Separation and characterization of highly purified forms of liver microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls, phenobarbital, and 3-methylcholanthrene

Recollection of the early years of the research on cytochrome P450

Since the publication of the first paper on "cytochrome P450" in 1962, the biochemical research on this novel hemoprotein expanded rapidly in the 1960s and the 1970s as its principal roles in various important metabolic processes including steroid hormone biosynthesis in the steroidogenic organs and drug metabolism in the liver were elucidated. Establishment of the purification procedures of microsomal and mitochondrial P450s in the middle of the 1970s together with the introduction of molecular biological techniques accelerated the remarkable expansion of the research on P450 in the following years. This review paper summarizes the important developments in the research on P450 in the early years, for about two decades from the beginning, together with my personal recollections.

Induction of drug-metabolizing enzymes: a path to the discovery of multiple cytochromes P450

This article provides a personal account of the discovery of the induced synthesis of drug-metabolizing enzymes and of subsequent research that led to the discovery of multiple cytochromes P450 with different catalytic activities. The manuscript also emphasizes the role of environmental factors (in addition to genetic polymorphisms) in explaining person-to-person and day-to-day differences in rates and pathways of drug metabolism that occur in the human population.

Identification of repressor element 1 in cytochrome P450 genes and their negative regulation by RE1 silencing transcription factor/neuron-restrictive silencer factor

RE1 silencing transcription factor/neuron-restrictive silencing factor (REST/NRSF) mediates transcriptional repression in many neuron-specific genes by interaction with the repressor element 1/neuron-restrictive silencing element (RE1/NRSE). This element has been identified at least in 20 neuron specific genes. REST/NRSF is highly expressed in non-neuronal tissues, where it is thought to repress gene transcription. We performed a BLAST search to look for the presence of RE1/NRSE elements in the rat cytochrome P450 genes. We identified the presence of RE1/NRSE element in the cytochrome P450 genes CYP1A1, 2A2, 2E1 and 3A2. Electrophoretic mobility shift assay and supershift assays were carried out to prove functionality of these sites and detect the interaction of REST/NRSF with this sequence. Cotransfection studies in PC12 cells with a plasmid containing the RE1 element of the CYP genes, cloned upstream of the minimal type II sodium channel promoter, in the presence of REST/NRSF, showed a marked expression inhibition of the CAT reporter gene. These data suggest that the RE1 elements that exist in these four CYP genes might be a target for the REST/NRSF transcription factor and such an interaction might play a role in the negative regulation of these genes.

Metabolism of territrem a in liver microsomes from male wistar rats: 3. Cytochrome p-450 isoforms catalyzing tra metabolism

The cytochrome P-450 isoforms involved in territrem A (TRA) metabolism in liver microsomes of male Wistar rats have been characterized. Pretreatment with phenobarbital (PB) or dexamethasone (DEX) resulted in a similar significant increase in TRA metabolic activity. Although PB treatment resulted in a significant elevation in CYP2B, CYP2C11, and CYP3A levels, only CYP3A levels were significantly increased by DEX treatment. Cimetidine markedly reduced the formation of the TRA metabolites 4beta-hydroxymethyl-4beta-demethylterritrem A (MA(1)), 4beta-oxo-4beta-demethylterritrem A (MAX) and 2-dihydro-4beta-demethylterritrem A (MA(2)) in liver microsomes from 2-wk-old rats (mainly containing CYP3A2) and 7-wk-old rats (containing CYP2B, CYP2C11, and CYP3A2). SKF 525A, which inhibits CYP2B, CYP2C11, and CYP3A2, and orphenadrine, which inhibits CYP2B, also decreased MA(2) formation in liver microsomes from 7-wk-old phenobarbital-pretreated rats. The formation of MA(1) and MAX was not affected. Furthermore, an immunoinhibition study demonstrated that anti-CYP3A2 antibody reduced MA(1), MAX, and MA(2) formation to nondetectable levels in liver microsomes from 2- and 7-wk-old rats, whereas anti-CYP2C11 or anti-CYP2B antibody, respectively, had no marked effect on MA(1), MAX, and MA(2) formation in liver microsomes from 7-wk-old untreated or PB-treated rats. These results suggest that the CYP3A isoform is mainly responsible for MA(1), MAX, and MA(2) formation in liver microsomes in male Wistar rats.

Purification of cytochromes P-450(scc) and P-450(17 alpha) by steroid-binding affinity column chromatography

The preparation, testing and use of a variety of cholesterol-, deoxycorticosterone (DOC)- and pregnenolone-binding 1,6-diaminohexyl (EAH)-Sepharose 4B supports for affinity column chromatography of cytochromes P-450(scc) and P-450(17 alpha) from bovine adrenal and pig testis are described. EAH-Sepharose 4B has free amino groups at the end of a 10-atom spacer arm. Hydroxyl groups of cholesterol (3 beta), deoxycorticosterone (21 beta) and pregnenolone (3 beta) are linked to succinic anhydride in pyridine through an ester linkage. These coupling ligands of hemisuccinate were synthesized by a general procedure. Free amino groups of EAH-Sepharose 4B were used to couple ligands, containing carboxyl groups, by the carbodiimide coupling method. Both the purified cytochromes P-450(scc) and P-450(17 alpha) were found to be homogeneous and estimated to have a molecular weight of 52,000 on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The carbon monoxide difference spectra with peaks at 450 and 448 nm exhibit the absorption spectra of typical cytochromes P-450(scc) and P-450(17 alpha), respectively. Cytochromes P-450(scc) and P-450(17 alpha) were determined to have isoelectric points of 8.0 and 6.5 in isoelectric focusing on a pH gradient gel. Cytochrome P-450s can be purified between 425- and 1000-fold from the crude extracts.

Forging the links between metabolism and carcinogenesis

Metabolism plays important roles in chemical carcinogenesis, both good and bad. The process of carcinogen metabolism was first recognized in the first half of the twentieth century and developed extensively in the latter half. The activation of chemicals to reactive electrophiles that become covalently bound to DNA and protein was demonstrated by Miller and Miller [Cancer 47 (1981) 2327]. Today many of the DNA adducts formed by chemical carcinogens are known, and extensive information is available about pathways leading to the electrophilic intermediates. Some concepts about the stability and reactivity of electrophiles derived from carcinogens have changed over the years. Early work in the field demonstrated the ability of chemicals to modulate the metabolism of carcinogens, a phenomenon now described as enzyme induction. The cytochrome P450 enzymes play a prominent role in the metabolism of carcinogens, both in bioactivation and detoxication. The conjugating enzymes can also play both beneficial and detrimental roles. As an example of a case in which several enzymes affect the metabolism and carcinogenicity of a chemical, aflatoxin B1 (AFB1) research has revealed insight into the myriad of reaction chemistry that can occur even with a 1s half-life for a reactive electrophile. Further areas of investigation involve the consequences of enzyme variability in humans and include areas such as genomics, epidemiology, and chemoprevention.

Rat cytochromes P450 (CYP) specifically contribute to the reductive bioactivation of AQ4N, an alkylaminoanthraquinone-di-N-oxide anticancer prodrug

1. The bioreductive activation of the alkylaminoanthraquinone di-N-oxide prodrug AQ4N has been characterized in rat hepatic tissue using HPLC. 2. AQ4N was shown to be metabolized to two products, namely AQM, the two electron reduced mono-N-oxide, and AQ4, the four electron reduced active cytotoxic agent. 3. Metabolism was shown to occur in microsomes with an apparent Km = 30.29 microM and Vmax = 1.05 nmol/mg/min. 4. Bioreduction was dependent on anaerobic conditions and the presence of the reduced cofactor NADPH. Ketoconazole (100 microM) and carbon monoxide both inhibited AQ4N metabolism inferring a role for cytochrome P450 (CYP). 5. Microsomes from phenobarbitone and isoniazid-pretreated animals significantly (p < 0.05) enhanced the formation of AQ4 from AQ4N indicating a role for CYP2B and 2E respectively. The involvement of both CYP2B and 2E was confirmed by the use of CYP-specific inhibitors. 6. In conclusion, the involvement of rat hepatic CYP in the reductive bioactivation of the novel antitumour prodrug AQ4N has been established in detail for the first time. These findings highlight an important interspecies difference between the metabolism of AQ4N in rat and man which was shown earlier to be mediated by CYP3A enzymes. The pharmacological significance of this is discussed.

The proximate carcinogen trans-3,4-dihydroxy-3,4-dihydro-dibenz[c,h]acridine is oxidized stereoselectively and regioselectively by cytochrome 1A1, epoxide hydrolase and hepatic microsomes from 3-methylcholanthrene-treated rats

Metabolism of the proximate carcinogen trans-3,4-dihydroxy-3,4-dihydrodibenz[c,h]acridine has been examined with rat liver enzymes. The dihydrodiol is metabolized at a rate of 2.4 nmol/nmol of cytochrome P450 1A1/min with microsomes from 3-methylcholanthrene-treated rats, a rate more than 10-fold higher than that observed with microsomes from control or phenobarbital-treated rats. Major metabolises consisted of a diastereomeric pair of bis-dihydrodiols (68-83%), where the new dihydrodiol group has been introduced at the 8,9-position, tetraols derived from bay region 3,4-diol-1,2-epoxides (15-23%), and a small amount of a phenolic dihydrodiol(s) where the new hydroxy group is at the 8,9-position of the substrate. A highly purified monooxygenase system reconstituted with cytochrome P450 1A1 and epoxide hydrolase (17 nmol of metabolites/nmol of cytochrome P450 1A1/min) gave a metabolite profile very similar to that observed with liver microsomes from 3-methylcholanthrene-treated rats. Study of the stereoselectivity of these microsomes established that the (+)-(3S,4S)-dihydrodiol gave mainly the diol epoxide-1 diastereomer, in which the benzylic 4-hydroxyl group and epoxide oxygen are cis. The (-)-(3R,4R)-dihydrodiol gave mainly diol epoxide-2 where these same groups are trans. The major enantiomers of the diastereomeric bis-dihydrodiols are shown to have the same absolute configuration at the 8,9-position. Correlations of circular dichroism spectra suggest this configuration to be (8R,9R). The (8R,9S)-oxide may be their common precursor.

Purification of cytochrome P-450 from adult pig testis by hydroxylapatite and deoxycorticosterone affinity column chromatography

Adult testicular cytochrome P-450 was purified by a two-step procedure utilizing hydroxylapatite and deoxycorticosterone affinity column chromatography. Cytochrome P-450 was determined to have an isoelectric point of 6.5 on analytical isoelectric focusing. The purified cytochrome P-450 was found to be homogeneous and its molecular mass was estimated to be 52000 on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The carbon monoxide difference spectrum with a peak at 448 nm exhibited the absorption spectrum of a typical cytochrome P-450. A 1000-fold purification was achieved with a yield of 5%.

Two-step purification of cytochrome P-450 from adult pig testis by isoelectric focusing

Adult testicular cytochrome P-450 was purified by a two-step procedure utilizing preparative isoelectrofocusing. Purification was achieved 1132 times with a yield of 4.82%. 17alpha-hydroxylase activity was shown to be 14.5 nmol of product/min/nmol of P-450. The cytochrome P-450 was determined to have an isoelectric point of 6.45 on analytical isoelectric focusing. The purified cytochrome P-450 was found to be homogeneous and its molecular weight was estimated to be 52000 on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The carbon monoxide difference spectrum with a peak at 448 nm exhibited the absorption spectrum of a typical cytochrome P-450.

Beta carotene and its oxidation products have different effects on microsome mediated binding of benzo[a]pyrene to DNA

The effects of beta-carotene (betaC) and its oxidation products on the binding of benzo[a]pyrene (BaP) metabolites to calf thymus DNA was investigated in the presence of rat liver microsomes. Mixtures of betaC oxidation products (betaCOP) as well as separated, individual betaC oxidation products were studied. One set of experiments, for example, involved the use of the mixture of betaCOP obtained after a 2-h radical-initiated oxidation. For this data set, the incorporation of unoxidized betaC into microsomal membranes caused the level of binding of BaP metabolites to DNA to decrease by 29% over that observed in the absence of betaC; however, the incorporation of the mixture of betaCOP caused the binding of BaP metabolites to DNA to increase 1.7-fold relative to controls without betaC. Two variations of this experiment were studied: (1) When no NADPH was added, betaC decreased the binding of BaP metabolites to DNA by 19%, but the mixture of betaCOP increased binding by 3.3-fold relative to that observed in the absence of betaC. (2) When NADPH was added under near-anaerobic conditions, betaC caused an almost total (94%) decrease in binding whereas betaCOP had no effect on the amount of binding relative to that observed in the absence of betaC. Both betaCOP and cumene hydroperoxide caused BaP metabolites to bind to DNA even when NADPH was omitted from the incubation mixture. Separation of the mixture of betaC oxidation products into fractions by HPLC allowed preliminary testing of individual betaC oxidation products separately; of the various fractions tested, the products tentatively identified as 11,15'-cyclo-12,15-epoxy-11,12,15,15'-tetrahydro-beta-carotene and beta-carotene-5,6-epoxide appeared to cause the largest increase in BaP-DNA binding. Microsomes from rats induced with 3-methylcholanthrene (3MC) or Aroclor 1254 produced different levels of binding in some experimental conditions. We hypothesize that, under some conditions, the incorporation of betaC into microsomal membranes can be protective against P450-catalyzed BaP binding to DNA; however, the incorporation of betaCOP facilitates the formation of BaP metabolites that bind DNA, although only certain P450 isoforms catalyze the binding process.

Lipid peroxidation induced by adriamycin in linolenic acid-loaded cultured hepatocytes

Addition of more than 10 microM of adriamycin to cultured rat hepatocytes loaded with alpha-linolenic acid (linolenic acid-loaded hepatocytes) caused marked lipid peroxidation as measured by an accumulation of malondialdehyde during a 9 hr incubation. After addition of 50 microM of adriamycin to linolenic acid-loaded hepatocytes, malondialdehyde accumulation significantly increased at 3 hr, followed by cellular reduced glutathione decrease and lactate dehydrogenase leakage after 6 hr. Inhibition of adriamycin-induced lipid peroxidation by addition of N,N'-diphenyl-p-phenylenediamine or alpha-tocopherol, both lipid radical scavengers, or deferoxamine, which is a Fe ion chelator, prevented both glutathione decrease and lactate dehydrogenase leakage, indicating that lipid peroxidation caused cellular damage to linolenic acid-loaded hepatocytes exposed to adriamycin. The effect of SKF 525-A, which is a cytochrome P450 inhibitor, on adriamycin-induced lipid peroxidation and on 7-ethoxycoumarin O-deethylase activity was determined by 6 hr incubation of linolenic acid-loaded cells. Addition of SKF 525-A suppressed adriamycin-induced lipid peroxidation comparably with its 7-ethoxy-coumarin 0-deethylase inhibitory activity. These results suggest that cytochrome P450 contributes to the one-electron bioreduction of adriamycin into its semiquinone radical in rat hepatocytes.

Role of cytochrome P-450 in quinalphos toxicity: effect on hepatic and brain antioxidant enzymes in rats

Quinalphos (QP), an organophosphate pesticide, is used in controlling the pests of a variety of crops. To understand the mechanism of the metabolic basis of the toxicity of QP it was thought pertinent to study the role of cytochrome P-450 (P450) and antioxidant enzyme systems. Albino rats treated orally with QP (0.52 and 1.04 mg/kg body weight) for 60 days showed a significant decrease in body, brain and liver weights. Hepatic P450 content and its dependent monooxygenases, namely aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin-O-deethylase (ERD), were induced to 1.8-2.5-fold, while neuronal AHH was induced to 1.8-fold following QP treatment (1.04 mg/kg) to animals. The hepatic antioxidant defence system, comprising catalase, glutathione (GSH) reductase, superoxide dismutase (SOD) and GSH peroxidase, was also significantly increased in QP-treated animals, while in the brain only catalase was increased and GSH reductase decreased. There was no significant change in hepatic GSH content and lipid peroxide levels in QP treated animals at any dose group in comparison with the control group. Pretreatment of rats with phenobarbitone (PB) or 3-methylcholanthrene (MC) (P450 inducers) prevented mortality caused by the LD50 dose of QP, whereas pretreatment with cobalt chloride (a P450 inhibitor) enhanced the mortality rate to 100% within 3 days. From the above study it can be inferred that the toxicity of QP may be due to the parent compound or its metabolite(s) produced prior to P450 oxidation and that the induction of P450 system by QP may be a defence mechanism.

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.

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

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

Chromatographic separation and behavior of microsomal cytochrome P450 and cytochrome b5

The methods used for separation of the multiple mammalian cytochrome P450 enzymes by liquid chromatography are reviewed. In addition to the chromatographic techniques, preparation and handling of samples and prefractionation procedures are considered. Conditions that affect stability and chromatographic resolution of cytochromes P450 are also discussed. Special emphasis is put on useful methods which are not routinely used for P450 separation, such as immobilized metal affinity or hydrophobic-interaction chromatography. Applications of low- and high-pressure methods with regard to preparative and analytical separations are compared. It is shown that high- and medium-pressure ion-exchange chromatography are suitable tools for separation of closely related P450 enzymes, especially when specific detection methods are available. In addition to fractionation of cytochromes P450, the isolation and chromatographic behavior of cytochrome b5 is discussed.

Metabolite complex formation of orphenadrine with cytochrome P450. Involvement of CYP2C11 and CYP3A isozymes

Expression and inhibition of cytochrome P450 (CYP) isozymes capable of forming an orphenadrine metabolite complex were studied in microsomes of untreated and inducer-treated male and female rats. High levels of complex-forming isozymes were found in microsomes of untreated male as compared to female rats. Treatment of male rats with several P450 inducers did not considerably increase the extent of in vitro complex formation. In female rats, however, phenobarbital or dexamethasone treatments led to pronounced induction. The isozyme specificity of complex formation was investigated by several approaches including: 1. inhibition by orphenadrine of isozyme-specific P450 activities, such as hydroxylation of testosterone, O-dealkylation of pentoxy-and ethoxyresorufin and complex formation with triacetyloleandomycin (TAO), 2. inhibition of orphenadrine complex formation by metyrapone, TAO, and cimetidine, and 3. correlation of complex levels with immunochemically, enzymatically, or spectroscopically determined amounts of P450 isozymes. Our data suggest that CYP2C11, a CYP3A isozyme and an unidentified P450 species are involved in complex formation with orphenadrine, but exclude the involvement of CYP1A1/2 and CYP2B1/2. The capability of CYP2C11 to form a metabolite complex with orphenadrine is strongly suggested for the following reasons: 1. Efficient inhibition of testosterone 2 alpha- and 16 alpha-hydroxylation by complex formation with orphenadrine in microsomes of untreated male rats, 2. high expression of orphenadrine-complexing isozymes in untreated male compared to female rats, 3. specific inhibition of in vitro complex formation by cimetidine, 4. suppression of complex-forming isozymes by 3-methylcholanthrene and beta-naphthoflavone, and 5. concomitant induction of complex-forming isozymes, immunodetectable CYP2C11, and testosterone 2 alpha-hydroxylase by stanozolol. That at least one, but not all, CYP3A isozymes is involved in complex formation is concluded from inhibition experiments with TAO that show that orphenadrine complexation can be significantly inhibited in microsomes of dexamethasone-treated, but not in microsomes of untreated rats. Furthermore, complex formation with TAO is not inhibited by orphenadrine in microsomes of phenobarbital (PB)-treated rats. In PB-treated female rats, a further unidentified complex-forming isozyme can be detected that is not inhibited by complex formation with TAO.

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.

Purification and properties of a new beta-naphthoflavone inducible cytochrome P-450, aryl hydrocarbon hydroxylase from rat kidney

In rat kidney, beta-naphthoflavone induced 53 kDa and 55 kDa proteins, which were both recognized by the antibodies against rat liver cytochrome P-450 1A1 (55kDa). The major inducible 53 kDa protein was purified from the beta naphthoflavone-treated rat kidney and shown to be a new cytochrome P-450 having a high aryl hydrocarbon hydroxylase activity. Purified cytochrome P-450, named P-450KAh, was homogeneous on SDS-polyacrylamide gel electrophoresis, and the apparent molecular weight was estimated to be 53 kDa. The absorption spectra of the oxidized form of P-450KAh showed a Soret peak at 416 nm, a characteristic of low-spin hemoprotein, and the Soret peak of the reduced cytochrome P-450-CO complex was at 446 nm. In the reconstituted system, purified P-450KAh showed high catalytic activity for benzo[a]pyrene hydroxylation and 7-ethoxycoumarin O-deethylation. P-450KAh could activate genotoxicities of not only B[a]P, but also 2-acetylaminofluorene and aflatoxin B1 on the umu test. These catalytic properties of P-450KAh were almost the same as those of P-4501A1, a major P-450 form having arylhydrocarbon hydroxylase in liver microsomes of 3-methylcholanthrene-treated rats, and P-450KAh could not be distinguished from P-4501A1 even by immunochemical analysis. However, the electrophoretic peptide patterns after alpha-chymotrypsin or trypsin treatment of P-450KAh were different from those of P-4501A1, and the NH2-terminal 11 amino acid sequence of the P-450 was also different from that of P-4501A1 and any other P-450s of rat.

DNA adducts and P450 induction in human, rat and avian liver cells after exposure to polychlorobiphenyls

Polychlorinated biphenyls (PCBs) are industrial chemicals which have been detected in fish, birds and humans. They are known to exert marked effects on the liver. They induce hepatocellular carcinoma in rats and birds, and are suspected of being carcinogenic to humans. To better understand the genotoxic effects of PCBs, we used 32P-postlabelling to investigate DNA adduct formation, after exposure to PCBs (Aroclor 1254 and 3,3',4,4'-tetrachlorobiphenyl), in primary cultures of fetal hepatocytes from two animal species and in a human cell line (Hep G2). We also studied the induction of 7-ethoxyresorufin-O-deethylase (EROD) in these PCB-treated cells. The three cell types used are known to express different cytochrome P450 families. The aim was to see whether a correlation could be established between EROD activity (a CYP1A1-related activity) and DNA adduct formation. DNA adducts were found in all three models after exposure to 50 microM 3,3',4,4'-tetrachlorobiphenyl. The number of adducts was higher in quail hepatocytes (37 adducts per 10(9) nucleotides) than in rat hepatocytes or Hep G2 cells (20 adducts per 10(9) nucleotides in both cases). The major adduct was the same in all three cell types, but some adducts were found in only one or two species. These inter-species differences probably reflect metabolic differences leading to different ultimate carcinogens. Exposure to Aroclor 1254 failed to produce significant levels of DNA adducts, suggesting that pre-treated cells are required to magnify Aroclor 1254 metabolism. No correlation was found between adduct formation and the level of EROD induction.

Characterization of benzo[a]pyrene metabolism and related cytochrome P-450 isozymes in Syrian hamster livers

Cytochrome P-450 monooxygenases of golden Syrian hamsters were characterized with respect to benzo[a]pyrene metabolism. Male hamsters were treated with phenobarbital, 3-methylcholanthrene, dexamethasone, benzoflavone, or ethanol, and the activity of aryl hydrocarbon hydroxylase and benzo[a]pyrene activation was determined by mutagenicity testing in hepatic microsomes. Aryl hydrocarbon hydroxylase activity was induced markedly by treatment with phenobarbital but not with 3-methylcholanthrene, nor with other chemicals. The degree of benzo[a]pyrene activation on mutagenicity testing was significantly elevated by treatment with 3-methylcholanthrene and phenobarbital but was reduced with dexamethasone. Immunoinhibition of these activities and Western blotting of hepatic microsomes using antibodies against cytochrome P-450 isozymes suggested that the isozymes responsible for benzo[a]pyrene metabolism in Syrian hamsters belong to the CYP1A, CYP2A, and CYP3A families, a result that differs from observations in rats.

The cytotoxicity of mitomycin C and adriamycin in genetically engineered V79 cell lines and freshly isolated rat hepatocytes

The objective of the present study was to investigate the cytotoxicity of Adriamycin (ADR) and mitomycin C (MMC) in tumor and non-tumor cells with respect to the role of cytochrome P450 (P450). Therefore, genetically engineered V79 Chinese hamster fibroblasts expressing only single enzymes of P450 were used. SD1 and XEM2 cells expressed rat P450IIB1 and P450IA1, respectively, whereas the V79 parental cells contained no detectable P450 levels. The cytotoxicity of ADR and MMC in the V79 cell system was compared with that in freshly isolated hepatocytes from phenobarbital (PB-hepatocytes)- and beta-naphthoflavone (beta NF-hepatocytes)-induced rats. Following 24 h of exposure to ADR equal cytotoxicity was observed in V79, SD1 and XEM2 cells. Addition of metyrapone (MP, an inhibitor of P450IIB1) and alpha-naphthoflavone (alpha NF, an inhibitor of P450IA1) had no effect on the ADR-induced cytotoxicity in SD1 and XEM2 cells, respectively. Likewise, MMC was equitoxic in V79 and SD1 cells. Co-incubation of SD1 cells with MP did not alter MMC-induced cytotoxicity. MMC, however, showed a decreased cytotoxicity in XEM2 cells when compared to the parental V79 cells. Unexpectedly, the cytotoxicity of MMC in XEM2 cells was increased by alpha NF to the same level as observed in the parental V79 cells. In contrast to V79- and V79-derived cells, in freshly isolated hepatocytes from PB or beta NF-induced rats, MMC was cytotoxic (measured as lactate dehydrogenase leakage) within 3 h of incubation. ADR, however, was only cytotoxic to the hepatocytes when intracellular glutathione was first depleted by diethylmaleate. The MMC- and ADR-induced cytotoxicity was found to be more pronounced in PB-hepatocytes than in beta NF-hepatocytes. Contrary to the findings in the V79-derived cells, MP afforded complete protection against both MMC- and ADR-induced cytotoxicity in PB-hepatocytes, whereas alpha NF only partially inhibited the cytotoxicity of MMC in beta NF-hepatocytes. In conclusion, we have demonstrated that PB-inducible P450s play a role in the cytotoxicity of both MMC and ADR in freshly isolated PB-hepatocytes but that P450IIB1 does not in genetically reconstituted SD1 cells. P450IA1, however, decreased the cytotoxicity of MMC in the XEM2 cells. The ADR-induced cytotoxicity, which was observed in XEM2 cells, was not mediated by P450IA1. The present study underscores the complexity in the comparison of ADR- and MMC-induced cytotoxicities in normal and tumor cells.

Induction of Cyp1a-1 and Cyp1a-2 gene expression by a reconstituted mixture of polynuclear aromatic hydrocarbons in B6C3F1 mice

The potential non-additive interactions of polynuclear aromatic hydrocarbon (PAH) mixtures as inducers of Cyp1a-1 and Cyp1a-2 gene expression were investigated in B6C3F1 mice using a reconstituted PAH mixture. The chemical composition (% by weight) of the reconstituted PAH mixture was: 2-ring PAHs--indan (0.22), naphthalene (23.8), 2-methylnaphthalene (23.2) and 1-methylnaphthalene (13.3); 3-ring PAHs--acenaphthylene (7.7), acenaphthene (0.6), dibenzofuran (0.7), fluorene (4.3), phenanthrene (10.5) and anthracene (3.4); > or = 4-ring PAHs--fluoranthene (2.4), pyrene (4.3), benz[a]anthracene (1.4), chrysene (1.5), benzo[b]fluoranthene (0.8), benzo[k]fluoranthene (0.9) and benzo[a]pyrene (0.9). The composition of the 2-, 3- and > or = 4-ring PAH fractions were based on the relative concentration of individual PAHs as noted above. The > or = 4-ring PAH fractions were based on the relative concentration of individual PAHs as noted above. The > or = 4-ring PAH fraction and reconstituted mixture induced hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity and Cyp1a-1 mRNA levels, whereas the 2- and 3-ring PAHs were only weakly active. Direct comparison of the potencies of the reconstituted mixture and > or = 4-ring PAHs showed that the Cyp1a-1 induction activity of the reconstituted mixture was due to the > or = 4-ring PAHs. The reconstituted PAH mixture and > or = 4-ring PAHs also induced Cyp1a-2 hepatic mRNA levels and microsomal methoxyresorufin O-deethylase (MROD) activity; however, their dose-response curves indicated that the reconstituted PAH mixture was more potent as a Cyp1a-2 inducer than the > or = 4 ring PAHs. The differences in potency were due to 3-ring PAHs which were found to be strong inducers of hepatic Cyp1a-2 mRNA levels and microsomal MROD activity at the lowest dose administered (37 mg/kg). The 3-ring mixture caused a maximal 29-fold increase in hepatic MROD activity at a dose of 292 mg/kg, but only 28% of maximal induction of EROD activity. Northern analysis of liver mRNA from mice treated with 3-ring PAHs showed that there was minimal induction of Cyp1a-1 mRNA levels. The 3-ring PAHs did not competitively bind to the mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor suggesting that 3-ring PAHs are a new class of Cyp1a-2 inducers which do not act through the Ah receptor.

Stereo-selectivity and regio-selectivity in the metabolism of 7,8-dihydrobenzo[a]pyrene by cytochrome P450, epoxide hydrolase and hepatic microsomes from 3-methylcholanthrene-treated rats

The active site of cytochrome P450 1A1 has been probed with the substrate 7,8-dihydrobenzo[a]pyrene using a purified, reconstituted system composed of cytochrome P450 1A1, NADPH-cytochrome c reductase and lipid in the presence or absence of epoxide hydrolase. The turnover of the substrate was found to be 38 nmol/nmol of cytochrome P450/min. The metabolic products that were identified are: a phenolic 7,8-dihydrobenzo[a]pyrene (20-29%); 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (17-28%); benzo[a]pyrene (12-19%); 7-hydroxy-7,8-dihydrobenzo[a]pyrene (13-16%); 8-hydroxy-7,8-dihydrobenzo[a]pyrene (7-15%); 3-hydroxybenzo[a]pyrene (7-15%); 4,5-epoxy-4,5,7,8-tetrahydrobenzo[a]pyrene (0-4%); and a triol of 7,8,9,10-tetrahydrobenzo[a]pyrene (0-4%). 9,10-Epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene undergoes rapid hydrolysis to cis- and trans-9,10-dihydroxy-dihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (2:1) by benzylic attack of water at C-10. Approximately 71% of the trans diols are derived from (+)-(9S,10R)-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, indicating that cytochrome P450 1A1 has more than a 2:1 preference for selective epoxidation of an enantiotopic face of 7,8-dihydrobenzo[a]pyrene. This stereo-selectivity agrees with the postulated stereo-selectivity predicted by a previously described active site model for cytochrome P450 1A1. Epoxide hydrolase in pure form or in hepatic microsomes catalyzes the hydrolysis of 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, which is inhibited by 1,1,1-trichloropropane 2,3-oxide. The (+)-(9S,10R)-isomer of the epoxide is slightly preferred as a substrate over its enantiomer and is cleaved by benzylic and nonbenzylic attack. Only benzylic attack was found with (-)-(9R,10S)-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene.

P450 enzyme CYP2B catalyzes the detoxification of diisopropyl fluorophosphate

Phenobarbital and some other enzyme-inducers are known to reduce organophosphate toxicity. One suggested mechanism is the induction of liver cytochrome P450 enzymes catalyzing monooxygenation reactions. The aim of the present study was to elucidate the cytochrome P450 subfamily, or P450 isoenzyme(s), participating in the detoxification of diisopropyl fluorophosphate (DFP) in the rat. DFP resulted in a type I spectrum in liver microsomes from phenobarbital- or RP 52028-treated rats (binding constants 0.32 and 0.17 microM, respectively) and in a purified P450 preparation enriched with CYP2B. The spectrum was reversible by metyrapone, an inhibitor of the CYP2B enzyme subfamily. The 7-pentoxyresorufin O-dealkylase activity was inhibited by DFP in liver microsomes from phenobarbital- or RP 52028-treated rats and in a reconstituted system containing the purified CYP2B preparation. In microsomes from phenobarbital-pretreated rats, the inhibition was of a mixed type, i.e., competitive-non-competitive (Km = 0.5 microM; Ki = 6 microM). The microsomal fractions of livers from phenobarbital- or RP 52028-treated rats detoxified DFP effectively in vitro, as measured by a decrease in the DFP inhibition of cholinesterase activity. This detoxification was antagonized by metyrapone and by an antibody raised against purified CYP2B preparation. Clotrimazole, an inhibitor of P450 enzymes, inhibited the detoxification of DFP in rat liver in vivo. A genetically-modified hamster cell line expressing CYP2B1 oxidized NADPH in the presence of DFP. No such oxidation was detected in the parent cell line. These studies suggest that CYP2B1 metabolizes DFP and may significantly contribute to the detoxification of this organophosphate in vivo.

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.

Metabolism of lidocaine by rat pulmonary cytochrome P450

The metabolism of lidocaine was studied using microsomes from extrahepatic tissues of rats, including lung, kidney and brain, or using a reconstituted system with purified CYP2B1 and CYP4B. Rat pulmonary microsomes metabolized lidocaine to an N-deethylated metabolite, monoethylglycinexylidide (MEGX). Renal microsomes produced MEGX and 3-hydroxylidocaine (3-OH LID), although the rate of MEGX formation was much lower in renal than in pulmonary microsomes. Other metabolites were not detected. Lidocaine was not metabolized by brain microsomes. In extrahepatic tissues, pulmonary microsomes had the highest activity. Hence, two major forms of cytochrome P450 isozymes, CYP2B1 and CYP4B1, in rat pulmonary microsomes were used for further study. The study with a reconstituted system using purified cytochrome P450 isozymes revealed that only CYP2B1 showed lidocaine deethylation activity; the other form of cytochrome P450 in the lung, CYP4B1, did not. The Michaelis-Menten constant for lidocaine N-deethylation by rat pulmonary microsomes was 0.27 mM. Antibody against CYP2B1 completely inhibited the formation of MEGX by pulmonary microsomes. These results suggest that lidocaine is metabolized by rat lung, including CYP2B1.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Enhancement of the mutagenicity of amino acid pyrolysates by phthalate esters

The ability of phthalic acid, phthalic acid anhydride, and various phthalate esters to enhance the mutagenicity of many amino acid pyrolysates was observed with the Ames test (Salmonella typhimurium TA98), but not the SOS Chromotest. Phthalate enhancement of the mutagenicity of 4-nitroquinoline-1-oxide, 2-nitrofluorene, and benzo[a]pyrene was not observed with either test. The mutagenicity-enhancing ability may be related to the induction of enzymes such as P450IIB, that metabolize amino acid pyrolysates. By quantitative structure activity relationship (QSAR) analysis, a good correlation was observed between the mutagenicity-enhancing activity of phthalates and their octanol-water partition coefficients.

Synergistic activity of polynuclear aromatic hydrocarbon mixtures as aryl hydrocarbon (Ah) receptor agonists

The relative potencies of benzo[a]pyrene and a complex mixture of polynuclear aromatic hydrocarbons (PAHs) produced as by-products of manufactured gas plant (MGP) residues as inducers of hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity were determined in the B6C3F1 mouse. The ED50 values for the induction response were 78 and 65 mg/kg for benzo[a]pyrene and the MGP-PAH mixture, respectively. Analysis of the MGP-PAH mixture indicated that benzo[a]pyrene and other compounds containing four or more rings and which are known to induce EROD activity were only present as trace components of this mixture. A comparison of the EROD induction potencies of benzo[a]pyrene and the MGP-PAH mixture showed that the mixture was approximately 706 times more potent than expected based on its benzo[a]pyrene content (0.17%). This induced P-450 activity could significantly increase the metabolism of the carcinogenic PAHs and thereby modulate the overall carcinogenicity of the mixture. The apparent synergistic activity of the MGP-PAH mixture was further investigated by comparing the activities of this mixture and benzo[a]pyrene for several other aryl hydrocarbon (Ah) receptor-mediated responses including (i) induction of hepatic CYP1A1 mRNA levels, (ii) transformation of the rat cytosolic Ah receptor to a complex which binds to a dioxin responsive element, (iii) induction of EROD activity and (iv) antiestrogenicity in MCF-7 human breast cancer cells, and (v) inhibition of the splenic plaque-forming cell (PFC) response to both T cell-dependent and independent antigens in B6C3F1 mice. For the EROD and CYP1A1 mRNA induction and cytosolic transformation activities and immunosuppressive effects, the MGP-PAH mixture was approximately 100-900 times more potent as an Ah receptor agonist than expected based on its benzo[a]pyrene content. The synergistic activity was lower (19-fold) for the antiestrogenic response in MCF-7 cells. The reason for the synergistic effects of the MGP-PAH mixture were not due to contamination of the mixture by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds and the results suggest that the enhanced potency of the mixture is due to unknown interactions between the individual PAHs present in the mixture.

Specific induction of hepatic cytochrome P4501a-2 in C57BL/6 and DBA/2 mice treated with 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)

The food mutagen/carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) is activated by cytochrome p4501a-2 via N-hydroxylation; various P450s may contribute to detoxification via ring hydroxylation. Alterations in P450 levels by IQ treatment might therefore influence its toxicity. To examine the role of Ah locus genotype on the biochemical effects of IQ, C57BL/6 (AhbAhb; p450Ia-1/2 inducible) and DBA/2 (AhdAhd, noninducible) mice of both sexes were given IQ at varying doses, with different vehicles and routes of administration. Livers taken after 24 hours were assessed for total cytochrome p450 and activities of ethoxyresorufin-O-deethylase (EROD, a p4501a-1 activity, inducible in Ahb mice), methoxyresorufin-O-demethylase (MROD, a p4501a-2 activity), and benzyloxyresorufin-O-dealkylase (BzROD, an activity of p4502b). There was little effect on total cytochrome p450, but all three enzyme activities were often induced, a maximum of 2.5-fold, in both sexes and in DBA/2 as well as C57BL/6 mice. However, Western immunoblot analysis with monoclonal antibodies demonstrated an increase only in p4501a-2 protein. p4501a-1 remained undetectable. A monoclonal antibody to p4502-b recognized one protein band in liver microsomes from males and two bands in female mice of both strains. Amounts of these proteins were not altered by IQ treatment. Thus, IQ specifically, if moderately, induces its activating enzyme, p4501a-2, in a process that was not clearly related to Ah responsiveness.

Molecular cloning of guinea pig CYP1A1: complete primary structure and fast mobility of expressed protein on electrophoresis

Guinea pig CYP1A1 cDNA was isolated from a liver cDNA library of guinea pig treated with 3-methylcholanthrene. The cDNA, named GPc1, was 2674 bp long and contained an entire coding region for 516 amino acids. The amino acid sequence of guinea pig CYP1A1 shared 74-78% identity with those of the other mammalian CYP1A1s. RNA blot and immunoblot analyses revealed that CYP1A1 was constitutively expressed and was induced by 3-methylcholanthrene in guinea pig liver. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, guinea pig CYP1A1 expressed in yeast had a significantly smaller apparent molecular mass than expressed mouse CYP1A1. An alignment of the amino acid sequences of mammalian CYP1A1s demonstrated that guinea pig CYP1A1 was several residues shorter than the counterparts in the N-terminal region. Thus, to clarify the contribution of the N-terminal sequence of guinea pig CYP1A1 to the fast mobility on the electrophoresis, mouse-guinea pig chimeric CYP1A1 was prepared through cDNA-directed expression in yeast. The chimeric CYP1A1 protein had an intermediate molecular mass between mouse and guinea pig CYP1A1s indicating that the anomalous mobility of guinea pig CYP1A1 is in part due to the shortened N-terminal amino acid sequence of the protein.

Interaction of caffeine with acetaminophen. 1. Correlation of the effect of caffeine on acetaminophen hepatotoxicity and acetaminophen bioactivation following treatment of mice with various cytochrome P450 inducing agents

The combination of caffeine with acetaminophen (APAP) is used widely in the treatment of headache. The effects of caffeine on APAP-induced hepatotoxicity and APAP bioactivation by liver microsomes from uninduced mice and from mice pretreated with various agents that induce cytochrome P450 were studied. When 1 mM caffeine was included, the rate of glutathione-APAP conjugate (GS-APAP) formation was increased significantly by 33 and 39% in microsomes from phenobarbital (PB)- and dexamethasone (DEX)-treated mice, respectively, whereas this parameter was decreased 39 and 12% by caffeine in microsomes from beta-naphthoflavone (beta NF)- and acetone-treated mice, respectively. A 5 mM concentration of caffeine increased GS-APAP formation by 47, 107 and 117% in microsomes from control, PB-, and DEX-treated mice, respectively, and decreased it 39 and 25% in microsomes from beta NF- and acetone-treated mice, respectively. Caffeine was a competitive inhibitor of APAP bioactivation in microsomes from beta NF- and acetone-treated mice. While caffeine increased APAP bioactivation in microsomes from uninduced, PB-, and DEX-treated mice, the apparent Km values for APAP were increased by caffeine, indicating that this enhancement was not due to a direct effect of caffeine on APAP binding to cytochrome P450 but may be due to an effect of caffeine on the substrate-enzyme complex. The variable effect of caffeine on APAP hepatotoxicity correlated with the effect of caffeine on APAP bioactivation by liver microsomes, regardless of pretreatment. Lack of correlation of aminopyrine N-demethylase, but good correlation of erythromycin N-demethylase activity with the extent of caffeine enhancement of APAP bioactivation following PB or DEX treatment suggests that a murine P450 subfamily similar to the rat P450 3A subfamily may be the candidate in mediating the stimulatory effect of caffeine on APAP bioactivation and APAP-induced hepatotoxicity.

Monoclonal antibody-directed assessment of toluene induction of rat hepatic cytochrome P450 isozymes

Cytochrome P450 isozymes induced in rat liver by a range of concentrations of toluene were studied with monoclonal antibodies (MAbs) to specific P450 isozymes and by enzyme assays. Nitrosodimethylamine demethylase activity was significantly increased in microsomes from rats exposed to more than 1000 ppm of toluene, an increase that was dose-dependent. Anti-CYP2E1 significantly inhibited the metabolism of toluene to benzyl alcohol (BA) by about 50%, in microsomes from 1000 to 4000 ppm toluene-exposed rats, at low substrate concentration (0.2 mM). With anti-CYP2B1/2, the rate of BA formation was decreased by 15-17% in microsomes from rats of 2000 and 4000 ppm toluene exposures at high substrate concentration (5.0 mM). On the other hand, anti-CYP2C11/6 inhibited the rate of formation of BA in all of the microsomes, but the extent of inhibition was progressively decreased from 55% in control to 33% in 4000 ppm exposure. Immunoblot analysis with anti-CYP2E1 and anti-CYP2B1/2 revealed stronger immunoreactive bands in microsomes from rats exposed to more than 1000 and 2000 ppm of toluene, respectively. Stronger bands were also observed in microsomes from rats of 2000-4000 ppm toluene exposures with anti-CYP3A1/2, but no immunoreactivity appeared with anti-CYP1A1/2. These results suggest that toluene induces CYP2E1, CYP2B1/2 and CYP3A1/2, but reduces CYP2C11/6, and has no effect on CYP1A1/2.

Comparative metabolism of bis(2-methoxyethyl)ether in isolated rat hepatocytes and in the intact rat: effects of ethanol on in vitro metabolism

The metabolism of the reproductive and developmental toxicant bis(2-methoxyethyl)ether (diglyme) was studied in isolated rat hepatocytes and in the intact rat. Male Sprague-Dawley rats (190-220 g) were used in both studies. Hepatocytes, isolated by a two-step in situ collagenase perfusion of the liver, were cultured as monolayers and incubated with [14C]diglyme at 1, 10, 30, and 50 microM for up to 48 h. For the in vivo study, rats were given single oral doses of [14C]diglyme at 5.1 mmol/kg body wt, and urine was collected for up to 96 h. Radioactive compounds in the culture medium or in the urine were separated by high performance liquid chromatography and quantified with an in-line radioactivity monitor. Metabolites were identified by comparison of their chromatographic retention times and their mass spectra with those of authentic compounds. The principal metabolite from hepatocytes and in the urine was (2-methoxyethoxy)acetic acid (MEAA). This metabolite accounted for approximately 36% of the radioactivity in the 48-h culture medium and about 67% of the administered dose in the 48-h urine. Other prominent metabolites common to both systems included 2-(2-methoxyethoxy)ethanol, methoxyacetic acid (MAA), 2-methoxyethanol, and diglycolic acid. The diglyme metabolite profiles from urine and from hepatocytes were qualitatively similar, demonstrating that, in the rat, hepatocytes serve as a good model system for predicting the urinary metabolites of diglyme. Moreover, MEAA was shown to be the metabolite best suited for use as a short-term biological marker of exposure to diglyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ciprofloxacin and enrofloxacin on zoxazolamine kinetics, plasma concentration and sleeping times in mice

The treatment of CD1 male mice with either ciprofloxacin (CP) or enrofloxacin (EF) prior to zoxazolamine (ZX) administration increased the mean ZX sleeping times to, respectively, 162 and 156% of the control (ZX alone). At the end of the sleeping time, the mean ZX plasma concentration in controls was 27.2 micrograms/ml and was not different in EF- or CP-treated groups (87% and 95% of controls, respectively). The animals coadministered with CP or EF and ZX eliminated the latter more slowly than the controls. The estimated zero-time drug concentration of the disposition curves of both the CP- and EF-treated groups as well as the apparent half-life of elimination and apparent overall rate of elimination of the CP-treated group were different from the control values.

A comparative study of liver mixed function oxidases in camels (Camelus dromedarius), guinea pigs (Cavia porcellus) and rats (Rattus norvegicus)

1. The activities of the drug-metabolizing enzymes, benzphetamine N-demethylase, 7-ethoxycoumarin O-deethylase and dicoumarol oxidation have been measured in vitro in the liver of camels, guinea pigs and rats. 2. In these species, levels of hepatic microsomal parameters namely microsomal protein, cytochrome P450, cytochrome b5 and NADPH-cytochrome c reductase have also been determined. 3. In general, camels seemed to have the lowest enzyme activity when compared to rats and guinea pigs. 4. Some sex differences were observed in the levels of enzymes studied. In rats and guinea pigs, males had higher benzphetamine N-demethylase than females. However, in camels and guinea pigs, females had higher 7-ethoxycoumarin O-deethylase when compared to males.

Formation of ligand and metabolite complexes as a means for selective quantitation of cytochrome P450 isozymes

The suitability of triacetyloleandomycin (TAO) metabolite complex formation and metyrapone binding to reduced cytochrome P450 as a means for selective isozyme quantitation has been studied. Although isozymes of both subfamilies bind metyrapone in the reduced state, selective quantitation of 2B isozymes through the metyrapone complex is possible after complex formation of P450 3A with a TAO metabolite. Thus, consecutive application of both reactions allows the spectroscopic quantitation of P450 3A and 2B isozymes. Complete conversion of P450 3A into the complex, a precondition for P450 3A quantitation, requires NADH in addition to NADPH. A precise collective quantitation of 3A + 2B isozymes as metyrapone complexes alone is not possible because the corresponding complexes possess different molar extinction coefficients, i.e 71.5 and 52 mM-1 cm-1 at 446-490 nm, respectively. The formation of the TAO complex appears to be quite specific, since it correlates well with 3A-specific enzymatic activities, i.e. TAO N-demethylation and formation of 2 beta-hydroxy-, 15 beta-hydroxy- and 6-dehydrotestosterone. P450 3A levels in liver microsomes of male rats either untreated or treated with TAO, dexamethasone (DEX), phenobarbital or hexachlorobenzene amount to 13%, 78%, 66%, 24% and 11% of total P450, respectively. Good correlation between these values and P450 3A-specific enzymatic activities is obtained. By the spectroscopic method, P450 2B isozymes could not be detected in microsomes of untreated rats. With TAO, DEX and hexachlorobenzene the microsomal 2B level is elevated to about 20% of total P450, i.e. to 0.8, 0.4 and 0.4 nmol P450/mg protein, respectively. 2B levels of about 60% of total P450 (0.75 nmol P450/mg protein) are obtained by phenobarbital treatment. Immunoblotting with anti-P450 2B shows that the ratio of expressed 2B1 and 2B2 differs depending on the type of inducer. DEX predominantly leads to induction of 2B2, which may explain the low pentoxyresorufin O-depentylase activity in these microsomes.

Exposure to various benzene derivatives differently induces cytochromes P450 2B1 and P450 2E1 in rat liver

Benzene (B), toluene (T), ethylbenzene (EB), styrene (S) and xylene isomers (oX, mX, pX) are important environmental pollutants and B is a proved human carcinogen. Their inhalation by male Wistar rats (4 mg/l, 20 h/day, 4 days) caused cytochrome P450 (P450) induction. The degree of P450 2B1 induction increased and that of 2E1 decreased in the series B, T, EB, S, oX, mX and pX, as estimated by Western blots, while neither solvent was as effective for 2B1 induction as phenobarbital and B was more effective for 2E1 than ethanol. The levels of several other P450s decreased after exposure to these solvents, B being most effective. Exposure to these solvents increased in vitro hepatic microsomal oxidation of B and aniline (AN) (2E1 substrates) 3 to 6-fold, indicating induction of this P450. T oxidation was increased 2 to 4-fold and chlorobenzene (ClB) oxidation 3-fold. Sodium phenobarbital (PB, 80 mg/kg/day, 4 days, i.p.) did not increase ethylmorphine (EM) and benzphetamine (BZP) demethylation (2B1 substrates), neither of the B derivatives did so, and oX decreased it; however, pentoxyresorufin O-dealkylation was well related to the immunochemically detected 2B1 levels in control, PB and B microsomes. PB did not increase B, but increased T and ClB oxidation 2-4 and 3-fold, respectively, indicating possible 2B1 role in their oxidation. B oxidation after various inducers was related to immunochemical 2E1 levels, T and ClB oxidation to both 2B1 and 2E1 and AN oxidation to 2E1 and 1A2 levels.(ABSTRACT TRUNCATED AT 250 WORDS)