Cytochromes P-450 in rats: structures, functions, properties and relevant human forms

Lack of Effect of Medium Supplementation with Pyruvate and Hormones on Cytochrome P450-mediated Activity of Rat Hepatocytes in Primary Culture

The loss of cytochrome P450 (CYP)-dependent activity continues to be a problem in the use of cultured hepatocytes in xenobiotic toxicity studies. It has been reported that the inclusion of pyruvate and various hormones in the culture medium improves the maintenance of various hepatic functions, including that of CYP2C11 mRNA expression. We have studied this further, by investigating the effects of the addition of pyruvate and hormones on various CYP-dependent enzyme activities and on the CYP-dependent toxicity of precocene II in rat hepatocyte cultures. No beneficial effects of this medium supplementation could be demonstrated.

Preliminary in vitro toxicological evaluation of a series of 2-pyridylcarboxamidrazone candidate anti-tuberculosis compounds

We have investigated the toxicity of a series of 2-pyridylcarboxamidrazones in vitro using a rat liver metabolism system as well as human erythrocytes and mononuclear leucocytes (MNL) as target cells. Of the seven derivatives and four precursors tested, only minimal (<2.3%) metabolism-mediated methaemoglobin was formed by two analogues. However, one of these, a naphthylidene 2-pyridylcarboxamidrazone derivative (compound III), was also directly toxic to human MNLs. This toxicity was partially attenuated by the rat metabolising system and incubation of diethyldithiocarbamate or cimetidine together with compound III and the rat metabolising system suppressed the metabolism-dependent detoxification. This indicated that cytochrome P-450-mediated biotransformation of compound III was preventing its direct toxicity to the MNL. Of the seven derivatives tested, six were low in toxicity to MNL directly and in the presence of a metabolising system. The two compounds which were the most potent anti-mycobacterially, the dimethylpropyl and dimethylethyl benzylidene amidrazone derivatives, were also the least toxic to MNL and erythrocytes. This amidrazone series has shown promise for future development as antituberculosis drugs.

In vitro biotransformation of atrazine by rat liver microsomal cytochrome P450 enzymes

We studied atrazine (ATZ) metabolism in male and female rat liver microsomes in vitro, and the major metabolite was deisopropylatrazine (DeiPr-ATZ) with deethylatrazine (DeEt-ATZ) and 1-hydroxyisopropylatrazine (iPrOH-ATZ) as minor metabolites in both sexes. The enzyme kinetics of ATZ biotransformation were examined by means of Eadie-Hofstee analyses. Although no remarkable sex difference of Michaelis Menten values for each pathway was observed, Cl(int)S (Vmax/Km) for DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ were slightly higher in female than in male rats. The formation of DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ from ATZ was substantially inhibited by SKF-525A, metyrapone, diallyl sulfide, 7-ethoxycoumarin, benzphetamine, nicotine, testosterone and lauric acid in both sexes. Cimetidine effectively inhibited the formation of all metabolites in male rats. On the other hand, the inhibition rates of the formation of DeiPr-ATZ and iPrOH-ATZ by cimetidine in female rats were lower than those in male rats, and DeEt-ATZ was hardly affected by the chemicals. In contrast with the results for cimetidine, the inhibition of ATZ biotransformation by bufuralol was more effective in female than in male rats. Anti-rat CYP2B1 and CYP2E1 antibodies effectively inhibited DeiPr-ATZ, DeEt-ATZ and iPrOH-ATZ formations in both sexes. Anti-rat CYP2C11 antibody also inhibited the three metabolites in both sexes, with the inhibition rates higher in male than in female rats, similar to cimetidine. In the case of anti-rat CYP2D1 antibody, the inhibitory effect on ATZ biotransformation in male rats was less than that in female rats. On the other hand, anti-rat CYP1A2, CYP3A2 and CYP4A1 antibodies did not affect the ATZ biotransformation in either sex. There was no significant correlation between the formation rate of ATZ metabolites and P450 isoform levels in either sex. These results may mean that CYP2B2, CYP2C11, CYP2D1 (only iPrOH-ATZ formation) and CYP2E1 in male rats, and CYP2B2, CYP2D1 and CYP2E1 in female rats are involved ATZ metabolism in liver, and that the substrate specificity of P450 isoforms for ATZ is broad.

Interindividual variability in P450-dependent generation of neoantigens in halothane hepatitis

Halothane hepatitis occurs because susceptible patients mount immune responses to trifluoroacetylated protein antigens, formed following cytochrome P450-mediated bioactivation of halothane to trifluoroacetyl chloride. In the present study, an in vitro approach has been used to investigate the cytochrome P450 isozyme(s) which catalyze neoantigen formation and to explore the protective role of non-protein thiols (cysteine and reduced glutathione). Significant levels of trifluoroacetyl protein antigens were generated when human liver microsomes, and also microsomes from livers of rats pre-treated with isoniazid, phenobarbital or beta-naphtoflavone, were incubated with halothane plus a nicotinamide adenine dinucleotidephosphate (NADPH) generating system. Immunoblotting studies revealed that the major trifluoroacetyl antigens expressed in vitro exhibited molecular masses of 50-55 kDa and included 60 and 80 kDa neoantigens recognized by antibodies from patients with halothane hepatitis. Much lower concentrations of halothane were required to produce maximal antigen generation in isoniazid-induced rat microsomes, as compared with phenobarbital or isosafrole-induced microsomes (0.5 vs 12.5 microl/ml). In isoniazid-induced microsomes, antigen generation was inhibited > 90% by the nucleophiles cysteine and glutathione and by the CYP2E1-selective inhibitors diallylsulfide and p-nitrophenol, but was unaffected by inhibitors of other P450 isozymes (furafylline, sulfaphenazole or triacetyloleandomycin). Neoantigen formation in six human liver microsomal preparations was inhibited in the presence of diallylsulfide, but not by furafylline, sulfaphenazole or triacetyloleandomycin, and exhibited marked variability which correlated with CYP2E1 levels. These results suggest that the balance between metabolic bioactivation by CYP2E1 and detoxication of reactive metabolites by cellular nucleophiles could be an important metabolic risk factor in halothane hepatitis.

Rainbow trout cytochrome P450s: purification, molecular aspects, metabolic activity, induction and role in environmental monitoring

Cytochromes P450 (P450s or CYPs) constitute a superfamily of heme-thiolate proteins that play important roles in oxidative metabolism of endogenous and exogenous compounds. This review provides some limited history but addresses mainly the research progress on the cytochrome P450s in rainbow trout (Oncorhynchus mykiss), their purification, structures at the primary level, role in metabolism, responses to chemicals and environmental pollutants, application to biomonitoring and the effect of various factors on their expression or activities. Information obtained to date suggests that the rainbow trout P450 systems are as complex as those seen in mammals. Fourteen P450s have been purified from liver or trunk kidney to relatively high specific content. cDNAs belonging to seven different P450 families have been documented from trout liver, kidney and ovary. Two CYP1A genes, nine cDNAs containing open reading frames, and a cDNA fragment were entered into GenBank. Among them, CYP2K1, CYP2K3, CYP2K4, CYP2M1, CYP3A27 and CYP4T1 are the most recently described forms. CYP2K1, CYP2M1 and CYP4T1 represent newly identified P450 subfamilies first described in the rainbow trout. In many cases, the cloned rainbow trout P450s have subsequently been expressed in heterologous expressions systems such as COS-7 cells, yeast and baculovirus infected insect cells. Some of the overexpressed P450 isoforms have been partially characterized. Potential future research directions are discussed.

Metabolism of nonylphenol by rat and human microsomes

The in vitro metabolism of [14C]-nonylphenols (NPs) by rat hepatic microsomes in vitro was examined. Product formation was NADPH dependent and inhibited by the cytochrome P450 inhibitors, piperonyl butoxide and SKF525. Hepatic microsomes isolated from various inducer-treated rats (including beta naphthoflavone, phenobarbital, ethanol, dexamethasone, and clofibrate which selectively induce CYP1A, 2B, 2E, 3A and 4A, respectively) all metabolized NPs. Only microsomes from phenobarbital-treated rats exhibited a significantly higher activity towards NPs and showed a different profile of NP metabolites compared to control, untreated rats. Microsomes from human CYP2B6 transfected cells with endogenous NADPH-P450 reductase activity but not microsomes from the non-transfected parent cells metabolized NPs. The metabolism of NPs using microsomes from phenobarbital-treated rats was inhibited by 4-amino-2, 6-dinitro-1-t-butylxylene, a specific CYP2B enzyme inhibitor. Addition of a general anti-CYP2B sera to the reaction mixture attenuated the enzyme activity of microsomes from phenobarbital-treated rats to metabolize NPs. This metabolic reaction was, however, insensitive to a specific anti-CYP2B1 sera that had been shown to inhibit enzyme activities attributed only to CYP2BI suggesting that the CYP2B2 pathway is predominant in NP metabolism. The results indicate that hepatic cytochrome P450 enzyme(s) can metabolize NPs and that CYP2B isozymes are probably involved.

Metabolism and drug interactions of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in transplant patients: are the statins mechanistically similar?

3-Hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) inhibitors are the most effective drugs to lower cholesterol in transplant patients. However, immunosuppressants and several other drugs used after organ transplantation are cytochrome P4503A (CYP3A, EC 1.14.14.1) substrates. Pharmacokinetic interaction with some of the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, specifically lovastatin and simvastatin, leads to an increased incidence of muscle skeletal toxicity in transplant patients. It is our objective to review the role of drug metabolism and drug interactions of lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, and cerivastatin. In the treatment of transplant patients, from a drug interaction perspective, pravastatin, which is not significantly metabolized by CYP enzymes, and fluvastatin, presumably a CYP2C9 substrate, compare favorably with the other statins for which the major metabolic pathways are catalyzed by CYP3A.

Medicinal herb, Thonningia sanguinea protects against aflatoxin B1 acute hepatotoxicity in Fischer 344 rats

1. Thonningia sanguinea, a plant used prophylactically against bronchial asthma in Ghana was recently found to have antioxidative and hepatoprotective actions in our laboratory. 2. In this study, the effect of T. sanguinea extract on certain biochemical indices in serum and liver of Fischer 344 rats given a single intraperitoneal (i.p.) dose (1 mg/kg) of aflatoxin B1 (AFB1) was investigated. 3. Administration of AFB1 resulted in significant increases in serum alanine aminotransferase (ALT) and glutathione S-transferase (GST) levels and a significant decrease in aniline hydroxylase activity in liver microsomes. When T. sanguinea (5 ml/kg) was intraperitoneally administered to rats 12 h and 1 h before AFB1, liver injury was significantly reduced as seen in the decreased levels of serum ALT and serum GST. However, the decrease in aniline hydroxylase activity by AFB1 was not recovered but enhanced by T. sanguinea pre-treatment. 4. Kinetic analysis of cytochrome P450 activity of rat liver microsomes in vitro demonstrated that T. sanguinea inhibited aniline hydroxylase non-competitively suggesting depression of biotransformation of AFB1 to toxic metabolites. 5. The data indicate a hepatoprotective action of T. sanguinea against AFB1-induced liver injury.

Implication of cytochrome P-450 1A isoforms and the AH receptor in the genotoxicity of coal-tar fume condensate and bitumen fume condensates

During the hot application of bitumen- or coal-tar-containing materials, fumes are emitted that contain polycyclic aromatic compounds. Although workers' exposure to these fumes is low, it might lead to health problems. No study has reported the metabolic pathways involved in the genotoxicity of coal tar or bitumen fume condensates (CTFC, BFCs). We have therefore studied the DNA adducts formed by incubation of CTFC or BFCs with liver microsomes from several type of mice and with yeast microsomes expressing individual human CYP enzymes. Our results demonstrates that: (1) the aryl hydrocarbon receptor (AHR) plays an important role in the biotransformation of BFCs and to a lesser extent of CTFC; (2) for CTFC, both cytochrome P450 (CYP) 1A isoforms are involved in the formation of genotoxic compounds, and the reactive metabolites formed via CYP 1A1, are substrates for epoxide hydrolase (mEH); (3) for BFCs, the genotoxicity is partially dependent upon CYP 1A1 and the reactive metabolites are not substrates for mEH; (4) CYP 1A isoforms are not exclusively responsible for the genotoxicity of the CTFC and BFCs as other CYPs and also enzymes of the [AH] gene battery, may play an important role.

Clinical importance of non-genetic and genetic cytochrome P450 function tests in liver disease

Liver disease is associated with reduced metabolic capacity for drugs that are metabolized by oxidative biotransformation. Three cytochrome P450 (P450 or CYP) gene families in liver microsomes (CYP 1, CYP2 and CYP3) appear to be responsible for much of the drug metabolism that takes place. The genetic polymorphism of the CYPs responsible for debrisoquine/ sparteine (CYP2D6) metabolism and S-mephenytoin (CYP2C19) metabolism has been well documented, but information on the impairment of each isoform in liver disease is still limited. There are two types of hepatic P450 function tests. One type consists of non-genetic P450 function tests (CYP1A2, 2A6, 2C9/10, 2E1 and 3A3/4), and probe drugs include caffeine, catalysed by CYP1A2, coumarin by CYP2A6, phenytoin by CYP2C6, chlorzoxazone by CYP2E1, and nifedipine, erythromycin and lidocaine by CYP3A3/4. The second type of genetic P450 function tests (CYP2C19 and CYP2D6) involves probe drugs such as S-mephenytoin, catalysed by CYP2C19, and debrisoquine and sparteine, catalysed by CYP2D6. The metabolism of the probe drugs used in non-genetic P450 function tests in patients with liver disease falls into two categories: reduced (CYP1A2, CYP2C, 2E1 and 3A) and unchanged (CYP2C). In genetic P450 function tests there seems to be a lesser degree of inhibition in poor metabolizers (PMs) than extensive metabolizers (EMs) among patients with liver disease. There have been very few reports on changes in metabolism of the probe drugs used in genetic P450 function tests in liver disease. In this paper the subject is reviewed.

Differential selectivity of cytochrome P450 inhibitors against probe substrates in human and rat liver microsomes

AIMS

Chemical inhibitors of cytochrome P450 (CYP) are a useful tool in defining the role of individual CYPs involved in drug metabolism. The aim of the present study was to evaluate the selectivity and rank the order of potency of a range of isoform-selective CYP inhibitors and to compare directly the effects of these inhibitors in human and rat hepatic microsomes.

METHODS

Four chemical inhibitors of human cytochrome P450 isoforms, furafylline (CYP1A2), sulphaphenazole (CYP2C9), diethyldithiocarbamate (CYP2E1), and ketoconazole (CYP3A4) were screened for their inhibitory specificity towards CYP-mediated reactions in both human and rat liver microsomal preparations. Phenacetin O-deethylation, tolbutamide 4-hydroxylation, chlorzoxazone 6-hydroxylation and testosterone 6beta-hydroxylation were monitored for enzyme activity.

RESULTS

Furafylline was a potent, selective inhibitor of phenacetin O-deethylation (CYP1A2-mediated) in human liver microsomes (IC50 = 0.48 microM), but inhibited both phenacetin O-deethylation and tolbutamide 4-hydroxylation (CYP2C9-mediated) at equimolar concentrations in rat liver microsomes (IC50 = 20.8 and 24.0 microM respectively). Sulphaphenazole demonstrated selective inhibition of tolbutamide hydroxylation in human liver microsomes but failed to inhibit this reaction in rat liver microsomes. DDC demonstrated a low level of selectivity as an inhibitory probe for chlorzoxazone 6-hydroxylation (CYP2E1-mediated). DDC also inhibited testosterone 6beta-hydroxylation (CYP3A-mediated) in man and rat, and tolbutamide 4-hydroxylase activity in rat. Ketoconazole was a very potent, selective inhibitor of CYP3A4 activity in human liver (IC50 = 0.04 microM). Although inhibiting CYP3A in rat liver it also inhibited all other reactions at concentrations < or = 5 microM.

CONCLUSIONS

It is evident that CYP inhibitors do not exhibit the same selectivity in human and rat liver microsomes. This is due to differential selectivity of the inhibitors and/or differences in the CYP isoform responsible for metabolism in the different species.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.

Identification of cytochrome P4503A as the major enzyme sub-family responsible for the metabolism of 22,23-dihydro-13-O-[(2-methoxyethoxy)methyl]-avermectin B1 aglycone by rat liver microsomes

1. Metabolism of 22,23-dihydro-13-O-[(2-methoxyethoxy)methyl]-avermectin B1 aglycone (MEM-H2B1), a new avermectin, by rat liver microsomes has been studied. Metabolites identified were formed by demethylation of the methoxyethoxymethoxy (MEM) side chain, loss of the MEM side chain, partial cleavage and further oxidation of the MEM side chain, and oxidation of the aglycone after cleavage of the MEM side chain. 2. The specific cytochrome P450 isoforms involved in the metabolism of MEM-H2B1 were identified through immunoinhibition studies. Among several antibodies prepared against various cytochrome P450s, only anti-rat P4503A IgG inhibited MEM-H2B1 metabolism by liver microsomes from the untreated rat. Moreover, troleandomycin, a selective suicide inhibitor for enzymes of the cytochrome P4503A family, inhibited the total metabolism by > 80%. These results clearly indicate that cytochrome P4503A is primarily responsible for the metabolism of MEM-H2B1. 3. Secondary metabolism was evident in the metabolism of MEM-H2B1 by dexamethasone and phenobarbital induced liver microsomes, where different isoform(s) of cytochrome P4503A could be involved in these multiple step reactions.

Genotoxicity of industrial dyes under the inductive effect of ethanol on monooxygenase system in mice

The genotoxic effects of triarylmethane (Acid Green 16, C.I.44025) and arylmonoazo (Basic Orange 28, developed by Boruta Pigment Plant, Poland, C.I. undisclosed) dyes, were evaluated in Balb/C mice. Animals were fed for 6 days nutritionally adequate Portagen liquid diet (1 kcal/ml) or isocaloric alcoholic diet containing 5% (w/v) ethanol (36% of total calories) in order to induce the cytochrome P-4502E1 monooxygenase. Dye compounds were administered intraperitoneally 30 h before the test at doses: 90 mg/kg of Acid Green 16 and 70 mg/kg of Basic Orange 28. Bone marrow micronucleus test was used for evaluation of genotoxicity of the dyes. Ethanol caused an increase of the level of cytochrome P-450 by 200% and activities of 7-ethoxycoumarin O-deethylase (ECOD) by 650%, 7-ethoxyresorufin O-deethylase (EROD) by 460% and glutathione (GSH)-S-transferase by 60% in the liver. Both dyes exerted genotoxic effect as inferred from a 3-fold increase of frequency of micronucleated polychromatic erythrocytes in bone marrow, and a further increase (2-fold) was caused by ethanol liquid diet combined with Acid Green 16 treatment. Basic Orange 28 genotoxicity remained unaffected by ethanol. It is concluded that: (1) enhancement of genotoxic effect of Acid Green 16 by ethanol is caused by induction of cytochrome P-4502E1 monooxygenases resulting in an increased bioactivation of the dye; (2) lack of enhancement of the genotoxic effect of Basic Orange 28 by ethanol probably results from the dye- and ethanol-mediated stimulation of GSH-S-transferase, bypassing the cytochrome P-4502E1 bioactivation step.

Molecular modelling of mammalian CYP2B isoforms and their interaction with substrates, inhibitors and redox partners

1. The construction of three-dimensional models of CYP2B isozymes from rat (CYP2B1), rabbit (CYP2B4) and man (CYP2B6), based on a multiple sequence alignment with CYP102, a unique eukaryotic-like bacterial P450 (in terms of possessing an NADPH-dependent FAD- and FMN-containing oxidoreductase redox partner) of known crystal structure, is reported. 2. The enzyme models described are shown to be consistent with experimental evidence from site-directed mutagenesis studies, antibody recognition sites and amino acid residues identified as being associated with redox partner interactions, together with the location of a key serine residue (Ser-128) likely to be involved in protein kinaseA-mediated phosphorylation. 3. A substantial number of known substrates and inhibitors of CYP2B isozymes are shown to fit the putative active sites of the enzyme models in agreement with their reported position of metabolism or mode of inhibition respectively. In particular, there is complementarity between the characteristic non-planar geometries of CYP2B substrates and key groups in the enzymes' active sites. 4. Molecular modelling of CYP2B isozymes appears to rationalize a number of the reported findings from quantitative structure-activity relationship investigations on series of CYP2B substrates and inhibitors.

Evidence for nitric oxide participation in down-regulation of CYP2B1/2 gene expression at the pretranslational level

Septic or inflammatory stimuli suppress drug metabolism by cytochrome P-450 in the liver, presumably at the pretranslational level. We have shown previously that nitric oxide is responsible at least in part for the inhibition by bacterial lipopolysaccharide of phenobarbital-induced CYP2B1/2 activity in vivo. This was attributed to the interaction of nitric oxide with heme in the active-center of cytochrome P450, leading to enzyme inactivation. Here, we report that endogeneous nitric oxide also contributes to LPS-induced suppression of CYP2B1/2 in vivo by down-regulating the expression of CYP2B1/2 protein and mRNA.

Strain differences in adrenal CYP2D16 expression in guinea pigs. Relationship to xenobiotic metabolism

Experiments were done to determine the mechanisms responsible for differences in adrenal microsomal xenobiotic metabolism between Strain 13 and English Short-Hair (ESH) guinea pigs. The rates of adrenal xenobiotic metabolism (bufuralol 1'-hydroxylase, benzo[a]pyrene hydroxylase, benzphetamine N-demethylase) were 2-3 times greater in microsomes from the Strain 13 animals. In both strains, xenobiotic-metabolizing activities were far greater in the inner zone (zona reticularis) than in the outer zones (zona fasciculata and zona glomerulosa) of the adrenal cortex. Northern blot analyses of total adrenal RNA with a CYP2D16 cDNA as the probe revealed significantly greater amounts of CYP2D16 mRNA in the Strain 13 guinea pigs. In addition, SDS-PAGE and Western blotting of adrenal microsomes demonstrated higher concentrations of CYP2D16 protein in Strain 13 than in ESH animals. Expression of CYP2D16 was predominantly in the inner zone of the adrenal, coinciding with the major site of xenobiotic metabolism. The results demonstrated higher levels of expression of CYP2D16 in adrenal glands from Strain 13 than from ESH guinea pigs, which may account for the strain differences in adrenal xenobiotic metabolism. Strain 13 guinea pigs should serve as a good experimental model for further studies on the regulation of adrenal CYP2D16.

Studies on the toxicity of analogues of dapsone in-vitro using rat, human and heterologously expressed metabolizing systems

Three metabolizing systems (rat, heterologously expressed CYP3A4 and human liver) were used to evaluate 12 analogues of dapsone (4,4'diaminodiphenylsulphone) in-vitro. Methaemoglobin formation in a two-compartment and cytotoxicity in a single-compartment model were studied using human erythrocytes and neutrophils, respectively, as target cells. In the two-compartment system using rat microsomes as a generating system and methaemoglobin as an endpoint, the least potent methaemoglobin formers tested were the 2-methyl-4-propylamino (AXDD14), 2-hydroxy-4-4'amino (ABDD5) derivatives and a sulphone/trimethoprim derivative (K-130). Dapsone itself, a 2-methoxy-4-ethylamino (W10) and a 2-hydroxyl-4-ethylamino compound (ABDD39) were the most toxic. In the single-compartment cytotoxicity test using rat microsomes, AXDD14 was again among the least toxic, as was a 2-methyl 4-cyclopentyl derivative (AXDD17) and surprisingly ABDD39. The most cytotoxic compounds again included dapsone itself as well as two 2-trifluoromethyl derivatives. The only significant methaemoglobin formation and cytotoxicity shown with the heterologously expressed human CYP 3A4 was with AXDD14, which was extensively activated. Interestingly, metabolism of dapsone was low using the expressed CYP 3A4. In the two-compartment system using human liver microsomes, AXDD14, K-130 and ABDD5 were oxidized to a significantly lesser extent compared with dapsone and these preliminary findings indicate that future development of these compounds may be worthwhile.

Molecular modelling of CYP1A subfamily members based on an alignment with CYP102: rationalization of CYP1A substrate specificity in terms of active site amino acid residues

1. Using a novel amino acid sequence alignment, proteins of the CYP1A subfamily have been produced from the CYP102 crystal structure template via residue replacement and energy minimization procedures. 2. Known substrates and inhibitors of CYP1A1 and CYP1A2 are shown to fit their respective active sites via key interactions with complementary amino acid residues. Substrates used in the modelling studies include: caffeine, PhIP, oestradiol, 2,4- and 2,5-diaminotoluenes, Glu-P-1, phenacetin, acetanilide, 7-methoxy and 7-ethoxyresorufins, 11-methyl cyclopenta[a]phenanthren-17-one, 7-ethoxycoumarin, aflatoxin B1, benzo[a]pyrene, benzo[a]pyrene-7,8-diol and 1'-hydroxy 3-methylcholanthrene. 3. A number of aspects relating to CYP1A substrate specificity and metabolism can be explained in terms of the enzyme models, as it is found that key interactions with active site amino acid residues direct CYP1A-mediated metabolism in the known positions.

Multiple forms of human P450 expressed in Saccharomyces cerevisiae. Systematic characterization and comparison with those of the rat

We systematically characterized the levels and substrate specificity of P450s from humans and rats to extrapolate drug metabolism data from experimental animals to humans. Human P450s (CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C18, 2D6, 2E1, and 3A4) were expressed in Saccharomyces cerevisiae and purified. Rat P450s were purified from hepatic microsomes of rats. We investigated the catalytic activities of purified P450s in a reconstituted system. Human CYP2B6 and rat CYP2B1 had high lidocaine N-deethylation activity. Human and rat CYP2D forms had high debrisoquine 4-hydroxylation activity. Human CYP3A4 and rat CYP3A2 had high testosterone 2 beta- and 6 beta-hydroxylation activities in a modified reconstituted system with a lipid mixture. The hydroxylation site of testosterone by CYP2B6 (16 alpha- and 16 beta-positions) agreed with that by rat CYP2B1. Human CYP2E1 had the highest lauric acid (omega-1)-hydroxylation activity and also had catalytic properties similar to those of rat CYP2E1. Human CYP2A and 2C forms had catalytic properties in testosterone metabolism different from those of rats. Antibodies raised against purified P450s were used to measure the levels of hepatic P450s. The level of CYP3A4 was the highest in human hepatic microsomes, comprising 30-40% of the total P450. CYP2C9 comprised 10-20% of the total. The levels of CYP1A2, 2A6, 2C8, 2D6, and 2E1 were moderate (5-15% of total P450). CYP2B6 content was very low. The information of this study is useful for drug metabolism and toxicological studies.

Alternative cyclosporine metabolic pathways and toxicity

There are some indications from clinical studies (41,43) for aberrant cyclosporine metabolism resulting in formation of potentially toxic metabolites. When the activity of cytochrome P450 3A enzymes is low, more substrate is available for hypothetical alternative pathways of cyclosporine. There are several reasons for low P450 3A activity in a liver graft such as inter-individual genetic variability (43,49,84), cold ischemia and reperfusion damage, changes of the P450 activity during cholestasis (85) or other liver diseases (86), the influence of cytokines (87) and drug interactions such as inhibition or enzyme induction (88). Furthermore, low concentrations of cytochrome P450 3A influence the cyclosporine blood trough concentrations. The P450 3A concentration as estimated by the erythromycin breath test can be used to calculate the initial cyclosporine dose required to obtain cyclosporine blood trough concentrations in the therapeutic window (89). In vitro such alternative pathways comprising 3-methylcholanthrene-inducible (44,46,47) and/or ethinyl estradiol-inducible cytochrome P450 enzymes (48) could be identified and resulted in production of cyclized cyclosporine metabolites. The exact identification of the P450 enzymes involved requires metabolism of cyclosporine using reconstituted purified enzymes or single P450 enzymes expressed in cell lines. In addition, it remains to be clarified whether cyclosporine itself or its metabolite AM1 is the substrate for cyclization. Because cyclized metabolites have a low affinity to cyclophilin (58,59) they are mainly found in plasma. When more cyclized metabolites are formed primarily the concentration of cyclosporine metabolites in plasma increases. The free fraction of cyclosporine at 37 degrees C was found to be 1%-1.5% (90,91) of the cyclosporine concentration in blood. To date, nothing is known about the free fraction of cyclosporine metabolites. Because distribution characteristics of the cyclized metabolites in blood and urine are different from those of cyclosporine, it can be speculated that the free fraction of the cyclized metabolites is higher than that of cyclosporine. This might be reflected by a higher renal clearance resulting in relatively higher concentrations in urine compared with blood (61; Figure 3). If this is the case, a shift in the metabolite pattern with increased concentrations of cyclized metabolites will lead to an overproportional increase of the free fraction of cyclosporine metabolites. Although it is tempting to assume that cyclization is the alternative pathway explaining cyclosporine toxicity in patients with low concentrations of P450 3A enzymes in the liver (Figure 6), this has not yet been proven and will require not only quantification of P450 3A but of the complete P450 enzyme pattern in the liver in combination with characterization of the cyclosporine metabolite pattern by HPLC with special respect to the cyclized metabolites AM1c and AM1c9. Also, it is still unclear whether or not the cyclized metabolites contribute to cyclosporine toxicity. At least, it is unlikely that they are involved in covalent binding to macromolecules in the liver and kidney (44,71). In a clinical study using an HPLC method which allowed the specific quantification of 16 cyclosporine metabolites it was shown that the blood trough concentrations of the cyclized metabolite AM1c9 is elevated during early nephrotoxicity in liver graft recipients (82) and it was shown in an in vitro model that AM1c9 increases endothelin production and therefore might have a negative effect on renal hemodynamics.(ABSTRACT TRUNCATED)

Effect of cyclophosphamide administration on the activity and relative content of hepatic P4502D1 in rat

1. The effects of the administration of the anticancer and immunosuppressive drug, cyclophosphamide, to the rat on hepatic P4502D1 activity and content in the microsomal fraction have been examined. 2. Liver microsomes were obtained from male Hooded Wistar rats administered a single dose (i.p.) of saline or cyclophosphamide (200 mg/kg). Rats receiving cyclophosphamide were killed 1, 4, 7, 10 or 14 days after cyclophosphamide administration. The O-demethylation of dextromethorphan to dextrorphan was used to monitor 2D1 activity. 3. The mean Vmax for dextrorphan formation was reduced significantly (p < 0.0001) 7, 10 and 14 days after cyclophosphamide administration compared with the control group (control, 0.32 +/- 0.07; 7-day, 0.20 +/- 0.08; 10-day, 0.11 +/- 0.02; and 14-day group, 0.15 +/- 0.02 nmol/mg/min). 4. Western blotting revealed that there was a significant reduction (p < 0.0005) in the microsomal relative 2D1 content 10 days after cyclophosphamide administration compared with the control group (control, 1.25 +/- 0.44; and 10-day group, 0.65 +/- 0.14). 5. The activity of reduced nicotinamide adenine dinucleotide phosphate P450 reductase was significantly reduced (p < 0.0001) 7, 10 and 14 days following cyclophosphamide administration (control, 215 +/- 24; 7-day, 102 +/- 20; 10-day, 59 +/- 4 and 14-day group, 76 +/- 8 nmol/mg/min). Cytochrome b5 content was significantly reduced (p < 0.0001) 7 and 10 days following cyclophosphamide administration (control, 0.46 +/- 0.13; 7-day, 0.28 +/- 0.07 and 10-day group, 0.20 +/- 0.03 nmol/mg). 6. The significant reductions in the activity of rat hepatic microsomal 2D1 following cyclophosphamide administration, as seen by the alterations in mean Vmax for dextrorphan formation, do not appear to be due to a single factor, but may result from a combination of several events, including reductions in relative 2D1 content, reduced nicotinamide adenine dinucleotide phosphate P450-reductase activity and cytochrome b5 content.

Changes in the metabolism of three model substrates catalysed by different P450 isozymes when administered as a cocktail to the carbon tetrachloride-intoxicated rat

1. Caffeine (CA) metabolism in animal and man is mainly catalysed by P4501A2. Lidocaine (LID) and phenytoin (PHT) are metabolized by P4503A2 and 2B1/2 in animals and 3A4 and 2C9 in man, respectively. 2. We investigated the possibility of predicting liver damage from changes in blood concentrations after simultaneous i.v. administration (cocktail study) of the three probe drugs CA (10 mg/kg), LID (4 mg/kg) and PHT (4 mg/kg), and their main metabolites, paraxanthine (PX), monoethylglycinexylide (MEGX) and 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) respectively. 3. The metabolism of CA, LID and PHT and production of their metabolites (PX, MEGX and p-HPPH) in the carbon tetracholride (CCl4 0.25 and 0.5 ml/kg)-treated rat were reduced in comparison with the control group. 4. The ratios (PX/CA and p-HPPH/PHT) of CA and PHT to the serum levels of the their metabolites 2 h after i.v. administration of three drugs to the CCl4-treated rat were significantly reduced. 5. The correlation coefficients among CLs of CA, LID and PHT, and the PX/CA and p-HPPH/PHT ratios in rat pretreated with different doses of CCL4 are very high. 6. These results suggest that different estimates of hepatic oxidizing capacity, catalysed mainly by P4501A2, 2C and 3A, may be related to the extent of liver disease in the CCl4-intoxicated rat. Therefore a 'cocktail' is not in fact needed to assess hepatic damage in the CCl4-intoxicated rat.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Comparison of hepatic and extrahepatic drug-metabolizing enzyme activities in rats given single or multiple challenge infections with Fasciola hepatica

The activity of drug-metabolizing enzymes was compared in liver, kidneys and lungs of rats given single or repetitive fluke infections. Fascioliasis was induced by oral administration of 20 metacercariae of F. hepatica to rats, either 6, or 12 and 6, or 12, 9 and 6 weeks before sacrifice. In the liver of mono-infected rats, significant reductions (24-67%) were observed in microsomal content of cytochrome P450 and all P450-dependent monooxygenases investigated. Conjugations to glutathione or acetate were lowered by 34-50% in these animals. In multiply infected animals, a majority of specific enzymatic activities were unchanged, while some monooxygenase activities such as aminopyrine demethylation or benzo(a)pyrene hydroxylation were increased by 26-76% in the liver of tri-infected rats. A renal compensatory process occurred in all infected groups, since cytochrome P450, benzphetamine demethylation and glutathione conjugation were significantly increased. By contrast, dealkylation of benzphetamine and pentoxyresorufin were decreased in the lungs of monoinfected rats. The development of parasite resistance would account for the recovery of liver drug metabolizing capabilities in multi-infected animals.

The hepatoprotective cytochrome P-450 enzyme inhibitor isolated from the Nigerian medicinal plant Cochlospermum planchonii is a zinc salt

Aqueous extracts of Cochlospermum planchonii Hook.f. (Cochlospermaceae) rhizomes are used by native medical practitioners in northern Nigeria to treat jaundice. An extract prepared by a laboratory adaptation of their method was hepatoprotective in carbon tetrachloride-treated rats (CCl4), and it inhibited cytochrome P-450 enzymes, which constitutes a plausible hepatoprotective mechanism. A crystalline inhibitor (0.3% of dry weight of rhizomes) was isolated using inhibition of two rat cytochrome P-450 enzymes, aminopyrine-N-demethylase and aniline hydroxylase, as bioassays to guide fractionation by solvent partitioning, polyamdie column chromatography, preparative thin layer chromatography and fractional crystallization. The inhibitor was identified as zinc formate by inductively coupled plasma atomic emission spectroscopy, nuclear magnetic resonance spectroscopy and comparison with synthetic material by power X-ray diffraction crystallography. Synthetic and plant-derived zinc formate were equally effective as inhibitors of cytochrome P-450 enzymes and as hepatoprotective agents in carbon tetrachloride-treated rats. Cochlospermum planchonii rhizomes contain unusually high levels of manganese and zinc, although much higher levels have been observed in plants considered to be hyperaccumulators of these metals.

Effect of fluvastatin, an inhibitor of 3-hydroxy-3-methyl glutaryl CoA reductase, on drug-metabolizing enzymes in rats

Fluvastatin (FV), a new cholesterol-lowering agent, has been studied for its effects on hepatic microsomal drug-metabolizing enzymes in male rats. FV was orally administered in dosages of 1, 5, and 30 mg/kg/day for 7 consecutive days. 3-Hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase activities were markedly decreased at all dose levels. The amount of microsomal protein and the contents of cytochromes P450 and b5 did not change. No induction of aniline hydroxylase, aminopyrine N-demethylase, testosterone hydroxylases (15 alpha-, 7 alpha-, 6 beta-, 16 alpha-, and 16 beta-), and UDP-glucuronosyltransferase were found. On the other hand, 7-ethoxycoumarin o-deethylase activity was slightly increased and lauric acid omega-1-hydroxylase activity tended to be decreased after treatment with FV.

Effect of dimethyl sulfoxide on N-(3,5-dichlorophenyl)succinimide (NDPS) and NDPS metabolite nephrotoxicity

Dimethyl sulfoxide (DMSO) is frequently used as a solvent to assist in dissolving compounds which are not readily soluble in other injection vehicles. The purpose of this study was to determine the suitability of DMSO as a vehicle for administering the nephrotoxicant, N-(3,5-dichlorophenyl)succinimide, (NDPS) and two nephrotoxicant NDPS metabolites, N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (NDHSA). Male Fischer 344 rats (4/group) were administered a single intraperitoneal injection of NDPS (0.4 or 0.8 mmol/kg), NDHS (0.1 or 0.2 mmol/kg), or NDHSA (0.1 or 0.2 mmol/kg) dissolved in 25% DMSO in sesame oil or 100% sesame oil (2.5 ml/kg), while control rats received vehicle only. Renal function was then monitored at 24 and 48 h. Including DMSO in the vehicle markedly attenuated NDPS 0.4 mmol/kg-induced nephrotoxicity and reduced NDPS 0.8 mmol/kg-induced renal effects. Thus, the magnitude of the attenuating effect of DMSO depended in part on the nephrotoxicant dose of NDPS. In addition, NDHS nephrotoxicity was not altered by DMSO and only slight effects on NDHSA nephrotoxicity were observed. These results suggest that DMSO is capable of attenuating NDPS nephrotoxicity, and that the primary mechanism of this interaction might be due to an inhibition of the biotransformation of NDPS to NDHS.

The subcellular distribution of NADPH-cytochrome P450 reductase and isoenzymes of cytochrome P450 in the lungs of rats and mice

NADPH-cytochrome P450 reductase was localized by ultrastructural immunocytochemistry in the cytoplasm of Clara cells, ciliated cells and type II pneumocytes of young female Wistar rats and MF1 mice. Immunogold labelling was particularly dense over the apical cytoplasm of Clara cells. CYP1A was only detected in lungs of animals treated with Aroclor 1254 or 3-methylcholanthrene; it was localized in the cytoplasm of Clara cells of rats and in the endothelial cells of both rats and mice. In rat lung the labelling of cytochrome P450 CYP2B was most intense in the apical cytoplasm of Clara cells, but it was also found in both the type I and type II cells of the alveolar epithelium. Similar immunoreactivity was found in the Clara and type II cells of mice, it was also found in the ciliated cells of this species. CYP4B was localized in the cytoplasm of Clara and type II cells of rats. The cell-specific concentration of these isoenzymes, particularly that of CYP2B in the cytoplasm of type I cells, probably contributes to the particular susceptibility of these cells to pneumotoxins.

Induction of CYP2B1 mediated pentoxyresorufin O-dealkylase activity in different species, sex and tissue by prototype 2B1-inducers

The induction of CYP2B1 mediated pentoxyresorufin O-dealkylase (PROD) activity by various xenobiotics was explored in liver, kidney and lung from a variety of animal species of both sexes, in order to gain insights into the substrate specificity of induced CYPs. Marked species- and sex-related differences in the inducibility of PROD activity by tested chemicals were observed, the mouse being always more responsive when compared to hamster or rat. Induction by sodium phenobarbital (NaPB) led to a conspicuous increase in all situations, up to approximately 38-fold in female rat and mouse liver, with the exception of hamster kidney where PROD activity was only slightly affected. Unexpectedly, both sodium barbital (NaB) and phorone (PHR) moderately induce CYP2B1 isoforms in rat, the extent being highest in female kidney (PHR, 14-fold increase) and male lung (NaB, 4.5-fold). The degree of induction was maximal in the liver with some exceptions occurring in male mice where NaB induced up to 46- and 115-fold increases in lung and kidney and PHR up to 115-fold in kidney. Minimal, although significant induction of PROD activity following treatment with trans-1,2-dichloroethylene (1,2-DCE) occurred in all situations with the exception of hamster kidney and lung. Therefore, caution should be exercised when using PROD activity as specific enzymatic assay to probe CYP2B1-like induction.

Determination of theophylline and its metabolites in rat liver microsomes and human urine by capillary electrophoresis

A capillary electrophoretic (CE) method has been developed for the determination of theophylline and all of its identified and potential metabolites. The method is rapid, resolves all metabolites to baseline, and requires extraction of only some biological fluids. It has been applied to the analysis of theophylline metabolism by hepatic microsomes from rats treated with a variety of inducing agents for different forms of P450 enzymes which metabolize theophylline, and to human urine spiked with theophylline and its metabolites, and concentrated by solid-phase extraction.

Effect of d-fenfluramine on the indole contents of the rat brain after treatment with different inducers of cytochrome P450 isoenzymes

The effects of pretreatment with inducers of hepatic cytochrome P450 isoenzymes (phenobarbital, dexamethasone and beta-naphthoflavone) on the metabolism of d-fenfluramine (d-F) and its acute and long-lasting indole-depleting effects were studied in rats, in an effort to obtain further information on the importance of hepatic drug metabolism in relation to its neurochemical actions. Twenty-four hours after the last dose of each inducer, rats were injected with d-F hydrochloride (5 mg/kg, IP) and killed at various times thereafter for parallel determination of indoles and drug concentrations in plasma and brain. Additional rats were treated as above and killed 1 week after d-F hydrochloride (5 and 10 mg/kg) to study the recovery of indole in the cortex, a particularly sensitive brain area. Phenobarbital and beta-naphthoflavone and, to a lesser degree, dexamethasone, stimulated the metabolism of d-F, as evidenced by a decrease in plasma and brain areas under the curve (AUC) compared to vehicle-treated rats. This indicated that multiple isoenzymes are capable of mediating the drug's metabolism, primarily by N-dealkylation to d-norfenfluramine (d-NF). None of the inducers raised plasma and brain AUC of the nor-derivative, and in fact phenobarbital and particularly beta-naphthoflavone reduced it. These different effects were even apparent in rats given d-NF (2.5 mg/kg), indicating that both phenobarbital and beta-naphthoflavone also stimulate the sequential metabolism of the nor-metabolite (by N-deamintaion) which, however, is apparently enhanced most actively by beta-naphthoflavone-inducible forms of P-450.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of rat liver microsomal epoxide hydrolase by its endogenous substrate 16 alpha, 17 alpha-epoxyestra-1,3,5-trien-3-ol

1. The influence of the endogenous steroid epoxides 16 alpha, 17 alpha-epoxyestra-1,3,5(10)-trien-3-ol (estroxide) and 16 alpha, 17 alpha-expoxiandrost-4-en-3-one (androstene oxide) and their metabolic precursors estra-1,3,5(10), 16-tetraen-3-ol (estratetraenol) and androsta-4, 16-dien-3-one (androstadienone) on the specific activities of hepatic microsomal and soluble epoxide hydrolase, glutathione S-transferase, dihydrodiol dehydrogenase, and 7-ethoxycoumarin deethylase was investigated in the male Sprague-Dawley rat. 2. Both estroxide and estratetraenol induced microsomal epoxide hydrolase activity towards styrene oxide and estroxide itself 2-2.5-fold and glutathione conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) 1.6-fold after intraperitoneal administration of a high dose of compound (300 mg per kg of body weight). 3. In addition, estroxide decreased 7-ethoxycoumarin deethylation down to 20% of the activity observed in the untreated rat, whereas estratetraenol enhanced the activity of soluble epoxide hydrolase towards trans-stilbene oxide by a factor of 1.7. 4. In contrast, neither androstene oxide nor androstadienone showed a significant influence on any of the parameters under investigation. Dihydrodiol dehydrogenase was not significantly changed by any of the treatments.

Induction of cytochrome P-450 isoenzymes in cultured monkey hepatocytes

The effect of phenobarbital (PB), beta-naphthoflavone (beta-NF) and rifampicin (Rif) on the drug-metabolizing activity of cultured squirrel monkey hepatocytes was examined. The drug metabolizing activity (e.g. alkoxycoumarin dealkylase or steroid hydroxylase) gradually decreased during the culture period with 40-70% activity remaining at 72 hr. When 0.5 mM PB was added to the culture, the activities of 7-methoxycoumarin O-demethylase (MCOD) and 7-ethoxycoumarin O-deethylase (ECOD) increased to 6-7 fold higher level than those of control at 72 hr. Testosterone 6 beta-hydroxylase (6 beta-OH-T) and testosterone 16 beta-hydroxylase (16 beta-OH-T) activities were approx. 3-fold higher than those of the control. Addition of beta-NF significantly increased the activities of 7-ethoxyresorufin O-deethylase (EROD) and ECOD. Though statistically insignificant, Rif slightly increased 6 beta-OH-T activity. Western blot analysis indicated PB induced production of the CYP 2B and 3A subfamilies, while beta-NF and Rif induced that of the CYP 1A and the CYP 3A subfamily, respectively.

Induction of rat hepatic P4501A1 by organic extracts from airborne particulate matter in Santiago, Chile

1. Organic extracts from particulate matter collected in downtown Santiago, Chile, in 1990 were administered to female Wistar rats. 2. Extracts shifted the maximal absorption wavelength P450 spectra of the reduced-CO complex of hepatic microsomal P450 from 450 to 448 nm, and enhanced the total content of P450. In addition, substantial increases in aryl hydrocarbon hydroxylase, ethoxyresorufin O-deethylase and ethoxycoumarin O-deethylase activities were observed, whereas aminopyrine N-demethylase activity was not affected by treatment. 3. Western blotting using polyclonal antibodies against P4501A isozymes showed the appearance of a distinct and intense P4501A1 band in microsomes from rat pretreated with air particle extracts, and was not observed in microsomes from control rat. On the other hand, the intensity of the P4501A2 isoenzyme was apparently not affected. 4. These findings suggest that the organic extracts from airborne particulate matter modify the composition of rat liver P450 isozymes by inducing those isoforms responsible for the activation of precarcinogenic to carcinogenic agents.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.

P450 in the rat and man: methods of investigation, substrate specificities and relevance to cancer

1. Considerable evidence has been accumulated that orthologous rat and human P450 forms oxidize numerous chemicals in a highly similar manner, including the detoxication and activation of mutagens and carcinogens. 2. Nevertheless, certain specific substrates of rat P450s are not so well oxidized by the orthologous human forms, and vice versa. 3. Certain mutagens and carcinogens can be activated in a similar way by different (non-orthologous) forms in rat and man, confirming that studies on animals, directed ultimately to man, can be indicative but not predicative of chemical mutagenesis and carcinogenesis.

Characterisation of praziquantel metabolism by rat liver microsomes using cytochrome P450 inhibitors

The metabolism of praziquantel (PZQ) was studied in microsomes isolated from livers of differently pretreated rats and in the presence of various inhibitors of cytochrome P450 (P450) isoforms. Microsomes from phenobarbitone (PB)-pretreated rats metabolised PZQ to its major metabolite 4OH-praziquantel (4OH-PZQ) at a greater rate than those from 20-methylcholanthrene (MC) and saline (SA) pretreated rats. The Vmax for the PB microsomes was 600 nmol 4OH-PZQ formed/mg/min x 10(-3) compared to 91.4 nmol/mg/min x 10(-3) for MC and 238 nmol/mg/min x 10(-3) for SA microsomes. These results indicate that PZQ is metabolised by PB-inducible isoforms of P450. Inhibitor studies were conducted with microsomes from SA-pretreated animals. In these studies, caffeine, disulfuram, and tolbutamide were poor inhibitors of the metabolism of PZQ to 4OH-PZQ, with I50 values not determinable. Quinidine and quinine inhibited the hydroxylation of PZQ but with high Ki values. 17 alpha-Ethynylestradiol, cimetidine and diphenylhydramine were effective inhibitors of the formation of 4OH-PZQ, with 17 alpha-ethynylestradiol being the most potent with a Ki of 0.5 +/- 0.05 microM. From the known specificities of these P450 inhibitors, it is therefore concluded that cytochromes P450 1A2, 2E1, 2C9-10, and 2D6 probably do not contribute significantly to the metabolism of PZQ to its major metabolite in rats. It is likely that cytochromes P450 2B1 and 3A, both inducible by PB, are predominantly responsible for the formation of 4OH-PZQ.

Cytochrome P450 destruction and radical scavenging by benzene and its metabolites. Evidence for the key role of quinones

Exposure to benzene was reported to lower the cytochrome P450 (CYP; EC 1.14.14.1) content in phenobarbital-pretreated (PB) rats in vivo (Gut I, Zbl Pharm 122: 1139-1161, 1983). In this paper we followed the ability of benzene and its metabolites, phenol, catechol, hydroquinone and benzoquinone to destroy CYP in liver microsomes from PB rats in vitro. The spectrophotometric determinations of the total CYP content, 7-pentoxyresorufin O-depentylase and aniline hydroxylase activities, electrophoresis and western blot analysis after incubation of PB-microsomes with benzene or its metabolites revealed that: (1) benzene is metabolically activated to intermediates causing CYP destruction; phenol is not responsible for this effect. (2) Quinonic metabolites of benzene cause CYP destruction with different potency (30% CYP was destroyed by 3 mM catechol, 0.3 mM hydroquinone and 0.03 mM benzoquinone). (3) Low concentrations of quinones are capable of protecting CYP against reactive oxygen species produced in the CYP futile cycle. (4) Ascorbate effectively protects CYP against quinones, apparently by maintaining them in the reduced state. (5) Quinones attack both heme and protein of CYP. (6) CYP activities differ in the sensitivity to quinone-mediated destruction. In conclusion, we suggest that quinones may be responsible for CYP destruction by benzene in vivo.

Induction of cytochrome CYPIA1 and formation of toxic metabolites of benzo[a]pyrene by rat aorta: a possible role in atherogenesis

Cigarette smoking is a leading risk factor for atherosclerosis. Endothelial injury may be the initial event in this process. The carcinogenic metabolites of the polycyclic aromatic hydrocarbons found in cigarette smoke tars could cause this injury. We tested this model by examining the effect of 3-methylcholanthrene administration on aortic polycyclic aromatic hydrocarbon metabolism. Immunoblotting with a monoclonal antibody (mAb 1-7-1) specific for cytochromes CYPIA1 and CYPIA2 showed that aortic microsomes from treated, but not from control, animals contained CYPIA1; the CYPIA1 was primarily in the endothelium. Aortic microsomes from induced animals metabolized benzo[a]pyrene (BaP) to the 7R,8S,9,10-tetrahydrotetrol-, 7,8-dihydrodiol-, 1,6 quinone-, 3,6 quinone-, 6,12 quinone-, 3-hydroxy-, and 9-hydroxy-BaP. mAb 1-7-1 inhibited the formation of the tetrahydrotetrol, the dihydrodiol-BaP, and the 3-hydroxy-BaP but did not inhibit the quinones or the 9-hydroxy-BaP. Arachidonic acid did not affect metabolism. These data suggest that the aortas of induced animals metabolize the BaP in cigarette smoke to carcinogenic and toxic products and that this metabolism may initiate vessel injury and lead to the accelerated atherosclerosis seen in cigarette smokers.

Evidence for a new cytochrome P450 form induced by 3-methylcholanthrene in rats

Evidence is presented for a new 3-methylcholanthrene (3MC)-induced form of cytochrome P450, P450MCX, in rat liver microsomes. P450MCX was co-purified with CYP1A1 from 3MC-treated male Sprague-Dawley rats but was resolved by gel electrophoresis. The M(r) of P450MCX (56,700) was intermediate between CYP1A1 (57,000) and CYP1A2 (54,800). Monoclonal antibodies showed that P450MCX was immunorelated to both CYP1A1 and CYP1A2 but not to CYP2B1, CYP2C6 or CYP3A1. Immunoreactive P450MCX was not detectable in liver microsomes from untreated rats but was highly induced by 3MC and Aroclor 1254, although not induced by isosafrole. The N-terminal amino acid sequence of P450MCX, obtained from an electroblotted sample resolved on SDS-PAGE, did not match any known cytochrome P450 or other protein. P450MCX may be a new member of the CYP1 family.

A comparative study of constitutive and induced alkoxyresorufin O-dealkylation and individual cytochrome P450 forms in cynomolgus monkey (Macaca fascicularis), human, mouse, rat and hamster liver microsomes

The expression of constitutive and inducible cytochrome P450 forms was measured in cynomolgus monkey liver and compared with man, rat, mouse and hamster. Four alkoxyresorufin O-dealkylation (AROD) activities widely used as indicators of P450 induction were measured: methoxyresorufin O-demethylation (MROD), ethoxyresorufin O-deethylation (EROD), pentoxyresorufin O-dealkylation (PROD) and benzyloxyresorufin O-dealkylation (BROD). In monkeys there were no sex-differences in untreated, phenobarbitone (PB)- or beta-naphthoflavone (BNF)-treated animals in AROD activities, or in individual P450 proteins detected by immunoblotting. Basal MROD and EROD activities varied by less than 7-fold between the five species, but the comparative pattern of basal MROD, EROD, PROD and BROD activities (the "MEPB profile") was very species-specific, with monkeys being similar to rats but different from man, mouse and hamster. The induction of AROD activities by PB and BNF was also highly species-specific. Monkeys expressed constitutive proteins immunorelated to the CYP1A, CYP2A, CYP2B, CYP2C and CYP3A sub-families (human CYP2A6 cross-reacted with the anti-rat CYP2B1 antibodies used, and so CYP2A and CYP2B forms could not be separately identified in the monkey). Single constitutive immunoblot bands were identified in monkey for CYP1A (54 kDa), CYP2A/CYP2B (51 kDa) and CYP3A (51 kDa), respectively, but two strong (51 and 52 kDa) plus two weak (49 and 49.5 kDa) bands were shown for CYP2C. Human liver expressed CYP1A2 (54 kDa), CYP2A6 (51 kDa), CYP3A4 (50.5 kDa) and three CYP2C9-immunorelated protein bands (48, 50 and 54 kDa). In monkeys BNF induced the 54 kDa CYP1A protein and CYP1A-dependent MROD, EROD and PROD activities (18-, 15- and 6-fold increases in activity, respectively), whereas PB strongly induced the 51 kDa CYP2A/CYP2B protein but did not induce PROD activity. PB also induced non-constitutive CYP2A/CYP2B protein bands at 49 and 52 kDa in some monkeys. BROD activity was induced less that four-fold by either PB or BNF in monkeys. In conclusion, cynomolgus monkeys expressed a range of constitutive CYP1A, CYP2A or CYP2B, CYP2C and CYP3A proteins similar to man, and a range of AROD monooxygenase reaction rates similar to both man and rat, but the basal MEPB profile of AROD activities in monkeys was more similar to rat than to man. MROD and EROD were good measures of CYP1A induction by polycyclic aromatic hydrocarbons in cynomolgus monkeys, but neither PROD nor BROD were indices of CYP2B induction by PB.

Effect of E-5110, a novel non-steroidal anti-inflammatory drug, on trimethadione metabolism as an indicator of hepatic drug-oxidizing capacity in beagle dog

1. We examined the effects of N-methoxy-3-(3,5-di-tert-butyl-4- hydroxybenzylidene pyrrolidin-2-one (E-5110), a novel non-steroidal anti-inflammatory drug, on the pharmacokinetics of trimethadione (TMO) and characterized the P450 isozymes involved in the metabolism of TMO in beagle dog. 2. In the E-5110-treated dog (50 mg/kg/day for 7 days: oral) the plasma half-life (t1/2) and the area under the curve (AUC) of TMO (4 mg/kg, i.v.) in vivo were decreased, and total body clearance (CL) was increased; the apparent volume of distribution (Vd) was relatively unchanged. 3. Contents of P450 and b5, and the activity of p-nitroanisole O-demethylase and benzphetamine N-demethylase in vitro were significantly increased compared with controls by repeated E-5110 treatment in dog. 4. Contents of CYP2B and 3A were increased by E-5110 pretreatment in dog. 5. TMO N-demethylation was inhibited by the anti-CYP2B and 3A IgG fractions in liver microsomes obtained from the E-5110-treated dog. 6. Results of both the in vivo and in vitro studies of the effects of E-5110 treatment in dog on TMO indicate that these effects may be attributed to the induction of CYP2B and 3A.

Collagen gel immobilization: a useful cell culture technique for long-term metabolic studies on human hepatocytes

1. Primary cultures of human hepatocytes have already been employed in various applications for the study of xenobiotic metabolism. Most of these approaches were performed either on freshly isolated cells or on short-term primary cultures. Standard culture techniques do not maintain functional stability of P450 enzymes for > 1 week in vitro. 2. The aim of this study was to demonstrate the beneficial effect of an easy to apply, extracellular matrix configuration on the long-term performance of cultured human liver cells. Light microscopical examination of the cultures indicated that the cells remained viable over 1 month. As revealed by electron microscopy, hepatocytes exhibited bile canaliculi and desmosomes and were rich in mitochondria and endoplasmatic reticulum, indicating metabolic activity. 3. An early culture phase (3 days after isolation) could be described with decreasing DNA content of the cultures, peak values of alanine-amino-transferase (ALAT), and increasing albumin synthesis. After this adaptive period stable levels for DNA content and albumin synthesis were noted; ALAT returned to low values. 4. Functional activity was monitored by measurements of P450 1A1-dependent O-demethylation of p-nitroanisole to p-nitrophenol, which appeared to be constant over 3 weeks and weakly inducible by 1 mM phenobarbital. Another set-up examined conjugation of acetaminophen at subtoxic concentrations: acetaminophen was metabolized to its glucuronide and sulphate; 3-(glutathione-S-yl)-acetaminophen was not detected. Almost identical metabolism was found, comparing day 3 with 16 of culture. 5. We concluded that collagen gel immobilization not only provides mechanical support to cultured hepatocytes, but also supports long-term differentiated function of the cells for metabolic studies.

Comparative metabolism of SC-42867 and SC-51089, two PGE2 antagonists, in rat and human hepatocyte cultures

1. The metabolism of SC-42867 and SC-51089, two PGE2 antagonists, was studied in cultured rat and human hepatocytes. Both compounds possess an 8-chlorodibenzoxazepine moiety, but differ from each other by the nature of the side chain connected to the nitrogen atom. SC-42867 and SC-51089 and their in vitro metabolites were separated by reversed-phase hplc. The major metabolites of both compounds were identified by mass spectrometry (ms) analysis. 2. SC-42867 was metabolized on the tricyclic moiety only. Oxidative N-dealkylation with opening of the oxazepine ring was the major metabolic pathway obtained in rat hepatocytes. The metabolic profile obtained in cultured human hepatocytes was comparable with that of cultured rat hepatocytes. However, the compound was metabolized to a much lower extent by the human cells. 3. SC-51089 was extensively metabolized by both cultured rat and human hepatocytes. Human cells metabolized this compound quite differently than cultured rat hepatocytes. Aromatic hydroxylation with consequent glucuronidation and sulphation were the main metabolic pathways observed in cultured human hepatocytes. Oxidative N-dealkylation with opening of the oxazepine ring and consequent glucuronidation was the major metabolic pathway observed in rat hepatocytes. Further metabolism occurred, in contrast with the human hepatocytes, mainly on the side chain. 4. The present in vitro results are compared with data of previous in vivo studies performed in rat.