Molecular cloning and functional analysis of cynomolgus monkey CYP1A2

Phase I Metabolism of Pterostilbene, a Dietary Resveratrol Derivative: Metabolite Identification, Species Differences, Isozyme Contribution, and Further Bioactivation

Pterostilbene (PTE), a dietary derivative of resveratrol, displayed pleiotropic health-promoting activities. This study aimed to explore the metabolic profiles and species differences of the phase I metabolism of PTE and to investigate subsequent detoxification after PTE bioactivation. PTE was found to be biotransformed to two pharmacologically active metabolites, pinostilbene and 3'-hydroxypterostilbene, in vivo and in vitro with substantial species differences. Human CYP1A2 was proved to be mainly responsible for the demethylation and 3'-hydroxylation of PTE, with its contribution to a demethylation of 94.5% and to a 3'-hydroxylation of 97.9%. An in vitro glutathione trapping experiment revealed the presence of an ortho-quinone intermediate formed by further oxidation of 3'-hydroxypterostilbene. Human glutathione S-transferase isoforms A2, T1, and A1 inactivated the ortho-quinone intermediate by catalyzing glutathione conjugation, implicating a potential protective pathway against PTE bioactivation-derived toxicity. Overall, this study provided a comprehensive view of PTE phase I metabolism and facilitated its further development as a promising nutraceutical.

Polymorphic cytochromes P450 in non-human primates

Cynomolgus macaques (Macaca fascicularis, an Old World monkey) are widely used in drug development because of their genetic and physiological similarities to humans, and this trend has continued with the use of common marmosets (Callithrix jacchus, a New World monkey). Information on the major drug-metabolizing cytochrome P450 (CYP, P450) enzymes of these primate species indicates that multiple forms of their P450 enzymes have generally similar substrate selectivities to those of human P450 enzymes; however, some differences in isoform, activity, and substrate specificity account for limited species differences in drug oxidative metabolism. This review provides information on the P450 enzymes of cynomolgus macaques and marmosets, including cDNA, tissue expression, substrate specificity, and genetic variants, along with age differences and induction. Typical examples of important P450s to be considered in drug metabolism studies include cynomolgus CYP2C19, which is expressed abundantly in liver and metabolizes numerous drugs. Moreover, genetic variants of cynomolgus CYP2C19 affect the individual pharmacokinetic data of drugs such as R-warfarin. These findings provide a foundation for understanding each P450 enzyme and the individual pharmacokinetic and toxicological results in cynomolgus macaques and marmosets as preclinical models. In addition, the effects of induction on some drug clearances mediated by P450 enzymes are also described. In summary, this review describes genetic and acquired individual differences in cynomolgus and marmoset P450 enzymes involved in drug oxidation that may be associated with pharmacological and/or toxicological effects.

Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands

Tracer development for positron emission tomography (PET) requires thorough evaluation of pharmacokinetics, metabolism, and dosimetry of candidate radioligands in preclinical animal studies. Since variations in pharmacokinetics and metabolism of a compound occur in different species, careful selection of a suitable model species is mandatory to obtain valid data. This study focuses on species differences in the in vitro metabolism of three xanthine-derived ligands for the A₁ adenosine receptor (A₁AR), which, in their ¹⁸F-labeled form, can be used to image A₁AR via PET. In vitro intrinsic clearance and metabolite profiles of 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), an established A₁AR-ligand, and two novel analogs, 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX), were determined in liver microsomes from humans and preclinical animal species. Molecular mechanisms leading to significant differences between human and animal metabolite profiles were also examined. The results revealed significant species differences regarding qualitative and quantitative aspects of microsomal metabolism. None of the tested animal species fully matched human microsomal metabolism of the three A₁AR ligands. In conclusion, preclinical evaluation of xanthine-derived A₁AR ligands should employ at least two animal species, preferably rodent and dog, to predict in vivo behavior in humans. Surprisingly, rhesus macaques appear unsuitable due to large differences in metabolic activity towards the test compounds.

The marmoset cytochrome P450 superfamily: Sequence/phylogenetic analyses, genomic structure, and catalytic function

The common marmoset (Callithrix jacchus) is a New World monkey that has attracted much attention as a potentially useful primate model for preclinical testing. A total of 36 marmoset cytochrome P450 (P450) isoforms in the P450 1-51 subfamilies have been identified and characterized by the application of genome analysis and molecular functional characterization. In this mini-review, we provide an overview of the genomic structures, sequence identities, and substrate selectivities of marmoset P450s compared with those of human P450s. Based on the sequence identity, phylogeny, and genomic organization of marmoset P450s, orthologous relationships were established between human and marmoset P450s. Twenty-four members of the marmoset P450 1A, 2A, 2B, 2C, 2D, 2E, 3A, 4A, and 4F subfamilies shared high degrees of homology in terms of cDNA (>89%) and amino acid sequences (>85%) with the corresponding human P450s; P450 2C76 was among the exceptions. Phylogenetic analysis using amino acid sequences revealed that marmoset P450s in the P450 1-51 families were located in the same clades as their human and macaque P450 homologs. This finding underlines the evolutionary closeness of marmoset P450s to their human and macaque homologs. Most marmoset P450 1-4 enzymes catalyzed the typical drug-metabolizing reactions of the corresponding human P450 homologs, except for some differences of P450 2A6 and 2B6. Consequently, it appears that the substrate specificities of enzymes in the P450 1-4 families are generally similar in marmosets and humans. The information presented here supports a better understanding of the functional characteristics of marmoset P450s and their similarities and differences with human P450s. It is hoped that this mini-review will facilitate the successful use of marmosets as primate models in drug metabolism and pharmacokinetic studies.

CpG site degeneration triggered by the loss of functional constraint created a highly polymorphic macaque drug-metabolizing gene, CYP1A2

Background

Elucidating the pattern of evolutionary changes in drug-metabolizing genes is an important subject not only for evolutionary but for biomedical research. We investigated the pattern of divergence and polymorphisms of macaque CYP1A1 and CYP1A2 genes, which are major drug-metabolizing genes in humans. In humans, CYP1A2 is specifically expressed in livers while CYP1A1 has a wider gene expression pattern in extrahepatic tissues. In contrast, macaque CYP1A2 is expressed at a much lower level than CYP1A1 in livers. Interestingly, a previous study has shown that Macaca fascicularis CYP1A2 harbored unusually high genetic diversity within species. Genomic regions showing high genetic diversity within species is occasionally interpreted as a result of balancing selection, where natural selection maintains highly diverged alleles with different functions. Nevertheless many other forces could create such signatures.

Results

We found that the CYP1A1/2 gene copy number and orientation has been highly conserved among mammalian genomes. The signature of gene conversion between CYP1A1 and CYP1A2 was detected, but the last gene conversion event in the simian primate lineage occurred before the Catarrhini-Platyrrhini divergence. The high genetic diversity of macaque CYP1A2 therefore cannot be explained by gene conversion between CYP1A1 and CYP1A2. By surveying CYP1A2 polymorphisms in total 91 M. fascicularis and M. mulatta, we found several null alleles segregating in these species, indicating functional constraint on CYP1A2 in macaques may have weakened after the divergence between humans and macaques. We propose that the high genetic diversity in macaque CYP1A2 is partly due to the degeneration of CpG sites, which had been maintained at a high level by purifying selection, and the rapid degeneration process was initiated by the loss of functional constraint on macaque CYP1A2.

Conclusions

Our findings show that the highly polymorphic CYP1A2 gene in macaques has not been created by balancing selection but by the burst of CpG site degeneration after loss of functional constraint. Because the functional importance of CYP1A1/2 genes is different between humans and macaques, we have to be cautious in extrapolating a drug-testing data using substrates metabolized by CYP1A genes from macaques to humans, despite of their somewhat overlapping substrate specificity.

Expression of cytochromes p450 in fetal, infant, and juvenile liver of cynomolgus macaques

Preclinical data of fetal, infant, and juvenile animals are important for the prediction of drug toxicity in fetuses and children. However, expression of drug-metabolizing enzymes, including cytochromes P450 (CYPs), have not been fully investigated in fetal, infant, or juvenile liver of the cynomolgus macaque, an animal species important for preclinical studies. In this study, hepatic expression of 20 cynomolgus macaque CYPs (mfCYPs) in the CYP1-4 subfamilies that are relevant to drug metabolism was measured in fetuses, infants, and juveniles using DNA microarrays. Expression of most mfCYPs, including those moderately or abundantly expressed in postnatal livers such as mfCYP2A23, mfCYP2A24, mfCYP2B6, mfCYP2C9, mfCYP2C19, mfCYP2C76, mfCYP2D17, mfCYP2E1 mfCYP3A4, and mfCYP3A5, was much less abundant in fetal livers, but increased substantially after birth. In contrast, expression of mfCYP2C8 in fetal livers was not substantially different from postnatal livers. Since human CYP3A7 is expressed more abundantly in fetal livers than in adult livers, mfCYP3A7, an ortholog of human CYP3A7, was analyzed by quantitative polymerase chain reaction. Expression of mfCYP3A7 in fetal livers was much lower than that in postnatal livers, and greatly increased after birth, unlike the expression of human CYP3A7. These results indicate that expression of most mfCYPs examined was low in fetal livers, but increased greatly in postnatal livers, with a few exceptions such as mfCYP2C8.

Regional distribution of cytochrome p450 mRNA expression in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is used to study drug metabolism because of its evolutionary closeness to humans. Despite their importance, regional distribution of cytochrome P450 (CYP) enzymes including CYP3As in the liver and small intestine, the major sites of drug metabolism, has not been fully investigated in cynomolgus monkeys. In this study, we measured mRNA expression levels of 14 CYPs in the CYP1, 2, and 3 subfamilies, including orthologs of human CYP3A4 and CYP3A5, in the liver and small intestine of cynomolgus monkeys. Expression levels of each CYP mRNA in various regions of the liver were quantified and comparisons were made between the right lobe, quadrate lobe, left medial lobe, left lateral lobe, and caudate lobe and with four different sections of the right lobe. In the small intestine, the same mRNAs were measured in the duodenum and six different sections from the proximal jejunum to the distal ileum. Expression levels of the CYP mRNAs were not substantially different between liver samples, but varied between the different sections of the small intestine, including CYP3A4. These results suggest that analysis of distinct sections is required for a better understanding of cynomolgus monkey CYPs in the small intestine.

Cynomolgus monkey CYPs: a comparison with human CYPs

Characteristics of twelve cytochromes P450 (CYPs) from cynomolgus monkeys were compared with those of human CYPs that play an important role in drug metabolism. Eleven members of CYP1A, CYP2A, CYP2C, CYP2D, CYP2E, and CYP3A subfamilies from cynomolgus monkeys exhibited a high degree of homologies (more than 90%) in cDNA and amino acid sequences with corresponding human CYPs, and catalysed typical reactions of corresponding human CYPs. One member of the cynomolgus monkey CYP2C subfamily, CYP2C76, exhibited a lower homology (around 70%) in amino acid sequences with other cynomolgus monkey and human CYP2C subfamilies. CYP2C76 catalysed typical CYP2C substrates with low activities, and has not been found in humans. CYPs identified in cynomolgus monkeys were similar to CYP1A1, CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 in humans. These results indicate that cynomolgus monkeys express CYPs similar to human CYPs that are important in drug metabolism.

Differentiation of monkey embryonic stem cells into hepatocytes and mRNA expression of cytochrome p450 enzymes responsible for drug metabolism: comparison of embryoid body formation conditions and matrices

We investigated the effects of embryoid body (EB) forming conditions on the expression of hepatocyte marker genes such as alpha-fetoprotein, albumin and CYP7A1 in cells cultured on Matrigel-coated plates for 15 d. The expression levels of hepatocyte marker genes in the cells cultured for 2 d for EB formation from cynomolgus monkey embryonic stem (cmES) cells was higher than those in cells cultured for 5 d. However, the fragment-size of cmES colonies did not markedly affect the expression levels. The expression levels of hepatocyte marker genes, and CYP1A1 and CYP2C43 in cells cultured on Matrigel were considerably higher than those on Matrigel reduced and collagen I. CYP1A1 and CYP3A8 mRNAs were significantly induced by 3-methylcholanthrene and rifampicin, respectively. However, CYP2C43 and CYP2D17 were not induced by these compounds. These results suggested that the differentiation into hepatocytes is affected by the incubation period for EB formation, and that Matrigel successfully promoted in vitro differentiation of cmES cells to hepatocytes.

Comparison of inducibility of CYP1A and CYP3A mRNAs by prototypical inducers in primary cultures of human, cynomolgus monkey, and rat hepatocytes

This study was conducted to investigate the effects of treatment with the prototypical inducers rifampicin (Rif), dexamethasone (Dex), and omeprazole (Ome) on the mRNA levels of drug-metabolizing enzymes in primary cultures of cryopreserved human, cynomolgus monkey, and rat hepatocytes. Analysis was performed by quantitative real-time RT-PCR using primers and TaqMan probes. Treatment with Ome substantially increased the mRNA levels of both CYP1A1 and CYP1A2 in human hepatocytes, but increased only the mRNA level of CYP1A1 in monkey hepatocytes, whereas it had no marked effect on the mRNA levels of CYP1A1 or CYP1A2 in rat hepatocytes. Treatment with Rif or Dex did not markedly affect the mRNA level of CYP1A in any of the hepatocyte cultures under the conditions used. All three inducers increased the mRNA level of CYP3A8 in monkey hepatocytes (in the order Rif>Dex>or=Ome), and a similar profile was observed for the mRNA level of CYP3A4 in human hepatocytes, but the potency of induction was markedly attenuated. In contrast, only Dex substantially increased the mRNA level of CYP3A1 in rat hepatocytes, with Rif and Ome showing no effects. These results indicate that the molecular mechanisms responsible for the regulation of CYP1A2 genes differ between humans and cynomolgus monkeys, although the regulatory mechanisms for CYP1A1 and CYP3A genes are similar.

Species differences between mouse, rat, dog, monkey and human CYP-mediated drug metabolism, inhibition and induction

Animal models are commonly used in the preclinical development of new drugs to predict the metabolic behaviour of new compounds in humans. It is, however, important to realise that humans differ from animals with regards to isoform composition, expression and catalytic activities of drug-metabolising enzymes. In this review the authors describe similarities and differences in this respect among the different species, including man. This may be helpful for drug researchers to choose the most relevant animal species in which the metabolism of a compound can be studied for extrapolating the results to humans. The authors focus on CYPs, which are the main enzymes involved in numerous oxidative reactions and often play a critical role in the metabolism and pharmacokinetics of xenobiotics. In addition, induction and inhibition of CYPs are compared among species. The authors conclude that CYP2E1 shows no large differences between species, and extrapolation between species appears to hold quite well. In contrast, the species-specific isoforms of CYP1A, -2C, -2D and -3A show appreciable interspecies differences in terms of catalytic activity and some caution should be applied when extrapolating metabolism data from animal models to humans.

The relationship among microsomal enzyme induction, liver weight, and histological change in cynomolgus monkey toxicology studies

The purpose of this investigation was to examine the relationship among hepatic microsomal enzyme induction, liver weight, histological evidence of hepatic injury, and serum clinical chemistry markers of hepatic origin in the cynomolgus monkey. We report here the results from independent toxicology studies for 10 investigative drug candidates representing four therapeutic classes. Study conditions were selected to elicit target organ toxicity. We found that six of the 10 compounds altered cytochrome P450-associated activities in both male and female monkeys, two in females only, and one altered similar activities in males only. Frequently, significant treatment-related elevations in NADPH cytochrome c reductase and ethylmorphine N-demethylase were noted. When the results from all 10 studies were pooled, 14 cytochrome P450-associated activities were significantly increased and five were decreased in males compared to 15 significantly increased and three decreased in the females. Treatment-associated liver weight increases were noted in four studies. Except for hepatocellular hypertrophy in one study, no significant treatment-related microscopic changes in liver and no elevations of serum biomarkers commonly associated with liver toxicity were observed in any of the studies that demonstrated significant hepatic enzyme induction. Compared to parallel rat studies, one compound was an inducer only in monkeys and one was an inducer only in rats. Significant elevations of microsomal drug-metabolizing enzymes in the cynomolgus monkey liver are not accompanied by substantial hepatic changes except for hepatomegaly. These alterations in the hepatic drug-metabolizing enzyme system were benign based the absence of histopathological lesions and serum biomarkers of hepatobiliary toxicity.

Antagonism of TCDD-induced ethoxyresorufin-O-deethylation activity by polybrominated diphenyl ethers (PBDEs) in primary cynomolgus monkey (Macaca fascicularis) hepatocytes

Polybrominated diphenyl ethers (PBDEs) are widespread environmental pollutants, and the levels of certain congeners have been increasing in biota and abiota in recent decades. Some PBDEs are lipophilic and persistent, resulting in bioaccumulation in the environment. Their structural similarity to other polyhalogenated aromatic hydrocarbons (PHAHs) such as polychlorinated biphenyls (PCBs) has raised concerns that PBDEs might act as agonists for the aryl hydrocarbon receptor (AhR). Recent studies in our laboratory with human and rat cell lines indicated no AhR mediated CYP1A1 induction for PBDEs. However, an earlier in vitro study by Van der Burght et al. (1999) [Van der Burght, A.S., Clijsters, P.J., Horbach, G.J., Andersson, P.L., Tysklind, M., van den Berg, M., 1999. Structure-dependent induction of CYP1A by polychlorinated biphenyls in hepatocytes of cynomolgus monkeys (Macaca fascicularis). Toxicol. Appl. Pharmacol. 155, 13-23] indicated that in cynomolgus monkey (M. fascicularis) hepatocytes PCBs with a non-planar configuration could induce CYP1A. As PBDEs show a structural similarity with non-planar (ortho substituted) PCBs, our present study focused on the possible CYP1A induction by PBDEs (BDE-47, -99, -100, -153, -154, -183, and -77) in individual preparations (n=4) of primary hepatocytes of cynomolgus monkeys (M. fascicularis). 7-Ethoxyresorufin-O-deethylase (EROD) was used as a marker for CYP1A-mediated catalytic activity. Cells were exposed for 48 h to various PBDE concentrations (0.01-10 microM), positive controls 2,3,7,8-TCDD (0.001-2.5 nM) and PCB-126 (0.01-10nM), and negative control (DMSO vehicle alone). No statistically significant induction of CYP1A was observed in the hepatocytes after 48 h of exposure to all environmentally relevant PBDEs. After exposing hepatocytes to PBDEs in combination with TCDD, a concentration-dependent decrease in TCDD-induced EROD activity was observed. All PBDEs tested showed a similar reduction in each of four experiments, though quantitative differences were observed. The observed antagonism of TCDD-induced EROD activity by PBDEs occurred in both male (n=3) and female (n=1) hepatocytes and was not due to catalytic inhibition of EROD activity or cytotoxicity. However, based on the results of this study we do not expect these antagonistic effects of PBDEs on CYP1A induction at environmental relevant levels, since these in vitro interactive effects with TCDD were observed only at relatively high concentrations that are normally not seen, e.g. in the human body.

Molecular cloning and functional analysis of cytochrome P450 1A2 from Japanese monkey liver: comparison with marmoset cytochrome P450 1A2

A cDNA encoding a novel cytochrome P450 1A2 (CYP1A2) was cloned from the liver of an adult female Japanese monkey. The CYP1A2 protein was expressed in yeast cells and its enzymatic properties were compared with those of marmoset CYP1A2 using ethoxyresorufin (ER) and phenacetin (PN) as substrates. The nucleotide sequence of Japanese monkey CYP1A2 revealed 94.7, 99.5 and 93.5% identities to those of human, cynomolgus monkey and marmoset monkey CYP1A2, respectively. Multiple amino acid sequence alignment of Japanese monkey CYP1A2 with CYP1A2 of humans, cynomolgus monkeys and marmosets showed that Japanese monkey CYP1A2 had 92.4, 99.0 and 91.9% identities to the human, cynomolgus monkey and marmoset enzymes, respectively. Kinetic studies demonstrated that the enzymatic properties as ER and PN O-deethylases were considerably different between the Japanese monkey and the marmoset CYP1A2. Furthermore, both of these reactions in liver microsomal fractions from the Japanese monkey and marmoset showed biphasic kinetics. On the basis of the kinetic parameters, it is suggested that Japanese monkey CYP1A2 is a high-K(m) enzyme in both ER and PN O-deethylations, whereas marmoset CYP1A2 is a high-K(m) and low-K(m) enzyme in ER and PN O-deethylations, respectively. alpha-Naphthoflavone, an inhibitor of human CYP1A1 and CYP1A2, did not completely inhibit the liver microsomal oxidations of ER and PN even at the highest concentration (50muM), supporting the notion that CYP1A2 enzymes are not the sole ER or PN O-deethylase in Japanese monkey and marmoset liver microsomes. Inhibitory effects of furafylline, an inhibitor of human CYP1A2, on ER O-deethylation by recombinant CYP1A2 enzymes were much lower than those of alpha-naphthoflavone, but marmoset CYP1A2 was more sensitive to furafylline than Japanese monkey CYP1A2. These results indicate that the properties of Japanese monkey CYP1A2 are considerably different from those of marmoset CYP1A2.

Molecular cloning, expression, and characterization of CYP2D17 from cynomolgus monkey liver

The cynomolgus monkey is a species used in drug-safety evaluation and biotransformation studies by the pharmaceutical industry. Relatively little is known, however, about the catalytic activities and specificities of cytochromes P450 (CYP) in this species. As a first step in characterizing monkey CYPs, a cDNA was cloned by reverse-transcriptase PCR from cynomolgus monkey liver mRNA using oligonucleotide primers based on the human CYP2D6 sequence. The full-length cDNA (called CYP2D17) encoded a 497-amino-acid protein that is 93% identical to human CYP2D6 and 90% identical to marmoset CYP2D19. The CYP2D17 cDNA was cloned into a baculovirus expression vector, and microsomes prepared from CYP2D17-infected insect cells were used to determine the catalytic properties of the recombinant enzyme. The recombinant CYP2D17 results were compared to data generated with monkey liver microsomes, human liver microsomes, and recombinant CYP2D6 and demonstrated catalytic similarity using probe substrates and inhibitors. Recombinant CYP2D17 catalyzed the oxidation of bufuralol to 1'-hydroxybufuralol and dextromethorphan to dextrorphan, reactions shown to be mediated by CYP2D6 in humans; the apparent K(m) values for bufuralol and dextromethorphan were 1 and 0.8 microM, respectively. Moreover, both of these reactions were more strongly inhibited by quinidine than by quinine. A more complete understanding of the substrate specificities and activities of monkey CYPs will be advantageous in delineating species differences in metabolite profiles and metabolic activation of new chemical entities in the pharmaceutical industry.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Photoaffinity labeling of the aglycon binding site of the recombinant human liver UDP-glucuronosyltransferase UGT1A6 with 7-azido-4-methylcoumarin

7-Azido-4-methylcoumarin (AzMC) is a fluorescent photoactive compound structurally related to 4-methylumbelliferone (4-MU), a marker substrate of the human liver recombinant UDP-glucuronosyltransferase (UGT) 1A6. AzMC was synthesized and utilized to label the substrate binding site of UGT1A6. AzMC exhibits a fluorescence spectrum with maximum excitation and emission wavelengths of 380 and 442 nm, respectively. Upon irradiation, the probe irreversibly inhibited glucuronidation activity measured with para-nitrophenol (pNP) as substrate and interacted with UGT1A6 according to a saturable process indicative of reversible binding before covalent incorporation of the photoaffinity label. This inhibition was both time and concentration dependent and led to the calculation of an inhibition constant, k(2) = 0.113 mM min(-1), and dissociation constant, K(d) = 2.89 mM, for the reaction. Partial photoinactivation of UGT1A6 with AzMC revealed that the probe decreased the apparent V(max) of the pNP glucuronidation reaction, but not the K(m). Moreover, inhibition was partially prevented by 1-naphthol, a surrogate substrate for the enzyme, or by preincubation with an active-site directed inhibitor, 5'-O-[[(2-decanoylamino-3-phenyl-propyloxycarbonyl)amino]-su lfonyl]-2 ',3'-O-isopropylideneuridine. In contrast, UDP-glucuronic acid (UDP-GlcUA) did not have any protective effect against photoinactivation and AzMC did not affect the photoaffinity labeling of UGT1A6 by 5-[beta-(32)P]N(3)UDP-GlcUA, a photoaffinity analog of UDP-GlcUA. Additionally, in the absence of irradiation, AzMC was found to be a competitive inhibitor of 4MU glucuronidation. Collectively, these results strongly indicate that AzMC specifically binds to the UGT1A6 aglycon binding site. Amino acid alignment of phenol-binding proteins revealed a conserved motif, YXXXKXXPXP. It is possible that this motif is involved in phenol binding to UGT1A6 and other phenol-accepting proteins.

A comparison of basal and induced hepatic microsomal cytochrome P450 monooxygenase activities in the cynomolgus monkey (Macaca fascicularis) and man

1. The specific activities of hepatic microsomal cortisol 6beta-hydroxylase, coumarin 7-hydroxylase, S-mephenytoin 4'-hydroxylase and phenoxazone hydroxylase and the O-dealkylations of seven homologous alkoxyresorufins were < 3-fold different between the untreated (UT) cynomolgus monkey and man. 2. Heptoxy- and octoxyresorufin O-dealkylase, S-mephenytoin N-demethylase and dextromethorphan O-demethylase specific activities were > 6-fold higher, whereas tolbutamide hydroxylase was almost 5-fold lower in the UT monkey than in man. 3. Phenobarbitone induced (2-6-fold) coumarin 7-hydroxylase, cortisol 6beta-hydroxylase, S-mephenytoin N-demethylase, phenoxazone hydroxylase and benzyloxyresorufin O-dealkylase activities, but not the O-dealkylations of pentoxyresorufin or other alkoxyresorufins, in monkey. 4. Rifampicin induced (2-3-fold) cortisol 6beta-hydroxylase, S-mephenytoin 4'-hydroxylase, S-mephenytoin N-demethylase and tolbutamide hydroxylase activities, the O-dealkylations of methoxy-, ethoxy- and propoxyresorufin and CYP2C- and CYP3A-immunorelated proteins in monkey. 5. Dextromethorphan O-demethylase was significantly reduced by both phenobarbitone and rifampicin treatment in monkey. 6. Beta-naphthoflavone induced (8-39-fold) the O-dealkylations of several alkoxyresorufins, the greatest effect being on propoxyresorufin, but had no effect on the other activities measured in monkey. 7. Constitutive hepatic microsomal CYP2D6-immunorelated proteins were expressed at apparently much higher levels in monkey than in man.