. However, the difference was that while the strongest inhibitory effect was observed for iso-menthol in HLM, borneol was the strongest inhibitor of the activity mediated by RLM. Although zidovudine is a typical substrate of UGT2B7, the inhibition of morphine glucuronidation by zidovudine was far weaker than that of monoterpenoid alcohols. These results demonstrate that dietary and supplementary monoterpenoid alcohols modify the pharmacokinetics and pharmacodynamics of morphine through inhibition of UGT2B7.

Cytotoxic effects of polychlorinated biphenyl hydroquinone metabolites in rat hepatocytes

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that exhibit various toxic effects in animals and exposed human populations. The molecular mechanisms of PCB toxicity have been attributed to the toxicological properties of its metabolites, such as hydroquinones, formed by cytochrome-P-450 oxidation. The effects of PCB hydroquinone metabolites towards freshly isolated rat hepatocytes were investigated. Hydroquinones can be oxidized to semiquinones and/or quinone metabolites. These metabolites can conjugate glutathione or can oxidize glutathione as a result of redox cycling. This depletes hepatocyte glutathione, which can inhibit cellular defence mechanisms, causing cell death and an increased susceptibility to oxidative stress. However in the following, glutathione-depleted hepatocytes became more resistant to the hydroquinone metabolites of PCBs. This suggested that their glutathione conjugates were toxic and that there was a third type of quinone toxicity mechanism which involved a hydrogen peroxide-accelerated autoxidation of the hydroquinones to form toxic electrophilic quinone and semiquinone-glutathione conjugates.

Percutaneous absorption of the montoterpene carvone: implication of stereoselective metabolism on blood levels

The purpose of this study was to determine whether an enantioselective difference in the metabolism of topically applied R-(—)- and S-(+)-carvone could be observed in man. In a previous investigation we found that R-(—)- and S-(+)-carvone are stereoselectively biotransformed by human liver microsomes to 4R,6S-(—)- and 4S,6S-(+)-carveol, respectively, and 4R,6S-(—)-carveol is further glucuronidated. We therefore investigated the metabolism and pharmacokinetics of R-(—)- and S-(+)-carvone in four healthy subjects using chiral gas chromatography as the analytical method. Following separate topical applications at a dose of 300 mg, R-(—)- and S-(+)-carvone were rapidly absorbed, resulting in significantly higher Cmax levels for S-(+)-carvone (88.0 vs 23.9 ng mL−1) and longer distribution half-lives (t2α1) (19.4 vs 7.8 min), resulting in 3.4-fold higher areas under the blood concentration-time curves (5420 vs 1611 ng min mL−1). The biotransformation products for both enantiomers in plasma were below detection limit. Analysis of control- and β-glucuronidase pretreated urine samples, however, revealed a stereoselective metabolism of R-(—)-carvone to 4R,6S-(—)-carveol and 4R,6S-(—)-carveol glucuronide. No metabolites could be found in urine samples after S-(+)-carvone application. These data indicate that stereoselectivity in phase-I and phase-II metabolism has significant effects on R-(—)- and S-(+)-carvone pharmacokinetics. This might serve to explain the increased blood levels of S-(+)-carvone.

Protein-protein interactions between rat hepatic cytochromes P450 (P450s) and UDP-glucuronosyltransferases (UGTs): evidence for the functionally active UGT in P450-UGT complex

The interaction between cytochrome P450s (CYP, P450) and UDP-glucuronosyltransferases (UGTs) was studied by co-immunoprecipitation. P450 isoform-selective antibody was used as a probe to co-precipitate UGTs with the P450s from solubilized rat liver microsomes. Antibodies toward CYP3A2, CYP2B2, CYP2C11/13 and CYP1A2 co-precipitated UGTs with corresponding P450s. However, calnexin, a type-I membrane protein, in the endoplasmic reticulum was not co-precipitated by anti-P450 antibodies. UGT activity toward 4-methylumbelliferone was detected in all co-precipitates, suggesting that UGT in the complex with P450s is functionally active. Repeated washing of co-immunoprecipitates revealed differences among P450 isoforms with regard to the affinity for UGT. Larger amounts of UGT1A1 and UGT1A6, compared with UGT2B1, were washed out from UGTs-CYP2C11/13 co-precipitates, whereas UGT-CYP3A2 and UGT-CYP2Bs complexes were resistant to thorough washing. Thus, CYP2C11/13 could associate with UGTs, but the affinity is assumed to be weaker than that of CYP2B/3As. These results suggest that there is isoform specificity in the interaction between P450s and UGTs.

Molecular cytotoxic mechanisms of catecholic polychlorinated biphenyl metabolites in isolated rat hepatocytes

Polychlorinated biphenyl (PCB) and PCB metabolites are highly lipophilic and accumulate easily in the lipid bilayer and fat deposits of the body. The molecular cytotoxic mechanisms of these metabolites are still not understood. The aim of the present study was to compare the cytotoxicity and toxicological properties of six dihydroxylated metabolites using isolated rat hepatocytes. All of the metabolites were more cytotoxic than 4-chlorobiphenyl (4-ClBP) and less cytotoxic than phenyl hydroquinone (PHQ). The order of cytotoxic effectiveness of catecholic metabolites expressed as LC(50) (2h) was 3',4'-diCl-2,3-diOH-biphenyl>PHQ>4'-Cl-2,5-diOH-biphenyl, 4'-Cl-2,3-diOH-biphenyl>2',5'-diCl-3,4-diOH-biphenyl>2',3'-diCl-3,4-diOH-biphenyl>3',4'-diCl-3,4-diOH-biphenyl>4'Cl-3,4-diOH-biphenyl>4'-Cl-biphenyl; showing that the positions of hydroxyl and chlorine groups were important for their hepatotoxicity and that the two 2,3-diOH congeners were the most cytotoxic. Cytotoxicity for 3,4-diOH metabolites correlated with the number and position of chlorine atoms with the more chlorine atoms being more cytotoxic. The cytotoxic order of metabolites with two chlorine atoms being 2',5'>2',3'>3',4'. Borneol, an uridine diphosphate glucuronosyltransferases (UGT) inhibitor, increased the cytotoxicity of all tested metabolites; suggesting that glucuronidation was a major mechanism of elimination of these compounds. On the other hand entacapone, a catechol-O-methyl transferase (COMT) inhibitor, only increased the cytotoxicity of 3',4'-diCl-3,4-diOH-biphenyl, 3',4'-diCl-2,3-diOH-biphenyl and 4'-Cl-2,3-diOH-biphenyl. Hepatocyte GSH was depleted (oxidized and conjugated) by these metabolites before cytotoxicity ensued in a similar order of effectiveness to their cytotoxicity with PHQ being the most effective. Hepatocyte mitochondrial membrane potential also decreased before cytotoxicity ensued with a similar order of effectiveness as their cytotoxicity. These results suggest that catecholic cytotoxicity can be attributed to mitochondrial toxicity and oxidative stress. Semiquinone or benzoquinone species were also important in the cytotoxicity of catecholic metabolites.

Induction of Rat UDP-Glucuronosyltransferases in Liver and Duodenum by Microsomal Enzyme Inducers That Activate Various Transcriptional Pathways

Microsomal enzyme inducers (MEIs) up-regulate phase I biotransformation enzymes, most notably cytochromes P450. Transcriptional up-regulation by MEIs occurs through at least three nuclear receptor mechanisms: constitutive androstane receptor (CAR; CYP2B inducers), pregnane X receptor (PXR; CYP3A inducers), and peroxisome proliferator-activated receptor alpha (PPARalpha; CYP4A inducers). Other mechanisms include transcription factors aryl hydrocarbon receptor (AhR; CYP1A inducers), and nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2; NADPH-quinone oxidoreductase inducers). UDP-glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that are predominantly expressed in liver and intestine. MEIs increase UGT activity; however, transcriptional regulation of individual UGT isoforms is not completely understood. The purpose of this study was to examine inducibility of individual UGT isoforms and potential mechanisms of transcriptional regulation in rat liver and duodenum. UGT mRNA levels were assessed in liver and duodenum of rats treated with MEIs that activate various transcriptional pathways. All four CAR activators induced UGT2B1 in liver, but not duodenum. UGT1A1, 1A5, 1A6, and 2B12 were induced by at least two CAR activators in liver only. Two PXR ligands induced UGT1A2, but only in duodenum. Two PPARalpha ligands induced UGT1A1 and 1A3 in liver only. AhR ligands induced UGT1A6 and 1A7 in liver, but not duodenum. Nrf2 activators increased UGT2B3 and 2B12 in both liver and duodenum, and UGT1A6, 1A7, and 2B1 in liver only. In summary, only UGT1A2 and 1A8 were not inducible in liver by MEIs. MEIs differentially regulate hepatic expression of individual UGT isoforms, although no one transcriptional pathway dominated. In duodenum, MEIs had minimal effects on UGT expression.

N-glucuronidation of perfluorooctanesulfonamide by human, rat, dog, and monkey liver microsomes and by expressed rat and human UDP-glucuronosyltransferases

N-Alkylperfluorooctanesulfonamides have been used in a range of industrial and commercial applications. Perfluorooctanesulfonamide (FOSA) is a major metabolite of N-alkylperfluorooctanesulfonamides and has a long half-life in animals and in the environment and is biotransformed to FOSA N-glucuronide. The objective of this study was to identify and characterize the human and experimental animal liver UDP-glucuronosyltransferases (UGTs) that catalyze the N-glucuronidation of FOSA. The results showed that pooled human liver and rat liver microsomes had high N-glucuronidation activities. Expressed rat UGT1.1, UGT2B1, and UGT2B12 in HK293 cells catalyzed the N-glucuronidation of FOSA but at rates that were lower than those observed in rat liver microsomes. Of the 10 expressed human UGTs (1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B15, and 2B17) studied, only hUGT2B4 and hUGT2B7 catalyzed the N-glucuronidation of FOSA. The kinetics of N-glucuronidation of FOSA by rat liver microsomes and by hUGT2B4/7 was consistent with a single-enzyme Michaelis-Menten model, whereas human liver microsomes showed sigmoidal kinetics. These data show that rat liver UGT1.1, UGT2B1, and UGT2B12 catalyze the N-glucuronidation of FOSA, albeit at low rates, and that hUGT2B4 and hUGT2B7 catalyze the N-glucuronidation of FOSA.

Herbal monoterpene alcohols inhibit propofol metabolism and prolong anesthesia time

2,6-Diisopropylphenol (Propofol) is a short-acting intravenous anesthetic that is rapidly metabolized by glucuronidation and ring hydroxylation catalyzed by cytochrome P450. The goal of this research was to determine whether dietary monoterpene alcohols (MAs) could be used to prolong the anesthetic effect of propofol by inhibiting propofol metabolism in animals. Mice were injected intraperitoneally (i.p.) with MAs (100-200) mg/kg followed by the administration of 100 mg/kg propofol 40 min later via an i.p. injection. The time of the anesthesia of each mouse was recorded. It was found that (+/-)-borneol, (-)-carveol, trans-sobrerol, and menthol significantly extended the anesthetic effect of propofol (>3 times). The concentration of propofol in the mouse blood over time (up to 180 min) also increased in mice pre-treated with (-)-borneol, (-)-carveol, and trans-sobrerol. The volume of distribution of propofol decreased in the (-)-borneol (p<0.05), pre-treated group as compared to the propofol control group. Moreover, the maximum blood concentration of propofol and the concentration of propofol in the blood as indicated by the area under the curve were significantly increased in (-)-borneol and (-)-carveol pre-treated groups. Additional evidence using rat hepatocytes showed that (-)-borneol inhibited propofol glucuronidation whereas trans-sobrerol and (-)-carveol inhibited cytochrome P450 dependent microsomal aminopyrine N-demethylation. These results suggest that (-)-borneol extends propofol-induced anesthesia by inhibiting its glucuronidation in the mouse whereas trans-sobrerol (-)-carveol extends propofol-induced anesthesia by inhibiting P450 catalyzed propofol metabolism.

Activity and expression of various isoforms of uridine diphosphate glucuronosyltransferase are differentially regulated during hepatic regeneration in rats

PURPOSE

Glucuronidation pathway is very important in the detoxification of endogenous and exogenous compounds. The objective of this study was to evaluate the activity and expression of various hepatic uridine diphosphate glucuronosyltransferases (UGTs) in rats at various time points after initiation of hepatic regeneration by partial hepatectomy (PHx).

METHODS

The mRNA expression of various UGTs was evaluated using real-time polymerase chain reaction (real-time PCR) with specific primers. The in vitro activity of UGTs was evaluated using different substrates such as estradiol (UGT1A1), acetaminophen (UGT1A6/7), morphine (UGT2B1), testosterone (UGT2B1/3/6), androsterone (UGT2B2), and (-)-borneol (UGT2B12).

RESULTS

Whereas the activity and mRNA expression of UGT1A1, UGT2B1, UGT2B1/3/6, UGT2B2, and UGT2B12 were lower, the activity and mRNA expression of UGT1A6/7 were preserved during hepatic regeneration. The mRNA expression of UGT2B8 was down-regulated, whereas the mRNA expression of UGT1A5 and UGT1A8 was not altered by PHx. The mRNA expression of UGT1A2 and UGT1A3 was increased during hepatic regeneration.

CONCLUSION

UGT-mediated drug-metabolizing ability of the liver was altered differentially in the regenerating rat liver. Individualized dosing regimen for different UGT substrates may be needed when using such substrates of these enzymes in patients with a regenerating liver, especially during the early postoperative period. However, the glucuronide conjugating capacity of the liver in the donor of a living donor liver transplantation is expected to completely return to normal with time after surgery.

Glucuronidation of haloperidol by rat liver microsomes: involvement of family 2 UDP-glucuronosyltransferases

The purposes of this study were to develop a HPLC method to assay for haloperidol glucuronide (HALG); to apply this assay method to the in vitro determination of haloperidol (HAL) UDP-glucuronosyltransferase (UGT) enzyme kinetics in rat liver microsomes (RLM); and to identify the UGT isoforms catalyzing glucuronidation of HAL in rats. Incubation of Brij-activated RLM with HAL and UDP-glucuronic acid (UDPGA) in TRIS pH 7.4 buffer resulted in the formation of a single peak in the HPLC chromatogram at 270 nm. The identity of this peak was confirmed to be that of HALG by 1) beta-glucuronidase hydrolysis; 2) incubation without UDPGA; 3) UV spectral analysis; and 4) LC/MS/MS to yield the expected mass of 552.1. Enzyme kinetic studies using single enzyme Michaelis-Menton model showed an apparent Vmax = 271.9 +/- 10.1 pmoles min(-1) mg protein(-1) and Km = 61 +/- 7.2 microM. Glucuronidation activity in homozygous Gunn (j/j) rats was approximately 80% as compared to Sprague-Dawley RLM. HALG formation was approximately doubled in PB-induced RLM. There was no increase in glucuronidation activities in 3MC-induced RLM. The Gunn rat and the PB-induced RLM data suggest predominant but not exclusive involvement of the UGT2B family in the formation of HALG. Because the UGTs exhibit overlapping substrate specificities and most substrates are glucuronidated by more than one isoform, inhibition studies with UGT2B1 substrate probe testosterone and the UGT2B12 substrate probe borneol were conducted. UGT2B1 and UGT2B12 exhibited 40% and 90% inhibition of HAL glucuronidation, respectively. Thus, UGT2B12 and UGT 2B1 isoforms are responsible for catalyzing HAL glucuronidation in rats. Our HPLC assay provides a specific and sensitive technique for the measurement of in vitro HAL-UGT activity.

Glucuronidation of 4-methylumbelliferone and 4-hydroxybiphenyl and in vitro induction of UDP-glucuronosyltransferase 2B12-mRNA in precision-cut rat liver slices

Fresh rat liver slices were used to demonstrate the glucuronidation of the model substrates 4-methylumbelliferone (MU) and 4-hydroxybiphenyl (HB). Both glucuronidation reactions proved to be more stable than cytochrome P450-dependent monooxygenations. After an incubation time of 48 h there was no decrease in MU glucuronidation rate, whereas HB glucuronidation was stable until 24 h, and then decreased by about 50% until 48 h. The technique of quantitative competitive RT-PCR was used to determine the expression of UDP-glucuronosyltransferase 2B12-mRNA (UGT2B12-mRNA) in precision-cut rat liver slices. Constitutive levels of UGT2B12-mRNA were measurable. Following 24 h culture of rat liver slices in the presence of phenobarbital, the level of UGT2B12-mRNA increased about twofold, which corresponds to the inducibility in vivo. The addition of beta-naphthoflavone had no influence. The results show that precision-cut liver slices are not only suitable for the detection of an in vitro induction of cytochrome P450-mRNAs, which is characterized by high induction factors, but also of poor induction effects, e.g. on UGT2B12-mRNA, provided that the respective mRNA is exactly quantified.

New hepatocyte in vitro systems for drug metabolism: metabolic capacity and recommendations for application in basic research and drug development, standard operation procedures

Primary hepatocytes represent a well-accepted in vitro cell culture system for studies of drug metabolism, enzyme induction, transplantation, viral hepatitis, and hepatocyte regeneration. Recently, a multicentric research program has been initiated to optimize and standardize new in vitro systems with hepatocytes. In this article, we discuss five of these in vitro systems: hepatocytes in suspension, perifusion culture systems, liver slices, co-culture systems of hepatocytes with intestinal bacteria, and 96-well plate bioreactors. From a technical point of view, freshly isolated or cryopreserved hepatocytes in suspension represent a readily available and easy-to-handle in vitro system that can be used to characterize the metabolism of test substances. Hepatocytes in suspension correctly predict interspecies differences in drug metabolism, which is demonstrated with pantoprazole and propafenone. A limitation of the hepatocyte suspensions is the length of the incubation period, which should not exceed 4hr. This incubation period is sufficiently long to determine the metabolic stability and to allow identification of the main metabolites of a test substance, but may be too short to allow generation of some minor, particularly phase II metabolites, that contribute less than 3% to total metabolism. To achieve longer incubation periods, hepatocyte culture systems or bioreactors are used. In this research program, two bioreactor systems have been optimized: the perifusion culture system and 96-well plate bioreactors. The perifusion culture system consists of collagen-coated slides allowing the continuous superfusion of a hepatocyte monolayer with culture medium as well as establishment of a constant atmosphere of 13% oxygen, 82% nitrogen, and 5% CO2. This system is stable for at least 2 weeks and guarantees a remarkable sensitivity to enzyme induction, even if weak inducers are tested. A particular advantage of this systemis that the same bioreactor can be perfused with different concentrations of a test substance in a sequential manner. The 96-well plate bioreactor runs 96 modules in parallel for pharmacokinetic testing under aerobic culture conditions. This system combines the advantages of a three-dimensional culture system in collagen gel, controlled oxygen supply, and constant culture medium conditions, with the possibility of high throughput and automatization. A newly developed co-culture system of hepatocytes with intestinal bacteria offers the possibility to study the metabolic interaction between liver and intestinal microflora. It consists of two chambers separated by a permeable polycarbonate membrane, where hepatocytes are cultured under aerobic and intestinal bacteria in anaerobic conditions. Test substances are added to the aerobic side to allow their initial metabolism by the hepatocytes, followed by the metabolism by intestinal bacteria at the anaerobic side. Precision-cut slices represent an alternative to isolated hepatocytes and have been used fo the investigation of hepatic metabolism, hepatotoxicity, and enzyme induction. A specific advantage of liver slices is the possibility to study toxic effects on hepatocytes that are mediated or modified by nonparenchymal cells (e.g., by cytokine release from Kupffer cells) because the physiological liver microarchitecture is maintained in cultured slices. For all these in vitro systems, a prevalidation has been performed using standard assays for phase I and II enzymes. Representative results with test substances and recommendations for application of these in vitro systems, as well as standard operation procedures are given.

Tissue mRNA expression of the rat UDP-glucuronosyltransferase gene family

UDP-Glucuronosyltransferases (UGTs) are phase II biotransformation enzymes that glucuronidate numerous endobiotic and xenobiotic substrates. Glucuronidation increases the water solubility of the substrate and facilitates renal and biliary excretion of the resulting glucuronide conjugate. UGTs have been divided into two gene families, UGT1 and UGT2. Tissue distribution of UGTs has not been thoroughly examined, and such data could provide insight into the importance of individual UGT isoforms in specific tissues and to the pharmacokinetics and target organ toxicity of UGT substrates. Therefore, the aim of this study was to determine mRNA levels of rat UGT1 and UGT2 family members in liver, kidney, lung, stomach, duodenum, jejunum, ileum, large intestine, cerebellum, and cerebral cortex, as well as nasal epithelium for UGT2A1. Tissue levels of UGT mRNA were detected using branched DNA signal amplification analysis. Three UGT isoforms, UGT1A1, UGT1A6, and UGT2B12, were detected in many tissues, whereas distribution of other UGT isoforms was more tissue-specific. For example, UGT2A1 was detected predominantly in nasal epithelium. Additionally, UGT1A5, UGT2B1, UGT2B2, UGT2B3, and UGT2B6 were detected primarily in liver. Furthermore, detection of UGT1A2, UGT1A3, UGT1A7, and UGT2B8 was somewhat specific to gastrointestinal (GI) tract. However, not all of these UGTs were detected in all portions of the GI tract. UGT1A8 was unique in that it was barely detectable in any of the tissues examined. In conclusion, some UGT isoforms were expressed in multiple tissues, whereas other UGT isoforms were predominantly expressed in a certain tissue such as nasal epithelium, liver, or GI tract.

Sex differences in hepatic gene expression in a rat model of ethanol-induced liver injury

Sex differences in susceptibility to alcohol-induced liver injury have been observed in both humans and experimental animal models. Using a standard model of alcohol-induced fatty liver injury and microarray analysis, we have identified differential expression of hepatic genes in both sexes. The genes that exhibit differential expression are of three types: those that are changed only in male rats fed alcohol, those that change in only female rats fed alcohol, and those that change in both sexes, although not always in the same manner. Certain of the differentially expressed genes have previously been identified as participants in the induction of alcohol-induced liver injury. However, this analysis has identified a number of genes that heretofore have not been implicated in alcoholic liver injury; such genes may provide new areas of investigation into the pathogenesis of this disease.

Isolation and characterization of the monkey UGT2B30 gene that encodes a uridine diphosphate-glucuronosyltransferase enzyme active on mineralocorticoid, glucocorticoid, androgen and oestrogen hormones

The present study reports the genomic organization and the characterization of a novel cynomolgus monkey UDP-glucuronosyltransferase (UGT) enzyme, UGT2B30. UGT enzymes are microsomal proteins that catalyse the transfer of the glucuronosyl group from UDP-glucuronic acid (UDPGA) to a wide variety of lipophilic compounds, namely hormonal steroids. The 15 kb UGT2B30 gene amplified by PCR showed a genomic organization similar to those encoding UGT2B human enzymes. The cDNA encoding UGT2B30 was isolated from a cynomolgus monkey prostate cDNA library, and the deduced amino acid sequence showed an identity of 94% with UGT2B19, a monkey isoform previously characterized. Stable expression of UGT2B30 protein in human kidney 293 (HK293) cells was assessed by Western-blot analysis and its conjugating activity was screened using 39 potential substrates. The UGT2B30 enzyme is active on many compounds of different classes, including testosterone, dihydrotestosterone, 5alpha-androstane-3alpha,17beta-diol, androsterone, oestradiol, tetrahydroaldosterone and tetrahydrocortisone, with glucuronidation efficiencies (V(max)/K(m) ratios) ranging from 0.6 to 8.8 microl x min(-1) x mg of protein(-1). Reverse-transcriptase-PCR analysis revealed that the UGT2B30 transcript is expressed in several tissues, including prostate, testis, mammary gland, kidney, adrenals and intestine. The relative activity of UGT2B30 in comparison with other simian UGT2B isoforms, as well as its large variety of substrates, strongly suggest that this enzyme is essential to inactivation of several steroids.

Glucuronidation of acetaminophen catalyzed by multiple rat phenol UDP-glucuronosyltransferases

Gunn rats glucuronidate acetaminophen (APAP) at reduced rates and show increased susceptibility to APAP-induced hepatotoxicity. This defect is presumed to involve UDP-glucuronosyltransferase (UGT) 1A6, which is nonfunctional in Gunn rats, but it is currently unclear whether other 1A family members are also involved. In humans, two 1A isoforms are known to be active (1A6 and 1A9) but 1A6 form has a 25-fold lower apparent K(m) (2 mM). Rat liver microsomal APAP UGT activity is induced by in vivo treatment with beta-naphthoflavone or oltipraz, an effect correlating with induction of 1A6 and 1A7. To address a possible role of 1A7 in APAP glucuronidation relative to other 1A forms, cDNAs encoding UGTs 1A1, 1A5, 1A6, 1A7, and 1A8 were expressed in human embryonic kidney cells and the contents of expressed enzyme in prepared membrane fractions determined by quantitative immunoblotting. At 2.5 mM APAP, 1A7 showed the highest specific activity (2.8 nmol/min/nmol 1A7 protein), followed by 1A6 (1.1 nmol/min/nmol), and 1A8 (0.27 nmol/min/nmol). 1A1 and 1A5 were essentially inactive. Kinetic comparisons indicated 1A7 had a similar apparent K(m) as 1A6 (4.7 versus 3.9 mM, respectively) but a 2.4-fold higher catalytic activity. These data suggest that in rats, 1A7 plays a major role in APAP glucuronidation and contributes to protection against APAP-induced hepatotoxicity. The involvement of other UGTs besides 1A6 is further underscored by the presence of significant residual APAP-glucuronidating activity by Gunn rat hepatocytes, indicating the activity of an unknown UGT2 family member.

Increase in rat liver UDP-glucuronosyltransferase mRNA by microsomal enzyme inducers that enhance thyroid hormone glucuronidation

Treatment of rats with the microsomal enzyme inducers pregnenolone-16alpha-carbonitrile (PCN), 3-methylcholanthrene (3-MC), and Aroclor 1254 [PCB (polychlorinated biphenyl)] has been shown to decrease circulating levels of thyroid hormones as well as increase microsomal glucuronidation of thyroxine (T(4)). In addition, PCN increases triiodothyronine (T(3)) uridine diphosphate glucuronosyltransferase (UGT) activity. Members of the UGT1A family are believed to glucuronidate T(4), specifically UGT1A1 and UGT1A6, whereas the UGT2 family is believed to glucuronidate T(3), namely UGT2B2. The purpose of this study was to determine whether the aforementioned microsomal enzyme inducers increase the mRNAs that encode these and other UGT enzymes in rat liver. Male Sprague-Dawley rats were fed a control diet or a diet containing PCN (1000 ppm), 3-MC (250 ppm), or PCB (100 ppm) for 7 days, at which time livers were collected. Increases in mRNA were detected by QuantiGene branched DNA signal amplification. A 3-fold increase in UGT1A1 mRNA was produced by PCN in addition to increases in UGT1A2 (4-fold) and UGT1A5 (2-fold) mRNA. PCN affected neither UGT2B2 nor any other UGT2B mRNA level. 3-MC and PCB increased UGT1A6 mRNA 6- and 4-fold, respectively. 3-MC and PCB each increased UGT1A7 mRNA 4-fold but did not significantly increase any other UGT mRNAs. These findings suggest that PCN enhances T(4) UGT activity by increased expression of UGT1A1 and that 3-MC and PCB enhance T(4) UGT activity by increased expression of UGT1A6. These findings also suggest that increased T(3) UGT activity produced by PCN is due to a mechanism other than increased transcription of UGT2B2, possibly increased UGT2B2 protein or induction of another UGT enzyme.

Simultaneous expression of guinea pig UDP-glucuronosyltransferase 2B21 and 2B22 in COS-7 cells enhances UDP-glucuronosyltransferase 2B21-catalyzed morphine-6-glucuronide formation

Although UDP-glucuronosyltransferases (UGTs) act as an important detoxification system for many endogenous and exogenous compounds, they are also involved in the metabolic activation of morphine to form morphine-6-glucuronide (M-6-G). The cDNAs encoding guinea pig liver UGT2B21 and UGT2B22, which are intimately involved in M-6-G formation, have been cloned and characterized. Although some evidence suggests that UGTs may function as oligomers, it is not known whether hetero-oligomer formation leads to differences in substrate specificity. In this work, evidence for a functional hetero-oligomer between UGT2B21 and UGT2B22 is provided by studies on the glucuronidation of morphine in transfected COS-7 cells. Cells transfected with UGT2B21 cDNA catalyzed mainly morphine-3-glucuronide formation although M-6-G was also formed to some extent. In contrast, cells transfected with UGT2B22 cDNA did not show any significant activity toward morphine. When UGT2B21 and UGT2B22 were expressed simultaneously in different ratios in COS-7 cells, extensive M-6-G formation was observed. This stimulation of M-6-G formation was not observed, however, when microsomes containing UGT2B21were mixed with those containing UGT2B22 in the presence of detergent. Furthermore, this effect was not very marked when human UGT1A1 and UGT2B21 were coexpressed in COS-7 cells. This is the first report suggesting that UGT hetero-oligomer formation leads to altered substrate specificity.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Determination of the kinetics of rat UDP-glucuronosyltransferases (UGTs) in liver and intestine using HPLC

Uridinediphosphoglucuronosyl transferases (UGTs) are a group of membrane bound proteins which catalyze the transfer of glucuronic acid from UDP-glucuronic acid to a wide variety of xenobiotics and drug molecules enabling them to be eliminated. The major UGT isoforms found in the rat are 1A1, 1A6, 2B1 and 2B12. Conventional methods for the assay of glucuronides (GLs) include TLC, extraction and colorimetry or quantification of the aglycone, liberated after hydrolyzing the GL with beta-glucuronidase. However these techniques cannot distinguish between isomeric GLs or GLs of multiple acceptor site substrates. Therefore the purpose of this study was to develop simple and sensitive HPLC methods for the direct and simultaneous analysis of the GL(s) and their aglycones without the drawbacks of the conventional methods. The three classical substrates we chose were 4-methylumbelliferone (4MU), testosterone (TES) and 8-hydroxyquinoline (8HOQ) representing UGT isoforms 1A6, 2B1 and 2B12 of the rat family, respectively. Here we report the validated HPLC conditions, for the detection and separation of 4-methylumbelliferone glucuronide (4MUG), testosterone glucuronide (TESG) and 8-hydroxyquinoline glucuronide (8HOQG) and their aglycones in incubation media containing male Sprague-Dawley rat liver and intestinal microsomal preparations. The separations were achieved on a Zorbax SB-CN column (150 x 4.6 mm, 5 micron). The analysis time for the separation of TES, 8HOQ and 4MU and their glucuronides were 17, 12 and 30 min, respectively. The methods showed excellent linearity (r2 > 0.99) over the concentration ranges tested (0.25-5.0 nmoles of TESG; 0.125-18.75 nmoles of 8HOQG and 0.125-12.5 nmoles of 4MUG), good precision and accuracy (RSD<2.5%). Inter-day variability studies (n = 3) showed no significant difference between the regression lines obtained on the three days. Recoveries were good ( > 90%) at all three points (low, mid-point, high) of the standard curve. The limits of detection were 0.125, 0.1 and 0.1 nmole for TESG, 8HOQG and 4MUG. respectively. The above methods were used to estimate kinetic parameters such as Vmax and Km for the GLs of the three substrates in both liver and intestinal tissue preparations and the values were comparable with previously reported results. UGT2B1 was found primarily in the liver while UGTs 1A6 and 2B12 were present in comparable amounts in both tissues.

Stereoselective metabolism of the monoterpene carvone by rat and human liver microsomes

The large amounts of carvone enantiomers consumed as food additives and in dental formulations justifies the evaluation of their biotransformation pathway. The in-vitro metabolism of R-(-)- and S-(+)-carvone was studied in rat and human liver microsomes using chiral gas chromatography. Stereoselective biotransformation was observed when each enantiomer was incubated separately with liver microsomes. 4R, 6S-(-)-Carveol was NADPH-dependently formed from R-(-)-carvone, whereas 4S, 6S-(+)-carveol was produced from S-(+)-carvone. Metabolite formation followed Michaelis-Menten kinetics exhibiting a significant lower apparent Km (Michaelis-Menten Constant) for 4R, 6S-(-)-carveol compared with 4S, 6S-(+)-carveol in rat and human liver microsomes (28.4+/-10.6 microM and 69.4+/-10.3 microM vs 33.6+/-8-55 microM and 98.3+/-22.4 microM). The maximal formation rate (Vmax) determined in the same microsomal preparations yielded 30.2+/-5.0 and 32.3+/-3.9 pmol (mg protein)(-1) min(-1) in rat liver and 55.3+/-5.7 and 65.2+/-4.3 pmol (mg protein)(-1) min(-1) in human liver microsomes. Phase II conjugation of the carveol isomers by rat and human liver microsomes in the presence of UDPGA (uridine S'-diphosphogluaronic acid) only revealed glucuronidation of 4R, 6S-(-)-carveol. Vmax for glucuronide formation was more than 4-fold higher in the rat liver compared with human liver preparations (185.9+/-34.5 and 42.6+/-7.1 pmol (mg protein)(-1) min(-1), respectively). Km values, however, showed no species-related difference (13.9+/-4.1 microM and 10.2+/-2.2 microM). This study demonstrated stereoselectivity in phase-I and phase-II metabolism for R-(-)- and S-(+)-carvone and might be predictive for carvone biotransformation in man.

Conserved domains of glycosyltransferases

Glycosyltransferases catalyze the synthesis of glycoconjugates by transferring a properly activated sugar residue to an appropriate acceptor molecule or aglycone for chain initiation and elongation. The acceptor can be a lipid, a protein, a heterocyclic compound, or another carbohydrate residue. A catalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzyme. To elucidate the structural requirements for substrate recognition and catalytic reactions of glycosyltransferases, we have searched the databases for homologous sequences, identified conserved amino acid residues, and proposed potential domain motifs for these enzymes. Depending on the configuration of the anomeric functional group of the glycosyl donor molecule and of the resulting glycoconjugate, all known glycosyltransferases can be divided into two major types: retaining glycosyltransferases, which transfer sugar residue with the retention of anomeric configuration, and inverting glycosyltransferases, which transfer sugar residue with the inversion of anomeric configuration. One conserved domain of the inverting glycosyltransferases identified in the database is responsible for the recognition of a pyrimidine nucleotide, which is either the UDP or the TDP portion of a donor sugar-nucleotide molecule. This domain is termed "Nucleotide Recognition Domain 1 beta," or NRD1 beta, since the type of nucleotide is the only common structure among the sugar donors and acceptors. NRD1 beta is present in 140 glycosyltransferases. The central portion of the NRD1 beta domain is very similar to the domain that is present in one family of retaining glycosyltransferases. This family is termed NRD1 alpha to designate the similarity and stereochemistry of sugar transfer, and it consists of 77 glycosyltransferases identified thus far. In the central portion there is a homologous region for these two families and this region probably has a catalytic function. A third conserved domain is found exclusively in membrane-bound glycosyltransferases and is termed NRD2; this domain is present in 98 glycosyltransferases. All three identified NRDs are present in archaebacterial, eubacterial, viral, and eukaryotic glycosyltransferases. The present article presents the alignment of conserved NRD domains and also presents a brief overview of the analyzed glycosyltransferases which comprise about 65% of all known sugar-nucleotide dependent (Leloir-type) and putative glycosyltransferases in different databases. A potential mechanism for the catalytic reaction is also proposed. This proposed mechanism should facilitate the design of experiments to elucidate the regulatory mechanisms of glycosylation reactions. Amino acid sequence information within the conserved domain may be utilized to design degenerate primers for identifying DNA encoding new glycosyltransferases.

Antiviral properties of isoborneol, a potent inhibitor of herpes simplex virus type 1

Isoborneol, a monoterpene and a component of several plant essential oils, showed dual viricidal activity against herpes simplex virus 1 (HSV-1). First, it inactivated HSV-1 by almost 4 log10 values within 30 min of exposure, and second, isoborneol at a concentration of 0.06% completely inhibited viral replication, without affecting viral adsorption. Isoborneol did not exhibit significant cytotoxicity at concentrations ranging between 0.016% and 0.08% when tested against human and monkey cell lines. Isoborneol specifically inhibited glycosylation of viral polypeptides based on the following data: (1) the mature fully glycosylated forms of two viral glycoproteins gB and gD were not detected when the virus was replicated in the presence of isoborneol, (2) no major changes were observed in the glycosylation pattern of cellular polypeptides between untreated and isoborneol treated Vero cells, (3) isoborneol did not affect the glycosylation of gB produced from a copy of the gB gene resident in the cellular genome, and (4) other monoterpenes such as 1,8-cineole and borneol, a stereoisomer of isoborneol, did not inhibit HSV-1 glycosylation.

Isolation and characterization of a simian UDP-glucuronosyltransferase UGT2B18 active on 3-hydroxyandrogens

A monkey cDNA, UGT2B18, encoding a UDP-glucuronosyltransferase (UGT) active on 3-hydroxyandrogens, has been isolated and characterized. Previous results suggested that the monkey represents the most appropriate animal model for studying the physiologic relevance of steroid UGTs. UGT2B18 was isolated from a cynomolgus monkey prostate cDNA library using human UGT2B7, UGT2B10 and UGT2B15 cDNA as probes. The cDNA is 1748 bp in length and contains an open reading frame of 1587 bp encoding a protein of 529 residues. The UGT2B18 cDNA clone was transfected into HK293 cells and a stable cell line expressing UGT2B18 protein was established. Western blot analysis of the UGT2B18-HK293 cell line using a human UGT2B17 polyclonal antibody (EL-93) revealed high expression of a 53 kDa UGT2B protein. The transferase activity of UGT2B18 was tested with over 60 compounds and was demonstrated to be principally active on C19 steroids having an hydroxyl group at position 3alpha of the steroid molecule. UGT2B18 was also active on planar phenols and bile acids. Kinetic analysis revealed that UGT2B18 glucuronidates 3-hydroxyandrogens with high velocity and affinity. Using cell homogenates, Km values of 5.1, 7.8 and 23 microM for androsterone (ADT), etiocholanolone and androstane-3alpha, 17beta diol (3alpha-diol) were obtained, respectively. Specific RT-PCR analysis demonstrated the expression of UGT2B18 transcripts in several tissues including liver, prostate, kidney, testis, adrenal, bile duct, bladder, colon, small intestine, cerebellum and pancreas suggesting a contribution of this isoenzyme to the high plasma levels of glucuronidated ADT and 3alpha-diol found in the cynomolgus monkey.

Expression and characterization of a novel UDP-glucuronosyltransferase, UGT2B9, from cynomolgus monkey

Uridine diphosphate glucuronosyltransferases (UGTs) are important phase II detoxification enzymes. Despite the expression of UGT proteins in many species, previous results have suggested that simians represent the most appropriate animal model to study the glucuronidation of steroids in extrahepatic steroid target tissues. Northern blot analysis using a pool of human UGT2B cDNA probes demonstrated the expression of homologous UGT2B transcripts in several tissues including the liver, kidney, adrenal, breast, testis, and prostate of the cynomolgus monkey (Macacafascicularis). Western blot analyses using a polyclonal antibody raised against human UGT2B17 protein also demonstrated expression of homologous UGT2B proteins in monkey tissues. cDNA libraries were constructed from monkey liver and prostate mRNA and a novel UGT2B cDNA, UGT2B9, was isolated from both libraries. The UGT2B cDNA from the prostate library is 2,648 bp in length and contains an open reading frame of 1,587 bp encoding a protein of 529 residues. In vitro transcription/translation of the cDNA clone produced a protein of 52 kD. The UGT2B9 cDNA clone was transfected into HK293 cells and a stable cell line expressing UGT2B9 protein was established. The activity of UGT2B9 was tested with over 60 compounds and was demonstrated to be active on C18, C19, and C21 steroids, bile acids, and several xenobiotics including eugenol, 1-naphthol, and p-nitrophenol. Kinetic analysis revealed that UGT2B9 glucuronidates steroids with high affinity and efficiency with Km values of 0.2, 3.2, 0.2, and 1.8 microM for dihydrotestosterone, testosterone, androsterone, and 1,3,5,10-estratrien-3,4-diol-17-one, respectively. It is apparent that this simian UGT2B enzyme is specific for more different classes of steroids than any other UGT enzyme characterized to date, and may be related to the high plasma levels of glucuronidated C19 steroids found in the cynomolgus monkey.

High induction of phenol UDP-glucuronosyltransferase in the kidney medulla of beta-naphthoflavone-treated rats

Phenol UDP-glucuronosyltransferase activity was highly induced in the microsomes of the kidney medulla of rats by beta-naphthoflavone treatment. In the medulla, phenol UDP-glucuronosyltransferase and its mRNA were greatly increased in both immunoblotting and Northern blot analyses following beta-naphthoflavone treatment of the rats. In untreated rat kidneys, phenol UDP-glucuronosyltransferase was detected by immunohistochemical analysis only in proximal convolution tubular cells located in the cortex. After beta-naphthoflavone treatment of the rats, UDP-glucuronosyltransferase appeared in the epithelial cells in the straight portion of the distal tubules located in the medulla. In conclusion, the medullary distal tubular cells have high latent glucuronidation activity and are thought to play an important role in drug excretion.

Intestinal UDP-glucuronosyltransferase activities in rat and rabbit

1. Tissues other than the liver can contribute significantly to the drug-metabolizing capacity of an animal. In the current study, the glucuronidation of several aglycones in microsomes from the small intestinal mucosa of rat and rabbit has been investigated and compared with glucuronidation in liver microsomes. 2. UDP-glucuronosyltransferase activities in intestinal microsomes were generally higher in rabbit when compared with rat, ranging from 200 to 300% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl. 3. In contrast, hepatic activities were much higher in rat than in rabbit, ranging from 300 to 400% for 1-naphthol, 2-naphthol, 4-methylumbelliferone, 2-hydroxybiphenyl and testosterone; and from 150 to 250% for 4-nitrophenol and diclofenac. 4. In rabbit, activities in the small intestinal mucosa were comparable (70-100%) with hepatic activities for most aglycones. In rat, intestinal mucosa activities were much lower than in liver, with activities toward 1-naphthol, 2-naphthol, 4-nitrophenol, 4-methylumbelliferone, 2-hydroxybiphenyl and 4-hydroxybiphenyl in the small intestine representing 5-15% of hepatic activities. 5. With a higher intestine:liver activity ratio, intestinal UDP-glucuronosyltransferases could be anticipated to contribute more to overall drug glucuronidation in rabbit as compared with rat, thereby contributing more to reducing drug bioavailability.

Selective induction of phase II drug metabolizing enzyme activities by quinolines and isoquinolines

Rats treated with quinoline, and to a lesser extent, isoquinoline (75 mg/kg, daily for 3 days) showed induction of phase II drug metabolizing enzyme activities without inducing either cytochrome P450 concentration or CYP1A-, CYP2B-, CYP2E-, and CYP3A-selective activities. Elevations of UDP-glucuronosyltransferase activities towards 4-nitrophenol, 1-naphthol, and morphine elicited by quinoline (1.9- to 2.7-fold), were greater than those elicited by isoquinoline (1.4- to 1.8-fold). UDP-glucuronosyltransferase activities towards estrone and testosterone were not increased by either compound. Microsomal epoxide hydrolase activity was increased only by quinoline (2.7-fold). NAD(P)H quinone oxidoreductase activity was increased 2-fold by quinoline and isoquinoline. Cytosolic glutathione S-transferase (GST) activity was increased similarly (approximately 20%) by both agents. Similar treatment of rats with either quinine (75 mg/kg) or chloroquine (150 mg/kg) increased 1-naphthol glucuronidation and GST (quinine only) activities. At 75 mg/kg, chloroquine did not affect any phase II enzyme activities but caused a minor elevation of a phase I enzyme, CYP1A; ascertained from an elevation of 7-ethoxyresorufin deethylase activity and a small hypsochromic shift to the absorbance maximum of the cytochrome P450 CO-complex. With quinoline and isoquinoline treatments (n = 14), the correlation coefficients (R) between microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities towards 4-nitrophenol and morphine were 0.96 and 0.92 respectively, suggesting a highly coordinated induction. The highest NAD(P)H quinone oxidoreductase correlations were with microsomal epoxide hydrolase and UDP-glucuronosyltransferase activities towards 4-nitrophenol and morphine (R approximately 0.78). Correlation coefficients between GST and microsomal epoxide hydrolase and NAD(P)H quinone oxidoreductase activities were approximately 0.49. Quinoline and isoquinoline, nitrogen heterocyclic analogs of naphthalene, join the list of simple nitrogen-containing polycyclic aromatic agents capable of selective induction of phase II drug metabolizing enzymes. The position of the single heterocyclic nitrogen atom in the bicyclic ring influences the magnitude and breadth of the induction response. The addition of bulky ring substituents (quinine, chloroquine) reduced the induction response.