Cytochrome P450IID subfamily in non-human primates. Catalytical and immunological characterization

Cynomolgus monkey CYP2D44 newly identified in liver, metabolizes bufuralol, and dextromethorphan

The cynomolgus monkey is used in drug metabolism studies, because of its evolutionary closeness to human, including cytochrome P450. Cynomolgus monkey CYP2D17, highly homologous to human CYP2D6, has been identified and characterized. Here, we report characterization of another CYP2D, CYP2D44, identified in cynomolgus monkey liver. The CYP2D44 cDNA contained an open reading frame of 497 amino acids sharing high sequence identity (87-93%) with other primate CYP2Ds. CYP2D44 mRNA was predominantly expressed in liver, similar to CYP2D17 mRNA. CYP2D17 and CYP2D44 form a gene cluster in the genome, similar to human CYP2Ds. Metabolic assays of the CYP2D17 and CYP2D44 proteins heterologously expressed in Escherichia coli indicated that CYP2D44 metabolized human CYP2D6 substrates, bufuralol and dextromethorphan (bufuralol 1'-hydroxylation and dextromethorphan O-demethylation) but to a lesser extent than CYP2D17. Kinetic analysis of dextromethorphan metabolism indicated that the apparent K(m) and V(max) of CYP2D17 and CYP2D44 catalyzed O-demethylation were similar, and, the V(max) values of CYP2D17 and CYP2D44 catalyzed N-demethylation (which human CYP2D6 catalyzes much less effectively) were similar, but the apparent K(m) of the CYP2D44 reaction was higher. Western blot analysis showed that CYP2D proteins were expressed in cynomolgus and rhesus monkey liver as well as in human and marmoset liver. Similar to CYP2D6, CYP2D44 copy number varied among the eight cynomolgus monkeys and four rhesus monkeys used in this study. These results indicated that CYP2D44, together with CYP2D17, had functional characteristics similar to those of human CYP2D6 but measurably differed in dextromethorphan N-demethylation, suggesting its importance for CYP2D-dependent drug metabolism in macaque.

Species difference in enantioselectivity for the oxidation of propranolol by cytochrome P450 2D enzymes

We examined and compared enantioselectivity in the oxidation of propranolol (PL) by liver microsomes from humans and Japanese monkeys (Macaca fuscata). PL was oxidized at the naphthalene ring to 4-hydroxypropranolol, 5-hydroxypropranolol and side chain N-desisopropylpropranolol by human liver microsomes with enantioselectivity of [R(+)>S(-)] in PL oxidation rates at substrate concentrations of 10 microM and 1 mM. In contrast, reversed enantioselectivity [R(+)<S(-)] in PL 5-hydroxylation and N-desalkylation rates at the same substrate concentrations was observed in monkey liver microsomes, although the selectivity was the same for PL 4-hydroxylation between the two species. All oxidation reactions of the PL enantiomers in human liver microsomes showed biphasic kinetics, i.e. the reactions could be expressed as the summation of a low-K(m) phase and a high-K(m) phase. Inhibition studies using antibodies and characterization of CYP2D6 enzymes expressed in insect cells or human lymphoblastoid cells indicated that the enantioselectivity of PL oxidation, especially the ring 4- and 5-hydroxylations reflected the properties of CYP2D6 in human liver microsomes. In monkey liver microsomes, all of the oxidation reactions of S(-)-PL showed biphasic kinetics, whereas ring 4- and 5-hydroxylations were monophasic and side chain N-desisopropylation was biphasic for R(+)-PL. Similarly, from the results of inhibition studies using antibodies and inhibitors of cytochrome P450 (P450), it appears that the reversed selectivity [R(+)<S(-)] of PL oxidation rates is catalyzed by CYP2D enzyme(s) in monkey liver at low substrate concentrations. These results indicate that different properties of P450s belonging to the 2D subfamily cause the reversed enantioselectivity between human and monkey liver microsomes.

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.

Comparisons of catalytic selectivity of cytochrome P450 subfamily enzymes from different species

Historically there has been considerable interest in comparing patterns of biotransformation of xenobiotic chemicals in experimental animal models and humans, e.g. in areas such as drug metabolism and chemical carcinogenesis. With the availability of more basic knowledge it has become possible to attribute the oxidation of selected chemicals to individual cytochrome P450 (P450) enzymes in animals and humans. Further, these P450s can be characterized by their classification into distinct subfamilies, which are defined as having > 59% amino acid sequence identity. Questions arise about how similar these enzymes are with regard to structure and function. More practically, how much can be predicted about reaction specificity and catalysis? In order to address these issues, we need to consider not only the relatedness of P450s from different species but also (i) functional similarity within P450 subfamilies and (ii) the effects of small changes imposed by site-directed mutagenesis. Relationships in the P450 1A, 2A, 2B, 2C, 2D, 2E, 3A, and 17A subfamilies are briefly reviewed. Overall functional similarity is generally seen in subfamily enzymes but many examples exist of important changes in catalysis due to very small differences, even a single conservative amino acid substitution. Some general conclusions are presented about predictability within various P450 subfamilies.

Tobacco budworm P-glycoprotein: biochemical characterization and its involvement in pesticide resistance

Since pesticides have been shown to interact with P-glycoprotein (P-gp), the purpose of this study was to examine the possible role of P-gp in pesticide resistance in the tobacco budworm (Heliothis virescens). Using three P-gp antibodies, P-gp expression in various resistant populations of tobacco budworms was found to be 2-6-times that of the susceptible larvae. Tobacco budworm P-gp was glycosylated and localized primarily in the cuticle and fat body with little expression in the mid gut. To determine the role of P-gp in pesticide resistance, resistant tobacco budworm larvae were treated with a P-gp inhibitor, quinidine, and challenged with various doses of thiodicarb. Inhibition of P-gp decreased the LD50 for thiodicarb by a factor of 12.5. Quinidine treatment did not result in a significant inhibition of the P-450 system nor did it alter the feeding of the larvae, suggesting the potential involvement of P-gp in pesticide resistance. An age-dependent increase in P-gp expression was detected in resistant larvae as compared to control, susceptible larvae. This correlates with the reported age-dependent increase in resistance and is further evidence supporting the role of P-gp in the development of pesticide resistance.

Effects of cytochrome P450 inhibitors on potassium currents and mechanical activity in rat portal vein

1. The effects of the cytochrome P450 inhibitors, proadifen, clotrimazole and 17-octadecynoic acid (17-ODYA) on K-currents in freshly-isolated single cells derived from rat portal vein and on mechanical activity in whole veins were studied. 2. When cells were stepped from -90 mV to a series of test potentials (from -80 to +50 mV), a delayed rectifier current (IK(V)) and an A-type current (IK(A)) could be identified. Proadifen (10 microM), clotrimazole (30 microM) and 17-ODYA (5 microM) each inhibited IK(V) but had little effect on IK(A). 3. When cells were held at -10 mV to inactivate the time-dependent K-currents, IK(V) and IK(A), levcromakalim (3 microM) induced a time-independent outward K-current (IK(ATP)) which was totally inhibited by clotrimazole (30 microM) and almost fully inhibited by proadifen (10 microM). 17-ODYA (5 microM) had no effect on IK(ATP) and exerted only a minor inhibitory action on this current at 20 microM. 4. 17-ODYA (5 microM) potentiated current flow through the large conductance, Ca-sensitive K-channel (BKCa). In contrast, proadifen (10 microM) had no effect on IBK(Ca) whereas clotrimazole (30 microM) exerted a small but significant inhibitory action. 5. Proadifen (10 microM) and clotrimazole (30 microM) each inhibited the magnitude but increased the frequency of spontaneous contractions in whole portal veins. 17-ODYA (5 microM) had no effect on spontaneous contractions but these were inhibited when the concentration of 17-ODYA was increased to 50 microM. 6. The spasmolytic effect of levcromakalim on spontaneous contractions was antagonized by proadifen (10-30 microM) in a concentration-dependent manner but 17-ODYA (up to 50 microM) was without effect. 7. These results in portal vein show that cytochrome P450 inhibitors exert profound effects on a variety of K-channel subtypes. This suggests that enzymes dependent on this cofactor may be important regulators of K-channel activity in smooth muscle. The relevance of these findings for the identification of the pathway involved in the synthesis of the endothelium-derived hyperpolarizing factor is discussed.

(+)-bufuralol 1'-hydroxylation activity in human and rhesus monkey intestine and liver

(+)-Bufuralol 1'-hydroxylation, a commonly used marker of hepatic CYP2D6 activity, was investigated in human and rhesus monkey intestinal microsomes and compared with that in hepatic microsomes. The cumene hydroperoxide (CuOOH)-mediated metabolism of (+)-bufuralol suggested that at least two enzymes were responsible for bufuralol 1'-hydroxylation in both human and monkey intestinal microsomes. In contrast, the kinetics of the CuOOH-mediated metabolism in human and monkey livers were monophasic. The Km values for the higher affinity component of the intestinal enzyme(s) of both species were similar to, while the corresponding Vmax values were much lower than, those obtained with the livers. Bufuralol metabolism mediated by NADPH exhibited biphasic kinetics and was less efficient than that observed in the presence of CuOOH in both human and monkey intestines, in agreement with the observations in the livers. Inhibition of bufuralol hydroxylase activity in the intestine and liver preparations from the same species by known CYP2D6 inhibitors/substrates was qualitatively similar. Quinidine was the most potent inhibitor of (+)-bufuralol 1'-hydroxylation in all tissues studied. Western immunoblots using anti-CYP2D6 peptide antibody revealed a protein band in human and monkey intestinal microsomes of the same molecular weight as that observed in the liver preparations. The intestinal CYP2D protein content appeared to be much less than that of liver, and correlated with the (+)-bufuralol hydroxylase activity. Immunoinhibition studies indicated significant (up to 50%) inhibition of the CuOOH-mediated (+)-bufuralol metabolism in human and monkey intestines only by anti-CYP2D6, and not by anti-CYP2A6, or anti-CYP2E1. Inhibition of the bufuralol 1'-hydroxylase activity by anti-rat CYP3A1 was only slight (20%) in human, but marked (60-65%) in monkey intestinal microsomes. The hepatic metabolism of (+)-bufuralol in humans and monkeys was only inhibited (75%) by anti-CYP2D6, but not by anti-CYP3A1. Overall, the results suggest that (1) tissue and species differences exist in the catalysis of (+)-bufuralol 1'-hydroxylation, and (2) CYP2D6-related enzymes are partially or primarily responsible for the bufuralol hydroxylase activity in human and monkey intestines or monkey liver.

Conformational epitopes on CYP2D6 are recognized by liver/kidney microsomal antibodies

BACKGROUND/AIMS

Four linear antigenic sites have been shown on the CYP2D6 molecule that are recognized by serum positive for liver/kidney microsomal antibody (LKM) type 1. The aim of this study was to search for antibodies against CYP2D6 conformational antigenic sites in LKM-1-positive sera.

METHODS

The capacity of four LKM-1-positive sera, before and after absorption with synthetic peptides representing CYP2D6 linear antigenic sites, and rabbit sera against linear antigenic sites between CYP2D6 amino acids 254-271 and 373-389 to inhibit the O-demethylation of dextromethorphan by CYP2D6 was tested in vitro.

RESULTS

Inhibition of O-demethylation of dextromethorphan was not modified by absorption of antibodies against linear CYP2D6 antigenic sites. In addition, rabbit sera against two of these sites did not inhibit the reaction. These results strongly suggest that antibodies against CYP2D6 conformational antigenic sites were present in LKM-1-positive sera.

CONCLUSIONS

The autoimmune response against CYP2D6 is directed against linear and conformational antigenic sites. These results strengthen the argument that the LKM-1 response is polyclonal and antigen driven.

An evaluation of cytochrome P450 isoform activities in the female dark agouti (DA) rat: relevance to its use as a model of the CYP2D6 poor metaboliser phenotype

The female dark agouti (DA) rat lacks CYP2D1, the equivalent enzyme in the rat to human CYP2D6 (debrisoquine hydroxylase), and shows impaired metabolism of a number of CYP2D6 substrates. However, from the data available in the literature it is not entirely clear whether the enzyme deficiency in the DA rat is restricted to CYP2D1, and whether factors such as age and substrate concentration are important determinants of interstrain differences in the activity of this enzyme. Given that the female DA rat is used as a model of the human CYP2D6 poor metaboliser phenotype, there is a need for a systematic evaluation of the P450 activities in the DA rat, and of its suitability as a model of the PM phenotype. In the present study metoprolol was used as a probe substrate to investigate CYP2D1 activity since both the alpha-hydroxylation and O-demethylation of this drug are catalysed by CYP2D6 in man. Formation of alpha-hydroxymetoprolol (AHM) and O-demethylmetoprolol (ODM) was 10- and 2.5-fold lower in liver microsomes from female DA rats compared with microsomes from age-matched female Wistar rats, the latter representing the extensive metaboliser strain. Kinetic analysis suggested that in both strains of rat both the alpha-hydroxylation and O-demethylation of metoprolol were catalysed by more than one enzyme. By using quinine as a specific inhibitor of the enzyme, CYP2D1 was identified as an intermediate affinity site in the Wistar strain and was shown to have impaired activity in the DA strain. The activities of lower and higher affinity sites were similar in the two strains. Thus, the only difference between the two strains with respect to both routes of metoprolol metabolism appeared to be in the activity of CYP2D1. Interstrain differences were found to be highly dependent on the choice of substrate concentration, being more marked at lower concentrations. We have also investigated the metabolism of a number of probe compounds for some of the other P450 isoforms commonly involved in drug metabolism to determine the selectivity of the deficiency in the DA strain. p-Nitrophenol hydroxylation and erythromycin N-demethylation were catalysed at higher rates by DA than by Wistar liver microsomes, indicating higher levels of activity of CYP2E1 and CYP3A in the former strain. Felodipine oxidation, tolbutamide hydroxylation and both the hydroxylation and N-demethylation of S-mephenytoin were catalysed at similar rates by microsomes from the two strains, indicating similar activities of enzymes in the CYP2C and CYP3A families.(ABSTRACT TRUNCATED AT 400 WORDS)

Interleukin 2 induction of cytochrome P450-linked monooxygenase systems of rat liver microsomes

The effects of interleukin 2 (IL-2), a pivotal cytokine for generating an effective immune response, on rat liver microsomal cytochrome P450-linked monooxygenase systems were investigated by measuring the contents of cytochromes b5 and P450, and the activities of various xenobiotic-metabolizing enzymes [debrisoquine and bufuralol monooxygenases (CYP2D), 7-ethoxycoumarin O-deethylase, benzphetamine N-demethylase, aniline hydroxylase and p-nitroanisole N-demethylase]. The enzymatic activities except for p-nitroanisole N-demethylase and aniline hydroxylase were increased approximately to 1.3-fold of those of untreated liver microsomes following intraperitoneal injection of IL-2 (15 U/rat). However, the amount of immunoreactive b5 protein, and the activities of aniline hydroxylase and p-nitroanisole N-demethylase were not changed by injection of IL-2. To elucidate further the mechanism of the induction of CYP2D by IL-2, quantitative analyses of immunoreactive CYP2D protein and its mRNA were conducted by western blot and slot blot hybridization analyses. The results indicated that IL-2 induced an increase in the amounts of immunoreactive CYP2D protein and its mRNA. These enzymatic activities were thus up-regulated at the mRNA level.

Effects of interleukin 1 alpha on the activities and gene expressions of the cytochrome P450IID subfamily

The mechanism by which recombinant human interleukin 1 alpha (rhIL-1 alpha) inhibits the activities of drug-metabolizing enzymes of rat liver microsomes, especially debrisoquine monooxygenase and bufuralol monooxygenase (both cytochrome P450IID supported reactions), as well as other enzymes, was investigated by injecting IL-1 alpha into rats. rhIL-1 alpha suppressed the activities of various P450-linked monooxygenase systems such as aminopyrine N-demethylase, benzphetamine N-demethylase, and 7-ethoxycoumarin O-deethylase. It also suppressed the activities of debrisoquine monooxygenase and bufuralol monooxygenase. On the other hand, IL-1 alpha had little effect on the activity of p-nitroanisole N-demethylase. The suppression of debrisoquine monooxygenase and bufuralol monooxygenase activities was caused by a decrease in the amounts of immunoreactive P450IID protein and its mRNA. The reduction rates in the level of immunoreactive P450IID protein and its mRNA were comparable. These results suggest that at the mRNA level, the enzymatic activities of debrisoquine monooxygenase and bufuralol monooxygenase are down-regulated by IL-1 alpha.

Interspecies variations in caffeine metabolism related to cytochrome P4501A enzymes

1. Interspecies (including man, monkey, rabbit, rat and mouse) variations in caffeine metabolism by liver microsomes were studied. While N-3 demethylation was the major pathway in man (81% of total dimethylxanthines), N-7 demethylation was predominant in monkey (89%), and the three demethylation pathways were about equal in mouse, rabbit and rat. 2. Three monooxygenase activities (methoxyresorufin O-demethylase, phenacetin O-deethylase and acetanilide 4-hydroxylase) correlated significantly with the rate of metabolism of caffeine. 3. P4501A1 and 1A2 enzymes were immunodetected in different species. P4501A2 was the only isoform detected in liver of man, rat and mouse, while no polypeptide immunorelated to P4501A was detected in monkey and only a minor band of P4501A1 was detected in rat and rabbit. 4. All in vitro data indicate that paraxanthine formation is mediated mainly by P4501A2 in mammals while theophylline formation is mediated mainly by cytochromes P-450 other than those of the 1A family.