Comparative studies on coumarin and testosterone metabolism in mouse and human livers. Differential inhibitions by the anti-P450Coh antibody and metyrapone

The Coumarin 7-Hydroxylation Microassay in Living Hepatic Cells in Culture

Coumarin 7-hydroxylation was evaluated in hepatic cells from various species, cultured in 96-well plates. This microassay involved incubating living cultured cells with the substrate, followed by fluorimetric quantification of the product released into the culture supernatant, after hydrolysis of the conjugates of 7-hydroxycoumarin that were formed. Fluorescence was measured directly in the wells by using a microplate fluorescence reader. The major advantages of this technique are its simplicity and automation, the small number of cells required, the reduction in sample handling and assay time, and the possibility of performing repeated assays with the same cell monolayer, since no injury to cells is detectable during the assay. By using this microassay, it was shown that human hepatocytes hydroxylated coumarin at higher rates than did rabbit, dog or rat hepatocytes, and that no appreciable metabolic activity was observed in hepatoma cells (Hep G2 and FaO). In addition, methoxsalen was found to be a potent inhibitor of cytochrome P4502A6 activity in living human hepatocytes.

Selective inhibition of CYP2B6-catalyzed bupropion hydroxylation in human liver microsomes in vitro

Some inhibitory agents against CYP2B6 have been reported, but none of these has been extensively characterized or compared with others, as to the potency and selectivity of inhibition toward CYP2B6. The goal of this work was to find a selective and potent chemical in vitro inhibitor toward CYP2B6 using bupropion hydroxylation as a model reaction. At the initial screening of more than 30 substances, ticlopidine, triethylenethiophosphoramide (thioTEPA), metyrapone, xanthate C8, and benzylisothiocyanate displayed IC(50) values of <10 microM and were selected for a more detailed analysis. Metyrapone, xanthate C8, and benzylisothiocyanate inhibited several other cytochrome P450 activities rather effectively, some of them even more potently than CYP2B6, and consequently are unsuitable as CYP2B6-selective probes. Ticlopidine and thioTEPA were the most potent inhibitors of bupropion hydroxylation with K(i) values of 0.2 and 2.8 microM, respectively. The inhibition type of ticlopidine was found to be mixed type, with a component of mechanism-based inhibition, whereas thioTEPA inhibited CYP2B6 in a competitive manner. In addition to CYP2B6, ticlopidine also inhibited both mephenytoin 4-hydroxylation (CYP2C19) (IC(50), 2.7 microM) and dextromethorphan O-demethylation (CYP2D6) (IC(50), 4.4 microM). For thioTEPA the next sensitive P450 activity after CYP2B6 was coumarin 7-hydroxylation (IC(50), 256 microM). Thus, although both compounds proved to be relatively potent inhibitors of CYP2B6, thioTEPA was about 2 orders of magnitude more selective than ticlopidine. Thus, thioTEPA is a drug of choice when high CYP2B6 selectivity among major P450 enzymes is required. Ticlopidine is a useful alternative under a controlled experimental setup and when higher potency is needed.

The Gerhard Zbinden memorial lecture: application of biochemical and genetic approaches to understanding pathways of chemical toxicity

All cells have evolved a complex number of pathways, which allow them to survive in a chemically hostile environment. In multicellular organisms, these pathways are catalysed by a number of key enzymes, which inhibit the absorption of toxins or facilitate their elimination so that they do not accumulate to toxic levels within the cell. These cytoprotective pathways are also critical determinants of the effectiveness of drug therapy and are thought to have evolved from a limited number of biochemical pathways, such as those which allow cells to utilise molecular oxygen in respiration without suffering deleterious effects. The study of both simple and multicellular organisms has shown that many stress response pathways previously considered as distinct adaptive mechanisms in mammalian systems are interrelated coordinated responses to toxic challenge. Understanding the functions and mechanisms of regulation of the genes involved in these pathways has many applications in medical science-in evaluating the role of environmental factors in the pathogenesis of human disease, in chemoprevention, in drug development and in the application of drug therapy. The use of genetic approaches, coupled with new chip-based profiling technologies, will play a key role in the development of studies in this research area.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Assessment of P450 induction in the marmoset monkey using targeted anti-peptide antibodies

The identity and expression of hepatic P450 enzymes in marmosets was investigated using a panel of anti-peptide antibodies originally targeted against human P450 enzymes. In immunoblotting, of 12 antibodies examined, 10 bound specifically to bands in marmoset liver microsomal fraction corresponding to P450 enzymes. It is proposed that these represent marmoset CYP1A1, CYP1A2, CYP2A, CYP2B, CYP2C forms (CYP2C-1 and CYP2C-2), CYP2D19, CYP3A21 and another CYP3A form (CYP3A-m). The antibodies, together with an anti-marmoset CYP2E1 antibody, were used to investigate the expression of 10 P450 enzymes in marmosets treated with P450-inducing chemicals. Treatment with phenobarbitone caused CYP2B, CYP2C-2 and CYP3A21 levels to increase, rifampicin caused increases in CYP2B and CYP2C-1 and a decrease in CYP3A21 levels, whereas dioxin caused CYP1A1 and CYP1A2 levels to increase and CYP2E1 levels to decrease. Clofibric acid did not induce any P450. P450 enzyme activities were assessed using 8 different substrates and increases were found after treatment with phenobarbitone, rifampicin, and dioxin. However, due to species differences in substrate selectivity, it proved difficult to ascribe these changes to individual P450 enzymes. Thus, the use of anti-peptide antibodies provides a more informative way of assessing the levels of specific P450 enzymes than enzyme activity measurements.

Identification of in vitro cytochrome P450 modulators to detect induction by prototype inducers in the mallard duckling (Anas platyrhynchos)

Seven modulators of mammalian monooxygenase activity were screened for their ability to selectively stimulate or inhibit in vitro monooxygenase activities of hepatic microsomes from mallard ducklings treated with phenobarbital, beta-naphthoflavone, 3,3',4,4',5-pentachlorobiphenyl or vehicle. Microsomes were assayed fluorometrically for four monooxygenases: benzyloxy-, ethoxy-, methoxy-, and pentoxyresorufin-O-dealkylase, in combination with each of the seven modulators. Four combinations: alpha-naphthoflavone and 2-methylbenzimidazole with benzyloxyresorufin, and Proadifen with methoxy- and ethoxyresorufin, respectively, were evaluated further. beta-Naphthoflavone-treated groups were clearly distinguished from the corn oil vehicle control group by all of the assays and by the effects of the modulators in three of the four assay/modulator combinations. Enzyme activities of the phenobarbital and saline groups were statistically similar (P > or = 0.05) when assayed without modulator added, but each assay/modulator combination distinguished between these groups. The PCB-treated group was distinguished from the corn oil vehicle control group only for BROD activity, with or without the presence of modulator. Graphing of per cent modulation of BROD activity versus initial BROD activity provided the clearest distinction between all of the study groups. Identification of these selective in vitro modulators may improve detection and measurement of low level cytochrome P450 induction in avian species. Also, both the monooxygenase activities induced and the impacts of the modulators indicated differences between mammalian and avian cytochromes P450.

Cytochrome P4502A6 (CYP2A6) expression in human hepatocellular carcinoma

The hepatic cytochrome P4502A6 (CYP2A6) enzyme mediates the oxidative metabolism of several procarcinogens that have liver as their primary target. Mouse models indicate that liver tumors invariably overexpress CYP2A forms, and that inflammation and cirrhosis may regulate the CYP2A expression pattern. In this study, the distribution of the CYP2A6 protein was investigated in a series of 24 human hepatocellular carcinoma (HCC) samples by immunohistochemical analysis. A polyclonal antibody was raised in chicken against CYP2A5, the mouse orthologue of CYP2A6. The antibody was characterized and found to be specific for CYP2A members. In DBA/2 mouse liver, a strong increase of CYP2A5 protein amount, localized in the perivenous region, occurred in response to treatment with pyrazole. In human HCC samples, overexpression of CYP2A6 protein was associated with the presence of chronic inflammation and cirrhosis. CYP2A6 protein was observed in 9 of 16 (56%) of samples with non-neoplastic hepatocytes and in 10 of 24 (42%) HCC samples. The staining for CYP2A6 protein was very heterogeneous in tumor cells, suggesting that increased expression of CYP2A6 occurred in a distinct subpopulation of neoplastic cells. In Kaplan-Meyer survival analysis, there was a tendency toward a more favorable prognosis in patients with CYP2A6-positive tumors in comparison with patients with CYP2A6-negative tumors. These data suggest that, in human HCC, in contrast to mouse liver tumors, CYP2A6 overexpression is not an invariable phenotype.

Selective serotonin reuptake inhibitors and theophylline metabolism in human liver microsomes: potent inhibition by fluvoxamine

1. Fluvoxamine and seven other selective serotonin reuptake inhibitors (SRRI) were tested for their ability to inhibit a number of human cytochrome P450 isoforms (CYPs). 2. None of the drugs showed potent inhibition of CYP2A6 (coumarin 7-hydroxylase) or CYP2E1 (chlorzoxazone 6-hydroxylase), while norfluoxetine was the only potent inhibitor of CYP3A having IC50 values of 11 microM and 19 microM for testosterone 6 beta-hydroxylase and cortisol 6 beta-hydroxylase, respectively. 3. Norfluoxetine, sertraline and fluvoxamine inhibited CYP1A1 (7-ethoxyresorufin O-deethylase) in microsomes from human placenta (IC50 values 29 microM, 35 microM and 80 microM, respectively). Fluvoxamine was a potent inhibitor of CYP1A2-mediated 7-ethoxyresorufin O-deethylase activity (IC50 = 0.3 microM) in human liver. 4. In microsomes from three human livers fluvoxamine potently inhibited all pathways of theophylline biotransformation, the apparent inhibitor constant, Ki, was 0.07-0.13 microM, 0.05-0.10 microM and 0.16-0.29 microM for inhibition of 1-methylxanthine, 3-methylxanthine and 1,3-dimethyluric acid formation, respectively. Seven other SSRIs showed either weak or no inhibition of theophylline metabolism. 5. Ethanol inhibited the formation of 1,3-dimethyluric acid with K(i) value of 300 microM, a value which is consistent with inhibition of CYP2E1. Ethanol and fluvoxamine both inhibited 8-hydroxylation by about 45% and, in combination, the compounds decreased the formation of 1,3-dimethyluric acid by 90%, indicating that CYP1A2 and CYP2E1 are equally important isoforms for the 8-hydroxylation of theophylline. 6. It is concluded that pharmacokinetic interaction between fluvoxamine and theophylline is due to potent inhibition of CYP1A2.

The inhibition of CYP enzymes in mouse and human liver by pilocarpine

1. Pilocarpine is a cholinomimetic natural alkaloid. Its interactions with testosterone hydroxylations, coumarin 7-hydroxylase (COH), dimethylnitrosamine N-demethylase (DMNA), pentoxyresorufin O-dealkylase (PROD) and 7-ethoxyresorufin O-deethylase (EROD), which are indicative of the activities of cytochrome P4502A5 (CYP2A5) or 6, 2E1, 2B, 1A, were examined in mouse and human liver microsomes. 2. In mouse liver microsomes the IC50 values of pilocarpine were 6 microM for COH and testosterone 15 alpha-hydroxylase (T15 alpha OH) activities, 4 microM for PROD, approximately 100 microM for DMNA and testosterone 6 beta-hydroxylase (T6 beta OH) activities and > 1 mM for EROD activity. 3. In human liver microsomes, the IC50 value for COH was 6 microM and for DMNA 10 microM; T15 alpha OH and PROD activities were not detectable but T6 beta OH and testosterone 16 beta/2 beta-hydroxylase activities were moderately inhibited (IC50 70 microM). 4. These results suggest that pilocarpine has (i) a high affinity towards phenobarbitone-inducible CYP2A4/5 and CYP2B activities in mouse liver, (ii) a high affinity towards CYP2A6 in human liver microsomes and (iii) a moderate affinity towards CYP3A enzyme(s) in both microsomal preparations. 5. The low IC50 concentrations in vitro indicate potential metabolic interactions between pilocarpine and several P450 enzymes.

Activation of aflatoxin B1 by mouse CYP2A enzymes and cytotoxicity in recombinant yeast cells

The ability of three highly homologous mouse liver CYP2A enzymes to activate aflatoxin B1 was studied by expressing them in recombinant AH22 Saccharomyces cerevisiae yeast cells. The reconstituted monooxygenase complex with CYP2A5 purified from yeast cell microsomes produced epoxide at a rate of 17.2 nmol/min per nmol P450 in the presence of 50 microM aflatoxin B1 while CYP2A4 had about 10% and P4507 alpha only 1.5% of this activity. However, Km values were 530 and 10 microM and Vmax values 12.5 and 14.3 nmol/min per nmol P450 for CYP2A4 and CYP2A5, respectively. When recombinant yeast cells were exposed to aflatoxin B1 LC50 concentrations were 7.5 +/- 5.5 microM for CYP2A4, 0.45 +/- 0.10 microM for CYP2A5 and > 320 microM for P4507 alpha expressing yeast cells. Aflatoxin B1-DNA adduct levels in the same yeast cells were 50, 890 pmol/mg DNA and below detection limit when 3.0 microM aflatoxin B1 was used in the incubation mixture. Coumarin an inhibitor for CYP2A4 and a substrate for CYP2A5 diminished the toxicity of aflatoxin B1 in a dose-dependent manner for these recombinant yeast cells. These data demonstrate that (1) highly homologous mouse CYP2A enzymes activate aflatoxin B1 in a different manner and (2) that recombinant yeast cells expressing mammalian CYP enzymes are a useful and inexpensive system to test the role of different enzymes in aflatoxin B1 toxicity. The data also indicate that mouse CYP2A5 and its counterpart in other species could have a significant role in aflatoxin B1 toxicity in organs where it is expressed at high levels.

Catalytic roles of rat and human cytochrome P450 2A enzymes in testosterone 7 alpha- and coumarin 7-hydroxylations

Differences in the catalytic roles of rat and human cytochrome P450 2A enzymes in testosterone 7 alpha- and coumarin 7-hydroxylase activities were examined. Liver microsomes from 18 human samples catalyzed coumarin 7-hydroxylation at a mean rate of about 60 pmol/min/nmol P450, but did not show any measurable activity for testosterone 7 alpha-hydroxylation. In rats, both activities were found to be developmentally regulated; 3-week-old rats had the highest activities for these two reactions. Anti-P450 2A1 antibodies and methoxsalen, a potent inhibitor of P450 2A-dependent monooxygenase activities in several animal species, inhibited almost completely both testosterone 7 alpha- and coumarin 7-hydroxylations catalyzed by liver microsomes prepared from 3-week-old male rats. Interestingly, although Km values for coumarin 7-hydroxylation activities in liver microsomes from 3-week-old rats were not different from those of adult humans, the Vmax value in rats was only 1/30 of that obtained in 18 human samples. Thus, the present results support the view that marked differences exist in the catalytic roles of rat and human P450 2A enzymes, which, in turn, may sometimes cause species-related differences in susceptibilities toward drug actions and toxicities.

Metabolic interactions of methoxsalen and coumarin in humans and mice

Methoxsalen (8-methoxypsoralen) is a very potent inhibitor of human cytochrome P450 2A6 (CYP2A6) and mouse Cyp2a-5-mediated coumarin 7-hydroxylation in vitro. To determine the effect of methoxsalen on coumarin 7-hydroxylation in humans in vivo, five subjects were given 45 mg of methoxsalen and 5 mg of coumarin. Methoxsalen inhibited in vivo coumarin metabolism by 47 +/- 9.2% (mean +/- SEM). Methoxsalen was metabolized in human liver microsomes at the rate of 50-100 pmol/mg protein/min (approx. 30% of the activity in mouse liver microsomes). Metabolism was not inhibited by the anti-Cyp2a-5 antibody in human liver microsomes. NIH 3T3 cells stably expressing catalytically active CYP2A6 enzyme did not metabolize methoxsalen, indicating that CYP2A6 does not accept methoxsalen as a substrate. In pyrazole-induced mouse liver microsomes, methoxsalen metabolism was inhibited by the anti-Cyp2a-5 antibody. Cyp2a-5 protein expressed in the yeast Saccharomyces cerevisiae was capable of metabolizing methoxsalen, indicating that methoxsalen is a substrate of Cyp2a-5. Although kinetic studies indicated that the inhibition of coumarin 7-hydroxylation by methoxsalen is competitive in human liver microsomes, methoxsalen does not appear to be a substrate for CYP2A6. Methoxsalen and coumarin have the potential of strong metabolic interactions in man.

Tissue and sex-dependent differences in CYP2A activities in hamsters

Three enzymatic activities of the CYP2A subfamily, coumarin 7-hydroxylase (COH), testosterone 15 alpha-hydroxylase (T15 alpha OH) and testosterone 7 alpha-hydroxylase (T7 alpha OH), were characterized in liver, kidney and lung microsomes from control, pyrazole (PYR), 3-methylcholanthrene (MC) and phenobarbital (PB) treated female and male Syrian golden hamsters. Sex-dependent changes in the enzymatic activities were found. Among control animals COH and T15 alpha OH activities were higher in males. T7 alpha OH activity was five times higher in female kidneys than in males. Inducers changed this metabolic profile. MC and PB were potent CYP2A inducers in extrahepatic tissues: significant increases were found in COH (5-fold) and T15 alpha OH (12-fold) activities in female MC lung microsomes and T7 alpha OH (7-fold) in MC male kidney microsomes. PB increased significantly activities of COH (5-fold), T15 alpha OH (3-fold) and T7 alpha OH (10-fold) in male kidney microsomes. All inducers significantly increased T7 alpha OH activity in male kidney microsomes but decreased hepatic T7 alpha OH activity in both sexes. PYR treatment decreased hepatic CYP2A activities. Anti-mouse CYP2A4/5 antibody inhibited COH activity by a variable extent depending on the tissue and pretreatment and recognised three 52-, 49-, 48-kDa bands in liver and two major bands in kidney (48 and 49 kDa) and lung (49 and 52 kDa) microsomes. COH and T15 alpha OH activities correlated well with 49 kDa protein (r = 0.95 and r = 0.99, respectively) in lung microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cocaine N-demethylation and the metabolism-related hepatotoxicity can be prevented by cytochrome P450 3A inhibitors

Cocaine is eliminated and detoxified principally through the metabolism of nonspecific plasma and tissue esterases. Microsomal oxidative metabolism is of importance in cocaine N-demethylation, this being a principal pathway of cocaine bioactivation and hepatotoxicity. The contribution of different cytochrome P450 (CYP) enzymes to cocaine N-demethylase activity was studied in vitro with DBA/2 mouse and human liver microsomes, and cocaine hepatotoxicity was examined in vivo in DBA/2 male mice. Species dependent enzyme kinetics was observed. Cocaine N-demethylase displayed two Km values in murine liver (40-60 microM and 2-3 mM), whereas only one Km value was observed in human liver microsomes (2.3-2.7 mM). We suggest that CYP3A plays a prominent role in the N-demethylation of cocaine for the following reasons: (i) pregnenolone-16 alpha-carbonitrile, an inducer of CYP3As increases cocaine N-demethylase in parallel with testosterone 6 beta-hydroxylase activity and immunoreactive 3A protein in mouse liver; (ii) human and mouse cocaine N-demethylase and testosterone 6 beta-hydroxylase activities can be inhibited by triacetyloleandomycin, cannabidiol, or gestodene, all selective inhibitors of CYP3A P450s; (iii) antibodies directed against P450s within subfamilies 1A, 2A, 2B, 2C, or 2E inhibited cocaine N-demethylase activity only marginally, and finally, (iv) treatment of mice with triacetyloleandomycin or cannabidiol in vivo significantly attenuated the cocaine-elicited hepatotoxicity as assessed by the serum alanine aminotransferase activity and liver histology in parallel with decreased cocaine N-demethylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of hamster and mouse reveals interspecies differences in the regulation of hepatic CYP2A isozymes

Three CYP2A-related activities [coumarin 7-hydroxylase (COH), testosterone 7 alpha- (test7 alpha) and 15 alpha-hydroxylases (test15 alpha)], identified in hamster liver and analysed by immunoinhibition, and western and northern blotting, were found to be similar to mouse and human CYP2As. In the microsomal fractions, anti-mouse CYP2A5 antibody recognised three bands of about 48, 49 and 52 kDa, suggesting the presence of at least three proteins immunologically similar to mouse CYP2A5. The 49 kDa band migrated close to mouse CYP2A5 and changes in its expression followed COH and test15 alpha activities. Test7 alpha activity did not associate with any of the individual bands detected on western blots despite its strong inhibition by the antibody. Despite the immunological and catalytic similarities between mouse and hamster CYP2A enzymes, their regulation is different. In mice, the enzyme activities are higher in females than males, are induced by pyrazole (PY) and phenobarbital (Pb), and are not affected by 3-methylcholanthrene (MC). In hamsters, activities are not higher in females, induced by MC and reduced by PY. MC and PY appear to regulate expression at the mRNA level, while Pb seems to act post-transcriptionally by increasing either the synthesis or the stability of the protein. Our data indicate that the modes of expression and regulation of CYP2A-related enzymes make the hamster different from mice and humans with respect to the mechanism of metabolism of certain drugs and carcinogens.

Retrovirus-mediated stable expression of human CYP2A6 in mammalian cells

To study the pharmacological and toxicological significance of the human cytochrome P450 isoform CYP2A6, we expressed it in mammalian cells by retrovirus-mediated gene transfer. The LXSN vector and PA317 packaging cells were used to create amphotropic recombinant retroviruses containing CYP2A6 cDNA. NIH 3T3 and HeLa cells were infected with these retroviruses and cell clones expressing CYP2A6 were selected. The integration of the CYP2A6 construct was verified by PCR analysis and northern blot analysis showed that a 5 kb mRNA containing the CYP2A6 was present in the cells. The integrated cDNA directed the expression of catalytically active CYP2A6 enzyme which has remained stable over numerous cell passages. No oxidation of several other P450 substrates was detected. The promutagen aflatoxin B1 was metabolized to intermediates binding to the host cell genomic DNA by the 3T3 2A6 cells. These cell lines are thus well suited for the study of the catalytic profile and the biological consequences of promutagen activation by the human CYP2A6 isoform.

Initial characterization of the major mouse cytochrome P450 enzymes involved in the reductive metabolism of the hypoxic cytotoxin 3-amino-1,2,4-benzotriazine-1,4-di-N-oxide (tirapazamine, SR 4233, WIN 59075)

The benzotriazine di-N-oxide SR 4233 (tirapazamine, WIN 59075) is currently in phase I clinical trials as the lead compound in a series of novel and highly selective antitumour hypoxic cytotoxins. Reductive bioactivation is thought to proceed via a one-electron reduced, oxidizing nitroxide radical and also forms the inactive single N-oxide SR 4317 via radical disproportionation or a second one-electron reduction. In mouse liver microsomes reductive metabolism is catalysed predominantly by cytochrome P450 (70%) and cytochrome P450 reductase (30%). The aim of the present study was to examine which cytochrome P450 isozymes may be involved. Reduction of SR 4233 to SR 4317 was monitored by HPLC analysis. Metabolism by microsomes from both control and dexamethasone-induced BALB/c male mice was 70% inhibited by carbon monoxide. The cytochrome P450 inhibitor SKF 525A, following aerobic preincubation, also inhibited SR 4233 reduction by 58%. Reduction was induced 2-3-fold by dexamethasone and was not accountable by increases in cytochrome P450 reductase or DT-diaphorase. The induction data and the greater degree of inhibition of SR 4233 reduction by metyrapone compared to alpha-naphthoflavone suggested a possible involvement of Cyp2b, Cyp2c and Cyp3a cytochrome P450 subfamilies. Both Cyp3a (7.4-fold) and Cyp2b (1.8-fold) type enzymes were shown by western immunoblot analysis to be induced by dexamethasone, the latter correlating more closely with increased SR 4233 reductase activity and also with the 2-fold induction of benzphetamine N-demethylase, a Cyp2b-type enzyme. No inhibition of SR 4233 reduction was seen with erythromycin or cyclosporin A which act as substrates/inhibitors for Cyp3a-type enzymes, but inhibition was seen with p-nitrophenol and tolbutamide which are substrates for Cyp2el- and Cyp2c-type enzymes, respectively (11% and 25% inhibition in induced microsomes). SR 4233 itself inhibited benzphetamine N-demethylase, which is catalysed by Cyp2b-type enzymes but not erythromycin N-demethylase which is catalysed by Cyp3a-type isoforms. Immunoinhibition studies with epitope specific monoclonal antibodies were consistent with the major involvement of phenobarbitone- and steroid-inducible products of the Cyp2b and Cyp2c subfamilies. These forms contributed at least 53% and 26%, respectively, of the cytochrome P450-associated SR 4233 reductase activity in the induced microsomes. The findings support our earlier conclusion that cytochrome P450 is the major SR 4233 reductase in mouse liver and provides leads as to the possible involvement of specific isoforms in human tumours and normal tissues.

Differential inhibition of coumarin 7-hydroxylase activity in mouse and human liver microsomes

Coumarin is 7-hydroxylated by the P450 isoform Cyp2a-5 in mice and CYP2A6 in humans. Various drugs, endogenous substances, plant substances and carcinogens, altogether about 90 chemicals, were evaluated as possible inhibitors of coumarin 7-hydroxylase (COH) activity in mouse microsomes. The effects of selected compounds on COH activity in human liver microsomes were also tested. The furanocoumarin derivatives methoxsalen (8-methoxypsoralen) and psoralen proved to be the most potent inhibitors of mouse COH activity (IC50 values 1.0 and 3.1 microM, respectively). The furanocoumarins bergapten (5-methoxypsoralen), isopimpinellin (5,8-dimethoxypsoralen), imperatorin and sphondin also effectively inhibited mouse COH activity (IC50 values 19-40 microM). Methoxsalen, isopimpinellin and metyrapone were also inhibitors in mice in vivo. Methoxsalen was a potent inhibitor of COH activity also in human liver microsomes, (IC50 value 5.4 microM), whereas bergapten, isopimpinellin and imperatorin had no effect. The imidazole antimycotic miconazole was a potent but non-specific inhibitor of COH activity. Several known substrates and inhibitors of members in the CYP1A, CYP2B, CYP2C, CYP2D and CYP3A subfamilies were poor inhibitors of COH activity. These results suggest that (i) the coumarin-type compounds in particular interact with the active sites of Cyp2a-5 and CYP2A6, and (ii) the active sites of Cyp2a-5 and CYP2A6 are structurally different, since a number of compounds inhibited mouse, but not human COH activity.

Cytochrome P450 isoforms in human fetal tissues related to phenobarbital-inducible forms in the mouse

Four polyclonal antibodies raised against purified mouse liver cytochrome P450s representing Cyp1a, Cyp2a, Cyp2b and Cyp2c subfamilies were used to detect their related forms in human adult and fetal tissues. In immunoblot analysis, anti-Cyp2c antibody detected two to three proteins in adult livers and one to three proteins in 70% of the 18 fetal livers studied. Anti-Cyp2a-5 antibody recognized a 50-kDa protein in 50% of the fetal adrenals. Anti-Cyp1a-2 antibody reacted with a single protein (55 kDa) in adult liver. The anti-Cyp2b-10 antibody did not detect proteins in any of the tissues. No proteins were detected in fetal kidneys. There was no coumarin 7-hydroxylase activity (COH) in fetal liver or adrenals. The 7-ethoxycoumarin O-deethylase (ECOD) activities were slightly higher in fetal adrenals (mean 6.1 pmol/mg protein/min) vs livers. The fetal adrenal ECOD activity was not inhibited by the anti-Cyp2a-5 antibody. Aryl hydrocarbon hydroxylase (AHH) activities in fetal livers were about 5% of those in adult livers. AHH activity in fetal liver was not inhibited by the anti-Cyp2c antibody. Testosterone 6 beta-hydroxylase activity was much lower in fetal liver than in adult liver (about 20 and 1700 pmol/mg protein/min, respectively). No immunoinhibition occurred in fetal adrenal progesterone hydroxylation, hepatic benzphetamine N-demethylation and hepatic ethylmorphine N-demethylation. These data suggest that members of the P450 subfamilies 1A, 2A and 2B are expressed at a very low level in fetal liver, and that fetal liver may contain members of the 2C subfamily.

The role of cytochrome P450 3A (CYP3A) isoform(s) in oxidative metabolism of testosterone and benzphetamine in human adult and fetal liver

Testosterone metabolism was studied in human adult and fetal liver microsomes. In fetal livers 6 beta-hydroxylase (6 beta OH) activity (1-2% of adult activity) and 2 alpha-hydroxylase (2 alpha OH) activity (about 40% of adult activity) were present. Also some fetal livers produced two unknown metabolites. Androstenedione was formed in all fetal livers studied (10-20% of adult activity). Testosterone hydroxylations at 6 beta-, 2 beta-, 15 alpha- and 15 beta-positions were associated with CYP3A isoform(s) in adult liver, because they were strongly inhibited by midazolam, a known substrate for CYP3A4 and by anti-CYP3A4 antibody. Fetal liver activities were consistently inhibited less than the activities in adult livers. The formation of androstenedione was not affected by these inhibitors in fetal or adult liver microsomes. Benzphetamine N-demethylase activity in the fetal livers was about 40% of adult activity. Anti-CYP3A4 antibody had no effect on that activity in fetal or in adult liver microsomes, whereas a monoclonal antibody 1-68-11 (generated against rat CYP2C11) slightly inhibited benzphetamine N-demethylase activity in adult liver. This study indicates that human fetal and adult liver are dissimilar in their testosterone metabolism pattern. The formation of androstenedione from testosterone in fetal liver may have a physiological role. Testosterone hydroxylases are less inhibited by anti-CYP3A4 antibody, midazolam and progesterone in fetal than in adult liver.