Phenotyping of cytochromes P-450 in human tissues with monoclonal antibodies

Reaction Phenotyping of Low-Turnover Compounds in Long-Term Hepatocyte Cultures Through Persistent Selective Inhibition of Cytochromes P450

Recognizing the challenges of determining the relative contribution of different drug metabolizing enzymes to the metabolism of slowly metabolized compounds, a cytochrome P450 reaction phenotyping (CRP) method using cocultured human hepatocytes (HEPATOPAC) has been established. In this study, the emphasis on the relative contribution of different cytochrome P450 (P450) isoforms was assessed by persistently inhibiting P450 isoforms over 7 days with human HEPATOPAC. P450 isoform-selective inhibition was achieved with the chemical inhibitors furafylline (CYP1A2), tienilic acid (CYP2C9), (+)-N-3-benzylnirvanol (CYP2C19), paroxetine (CYP2D6), azamulin (CYP3A), and a combination of 1-aminobenzotriazole and tienilic acid (broad spectrum inhibition of P450s). We executed this CRP method using HEPATOPAC by optimizing for the choice of P450 inhibitors, their selectivity, and the temporal effect of inhibitor concentrations on maintaining selectivity of inhibition. In general, the established CRP method using potent and selective chemical inhibitors allows to measure the relative contribution of P450s and to calculate the fraction of metabolism (f _m) of low-turnover compounds. Several low-turnover compounds were used to validate this CRP method by determining their hepatic intrinsic clearance and f _m, with comparison with literature values. We established the foundation of a robust CRP for low-turnover compound test system which can be expanded to include inhibition of other drug metabolizing enzymes. This generic CRP assay, using human long-term hepatocyte cultures, will be an essential tool in drug development for new chemical entities in the quantitative assessment of the risk as a victim of drug-drug interactions. SIGNIFICANCE STATEMENT: An ongoing trend is to develop drug candidates which have limited metabolic clearance. The current studies report a generic approach to conducting reaction phenotyping studies with human HEPATOPAC, focusing on P450 metabolism of low-turnover compounds. Potent and selective chemical inhibitors were used to assess the relative contribution of the major human P450s. Validation was achieved by confirming hepatic intrinsic clearance and fraction of metabolism for previously reported low-turnover compounds. This approach is adaptable for assessment of all drug metabolizing enzymes.

Monoclonal Antibodies and Multifunctional Cytochrome P450: Drug Metabolism as Paradigm

Monoclonal antibodies are reagents par excellence for analyzing the role of individual cytochrome P450 isoforms in multifunctional biological activities catalyzed by cytochrome P450 enzymes. The precision and utility of the monoclonal antibodies have heretofore been applied primarily to studies of human drug metabolism. The unique and precise specificity and high inhibitory activity toward individual cytochrome P450s make the monoclonal antibodies extraordinary tools for identifying and quantifying the role of each P450 isoform in the metabolism of a drug or nondrug xenobiotic. The monoclonal antibodies identify drugs metabolized by individual, several, or polymorphic P450s. A comprehensive collection of monoclonal antibodies has been isolated to human P450s: 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C family, 2C19, 2D6, 2E1, 3A4/5, and 2J2. The monoclonal antibodies can also be used for identifying drugs and/or metabolites useful as markers for in vivo phenotyping. Clinical identification of a patient's phenotype, coupled with precise knowledge of a drug's metabolism, should lead to a reduction of adverse drug reactions and improved drug therapeutics, thereby promoting advances in drug discovery.

Potent inhibition of recombinant human cytochrome p-450 1A1 by pentamethoxystilbene

Previously it was reported that various hydroxystilbene compounds strongly inhibit human cytochrome P-450 1 enzymes and were postulated as candidate chemopreventive agents. In this study, the inhibitory potential of P-450 1 enzyme activities by 3,5,3,4,5-pentamethoxystilbene (PMS), a synthetic stilbene compound, was evaluated with the Escherichia coli (E. coil) membranes of recombinant human cytochrome P-450 1A1, 1A2, or 1B1 coexpressed with human NADPH-P-450 reductase. PMS produced a significant inhibition of ethoxyresorufin O-deethylation (EROD) activities with IC50 values of 0.14, 934, and 3.2 M for 1A1, 1A2, and 1B1, respectively. PMS did not significantly inhibit EROD activities in human liver microsomes. To elucidate the mechanism of inhibition by PMS, kinetic studies were performed. Analysis of the mode of inhibition indicated a mixed-type inhibition of P-450 1A1. The inhibition of P-450 1A1-mediated EROD activity by PMS was not irreversible-mechanism based. The loss of EROD activity of P-450 1A1 with PMS was blocked by trapping agents such as glutathione, N-acetylcysteine, or dithiothreitol. Moreover, PMS significantly suppressed P-450 1A1-mediated EROD activity and P-450 1A1 gene expression in HepG2 cells induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Taken together, the results suggested that PMS is a potent and selective inhibitor of human P-450 1A1 and may be considered for use as a cancer chemopreventive agent in humans.

Production of Inhibitory Polyclonal Antibodies against Cytochrome P450s

Nine different antibodies against P450 isoforms were prepared using purified cytochrome P450s (P450) expressed in E. coli. Purified isozymes were injected into rabbits to raise specific antibody. The resulting antibodies were characterized for their specificity and sensitivity through each particular P450 enzyme-mediated probe reaction.Anti-CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6, CYP2E1, and CYP3A4 antibodies proved to be strong immunoinhibitors with inhibitory effects specific to their corresponding antigen. Antiserum derived from the CYP2C19-immunized rabbits was reacted with CYP2C9 as well as CYP2C19 and immunoabsorbed with membrane-bound CYP2C9 expressed in E. coli. Antibody specific for CYP2C19 was obtained. Anti-CYP2C19 together with the anti-CYP2C8 and anti-CYP2C9 can be very useful for determining the contribution of a particular P450 in the metabolism of a drug. The developed inhibitory antibodies will serve as in vitro-specific tools for evaluating the quantitative contribution of individual P450 enzymes to drug metabolism.

Monoclonal antibody-directed analysis of benzo[a]pyrene metabolism in rat liver and extrahepatic tissues: effect of propyl and octyl gallate

Monoclonal antibody (MAb) 1-7-1 against 3-methylcholanthrene (MC)-induced forms of cytochrome P-450 (CYP) was used to characterize benzo[a]pyrene (B[a]P) metabolism in rat liver and extrahepatic tissues and its modulation by phenolic antioxidants, propyl and octyl gallates. Male Wistar rats were treated with these food additives (50 mg/kg body wt i.p.) twice weekly for 14 days alone or in combination with MC. Immunochemical inhibition of aryl hydrocarbon hydroxylase (AHH) and [14C]B[a]P metabolism (analyzed by high-performance liquid chromatography) were measured in liver, kidney, and lung microsomes. Organ-specific changes in levels of MAb-mediated inhibition of microsomal metabolism of B[a]P were observed. In liver microsomes from untreated rats, AHH was not affected by MAb, but in kidney and lung, there was 70% and 50% inhibition, respectively. In MC-treated rats, MAb reduced AHH activity by 43% in liver. Kidney and lung AHH was inhibited up to 80% by this MAb. Formation of B[a]P metabolites in MC-induced microsomes from liver and kidney was affected by MAb in a similar way. In lung, the total metabolism was inhibited by 50% by MAb treatment, but significant differences in inhibition of individual metabolites were observed. Treatment with propyl or octyl gallate alone had no effect on MAb inhibition of AHH activity in liver and lung but decreased the level of inhibition in kidney. Combined treatment with MC and propyl or octyl gallate slightly reduced the effect of MAb on AHH activity in liver and significantly reduced the level of inhibition in kidney but did not affect AHH activity in lung. The same treatment regimen dramatically reduced MAb inhibition of B[a]P metabolism in kidney but had no effect on B[a]P metabolite formation in liver. Inhibition by MAb of renal 3-hydroxy-B[a]P, 7,8-B[a]P-dihydrodiol, and 1,6-quinone-B[a]P was the most affected. In lung, treatment with gallates affected only formation of 7,8-B[a]P-dihydrodiol. These results suggest that treatment with gallates affects the CYP 1A and may change the CYP isozyme composition and, thus, alter the tissues' susceptibility to tumor induction by B[a]P.

Phenotyping cytochromes P450 with monoclonal antibodies

Monoclonal antibodies (MAbs) to cytochrome P-450 isozymes can be used to phenotype tissues for epitope-specific cytochrome P-450 content. MAbs that inhibit specific cytochrome P-450 dependent drug or carcinogen reactions are useful tools for quantitative measurement of the individual or classes of cytochromes P-450 that catalyze these reactions. This method has been applied successfully to animal as well as human tissues. Radioimmunoassays based on MAbs have been developed and provide a rapid and efficient means for detecting cytochromes P-450 independent of functional enzyme activity. In addition, MAbs coupled to a Sepharose support can be used to immunopurify cytochromes P-450 in a procedure that is more rapid and efficient than conventional purification schemes. MAbs add a new dimension to analyses of cytochrome P-450 multiplicity and will find numerous applications in elucidation of the relationship between cytochrome P-450 phenotype and carcinogen or drug metabolism.

Expression of multiple cytochrome p450 enzymes and multidrug resistance-associated transport proteins in human skin keratinocytes

Cytochrome P450 enzymes metabolize various endogenous and exogenous small molecular weight compounds. Transport-associated proteins, such as P-glycoprotein, multidrug resistance-associated protein and lung resistance protein are overexpressed in drug-resistant cell lines, as well as in human tumors from various histologic origins, including malignant melanoma. Little is known about the expression and function of cytochrome enzymes and multidrug resistance-associated transport proteins in human skin; therefore, the aim of this study was to analyze the expression pattern of cytochrome enzymes and multidrug resistance-associated transport proteins in proliferating human epidermal keratinocytes under constitutive conditions and after induction with various inducers. Reverse transcription-polymerase chain reaction revealed constitutive expression of cytochromes 1A1, 1B1, 2B6, 2E1, and 3A5 in keratinocytes and showed expression of cytochrome 3A4 after incubation with dexamethasone. The expression of cytochrome 1A1 was enhanced on the mRNA level after induction with benzanthracene. Reverse transcription-polymerase chain reaction analysis of the multidrug resistance-associated transport proteins revealed constitutive expression of multidrug resistance-associated proteins 1 and 3-6, and lung resistance protein in human epithelial keratinocytes and was negative for multidrug resistance 1 and 2. Expression of 1 was seen after induction with dexamethasone. Reverse transcription-polymerase chain reaction results were confirmed by immunoblots which showed expression of cytochromes 1A1, 2B6, 2E1, and 3A, multidrug resistance-associated proteins 1, 3, and 5 as well as multidrug resistance 1 after induction with dexamethasone. Immunohistology showed positive immunofluorescence in skin specimens for cytochromes 1A1, 2B6, 2E1, and 3A and multidrug resistance-associated protein 1 and multidrug resistance 1. Constitutive activity of cytochrome 1A1, 2B, 2E1, and 3A enzymes was measured by catalytic assays. These results show that keratinocytes of the human skin express various transport-associated enzymes and detoxifying metabolic enzymes. Previous studies have revealed that cytochrome enzymes and transport-associated proteins play complementary parts in drug disposition by biotransformation (phase I) and anti-transport (phase III) and act synergistically as a drug bioavailability barrier.

An inhibitory monoclonal antibody to human cytochrome P450 that specifically binds and inhibits P4502C9II, an allelic variant of P4502C9 having a single amino acid change Arg144 Cys

A monoclonal antibody (MAb 292-2-3) has been isolated that binds specifically to a single allele of three expressed human cytochrome P4502C9 alleles. The MAb binds to 2C9Cys144 (II), and does not bind to the wild-type 2C9Arg144 (I), or the third allele 2C9Ile-->Leu359 (III) and thus the MAb detects an allele with > 99% homology and differing from the wild-type 2C9Arg144 (I) by a single amino acid. The MAb 292-2-3 does not bind to the other 2C isoforms (2C8, 2C18, 2C19) or the other human cytochrome P450s, 1A1, 1A2, 2A6, 2B6, 2C8, 2D6, 2E1 or 3A4/5. MAb 292-2-3 inhibits the metabolism of tolbutamide, diclofenac and phenanthrene by the target 2C9Cys144 (II) allele by > 90% and does not inhibit the catalytic activity of the wild-type 2C9Arg144 (I), or 2C9Ile-->Leu359 (III) the other 2C isoforms 2C8, 2C18, 2C19, or the other non-2C human P450s listed above. The MAb 292-2-3 is thus a prototype of an ideal and extraordinarily specific reagent for the detection and measurement of the metabolic role of highly related isoforms and polymorphic alleles of human cytochrome P450s. MAbs of high specificity can also determine the amount of phenotypic expression of polymorphic alleles and their metabolic role in drug and non-drug xenobiotic metabolism in heterozygote individuals. The inhibitory MAb might also identify allele-specific substrates of polymorphic human cytochrome P450s.

The expression and regulation of drug metabolism in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears restricted. Maternal cigarette smoking or PCB exposure increase the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene into DNA-bound adducts, both in vitro and in vivo. Studies with RT-PCR technique have demonstrated that first trimester placentae express at the mRNA level CYP1A1, 1A2, 2C, 2D6, 2E1, 2F1, 3A4, 3A5, 3A7 and 4B1 and at full term CYP1A1, 2E1, 2F1, 3A3/4, 3A5 and 3A7. However, more detailed studies on cDNA probes or with specific antibodies or 'diagnostic' substrates for other than CYP1A1, 2E1 and 3A gene products have yielded negative results. Studies on human placenta and a chorioncarcinoma cell line, JEG 3 cells, boulster the concept that placental CYP1A1 and 1B1 - although their expression is Ah receptor and ARNT mediated - is controlled by distinct mechanisms. Aromatase, CYP19, and cholesterol side-chain cleaving, CYP11B, genes, proteins and activities are catalytically active in human placentae throughout the pregnancy and those parameters do not seem to be affected by maternal cigarette smoking but rather maternal health status. However, the substrate binding pocket of aromatase accepts as its substrate several xenobiotics and is responsible for constitutive xenobiotic biotransformations.Functional placental glutathione S-transferase, N-acetyl transferase and epoxide hydrolase are expressed via one gene each and their function reflects the placenta as an endocrine organ rather than a xenobiotic-metabolizing unit. However, markers for oxidative stress can be detected in decreased glutathione S-transferase activities.Because human placenta has quite well defined metabolic characteristics, and obtaining placental samples will not meet any drastic ethical difficulties, it could be used more intensively as a source of metabolizing enzymes in in vitro studies during the course of a drug development program. The human placenta, or its subcellular organelles, could serve as a real alternative model for an extrahepatic tissue in replacing recombinant expression systems especially if CYP11, 19, 1A1 or potentially 2E1 are target enzymes for potential metabolic interactions.

Cytochrome P450 1B1: a major P450 isoenzyme in human blood monocytes and macrophage subsets

In this study, cytochrome P450 (CYP; EC 1.14.14.1)-dependent activities and P450 isoenzyme patterns were determined in human monocytes and macrophages, which play a major role in antigen processing including small molecular weight compounds which cause contact dermatitis or drug-allergic reactions. Using reverse transcriptase-polymerase chain reaction (RT-PCR) we determined the mRNA expression of eight CYPs (1A1, 1A2, 1B1, 2B6/7, 2E1, 3A3/4, 3A7 and 4B1) in human blood monocytes and macrophage subsets 27E10 and RM3/1. To study the influence of known P450 inducers, monocytes were incubated in vitro with ethanol, dexamethasone, cyclosporin A (CSA), benzanthracene (BA), phenobarbital (PB), lipopolysaccharide (LPS) and 12-O-tetradecanoyl-phorbol-13-acetat (TPA) for 24 hr. Percoll density gradient isolated monocytes as well as the pro-inflammatory macrophage subtype 27E10 expressed 1B1, 2E1 and 2B6/7. On the other hand, in the anti-inflammatory macrophage subtype RM3/1, predominantly 1B1 and to some extent 2B6/7 were found. Treatment with cyclosporin A, phenobarbital, benzanthracene or ethanol resulted in induction of the expression of 3A3/4. CYP1B1 was the predominant isoenzyme in all monocytes and macrophages. In monocytes purified by adherence or induced by benzanthracene, lipopolysaccharide or 12-O-tetradecanoyl-phorbol-13-acetat, 1A1 was also expressed. Northern blot analysis confirmed the presence of CYP1B1 in monocytes and macrophages, a presence which was also demonstrated on the protein level by immunoblot and by immunohistochemical staining of the cells. The expression of several CYPs in monocytes/macrophages suggests that these cells may be important in the metabolism of small molecular weight compounds, which play a role in allergic contact dermatitis and drug reactions. Of particular interest is the remarkably strong expression of the recently identified dioxin inducible CYP1B1, known to be present in a wide range of malignant tumors.

Metabolism and activation of cyclopenta polycyclic aromatic hydrocarbons in liver tissue from rats and humans

The metabolism of radiolabelled benz(j)aceanthrylene (B(j)A) was studied by high performance liquid chromatography (HPLC) using suspensions of hepatocytes and liver microsomes from control- or Aroclor 1254 (PCB)-treated rats, or with human liver microsomes (five different donors) as activation systems. The major metabolites formed in hepatocytes were sulfate conjugates, indicating that sulfation is an important detoxication pathway for B(j)A. In incubations with B(j)A and rat or human liver microsomes, the major metabolite formed was B(j)A-1,2-diol. Studies with rat liver microsomes using antibodies (Ab) towards either P4501A1, 1A2 or 3A2, resulted in approximately 30% reduction in covalent binding with all Ab-using microsomes from control animals, whereas with microsomes from PCB-treated animals an 85% reduction was observed using Ab towards P4501A2, and only minor reductions were observed with 1A1 or 3A2. When compared to B(j)A and benzo(a)pyrene (B(a)P), benz(1)aceanthrylene (B(l)A) caused higher numbers of revertants in the Salmonella assay when plated with rat liver microsomes from control animals or human liver microsomes. The total DNA adduct levels in hepatocytes from control animals after 2 h exposure to 30 micrograms/ml (120 microM) B(j)A or B(l)A, as measured by the 32P-postlabelling technique, were 3.8 +/- 1.5 and 10.1 +/- 5.8 fmol/microgram DNA, respectively. PCB-treatment decreased the total level of B(j)A adducts slightly (1.8 +/- 0.5 fmol/microgram DNA), whereas in contrast the level of B(1)A adducts was increased (24.5 +/- 20.1 fmol/microgram DNA). The major DNA adduct formed in control hepatocytes exposed to B(j)A co-chromatographed with B(j)A-1,2-oxide, which also appeared to be the major adduct formed when rat or human liver microsomes were co-incubated with calf thymus DNA. The total DNA adduct levels in the modified calf thymus DNA after 30 min exposure to 30 micrograms/ml B(j)A, B(l)A or B(a)P using rat liver microsomes form control animals, were 3.6, 66.3 and 1.4 fmol/microgram DNA, respectively. These levels increased to 22.7, 93.3 and 7.4 fmol/microgram DNA, respectively, using microsomes from PCB-treated animals. With human liver microsomes, the total DNA adduct levels after exposure to B(j)A, B(l)A or B(a)P, ranged between 0.4-1.0, 0.3-4.3, and 0.1-0.3 fmol/microgram DNA, respectively. Overall, the present data supports the notion that oxidation at the cyclopenta-ring is an important activation pathway for B(j)A, and indicate that the activation mechanism for B(j)A is similar in rat and human liver tissue.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

Characterization of a fusion protein between human cytochrome P450 1A1 and rat NADPH-P450 oxidoreductase in Escherichia coli

A cDNA of fusion protein between human cytochrome P450 1A1 and rat NADPH-P450 reductase was genetically engineered and expressed in Escherichia coli DH5alpha cells under the control of an inducible tac promoter (Y. J. Chun, T. Shimada, and F. P. Guengerich, (1996) Arch. Biochem. Biophys. 330, 48-58). E. coli membranes of transformed cells showed much higher P450 1Al-dependent monooxygenase and NADPH-P450 reductase activities than pCW control vector or P450 1A1 expression vector-transformed cells. Ethoxyresorufin O-deethylase and methoxyresorufin O-demethylase were 22-fold and 11-fold higher than the control activity, respectively. alpha-Naphthoflavone and beta-naphthoflavone strongly inhibited P450 1A1 activity of the fusion protein, with alpha-naphthoflavone being more potent than beta-naphthoflavone. Divalent cations (e.g. Ca2+ and Mg2+) increased P450 1A1 activity as well as NADPH-P450 reductase activity. These results demonstrate that this fusion protein in E. coli membrane may be a useful model for elucidating details of protein-protein interactions between P450 and NADPH-P450 reductase in the endoplasmic reticulum of mammalian cells.

Specificity of cDNA-expressed human and rodent cytochrome P450s in the oxidative metabolism of the potent carcinogen 7,12-dimethylbenz[a]anthracene

7,12-Dimethylbenz[a]anthracene (DMBA), a potent carcinogen, requires metabolic activation by cytochrome P450s (P450s) to electrophilic metabolites that result in DNA modification, mutagenicity, and carcinogenicity. In this study, we used eight human forms, four rodent forms, and one rabbit form of P450 expressed from recombinant vaccinia or baculovirus vectors to define their specificity for metabolizing DMBA. Of the eight human P450s, 1A1 was the most active (specific activity = 14.7 nmol/min/nmol of P450) in total metabolism of DMBA and showed approximately 6- to 33-fold more activity than other P450s, 2B6, 2C9, and 1A2 were also capable of metabolizing DMBA (2.0-2.5 nmol/min/nmol of P450), whereas 2C8, 2E1, 3A4, and 3A5 exhibited relatively low activities. Among animal P450s, mouse 1A1 exhibited activity similar to that of human 1A1 and had 5.0- to 37-fold more activity than other rodent and rabbit P450s. In regard to enzyme regioselectivity, most human and rodent P450s predominantly formed the 8,9-diol, but human 2B6 and rat 2B1 preferentially formed the 5,6-diol. In the production of monohydroxymethyl metabolites, all the enzymes yielded more 7-hydroxymethyl-12-methylbenz[a]anthracene (7HOM12MBA) than 12-hydroxymethyl-7-methylbenz[a]anthracene (7M12HOMBA), except for human 1A1, which presented the reverse selectivity. Human liver microsomes from 10 organ donors were shown to metabolize DMBA and in most circumstances generated the metabolic profile DMBA trans-8,9-dihydrodiol > 7HOM12MBA > or = DMBA trans-5,6-dihydrodiol > or = 7,12-dihydroxymethylbenz[a]anthracene > 7M12HOMBA > DMBA trans-3,4-dihydrodiol. Thus, the combined activity of hepatic microsomal 2C9, 1A2, and 2B6 may contribute to the metabolic activation and the metabolism of DMBA in normal human liver.

Human liver S-9 metabolic activation: proficiency in cytogenetic assays and comparison with phenobarbital/beta-naphthoflavone or aroclor 1254 induced rat S-9

Induced rat liver S-9 is routinely used for metabolic activation in cytogenetic assays. When a compound gives a positive test result only with rat S-9, the significance for humans should be assessed. To evaluate the use of human S-9, we used sister-chromatid exchanges (SCEs) and chromosome aberrations (Abs) in Chinese hamster ovary cells to test five pro-mutagens, each preferentially activated by a different family of P-450: benzo(a)pyrene (BP), dimethylnitrosamine (DMN), diethylnitrosamine (DEN), aflatoxin B1 (AFB), and 2-acetylaminofluorene (2-AAF). We tested two human S-9 preparations, one from a single liver and a second pooled from two livers known to have good activity for several P-450s. Concentrations and ratios of NADP and isocitrate were adjusted to optimize NADPH generation by the S-9. Abs were scored 20 hr, and SCEs 29-45 hr, after the beginning of a 3 hr treatment. P-450 enzyme activities were generally higher in rat than human S-9. With the single-liver human S-9, increase in SCEs were seen with all chemicals; with both human S-9s, increases in Abs were seen with all chemicals except BP. (The level of P-450 1A1, required for BP activation, is very low in human liver.) Compared with rat S-9, generally higher concentrations of human S-9 and of promutagens were required to see positive results. However, human S-9 effectively activated 2-AAF, whereas neither of the two types of rat S-9 produced Abs with 2-AAF. We also compared rat S-9s induced with Aroclor 1254 or phenobarbital/ beta-naphthoflavone (PB/beta NF). Although there were some differences in P-450 enzyme activities, these did not translate into differences in Abs induction. At low doses of AFB and of BP, PB/beta NF induced S-9 appeared more effective than Aroclor 1254 induced S-9.

The expression and environmental regulation of P450 enzymes in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears somewhat restricted. Maternal cigarette smoking or PCB exposure increases the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene (B(a)P) into DNA-bound products, both in vitro and in vivo. Studies on adult human liver bolster the concept that CYP1A1 and -1A2 are differentially expressed in hepatic and extrahepatic tissues. Studies with cDNA probe or enzyme specific antibodies and substrates for CYP2A, -2B, -2C, -2D, and -2E gene products have yielded negative results. There are only minimal activities that can be found in substantial quantities in placentas without maternal smoking; one example is 7-ethoxycoumarin O-deethylase (ECOD). Aromatase and cholesterol side-chain cleaving P450 mRNAs, proteins, and activities are measurable in human placentas and do not seem to be affected by maternal cigarette smoking.

7-Alkoxyquinoline O-dealkylation by microsomes from human liver and placenta

1. The O-dealkylation of seven 7-alkoxyquinoline derivatives by human hepatic and placental microsomes and the effect of maternal cigarette smoking on placental 7-alkoxyquinoline metabolism was studied. 2. None of several monoclonal antibodies to isoenzymes of cytochrome P450 had a clear effect on metabolism of the compounds by liver microsomes. 3. Maternal cigarette smoking induced the O-dealkylation of all of the 7-alkoxyquinoline derivatives, being greatest for 7-butoxy- and 7-benzyloxyquinoline. 4. Placental 7-alkoxyquinoline metabolism induced by smoking was partially inhibited by the monoclonal antibody 1-7-1 raised against 3-methylcholanthrene-induced rat liver P450. 5. None of the 7-alkoxyquinoline O-dealkylations could be assigned specifically to any known P450 isoenzyme in human liver or placenta.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Immunochemical detection of cytochrome P450 isozymes induced in rat liver by n-hexane, 2-hexanone and acetonyl acetone

Cytochrome P450 isozymes induced in rat liver by treatment with n-hexane, 2-hexanone and acetonyl acetone (given intraperitoneally 5 mmol/kg for 4 days) were investigated using enzyme assays (benzene, toluene, 7-ethoxyresorufin and 7-pentoxyresorufin metabolism) and monoclonal antibodies (anti-P450IA1/2, anti-P450IIB1/2, anti-P450IIC11/6, anti-P450IIE1(91) and anti-P450IIE1(98)). n-Hexane treatment enhanced the activities of low-Km benzene aromatic hydroxylase and toluene side-chain oxidase, but not 7-ethoxyresorufin O-deethylase or 7-pentoxyresorufin O-depentylase. 2-Hexanone or acetonyl acetone treatment enhanced the activities of low- and high-Km benzene aromatic hydroxylases, toluene side-chain oxidase and 7-pentoxyresorufin O-depentylase, but not of 7-ethoxyresorufin O-deethylase. Immunoblot analysis showed that anti-P450IA1/2 did not bind liver microsomal protein from either control and treated rats in the region of cytochrome P450s, whereas with anti-P450IIE1(98) a clear-cut band was seen in liver microsomes from control and treated rats, with intensities in the following order: 2-hexanone = acetonyl acetone greater than or equal to n-hexane greater than control greater than phenobarbital. With anti-P450IIB1/2, a band was detected in microsomes from phenobarbital-treated rats, and to a lesser extent, in microsomes from 2-hexanone- and acetonyl acetone-treated rats. Like the immunoblot analysis, anti-P450IIE1(91) inhibited toluene side-chain hydroxylase activity in all microsomes, except in preparations from phenobarbital-treated rats and anti-P450IIB1 in microsomes from phenobarbital-, 2-hexanone- and acetonyl acetone-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibody-directed characterization of cytochrome P450 isozymes responsible for toluene metabolism in rat liver

Monoclonal antibodies (MAbs) were used to study the contribution of cytochromes P450IA1/IA2, P450IIB1/IIB2, P450IIC11/IIC6 and P450IIE1 to toluene side-chain (benzyl alcohol, BA, formation) and ring (o- and p-cresol formation) oxidation in liver microsomes from fed, one-day fasted, and phenobarbital (PB)-, 3-methylcholanthrene (MC)- and ethanol-treated rats. All rats were fed synthetic liquid diets. MAb 1-7-1 against P450IA1/IA2 inhibited markedly o-cresol formation and slightly p-cresol formation but not BA formation only in microsomes from MC-treated rats. MAbs 2-66-3, 4-7-1 and 4-29-5 against P450IIB1/IIB2 strongly inhibited BA, o-cresol and p-cresol formation only in PB-induced microsomes. MAb 1-68-11 against P450IIC11/IIC6 inhibited BA formation at high toluene concentration in the following order: fed greater than fasted greater than ethanol = MC greater than PB, and ethanol greater than or equal to fed = fasted greater than MC greater than PB on the basis of the percentage and net amount inhibition, respectively. MAb 1-91-3 against P450IIE1 inhibited BA formation at low toluene concentration, but not at high concentration, in the following order: ethanol greater than fasted = fed greater than MC, and ethanol greater than fasted greater than fed greater than MC on the basis of percentage and net inhibition, respectively. MAbs 1-68-11 and 1-91-3 also inhibited p-cresol formation at high and low toluene concentrations, respectively. These results indicate that (i) both P450IIE1 and P450IIC11/IIC6 are constitutive isozymes mainly responsible for the formation of BA and p-cresol from toluene as low- and high-Km isozymes, respectively; (ii) P450IIE1, but not P450IIC11/IIC6, is induced by one-day fasting and ethanol treatment; (iii) both P450IIE1 and P450IIC11/IIC6 are decreased by PB and MC treatments; (iv) P450IIE1 is inhibited by high concentration of toluene; (v) P450IIB1/IIB2 can contribute to the formation of BA, o- and p-cresol from toluene, while P450IAI/IA2 preferentially contributes to the formation of o-cresol.

A monoclonal antibody to rat liver cytochrome P450 IIC11 strongly and regiospecifically inhibits constitutive benzo[a]pyrene metabolism and DNA binding

The monoclonal antibody MAb 1-68-11, prepared to constitutive cytochrome P450 IIC11 (2c/RLM5) from male Sprague-Dawley rat liver, was used to study the contribution of the class of cytochrome P450s epitopically related to P450 IIC11 to the regiospecific metabolism of benzo[a]pyrene (BP) and its binding to DNA. The effect of MAb 1-68-11 was determined on the conversion of BP to BP-9,10-dihydrodiol, BP-7,8-dihydrodiol, BP-4,5-dihydrodiol, BP phenols, and BP quinones, and on the P450-dependent DNA binding catalyzed by P450 in microsomes from uninduced male and female Wistar and Sprague-Dawley rat livers, as well as 3-methylcholanthrene- and phenobarbital (PB)-induced male Wistar rat livers. In liver microsomes from untreated male rats, MAb 1-68-11 inhibited BP-9,10-dihydrodiol formation by 80%; in liver microsomes from untreated female rats, the inhibition was 100%. BP-7,8-dihydrodiol formation was inhibited from 38 to 77% in microsomes from males and 50% in those from females. In microsomes from PB-induced rats, inhibition of the 9,10-dihydrodiol and the 7,8-dihydrodiol was 90% and 73%, respectively, whereas BP-4,5-dihydrodiol formation was enhanced 80%. In microsomes from 3-methylcholanthrene-treated rats, no inhibition of MAb 1-68-11 was observed on either the metabolism of BP or its binding to DNA. In contrast, the binding of BP to DNA was completely inhibited by MAb 1-68-11 in microsomes from uninduced male Wistar rats and 70% in PB-induced microsomes. 32P-postlabeling analysis showed that formation of the major stable adduct, BP diol epoxide bound at C-10 to the 2-amino of deoxyguanosine, was strongly inhibited in uninduced and PB-induced microsomes. Formation of the major labile BP-DNA adduct 7-(benzo[a]pyren-6-yl) guanine (BP-N7Gua) was inhibited about 60% in microsomes from untreated male Wistar rats. These results show that MAb 1-68-11 regiospecifically inhibits cytochrome P450 IIC11 and epitopically related P450s that metabolize BP at the 7,8 and 9,10 positions. MAb 1-68-11 also inhibits enzyme-catalyzed binding of BP to DNA in the specific formation of BP-N7Gua and adducts detected by the 32P-postlabeling technique.

Xenobiotic and steroid-metabolizing monooxygenases catalysed by cytochrome P450 and glutathione S-transferase conjugations in the human placenta and their relationships to maternal cigarette smoking

Placentae from both smoking and non-smoking mothers were studied with respect to P450-dependent xenobiotic and steroid-metabolizing reactions, GSHt activity with different substrates and umbilical blood cotinine levels. Catalytic activities were determined in both freshly prepared homogenate subfractions and subfractions prepared after freezing the tissue sample. The results showed three correlation clusters: (i) AHH and ECDE (P less than 0.001), (ii) ECDE and ERDE (P less than 0.05), and (iii) Arom and CSCC reactions correlated with each other both in the mitochondria and microsomes. Among xenobiotic and steroid metabolizing activities, only CSCC and AHH showed a significant negative correlation. Our results agree with the earlier studies (Gottlieb and Manchester, 1986) reporting that xenobiotic metabolising MO and GSHt reactions did not show any statistically significant correlations, reflecting the fact that maternal cigarette smoking does not affect GSHt activities. However, in placentae from smoking mothers a statistically significant (P less than 0.01) positive correlation between GSHt and Arom activity was found. No plausible biological explanation is available for this finding. Among xenobiotic-metabolizing activities only ERDE correlated with plasma cotinine levels, suggesting that it is most closely related to the extent of maternal cigarette smoking. The present findings also suggest that cigarette smoke induced ERDE activity is a distinct one compared with the other xenobiotic-metabolizing P450s. Negative correlations between xenobiotic-metabolizing MO activities and CSCC lend some support to an earlier suggestion (Juchau et al, 1972) that cigarette smoking affects endogenous steroid-metabolizing CSCC reaction catalyzed by P450. Whether this finding is a true biological phenomenon and what is the mechanism behind it remain to be elucidated.

Monoclonal antibodies directed characterization of epidermal and hepatic cytochrome P-450 isozymes induced by skin application of therapeutic crude coal tar

A single application of crude coal tar (CCT) solution (USP) to the skin of neonatal rats was shown to induce epidermal and hepatic cytochrome P-450(P-450)-dependent monooxygenase activities. To further characterize the induction response, in this study we have utilized highly specific monoclonal antibodies (MoAb) 1-7-1, 2-66-3, and 1-98-1 directed against highly purified rat liver P-450s induced by 3-methyl-cholanthrene, phenobarbital and ethanol, respectively. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of hepatic microsomes prepared from CCT-treated animals showed a significant increase in the coomassie blue stainable proteins in the P-450 region; however, this was not evident in epidermal microsomes. Immunoblot analysis of epidermal and hepatic microsomes with MoAb 1-7-1 revealed strong immunoprecipitin bands in both tissues. MoAb 2-66-3 showed significant immunoreactivity only with hepatic microsomes. Interestingly, CCT treatment resulted in suppression of immunoreactivity with MoAb 1-98-1 in hepatic microsomes. MoAb 1-7-1 and 2-66-3 exhibited concentration-dependent inhibitory effects in aryl hydrocarbon hydroxylase and 7-ethoxycoumarin-O-deethylase activities induced by CCT application. MoAb 1-7-1 was substantially more effective in this respect. Epidermal and hepatic microsomes prepared from CCT-treated rats showed significantly greater metabolism of benzo(a)pyrene (BP). MoAb 1-7-1 and MoAb 2-66-3 inhibited BP metabolism in both the tissues. However, MoAb 1-7-1 was more inhibitory in this regard as compared to MoAb 2-66-3. These studies indicate that topical application of therapeutic CCT to the skin of neonatal rats results in induction of P-450 isozyme c in epidermis and isozymes b and c in liver, and that this induction is associated with the suppression of P-450 isozyme j in liver.

Erythromycin breath test as an assay of glucocorticoid-inducible liver cytochromes P-450. Studies in rats and patients

The major P-450IIIA gene family member present in human liver is HLp which, like its rat liver orthologue P-450p, is inducible by glucocorticoids and catalyzes erythromycin N-demethylation. To develop a practical method to estimate the amounts of HLp in patients [14C]N-methyl erythromycin was injected into rats that had been pretreated with dexamethasone or with inducers of other forms of cytochrome P-450. The rate of demethylation of this substrate, measured simply as 14CO2 in the breath, correlated well with the concentrations of immunoreactive P-450p protein (r = 0.70), holocytochrome P-450p (r = 0.70), or with erythromycin N-demethylase activity (r = 0.90) determined in the liver microsomes prepared from each rat. Next, [14C]N-methyl erythromycin was administered to 30 patients and there was a sixfold interindividual variation in breath 14CO2 production seemingly unrelated to medications, smoking status or age. However, the average breath test values were twofold greater in female as compared to male patients (P less than 0.01). Breath 14CO2 production rose in patients retested after treatment with the P-450IIIA inducers dexamethasone (P less than 0.05) or rifampicin (P less than 0.05) and was decreased after treatment with the HLp inhibitor triacetyloleandomycin (P less than 0.05). We conclude that the erythromycin breath test provides a convenient assay of P-450IIIA cytochromes in rats and in some patients.

Comparison of the formation of benzo[a]pyrene diolepoxide-DNA adducts in vitro by rat and human microsomes: evidence for the involvement of P-450IA1 and P-450IA2

The involvement of cytochrome P-450 isozymes in the activation of benzo[a]pyrene (BP) by human placental and liver microsomes was studied in vitro using monoclonal antibodies (Mab) toward the major 3-methylcholanthrene (MC)-inducible and phenobarbital-inductible rat liver P-450 isozymes (Mab 1-7-1 and Mab 2-66-3, respectively). Microsomes from human placenta and liver and rat liver were incubated with BP and DNA, and BP-diolepoxide-DNA (BPDE-DNA) adducts were measured by synchronous fluorescence spectrophotometry (SFS). The only BP metabolite giving the same fluorescence peak as chemically modified BPDE-DNA was BP-7,8-dihydrodiol. Five (smokers) out of 29 human placentas (smokers and nonsmokers), and five out of nine human livers were able to metabolically activate BP to BPDE-DNA adducts in this system. The Mab 1-7-1 totally inhibited the formation of BPDE-DNA adducts in placental microsomal incubations. Inhibition using rat or human liver microsomes was 50-60% and about 90%, respectively. The Mab 2-66-3 had no effect in any of the microsome types. Adduct formation was inhibited more strongly and at lower concentrations of Mab 1-7-1 compared with the inhibition of AHH activity. This study is a clear indication of the major role of P-450IA1 (P-450c) in human placenta and probably P-450IA2 (P-450d) in human liver in BP activation, while other isozymes also take part in the activation in rat liver. Furthermore, this clearly indicates that AHH activity and BP activation are not necessarily associated.

Inhibition of human hepatic and placental xenobiotic monooxygenases by imidazole antimycotics

Three imidazole antimycotic drugs, ketoconazole, clotrimazole and miconazole, were studied to characterize the inhibition of aryl hydrocarbon hydroxylase (AHH), 7-ethoxycoumarin O-deethylase (ECDE) and 7-ethoxyresorufin O-deethylase (ERDE) activities in human liver and placenta in vitro in comparison with liver enzymes from control, phenobarbital (PB) and 3-methylcholanthrene (MC) pretreated rats. All three compounds inhibited rat liver enzymes, although MC pretreatment seemed to lead to a resistance of inhibition relative to PB-treated and control animals. There were large differences in the extent of inhibition of human hepatic and placental activities. Furthermore, while the type of inhibition of the hepatic ERDE was competitive or mixed, that of the placental enzyme cannot be described in ordinary terms of inhibition kinetics. Ketoconazole and clotrimazole were relatively potent inhibitors of maternal cigarette smoking-induced placental ECDE activities (IC50 values from 0.5 microM to 5 microM), whereas much less inhibition of the placental AHH activity was obtained with ketoconazole and miconazole (IC50 values from 50 microM to 500 microM). In most cases, hepatic enzymes were less sensitive to antimycotics than placental activities. This was in contrast with results from rat enzyme studies, in which MC pretreatment seemed to decrease the inhibitory response.

7-Ethoxycoumarin and 7-ethoxyresorufin O-deethylase in human foetal and adult liver: studies with monoclonal antibodies

The 7-ethoxycoumarin and 7-ethoxyresorufin O-deethylase activities were investigated in the microsomal fractions from 5 human adult and 3 foetal livers and 5 human foetal adrenals. The enzyme activity expressed as pmol/min. per mg microsomal protein was higher with 7-ethoxyresorufin as substrate in all investigated specimens with average values (+/- S.E.M.) of 74 +/- 27, 13 +/- 3 and 12 +/- 1 in adult and foetal livers and foetal adrenals, respectively. Monoclonal antibodies raised against 3-methylchloranthrene or phenobarbital induced rat liver cytochrome P-450 were investigated with respect to their inhibiting effects on the rate of O-deethylation of both substrates in human adult liver. Only the monoclonal antibody against the 3-methylcholanthrene induced cytochrome P-450 inhibited the O-deethylation of 7-ethoxyresorufin to 64 to 79 percent of control values. The other antibody had no effect on this or the other O-deethylase activity. Thus, the 7-ethoxyresorufin O-deethylase is partly catalyzed in human adult liver by a cytochrome with an epitope that is recognized by the monoclonal antibody against 3-methylcholanthrene induced rat liver cytochrome P-450. With foetal liver the low activity of the enzyme became unmeasurable in the presence of this antibody.

Inhibitor panel studies of human hepatic and placental cytochrome P-450-associated monooxygenase activities

1. A panel of nine inhibitors displaying some P-450 isozyme specificity was used to characterize aryl hydrocarbon hydroxylase (AHH) and 7-ethoxyresorufin 0-deethylase (ERDE) activities in human liver and placenta in vitro in comparison with liver enzymes from control, phenobarbital (PB) and 3-methylcholanthrene (MC) treated rats. 2. SKF 525A and cimetidine inhibited more potently hepatic AHH than the placental enzyme. 7,8-Benzoflavone inhibited more efficiently placental AHH than the hepatic enzyme, whereas ERDE was inhibited at the same level in both tissues. Quinine, quinidine, SKF 525A and metyrapone inhibited ERDE almost to the same extent in both tissues, but the variability was larger with the liver enzyme. Aminoglutethimide, debrisoquine or tetrahydrofuran did not inhibit AHH or ERDE significantly in either tissue. 3. When compared with inhibition profiles obtained with rat liver microsomes, the human hepatic and placental ERDE resembled most that of MC-treated rat liver enzyme. Inhibition profile of placental AHH activity was also similar, but the inhibition characteristics of hepatic AHH activity resembled more closely control or PB-induced rat liver. It also seems that isozymes for alcohol induction or debrisoquine hydroxylation do not contribute significantly to hepatic or placental AHH or ERDE. 4. The inhibitor panel selected on the basis of known pretreatment and isozyme specificity might be useful in the characterization of enzymes and metabolic biotransformations participating in the metabolism of new substrates.

The immunocytochemical localisation and distribution of cytochrome P-450 in normal human hepatic and extrahepatic tissues with a monoclonal antibody to human cytochrome P-450

1. The localisation and distribution of cytochrome P-450 in human tissues has been studied by immunocytochemistry using a monoclonal antibody to a major form of human hepatic cytochrome P-450, P-450hA7, which is closely related to cytochromes P-450 HLp and P-450NF. 2. Strong immunoreactivity was identified in hepatocytes, columnar absorptive epithelial cells of the small intestine, polymorphonuclear leucocytes and their precursors in the bone marrow, and in mast cells. 3. Weak immunoreactivity was present in the proximal tubules of the kidney, pancreatic acini, gall bladder epithelium, squamous epithelium and sebaceous glands of the skin, interstitial cells of the testis and luteal cells of the ovary. 4. Immunoreactivity could not be demonstrated in the adrenal gland, placenta, colonic epithelium and alveolar type II cells and Clara cells of the lung.

Biochemical and molecular epidemiology of human cancer: indicators of carcinogen exposure, DNA damage, and genetic predisposition

The primary goal of biochemical and molecular epidemiology is to identify individuals at high cancer risk by obtaining evidence of high exposure to carcinogens, leading to pathobiological lesions in target cells, and/or increased oncogenic susceptibility due to either inherited or acquired host factors. This emerging and multidisciplinary area of cancer research combines epidemiological and laboratory approaches. Because DNA is considered to be an important target for modification by mutagens and carcinogens, damage to DNA can be used as an internal, molecular dosimeter of carcinogen exposure. The reactive species of these carcinogens may directly bind to DNA to form adducts and may indirectly cause secondary DNA lesions, e.g., via induction of free radicals and aldehydes. Highly sensitive and specific methods have been developed to measure the minute amounts of DNA lesions and DNA repair products found in biological specimens from humans exposed to carcinogens. For example, DNA adducts have been measured in cells and tissues from people occupationally exposed to carcinogenic polycyclic aromatic hydrocarbons. Antibodies recognizing carcinogen-DNA adducts have also been detected in human sera. Inherited predisposition to cancer has been revealed by recent advances in molecular genetics, including restriction-fragment-length polymorphism. For example, the hypothesis that rare alleles of the Ha-ras proto-oncogene are associated with an increased risk of lung cancer is currently being tested. These approaches afford the potential of biochemical and molecular epidemiology to predict disease risk for individual persons, instead of for populations, and before the onset of clinically evident disease.

The effect of cigarette smoking on 7-ethoxyresorufin O-deethylase and other monooxygenase activities in human liver: analyses with monoclonal antibodies

Four cytochrome P-450 enzyme activities, 7-ethoxyresorufin O-deethylase (ERDE), coumarin 7-hydroxylase (CH), 7-ethoxycoumarin O-deethylase (ECDE) and aryl hydrocarbon hydroxylase (AHH) were measured in human liver needle biopsy samples from smokers and non-smokers. Cigarette smoking was verified and quantitated by measuring plasma cotinine levels. Enzyme inhibitory monoclonal antibodies (MAb) to a 3-methylcholanthrene-induced (MAb 1-7-1) and phenobarbitone-induced (MAb 2-66-3) rat hepatic cytochrome P-450 were used to measure the contribution of MAb-defined, epitope-specific cytochromes P-450 to the total reaction measured for each of the above activities. ERDE activity was significantly elevated in the livers of cigarette smokers, whereas AHH, CH or ECDE activities were not affected by cigarette smoking. No correlation was observed between plasma cotinine concentration and ERDE activity. MAb 1-7-1 inhibited hepatic ERDE activity to a variable extent (from 0 to 65%), but had very little or no effect on AHH, CH or ECDE activities. The inhibitory effect of MAb 1-7-1 on ERDE activity was greater than 50% in the non-smokers. MAb 2-66-3 had no inhibitory effect on any of the enzyme activities studied. In contrast to liver both ERDE and AHH on human placental microsomes from cigarette smokers were inhibited by MAb 1-7-1. The MAb 2-66-3 was without effect. Cigarette smoking induces a form of P-450 in human liver, responsible for ERDE activity, that contains an epitope recognized by MAb 1-7-1. This form of cytochrome P-450 is insensitive to MAb 2-66-3 and is not contributing to AHH, CH or ECDE activities of human liver.

Inter-relatedness of some isoenzymes of cytochrome P-450 from rat, rabbit and human, determined with monoclonal antibodies

Monoclonal antibodies have been raised to rat liver cytochromes P-450 b and c, and rabbit liver cytochrome P-450 form 4. A total of six antibodies have been studied. Each antibody reacted strongly both with its homologous antigen and with microsomal fractions selectively enriched with that antigen by treatment of animals with inducing compounds. However, several of the antibodies showed cross-reactivity, either within or between species. A combination of enzyme-linked immunosorbent assay, immunoadsorption, Western blotting and competitive radioimmunoassay revealed that each of the antibodies reacted with a different epitope. Proteolytic digestion of antigen followed by Western blotting of the peptide fragments enabled antibodies, otherwise identical in their reactivity, to be distinguished. It is concluded that complex structural relationships exist amongst the different isoenzymes of cytochrome P-450 and that epitope mapping will help in characterizing both animal and human cytochromes P-450.

Amino-terminal sequence and structure of monoclonal antibody immunopurified cytochromes P-450

Six hepatic cytochromes P-450 were isolated from 3-methylcholanthrene-treated animals by immunopurification with monoclonal antibodies. The purified cytochromes P-450 include 57- and 56-kDa polypeptides from Sprague-Dawley rats, 57- and 56-kDa polypeptides from C57BL/6 mice, a 56-kDa polypeptide from DBA/2 mice, and a 53-kDa polypeptide from guinea pigs. These isozymes were structurally compared by peptide mapping using both sodium dodecyl sulfate--polyacrylamide gel electrophoresis and high-pressure liquid chromatography and by amino acid and NH2-terminal sequence analyses. The 57-kDa polypeptides from rats and mice have similar but nonidentical peptide maps and amino acid compositions and are about 80% homologous in their NH2-terminal amino acid sequence. The 56-kDa polypeptides from rats and both mice strains have very similar peptide maps and amino acid compositions and identical NH2-terminal sequences. The NH2-terminal sequence of the mice 56-kDa polypeptides corresponds to that reported for the mouse P1-450 isozyme except that we identified two additional residues, proline and serine, at the NH2 terminus in the 57-kDa polypeptide from C57BL/6 mice that were not deduced from the cDNA sequence of the mouse P1-450 isozyme. The guinea pig 53-kDa polypeptide has a distinct peptide map relative to the other polypeptides studied and an NH2-terminal sequence with only partial homology to the 56- and 57-kDa polypeptides from rats and mice. This report shows the varying degree of structural relatedness among the isozymes examined and demonstrates the suitability and advantage of immunopurified cytochromes P-450 for sequencing and structural studies.

Identification of human cytochromes P-450 analogous to forms induced by phenobarbital and 3-methylcholanthrene in the rat

Antibodies to four rat liver forms of cytochrome P-450, two phenobarbital-inducible (PB1 and PB2) and two 3-methylcholanthrene-inducible (MC1 and MC2) proteins, have been used to make a structural and functional comparison of rat and human cytochromes P-450. Proteins from both species were identified on Western blots by their reaction with these antibodies. In the human liver preparations, structurally related proteins to PB1 and to PB2 were identified in all the samples tested with apparent Mr values of 51 800 and 54 800 for PB1 and 53 600 and 57 200 for PB2. Considerable variation in the content of the lower-Mr proteins was measured between samples and, as with the rat enzymes, samples which reacted well with anti-PB1 also reacted with anti-PB2, indicating that these proteins are regulated at least to some degree, co-ordinately. The apparent Mr values of the major human proteins identified with anti-MC1 and anti-MC2 were 54 400 and 57 000 respectively. Only six (of 31) human samples contained significant amounts of these proteins. The same six samples which reacted with anti-MC1 also reacted with anti-MC2, again indicating co-ordinate regulation of these two proteins. Antibody inhibition of microsomal 7-ethoxycoumarin and 7-ethoxyresorufin metabolism demonstrated a degree of conservation of substrate specificity related to specific P-450 isoenzymes between the species. However, the contributions of the different P-450 isoenzymes to the human microsomal activity were not always related to the rat enzyme with the highest activity towards these substrates.

Epitope-relatedness and phenotyping of hepatic cytochromes P-450 with monoclonal antibodies

The epitope-specific cytochrome P-450 content of animal livers was analysed by radioimmunoassay using a panel of seven monoclonal antibodies (MAbs) made to a 3-methylcholanthrene-induced rat liver cytochrome P-450. Competitive radioimmunoassays utilizing a reference radiolabelled MAb and a series of unlabelled MAbs indicated that there are at least three distinct classes of MAbs to different epitopes on cytochrome P-450. In addition, a direct radioimmunoassay employing a radiolabelled second antibody detected MAb-specific cytochromes P-450 in livers from different animals. This radioimmunoassay detected large elevations in the levels of these cytochromes P-450 in the livers of 3-methylcholanthrene-treated rats and C57BL/6 mice compared with untreated rats, 3-methylcholanthrene-treated DBA/2 mice or guinea pigs. The two complementary radioimmunoassay methods are sensitive, efficient, and easily applicable for screening large number of tissue samples for MAb-defined cytochrome P-450 phenotype.

Monoclonal antibodies to cytochrome P-450 immunopurify a 45-kDa protein from a human lymphoblastoid cell line

Monoclonal antibodies (MAbs) to rat liver cytochromes P-450 have previously been used for successful immunopurification of cytochromes P-450 from animal tissues. We now report application of this MAb-based immunopurification technique to the human lymphoblastoid AHH-1 cell line. Immunopurification carried out with 3 different MAbs each yielded a 45-kDa polypeptide. The purified protein contains an MAb-specific epitope present on cytochromes P-450, and may therefore be a human cytochrome P-450.

Coumarin 7-hydroxylase activity in human liver microsomes. Properties of the enzyme and interspecies comparisons

Coumarin 7-hydroxylation and other cytochrome P-450-associated enzyme activities were studied in human liver biopsy homogenates and compared with activities in livers of other species. Coumarin 7-hydroxylation is extraordinarily active in human liver biopsy samples in vitro. Activity is lower in mouse, rabbit or guinea pig liver and essentially absent in rat liver. Cytochrome P-450 content and other associated enzyme activities were higher in animals. Coumarin 7-hydroxylation is induced by phenobarbitone in mouse liver, but no significant increase was seen in human or rat liver after exposure to inducers. Correlations amongst coumarin 7-hydroxylase, aryl hydrocarbon hydroxylase, 7-ethoxycoumarin O-deethylase and cytochrome P-450 are statistically significant (r values from 0.56 to 0.73), but do not permit the conclusion, that the same P-450 form catalyzes all the reactions studied. The correlations between coumarin hydroxylation and antipyrine half-life or clearance are statistically significant, but not good enough for predictive purposes. Coumarin 7-hydroxylase in human liver is inhibited by alpha-naphthoflavone, SKF 525A, metyrapone and aniline.

Enzymatic activation of chemicals to toxic metabolites

A variety of enzymes function in the oxygenation, oxidation-reduction, conjugation, and hydrolysis of drugs and other foreign chemicals. Often these enzymes detoxicate chemicals to prevent detrimental effects. In this review we will, however, concentrate on cases in which metabolism activates chemicals to reactive species which cause cellular damage. Particular attention will be given to mixed-function oxidases, which carry out a variety of oxygenations, as well as other reactions. (We will focus on cellular toxicity as opposed to initiation of tumorigenesis in this review.) In many cases, considerable circumstantial evidence exists linking these enzymes to enhanced toxicity of chemicals, although causal relationships have seldom been demonstrated. Further, in very few cases is the explicit cause of toxicity known. Modification of critical protein residues is suspected, although oxidative stress may also be involved in some cases. We discuss general aspects of mechanisms of toxic action, briefly list all cases in which metabolism is suspected to play a role in enhancing toxicity, and review a few examples in detail where substantial chemical and enzymatic information is available. The latter instances would involve knowledge of the enzymes involved, chemical evidence on the structures of the reactive metabolites, identification of adducts, and some inference into the biological processes which are effected to elicit toxicity. We consider, in this regard, vinyl halides (which have been a focus in our own laboratory), acetaminophen, pyrrolizidine alkaloids, and fluoroxene.

Genetic and environmental regulation of aryl hydrocarbon hydroxylase in man: studies with liver, lung, placenta, and lymphocytes

Properties and response smoking of aryl hydrocarbon hydroxylase (AHH) activity in various human tissues are reviewed. In the placenta, induction of AHH by smoking can be demonstrated unequivocally. AHH activity in lung samples is variable, but the relation to current or past smoking is unclear. The effect of cigarette smoking can be readily shown in the rate of antipyrine elimination, although there is no change in AHH activity in liver biopsy samples. The reason for this discrepancy is not known. In peripheral lymphocytes a sort of "memory-effect" of cigarette smoking is retained even after culturing, the nature of this phenomenon remaining unclear. Furthermore, there seem to be many factors affecting AHH induction in peripheral lymphocytes in culture. These data suggest that regulation of AHH activity and induction is tissue-specific, i.e. no systemic regulation is discernible. It is also possible that the P-450 isozyme composition in some tissues masks the induction. Reasons for discrepancies between animal and human data are not clear; however, genuine biological differences and technical difficulties in human studies can be postulated.