Regio- and stereo-selective metabolism of phenanthrene by twelve cDNA-expressed human, rodent, and rabbit cytochromes P-450

Construction of a fused grid-based CYP2C8-Template system and the application

A ligand-accessible space in the CYP2C8 active site was reconstituted as a fused grid-based Template∗ with the use of structural data of the ligands. An evaluation system of CYP2C8-mediated metabolism has been developed on Template with the introduction of the idea of Trigger-residue initiated ligand-movement and fastening. Reciprocal comparison of the data of simulation on Template with experimental results suggested a unified way of the interaction of CYP2C8 and its ligands through the simultaneous plural-contact with Rear-wall of Template. CYP2C8 was expected to have a room for ligands between vertically standing parallel walls termed Facial-wall and Rear-wall. Both the walls were separated by a distance corresponding to 1.5-Ring (grid) diameter size, which was termed Width-gauge. The ligand sittings were stabilized through contacts with Facial-wall and the left-side borders of Template including specific Position 29, left-side border of Rings I/J, or Left-end, after Trigger-residue initiated ligand-movement. Trigger-residue movement is suggested to force ligands to stay firmly in the active site and then to initiate CYP2C8 reactions. Simulation experiments for over 350 reactions of CYP2C8 ligands supported the system established.

Application of fused-grid-based CYP-Template systems for genotoxic substances to understand the metabolisms

Understanding of metabolic processes is a key factor to evaluate biological effects of carcinogen and mutagens. Applicability of fused-grid Template* systems of CYP enzymes (Drug Metab Pharmacokinet 2019, 2020, 2021, and 2022) was tested for three phenomena. (1) Possible causal relationships between CYP-mediated metabolisms of β-naphthoflavone and 3-methylcholanthrene and the high inducibility of CYP enzymes were examined. Selective involvement of non-constitutive CYP1A1, but not constitutive CYP1A2, was suggested on the oxidative metabolisms of efficient inducers, β-naphthoflavone and 3-methylcholanthrene. These results supported the view of the causal link of their high inducibility with their inefficient metabolisms due to the lack of CYP1A1 in livers at early periods after the administration of both inducers. (2) Clear differences exist between human and rodent CYP1A1 enzymes on their catalyses with heterocyclic amines, dioxins and polyaromatic hydrocarbons (PAHs). Reciprocal comparison of simulation results with experimental data suggested the rodent specific site and distinct sitting-preferences of ligands on Template for human and rodent CYP1A1 enzymes. (3) Enhancement of metabolic activation and co-mutagenicity have been known as phenomena associated with Salmonella mutagenesis assay. Both the phenomena were examined on CYP-Templates in ways of simultaneous bi-molecule bindings of distinct ligands as trigger and pro-metabolized molecules. α-Naphthoflavone and norharman served consistently as trigger-molecules to support the oxidations of PAHs and arylamines sitting simultaneously as pro-metabolized molecules on Templates of CYP1A1, CYP1A2 and CYP3A4. These CYP-Template simulation systems with deciphering capabilities are promising tools to understand the mechanism basis of metabolic activations and to support confident judgements in safety assessments.

Application of CYP1A2-Template System to Understand Metabolic Processes in the Safety Assessment

Cytochrome P450 (CYP)-mediated metabolisms of four chemicals have been investigated to understand their unresolved phenomena of their metabolisms using human CYP-Template systems developed in our previous studies (Drug Metab Pharmacokinet 2019, 2021, 2022). Simulation experiments of a topoisomerase-targeting agent, amonafide, offered a possible new inhibitory-mechanism as Trigger-residue inactivation on human CYP1A2 Template. N-Acetylamonafide as well as amonafide would inactivate CYP1A2 through the interference of Trigger-residue movement with their dimethylaminoethyl parts. The mechanism was also supported on the inhibition/inactivation of two other drugs, DSP-1053 and binimetinib. Both the drugs, after other CYP-mediated slight structural alterations, were expected to interact with Trigger-residue for the intense inhibition on CYP1A2 Template. Possible formation of reactive intermediates of amonafide and 3-methylindole was also examined on CYP1A2 Template. Placements of amonafide suggested the scare N-oxidation of the arylamine part due to the Trigger-residue interaction. Placements of 3-methylindole suggested the formation of a reactive intermediate, 3-methyleneindolenine, rather selectively on rodent CYP1A2 than on human CYP1A2, in consistent with the experimental data. These results suggest that CYP Template systems developed are effective tools to warn an appearance of unstable reactive intermediates. Our CYP-Template systems would support confident judgements in safety assessments through offering the mechanistic understandings of the metabolism.

The effect of alkyl substitution on the oxidative metabolism and mutagenicity of phenanthrene

Alkyl-substituted PAHs may be present in certain petroleum-derived products and in the environment and may eventually end up in consumer products, such as foodstuffs, cosmetics and pharmaceuticals. Safety concerns over possible exposure to alkylated PAHs have emerged. Bioactivation is a prerequisite for the mutagenicity and carcinogenicity of PAHs and has been extensively studied for non-substituted PAHs, while data on the bioactivation of alkyl-substituted PAHs are scarce. The present study investigated the effect of alkyl substitution on the CYP 450-mediated metabolism of phenanthrene and eight of its alkylated congeners by quantifying metabolite formation in rat and human liver microsomal incubations. Furthermore, the mutagenicity of four selected methylated phenanthrenes was compared to that of phenanthrene using the Ames test. The obtained results support the hypothesis that alkyl substitution shifts the oxidative metabolism from the aromatic ring to the alkyl side chain. Increasing the length of the alkyl chain reduced overall metabolism with metabolic conversion for 1-n-dodecyl-phenanthrene (C12) being negligible. 1- and 9-methyl-phenanthrene, in which the methyl group generates an additional bay region-like structural motif, showed mutagenicity toward Salmonella typhimurium TA98 and TA 100, whereas phenanthrene and also 2- and 3-methyl-phenanthrene, without such an additional bay region-like structural motif, tested negative. It is concluded that the position of the alkylation affects the metabolism and resulting mutagenicity of phenanthrene with the mutagenicity increasing in cases where the alkyl substituent creates an additional bay region-like structural motif, in spite of the extra possibilities for side chain oxidation.

Associations of polycyclic aromatic hydrocarbons exposure and its interaction with XRCC1 genetic polymorphism with lung cancer: A case-control study

Humans are extensively exposed to polycyclic aromatic hydrocarbons (PAHs) daily via multiple pathways. Epidemiological studies have demonstrated that occupational exposure to PAHs increases the risk of lung cancer, but related studies in the general population are limited. Hence, we conducted a case-control study among the Chinese general population to investigate the associations between PAHs exposure and lung cancer risk and analyze the modifications of genetic polymorphisms in DNA repair genes. In this study, we enrolled 122 lung cancer cases and 244 healthy controls in Wuhan, China. Urinary PAHs metabolites were determined by gas chromatography-mass spectrometry, and rs25487 in X-ray repair cross-complementation 1 (XRCC1) gene was genotyped by the Agena Bioscience MassARRAY System. Then, multivariable logistic regression models were performed to estimate the potential associations. We found that urinary hydroxynaphthalene (OH-Nap), hydroxyphenanthrene (OH-Phe) and the sum of hydroxy PAHs (∑OH-PAHs) levels were significantly higher in lung cancer cases than those in controls. After adjusting for gender, age, BMI, smoking status, smoking pack-years, drinking status and family history, urinary ∑OH-Nap and ∑OH-Phe levels were positively associated with lung cancer risk, with dose-response relationships. Compared with those in the lowest tertiles, individuals in the highest tertiles of ∑OH-Nap and ∑OH-Phe had a 2.13-fold (95% CI: 1.10, 4.09) and 2.45-fold (95% CI: 1.23, 4.87) increased risk of lung cancer, respectively. Effects of gender, age, smoking status and smoking pack-years on the associations of PAHs exposure with lung cancer risk were shown in the subgroup analysis. Furthermore, associations of urinary ∑OH-Nap and ∑OH-PAHs levels with lung cancer risk were modified by XRCC1 rs25487 (P_interaction ≤ 0.025), and were more pronounced in wild-types of rs25487. These findings suggest that environmental exposure to naphthalene and phenanthrene is associated with increased lung cancer risk, and polymorphism of XRCC1 rs25487 might modify the naphthalene exposure-related lung cancer effect.

Refined CYP2E1∗ Template∗∗ system to decipher the ligand-interactions

A Template system for a prediction of human CYP2E1-mediated reactions (Drug Metab Rev 2011) has been refined with the introduction of ideas of Trigger-residue and the residue-initiated movement of ligands in the active site. The refined system also includes ideas of bi-molecule binding and angled-placement, which allow to sit diverse types of ligands on Template. With the use of these ideas in common with other Template systems for human CYP1A1, CYP1A2 and CYP3A4 (Drug Metab Pharmacokinet 2016, 2017, 2019, and 2020), 349 reactions of 192 distinct chemicals published as CYP2E1 ligands were examined in the refined system. Verifications of good and poor substrates, regioselectivity and also inhibitory interaction were available faithfully for these ligands from their placements on the refined Template and rules for interaction modes, accompanied with their deciphering information to lead to the judgements. The refined CYP2E1 Template system will thus offer more reliable estimations of human CYP2E1 catalysis toward ligands of diverse structures.

Investigation of the presence in human urine of mercapturic acids derived from phenanthrene, a representative polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAH) are environmental carcinogens implicated as causes of cancer in certain industrial settings and in cigarette smokers. PAH require metabolic activation to exert their carcinogenic effects. One widely accepted pathway of metabolic activation proceeds through formation of "bay region" diol epoxides which are highly reactive with DNA and can cause mutations. Phenanthrene (Phe) is the simplest PAH with a bay region and an excellent model for the study of PAH metabolism. In previous studies in which [D10]Phe was administered to smokers, we observed higher levels of [D10]Phe-tetraols derived from [D10]Phe-diol epoxides in subjects who were null for the glutathione-S-transferase M1 (GSTM1) gene. We hypothesized that Phe-epoxides, the primary metabolites of Phe, were detoxified by glutathione conjugate formation, which would result ultimately in the excretion of the corresponding mercapturic acids in urine. We synthesized the four stereoisomeric mercapturic acids that would result from attack of glutathione on Phe-epoxides followed by normal processing of the conjugates. We also synthesized the corresponding dehydrated metabolites and sulfoxides. These 12 standards were used in liquid chromatography-nanoelectrospray ionization-high resolution tandem mass spectrometry analysis of urine samples from smokers and creosote workers, the latter exposed to unusually high levels of PAH. Only the sulfoxide derivatives were consistently detected in the urine of creosote workers; none of the compounds was detected in the urine of smokers. These results demonstrate a new pathway of PAH-mercapturic acid formation, but do not provide an explanation for the role of GSTM1 null status on Phe-tetraol formation.

Upgrading HepG2 cells with adenoviral vectors that encode drug-metabolizing enzymes: application for drug hepatotoxicity testing

INTRODUCTION

Drug attrition rates due to hepatotoxicity are an important safety issue considered in drug development. The HepG2 hepatoma cell line is currently being used for drug-induced hepatotoxicity evaluations, but its expression of drug-metabolizing enzymes is poor compared with hepatocytes. Different approaches have been proposed to upgrade HepG2 cells for more reliable drug-induced liver injury predictions. Areas covered: We describe the advantages and limitations of HepG2 cells transduced with adenoviral vectors that encode drug-metabolizing enzymes for safety risk assessments of bioactivable compounds. Adenoviral transduction facilitates efficient and controlled delivery of multiple drug-metabolizing activities to HepG2 cells at comparable levels to primary human hepatocytes by generating an 'artificial hepatocyte'. Furthermore, adenoviral transduction enables the design of tailored cells expressing particular metabolic capacities. Expert opinion: Upgraded HepG2 cells that recreate known inter-individual variations in hepatic CYP and conjugating activities due to both genetic (e.g., polymorphisms) or environmental (e.g., induction, inhibition) factors seems a suitable model to identify bioactivable drug and conduct hepatotoxicity risk assessments. This strategy should enable the generation of customized cells by reproducing human pheno- and genotypic CYP variability to represent a valuable human hepatic cell model to develop new safer drugs and to improve existing predictive toxicity assays.

Structure-Function Studies of Naphthalene, Phenanthrene, Biphenyl, and Their Derivatives in Interaction with and Oxidation by Cytochromes P450 2A13 and 2A6

Naphthalene, phenanthrene, biphenyl, and their derivatives having different ethynyl, propynyl, butynyl, and propargyl ether substitutions were examined for their interaction with and oxidation by cytochromes P450 (P450) 2A13 and 2A6. Spectral interaction studies suggested that most of these chemicals interacted with P450 2A13 to induce Type I binding spectra more readily than with P450 2A6. Among the various substituted derivatives examined, 2-ethynylnaphthalene, 2-naphthalene propargyl ether, 3-ethynylphenanthrene, and 4-biphenyl propargyl ether had larger ΔAmax/Ks values in inducing Type I binding spectra with P450 2A13 than their parent compounds. P450 2A13 was found to oxidize naphthalene, phenanthrene, and biphenyl to 1-naphthol, 9-hydroxyphenanthrene, and 2- and/or 4-hydroxybiphenyl, respectively, at much higher rates than P450 2A6. Other human P450 enzymes including P450s 1A1, 1A2, 1B1, 2C9, and 3A4 had lower rates of oxidation of naphthalene, phenanthrene, and biphenyl than P450s 2A13 and 2A6. Those alkynylated derivatives that strongly induced Type I binding spectra with P450s 2A13 and 2A6 were extensively oxidized by these enzymes upon analysis with HPLC. Molecular docking studies supported the hypothesis that ligand-interaction energies (U values) obtained with reported crystal structures of P450 2A13 and 2A6 bound to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, indole, pilocarpine, nicotine, and coumarin are of use in understanding the basis of possible molecular interactions of these xenobiotic chemicals with the active sites of P450 2A13 and 2A6 enzymes. In fact, the ligand-interaction energies with P450 2A13 4EJG bound to these chemicals were found to relate to their induction of Type I binding spectra.

A light in the darkness: new biotransformation genes, antioxidant parameters and tissue-specific responses in oysters exposed to phenanthrene

Phenanthrene (PHE), a major component of crude oil, is one of the most abundant polycyclic aromatic hydrocarbons (PAHs) in aquatic ecosystems, and is readily bioavailable to marine organisms. Understanding the toxicity of PAHs in animals requires knowledge of the systems for xenobiotic biotransformation and antioxidant defence and these are poorly understood in bivalves. We report, for the first time, new transcripts and tissue-specific transcription in gill and digestive gland from the oyster Crassostrea brasiliana following 24h exposure to 100 and 1000μgL(-1) PHE, a model PAH. Six new cytochrome P450 (CYP) and four new glutathione S-transferase (GST) genes were analysed by means of quantitative reverse transcription PCR (qRT-PCR). Different antioxidant endpoints, including both enzymatic and non-enzymatic parameters, were assessed as potential biomarkers of oxidative stress. GST activity was measured as an indicator of phase II biotransformation. Rapid clearance of PHE was associated with upregulation of both phase I and II genes, with more pronounced effects in the gill at 1000μgL(-1) PHE. After 24h of exposure, PHE also caused impairment of the antioxidant system, decreasing non-protein thiols and glutathione levels. On the other hand, no change in antioxidant enzymes was observed. PHE treatment (100μgL(-1)) significantly decreased GST activity in the gill of exposed oysters. Both CYP and GST were transcribed in a tissue-specific manner, reflecting the importance of the gill in the detoxification of PAHs. Likewise, the antioxidant parameters followed a similar pattern. The data provide strong evidence that these genes play key roles in C. brasiliana biotransformation of PHE and highlight the importance of gill in xenobiotic metabolism.

Urinary metabolites of a polycyclic aromatic hydrocarbon and volatile organic compounds in relation to lung cancer development in lifelong never smokers in the Shanghai Cohort Study

Exposures to polycyclic aromatic hydrocarbons (PAHs) from various environmental and occupational sources are considered a primary risk factor for lung cancer among lifelong never smokers, based largely on results from epidemiologic studies utilizing self-reported exposure information. Prospective, biomarker-based human studies on the role of PAH and other airborne carcinogens in the development of lung cancer among lifelong non-smokers have been lacking. We prospectively investigated levels of urinary metabolites of a PAH and volatile organic compounds in relation to lung cancer risk in a nested case-control study of 82 cases and 83 controls among lifelong never smokers of the Shanghai Cohort Study, a prospective cohort of 18 244 Chinese men aged 45-64 years at enrollment. We quantified three PAH metabolites: r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene (PheT), 3-hydroxyphenanthrene (3-OH-Phe) and total hydroxyphenanthrenes (total OH-Phe, the sum of 1-, 2-, 3- and 4-OH-Phe), as well as metabolites of the volatile organic compounds acrolein (3-hydroxypropyl mercapturic acid), benzene (S-phenyl mercapturic acid), crotonaldehyde (3-hydroxy-1-methylpropylmercapturic acid) and ethylene oxide (2-hydroxyethyl mercapturic acid). Urinary cotinine was also quantified to confirm non-smoking status. Compared with the lowest quartile, odds ratios (95% confidence intervals) for lung cancer risk for the highest quartile levels of PheT, 3-OH-Phe and total OH-Phe were 2.98 (1.13-7.87), 3.10 (1.12-7.75) and 2.59 (1.01-6.65) (all P trend < 0.05), respectively. None of the metabolites of the volatile organic compounds were associated with overall lung cancer risk. This study demonstrates a potentially important role of exposure to PAH in the development of lung cancer among lifelong never smokers.

Longitudinal study of [D10]phenanthrene metabolism by the diol epoxide pathway in smokers

The extent of metabolism of [D10]phenanthrene to [D(10)]r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetradeuterophenanthrene ([D10]PheT) could be a biomarker of human metabolic activation of carcinogenic polycyclic aromatic hydrocarbons, leading to identification of smokers particularly susceptible to lung cancer. The longitudinal stability of [D10]PheT was evaluated in 24 cigaret smokers given 7-8 oral doses of [D10]phenanthrene (10 µg) over 5.5 months. [D10]PheT in 6 h urine was quantified after each dose. The overall coefficient of variation for 24 subjects was (mean ± S.D.) 27.4% ± 8.83%. Thus, a single administration of [D10]phenanthrene is likely sufficient to determine a smoker's ability to metabolize it to [D10]PheT.

Construction of a CYP2E1-template system for prediction of the metabolism on both site and preference order

We have constructed an in silico system for the prediction of CYP2E1-mediated reaction using a two-dimensional template derived from substrate structures. Although CYP2E1 prefers small-size molecules for the substrates, the enzyme mediates oxidations of large-size molecules, such as benzo[a]pyrene. Overlays of these substrates, to assemble their sites of oxidation into a specific area, suggested a range of regions frequently occupied. The region, having a benzo[a]pyrene-like shape, was thus used as a CYP2E1 template. In this system, atoms in substrates, except for hydrogen atoms, were placed on corners of honeycomb structures of the template after having expanded the structures. Using published data for the metabolism on more than 80 substrates of CYP2E1, the core template was further refined to verify the adjacent area and to define the relative contribution of template positions for the catalysis. The positions on the template were classified into four different point (0-3) groups, depending on relative usage. In addition, we set independent points (-5 to 3) for specific positions to incorporate three-dimensional or functional information. Total scores from both position-occupancy and -function points were calculated for all the orientations of possible conformers of test substrates, and the scores were found to predict the relative abundance (i.e., order) as well as the regioselectivity of human CYP2E1 reactions with high fidelities.

A fungal P450 (CYP5136A3) capable of oxidizing polycyclic aromatic hydrocarbons and endocrine disrupting alkylphenols: role of Trp(129) and Leu(324)

The model white rot fungus Phanerochaete chrysosporium, which is known for its versatile pollutant-biodegradation ability, possesses an extraordinarily large repertoire of P450 monooxygenases in its genome. However, the majority of these P450s have hitherto unknown function. Our initial studies using a genome-wide gene induction strategy revealed multiple P450s responsive to individual classes of xenobiotics. Here we report functional characterization of a cytochrome P450 monooxygenase, CYP5136A3 that showed common responsiveness and catalytic versatility towards endocrine-disrupting alkylphenols (APs) and mutagenic/carcinogenic polycyclic aromatic hydrocarbons (PAHs). Using recombinant CYP5136A3, we demonstrated its oxidation activity towards APs with varying alkyl side-chain length (C3-C9), in addition to PAHs (3–4 ring size). AP oxidation involves hydroxylation at the terminal carbon of the alkyl side-chain (ω-oxidation). Structure-activity analysis based on a 3D model indicated a potential role of Trp¹²⁹ and Leu³²⁴ in the oxidation mechanism of CYP5136A3. Replacing Trp¹²⁹ with Leu (W129L) and Phe (W129F) significantly diminished oxidation of both PAHs and APs. The W129L mutation caused greater reduction in phenanthrene oxidation (80%) as compared to W129F which caused greater reduction in pyrene oxidation (88%). Almost complete loss of oxidation of C3-C8 APs (83–90%) was observed for the W129L mutation as compared to W129F (28–41%). However, the two mutations showed a comparable loss (60–67%) in C9-AP oxidation. Replacement of Leu³²⁴ with Gly (L324G) caused 42% and 54% decrease in oxidation activity towards phenanthrene and pyrene, respectively. This mutation also caused loss of activity towards C3-C8 APs (20–58%), and complete loss of activity toward nonylphenol (C9-AP). Collectively, the results suggest that Trp¹²⁹ and Leu³²⁴ are critical in substrate recognition and/or regio-selective oxidation of PAHs and APs. To our knowledge, this is the first report on an AP-oxidizing P450 from fungi and on structure-activity relationship of a eukaryotic P450 for fused-ring PAHs (phenanthrene and pyrene) and AP substrates.

Quantitation of benzo[a]pyrene metabolic profiles in human bronchoalveolar (H358) cells by stable isotope dilution liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants and are carcinogenic in multiple organs and species. Benzo[a]pyrene (B[a]P) is a representative PAH and has been studied extensively for its carcinogenicity and toxicity. B[a]P itself is chemically inert and requires metabolic activation to exhibit its toxicity and carcinogenicity. Three major metabolic pathways have been well documented. The signature metabolites generated from the radical cation (peroxidase or monooxygenase mediated) pathway are B[a]P-1,6-dione and B[a]P-3,6-dione, the signature metabolite generated from the diol-epoxide (P450 mediated) pathway is B[a]P-r-7,t-8,t-9,c-10-tetrahydrotetrol (B[a]P-tetrol-1), and the signature metabolite generated from the o-quinone (aldo-keto reductase mediated) pathway is B[a]P-7,8-dione. The contributions of these different metabolic pathways to cancer initiation and the exploitation of this information for cancer prevention are still under debate. With the availability of a library of [(13)C(4)]-labeled B[a]P metabolite internal standards, we developed a sensitive stable isotope dilution atmospheric pressure chemical ionization tandem mass spectrometry method to address this issue by quantitating B[a]P metabolites from each metabolic pathway in human lung cells. This analytical method represents a 500-fold increased sensitivity compared with that of a method using HPLC-radiometric detection. The limit of quantitation (LOQ) was determined to be 6 fmol on column for 3-hydroxybenzo[a]pyrene (3-OH-B[a]P), the generally accepted biomarker for B[a]P exposure. This high level of sensitivity and robustness of the method was demonstrated in a study of B[a]P metabolic profiles in human bronchoalveolar H358 cells induced or uninduced with the AhR ligand, 2,3,7,8-tetrachlorodibenzodioxin (TCDD). All the signature metabolites were detected and successfully quantitated. Our results suggest that all three metabolic pathways contribute equally in the overall metabolism of B[a]P in H358 cells with or without TCDD induction. The sensitivity of the method should permit the identification of cell-type differences in B[a]P activation and detoxication and could also be used for biomonitoring human exposure to PAH.

Urinary levels of cigarette smoke constituent metabolites are prospectively associated with lung cancer development in smokers

Polycyclic aromatic hydrocarbons (PAH) are believed to be among the principal causative agents for lung cancer in smokers, but no epidemiologic studies have evaluated the relationship of PAH uptake and metabolism to lung cancer. In this study, we quantified prediagnostic urinary levels of r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene (PheT), a validated biomarker of PAH uptake and metabolism, as well as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol and its glucuronides (total NNAL), and cotinine and its glucuronides (total cotinine), validated biomarkers of uptake of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, and nicotine, respectively, in relation to lung cancer risk among current smokers in a nested case-control study within a cohort of 18,244 Chinese men in Shanghai, China. Urinary levels of PheT, total NNAL, and total cotinine were significantly higher in cases than controls (N = 476 matched pairs). ORs (95% confidence intervals) for lung cancer in the second, third, fourth, and fifth quintiles of PheT were 1.70 (1.00-2.88), 1.07 (0.62-1.84), 1.48 (0.86-2.53), and 2.34 (1.33-4.11), respectively, relative to the lowest quartile (P(trend) = 0.023) after adjustment for self-reported smoking intensity and duration and urinary total NNAL and total cotinine. This study also confirmed that urinary total NNAL and total cotinine are independently related to lung cancer risk.

Metabolism of [D10]phenanthrene to tetraols in smokers for potential lung cancer susceptibility assessment: comparison of oral and inhalation routes of administration

Polycyclic aromatic hydrocarbons (PAHs) are believed to be among the causative agents for lung cancer in smokers. PAHs require metabolic activation for carcinogenicity. One pathway produces diol epoxides that react with DNA, causing mutations. Because diol epoxides are converted to tetraols, quantitation of tetraols can potentially be used to identify smokers who may be at higher risk for lung cancer. Our approach uses [D(10)]phenanthrene, a labeled version of phenanthrene, a noncarcinogenic PAH structurally analogous to carcinogenic PAH. Although smokers are exposed to PAH by inhalation, oral dosing would be more practical for phenotyping studies. Therefore, we investigated [D(10)]phenanthrene metabolism in smokers after administration by inhalation in cigarette smoke or orally. Sixteen smokers received 10 μg of [D(10)]phenanthrene in a cigarette or orally. Plasma and urine samples were analyzed for [D(10)]r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene ([D(10)]PheT), the major end product of the diol epoxide pathway, by gas chromatography-negative ion chemical ionization-tandem mass spectrometry. The ratios of [D(10)]PheT (oral dosing/inhalation) in 15 smokers were 1.03 ± 0.32 and 1.02 ± 0.35, based on plasma area under the concentration-time curve (0-∞) and total 48-h urinary excretion, respectively. Overall, there was no significant difference in the extent of [D(10)]PheT formation after the two different routes of exposure in smokers. A large interindividual variation in [D(10)]PheT formation was observed. These results demonstrate that the level of [D(10)]PheT in urine after oral dosing of [D(10)]phenanthrene can be used to assess individual capacity of PAH metabolism by the diol epoxide pathway.

Quantitation of a minor enantiomer of phenanthrene tetraol in human urine: correlations with levels of overall phenanthrene tetraol, benzo[a]pyrene tetraol, and 1-hydroxypyrene

Polycyclic aromatic hydrocarbons (PAH) are well established carcinogens that are likely to play a role in causing some human cancers. One accepted pathway of PAH metabolic activation is the formation of bay region diol epoxides. Some individuals may be particularly susceptible to PAH carcinogenesis because they metabolically activate PAH more effectively than others. We have used the measurement of urinary phenanthrene tetraols (Phe-tetraols) as a biomarker of PAH exposure plus metabolic activation since bay region diol epoxides are hydrolyzed to tetraols. Because of stereoselectivity in Phe metabolism, Phe-(1R,2S,3R,4S)-tetraol (4) results mainly from the bay region diol epoxide pathway, and Phe-(1S,2R,3S,4R)-tetraol (7) is formed mainly from the reverse diol epoxide pathway, not generally associated with carcinogenicity. The latter pathway accounts for more than 95% of human urinary Phe-tetraol. In most previous studies, Phe-tetraol was quantified without enantiomeric resolution, using a relatively rapid and practical method, applicable to large studies. It was not clear, however, whether measurement of overall unresolved Phe-tetraol would accurately represent the bay region diol epoxide metabolic activation pathway. Therefore, in this study we specifically quantified Phe-(1R,2S,3R,4S)-tetraol (4) by supplementing our usual analysis with chiral HPLC separations and using [(13)C(6)]Phe-(1R,2S,3R,4S)-tetraol as internal standard. We then investigated the relationship of urinary levels of 4 to those of Phe-tetraols (4 + 7), quantified without enantiomeric resolution. We applied these methods to urine samples from cigarette smokers and highly PAH-exposed creosote workers. The results were also compared to levels of benzo[a]pyrene-7,8,9,10-tetraol and 1-hydroxypyrene in the same samples. Levels of 4 were highly correlated with those of 4 + 7 (r > 0.9, P < 0.0001) in both types of urine samples. Strong correlations of 4 and 4 + 7 with benzo[a]pyrene-7,8,9,10-tetraol and 1-hydroxypyrene were also observed. The results of this study demonstrate therefore that practical and convenient measurement of overall Phe-tetraols (4 + 7) in human urine, without enantiomeric resolution, is an excellent indicator of PAH exposure and metabolism by the bay region diol epoxide metabolic activation pathway.

Immediate consequences of cigarette smoking: rapid formation of polycyclic aromatic hydrocarbon diol epoxides

Polycyclic aromatic hydrocarbons (PAH) are among the likely major causative agents for lung cancer in smokers. PAH require metabolic activation to exert their carcinogenic effects, and one important pathway proceeds through a three-step sequence resulting in the formation of diol epoxides, which react with DNA to produce adducts that can cause mutations and initiate the carcinogenic process. However, no previous published studies have examined this critical pathway in humans specifically exposed to PAH by inhalation of cigarette smoke. This study used a unique approach employing a stable isotope derivative of phenanthrene, the simplest PAH with a bay region, a feature closely associated with PAH carcinogenicity. Twelve subjects each smoked a cigarette to which [D(10)]phenanthrene had been added. Plasma was analyzed for [D(10)]r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene ([D(10)]PheT), the major end product of the diol epoxide metabolism pathway of phenanthrene. The analysis was performed by gas chromatography--negative ion chemical ionization--tandem mass spectrometry, using [(13)C(6)]PheT as internal standard. The results demonstrated that the three-step pathway resulting in the formation of diol epoxides, as monitored by [D(10)]PheT, occurred with remarkable rapidity. Levels of [D(10)]PheT in plasma of all subjects were maximal at the earliest time points examined, 15-30 min after smoking the cigarette containing [D(10)]phenanthrene, and decreased thereafter. These results demonstrate that the formation of a PAH diol epoxide occurs rapidly in smokers. Because PAH diol epoxides are mutagenic and carcinogenic, the results clearly demonstrate immediate negative health consequences of smoking, which should serve as a major warning to anyone contemplating initiating tobacco use.

Preferential glutathione conjugation of a reverse diol epoxide compared to a bay region diol epoxide of phenanthrene in human hepatocytes: relevance to molecular epidemiology studies of glutathione-s-transferase polymorphisms and cancer

Bay region diol epoxides are recognized ultimate carcinogens of polycyclic aromatic hydrocarbons (PAH), and in vitro studies have demonstrated that they can be detoxified by conjugation with glutathione, leading to the widely investigated hypothesis that individuals with low activity forms of glutathione-S-transferases are at higher risk of PAH induced cancer, a hypothesis that has found at most weak support in molecular epidemiology studies. A weakness in this hypothesis was that the mercapturic acids resulting from the conjugation of PAH bay region diol epoxides had never been identified in human urine. We recently analyzed smokers' urine for mercapturic acids derived from phenanthrene, the simplest PAH with a bay region. The only phenanthrene diol epoxide-derived mercapturic acid in smokers' urine was produced from the reverse diol epoxide, anti-phenanthrene-3,4-diol-1,2-epoxide (11), not the bay region diol epoxide, anti-phenanthrene-1,2-diol-3,4-epoxide (10), which does not support the hypothesis noted above. In this study, we extended these results by examining the conjugation of phenanthrene metabolites with glutathione in human hepatocytes. We identified the mercapturic acid N-acetyl-S-(r-4,t-2,3-trihydroxy-1,2,3,4-tetrahydro-c-1-phenanthryl)-L-cysteine (14a), (0.33-35.9 pmol/mL at 10 microM 8, 24 h incubation, N = 10) in all incubations with phenanthrene-3,4-diol (8) and the corresponding diol epoxide 11, but no mercapturic acids were detected in incubations with phenanthrene-1,2-diol (7), and only trace amounts were observed in incubations with the corresponding bay region diol epoxide 10. Taken together with our previous results, these studies clearly demonstrate that glutathione conjugation of a reverse diol epoxide of phenanthrene is favored over conjugation of a bay region diol epoxide. Since reverse diol epoxides of PAH are generally weakly or nonmutagenic/carcinogenic, these results, if generalizable to other PAH, do not support the widely held assumption that glutathione-S-transferases are important in the detoxification of PAH in humans.

Analysis of phenanthrene and benzo[a]pyrene tetraol enantiomers in human urine: relevance to the bay region diol epoxide hypothesis of benzo[a]pyrene carcinogenesis and to biomarker studies

One widely accepted metabolic activation pathway of the prototypic carcinogenic polycyclic aromatic hydrocarbon (PAH) benzo[a]pyrene (BaP) proceeds through the "bay region diol epoxide" BaP-(7R,8S)-diol-(9S,10R)-epoxide (2). However, few studies have addressed the analysis of human urinary metabolites of BaP, which result from this pathway. Phenanthrene (Phe) is structurally related to BaP, but human exposure to Phe is far greater, and its metabolites can be readily detected in urine. Thus, Phe metabolites have been proposed as biomarkers of PAH exposure and metabolic activation. Phe-tetraols in particular could be biomarkers of the diol epoxide pathway. While BaP-tetraols and Phe-tetraols have been previously quantified in human urine, no published studies have determined their enantiomeric composition. This is important because different enantiomers would result from the bay region diol epoxide and "reverse" diol epoxide pathways, the latter being associated with weak mutagenicity and carcinogenicity. We addressed this problem using chiral HPLC to separate the enantiomers of BaP-7,8,9,10-tetraol and Phe-1,2,3,4-tetraol. Urine samples from smokers were subjected to solid-phase extraction, chiral HPLC, and GC-NICI-MS/MS analysis for silylated Phe-1,2,3,4-tetraols. The results demonstrated that >96% of Phe-1,2,3,4-tetraol in smokers' urine was Phe-(1S,2R,3S,4R)-tetraol (12), resulting from the "reverse" diol epoxide pathway, whereas less than 4% resulted from the "bay region diol epoxide" pathway of Phe metabolism. Urine from creosote workers was similarly analyzed for BaP-7,8,9,10-tetraol enantiomers. In contrast to the results of the Phe-tetraol analyses, 78% of BaP-7,8,9,10-tetraol in these human urine samples was BaP-(7R,8S,9R,10S)-tetraol (3) resulting from the "bay region diol epoxide" pathway of BaP metabolism. These results provide further support for the bay region diol epoxide pathway of BaP metabolism in humans and demonstrate differences in BaP and Phe metabolism, which may be important when considering Phe-tetraols as biomarkers of PAH metabolic activation.

Identification of surrogate measures of diesel exhaust exposure in a controlled chamber study

Exposure to diesel exhaust (DE) has been associated with acute cardiopulmonary and vascular responses, chronic noncancer health effects, and respiratory cancers in humans. To better understand DE exposures and eventually their related health effects, we established a controlled chamber experiment wherein human volunteer subjects were exposed to approximately 100 microg/m3 DE. In general, human exposure assessment for DE is based on ambient air measurements of surrogates such as elemental carbon (EC) or total organic carbon (OC) collected on filters. As specific health effect mechanisms and dose-response are obscured bythe complex composition of DE, the linkage from exposure to internal dose can presumably be improved by use of specific biomarkers and metabolites in blood, breath, or urine. Because EC and OC are not suitable as biomarkers, in this study, we focus on identifying compounds that are demonstrated indicators of DE and can also be found in biological fluids. We measured an assortment of volatile, semivolatile, and particle-bound aromatic compounds in the chamber air and report their airborne concentrations in DE and purified air, as well as the estimated values of the corresponding exposure ratios (mean DE air concentration:mean purified air concentration). These estimated exposure ratios were used to identify naphthalene (Nap) and phenanthrene (Phe) as potentially useful surrogates for DE exposure that could also serve as biomarkers. Estimated mean levels of Nap and Phe associated with the nominal 100 microg/m3 DE were 2600 and 765 ng/m3 with estimated exposure ratios of 252 and 92.4, respectively. Nap levels were significantly correlated with OC and total particle-bound polycyclic aromatic hydrocarbons (PAHs); Phe levels were significantly correlated with total volatile + semivolatile PAHs. These results suggest that Nap and Phe may be particularly useful surrogates for DE concentrations. While Nap and Phe are not validated here as internal biomarkers of DE exposure, we are currently assessing human biological specimens collected during this study and will discuss those results in ensuing papers.

Urinary naphthalene and phenanthrene as biomarkers of occupational exposure to polycyclic aromatic hydrocarbons

OBJECTIVES

The study investigated the utility of unmetabolised naphthalene (Nap) and phenanthrene (Phe) in urine as surrogates for exposures to mixtures of polycyclic aromatic hydrocarbons (PAHs).

METHODS

The report included workers exposed to diesel exhausts (low PAH exposure level, n = 39) as well as those exposed to emissions from asphalt (medium PAH exposure level, n = 26) and coke ovens (high PAH exposure level, n = 28). Levels of Nap and Phe were measured in urine from each subject using head space-solid phase microextraction and gas chromatography-mass spectrometry. Published levels of airborne Nap, Phe and other PAHs in the coke-producing and aluminium industries were also investigated.

RESULTS

In post-shift urine, the highest estimated geometric mean concentrations of Nap and Phe were observed in coke-oven workers (Nap: 2490 ng/l; Phe: 975 ng/l), followed by asphalt workers (Nap: 71.5 ng/l; Phe: 54.3 ng/l), and by diesel-exposed workers (Nap: 17.7 ng/l; Phe: 3.60 ng/l). After subtracting logged background levels of Nap and Phe from the logged post-shift levels of these PAHs in urine, the resulting values (referred to as ln(adjNap) and ln(adjPhe), respectively) were significantly correlated in each group of workers (0.71 < or = Pearson r < or = 0.89), suggesting a common exposure source in each case. Surprisingly, multiple linear regression analysis of ln(adjNap) on ln(adjPhe) showed no significant effect of the source of exposure (coke ovens, asphalt and diesel exhaust) and further suggested that the ratio of urinary Nap/Phe (in natural scale) decreased with increasing exposure levels. These results were corroborated with published data for airborne Nap and Phe in the coke-producing and aluminium industries. The published air measurements also indicated that Nap and Phe levels were proportional to the levels of all combined PAHs in those industries.

CONCLUSION

Levels of Nap and Phe in urine reflect airborne exposures to these compounds and are promising surrogates for occupational exposures to PAH mixtures.

Analysis of phenanthrene diol epoxide mercapturic acid detoxification products in human urine: relevance to molecular epidemiology studies of glutathione S-transferase polymorphisms

Many studies have investigated the effects of glutathione S-transferase (GST) polymorphisms on cancer incidence in people exposed to carcinogenic polycyclic aromatic hydrocarbons (PAHs). The basis for this is that the carcinogenic bay region diol epoxide metabolites of several PAH are detoxified by GSTs in in vitro studies. However, there are no reports in the literature on the identification in urine of the mercapturic acid metabolites that would result from this process in humans. We addressed this by developing a method for quantitation in human urine of mercapturic acids which would be formed from angular ring diol epoxides of phenanthrene (Phe), the simplest PAH with a bay region, and a common environmental pollutant. We prepared standard mercapturic acids by reactions of syn- or anti-Phe-1,2-diol-3,4-epoxide and syn- or anti-Phe-3,4-diol-1,2-epoxide with N-acetylcysteine. Analysis of human urine conclusively demonstrated that the only detectable mercapturic acid of this type--N-acetyl-S-(r-4,t-2,3-trihydroxy-1,2,3,4-tetrahydro-c/t-1-phenanthryl)-L-cysteine (anti-PheDE-1-NAC)--was derived from the 'reverse diol epoxide', anti-Phe-3,4-diol-1,2-epoxide, and not from the bay region diol epoxides, syn- or anti-Phe-1,2-diol-3,4-epoxide. Levels of anti-PheDE-1-NAC in the urine of 36 smokers were (mean +/- SD) 728 +/- 859 fmol/ml urine. The results of this study provide the first evidence for a mercapturic acid of a PAH diol epoxide in human urine, but it was not derived from a bay region diol epoxide as molecular epidemiologic studies have presumed, but rather from a reverse diol epoxide, representative of metabolites with little if any carcinogenic activity. These results demonstrate the need for integration of genotyping and phenotyping information in molecular epidemiology studies.

The importance of cytochrome P450 2B6 in the human metabolism of environmental chemicals

Cytochrome P450 (CYP) 2B6 (CYP2B6) is a human CYP isoform found in variable amounts in the liver and other organs. It is known to be inducible and polymorphic and has a wide range of xenobiotic substrates. Studies of CYP2B6 to date have concentrated heavily on clinical drugs. In the present communication, however, we concentrate on its role in the metabolism of environmental xenobiotics. The term environment is used, in its broadest sense, to include natural ecosystems and agroecosystems as well as the industrial and indoor domestic environments. In essence, this excludes only clinical drugs and drugs of abuse. Many of these chemicals, including agrochemicals and industrial chemicals, can serve as substrates, inhibitors and/or inducers of CYP2B6, these activities being often modified by the existence of polymorphic variants. Metabolism-based interactions between environmental chemicals are discussed, as well as the emerging possibility of metabolic interactions between environmental chemicals and clinical drugs.

Comparison of Polymorphisms in Genes Involved in Polycyclic Aromatic Hydrocarbon Metabolism with Urinary Phenanthrene Metabolite Ratios in Smokers

The hypothesis that interindividual differences among smokers in the metabolism of polycyclic aromatic hydrocarbons (PAH) are related to lung cancer risk has been extensively investigated in the literature. These studies have compared lung cancer risk in groups of smokers with or without polymorphisms in genes involved in PAH metabolism. We believe that carcinogen metabolite phenotyping, involving the actual measurement of PAH metabolites, would be a better way to investigate differences in lung cancer risk. With this goal in mind, we have developed methods for quantifying phenanthrene metabolites in urine. Phenanthrene is the simplest PAH with a bay region, a feature closely associated with carcinogenicity. The urinary metabolite r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene (PheT) is a measure of metabolic activation, whereas phenanthrols (HOPhe) are a measure of detoxification. In this study, we quantified urinary PheT/HOPhe ratios in 346 smokers who were also genotyped for 11 polymorphisms in genes involved in PAH metabolism: CYP1A1MspI, CYP1A1I462V, CYP1B1R48G, CYP1B1A119S, CYP1B1L432V, CYP1B1N453S, EPHX1Y113H, EPHX1H139R, GSTP1I105V, GSTP1A114V, and GSTM1 null. The geometric mean molar PheT/3-HOPhe ratio was 4.08 (95% confidence interval, 3.79-4.39). Ten percent of the smokers had PheT/3-HOPhe ratios of > or =9.90. We found a significant association between the presence of the CYP1A1I462V polymorphism and high PheT/3-HOPhe ratios (P = 0.02). This effect was particularly strong in females and in combination with the GSTM1 null polymorphism. In contrast, the CYP1B1R48G and CYP1B1A119S polymorphisms were associated with significantly lower PheT/3-HOPhe ratios, particularly in Blacks. There were no consistent significant effects of any of the other polymorphisms on PheT/3-HOPhe ratios. The highest 10% of PheT/3-HOPhe ratios could not be predicted by the presence of any of the 11 polymorphisms individually or by certain combinations. The effects of the CYP1A1I462 polymorphism observed here, particularly in combination with GSTM1 null, are quite consistent with reports in the literature. However, the results of this study indicate that genotyping is not an effective way to predict PAH metabolism at least as represented by PheT/HOPhe ratios.

Longitudinal study of urinary phenanthrene metabolite ratios: effect of smoking on the diol epoxide pathway

We have proposed that urinary phenanthrene metabolites could be used in a carcinogen metabolite phenotyping approach to identify individuals who may be susceptible to cancer induction by polycyclic aromatic hydrocarbons (PAH). In support of this proposal, we have developed methods for quantitation of r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene (PheT) and phenanthrols (HOPhe) in human urine. PheT is the end product of the diol epoxide metabolic activation pathway of PAH, whereas HOPhe are considered as detoxification products. In this study, we investigated the longitudinal consistency of these metabolites over time in smokers and nonsmokers and compared their levels. Twelve smokers and 10 nonsmokers provided urine samples daily for 7 days, then weekly for 6 weeks. Levels of PheT, HOPhe, and PheT/HOPhe ratios were relatively constant in most individuals, with mean coefficients of variation ranging from 29.3% to 45.7%. There were no significant changes over time in levels of the metabolites or in ratios. These results indicate that a single urine sample should be sufficient when comparing phenanthrene metabolites in different groups. PheT/HOPhe ratios were significantly higher in smokers than in nonsmokers, showing that smoking induces the diol epoxide metabolic activation pathway of phenanthrene. This finding is consistent with previous studies indicating that inducibility of PAH metabolism contributes to cancer risk in smokers.

Analysis of Phenanthrols in Human Urine by Gas Chromatography-Mass Spectrometry: Potential Use in Carcinogen Metabolite Phenotyping

Phenanthrene is the simplest polycyclic aromatic hydrocarbon (PAH) containing a bay region, a feature closely associated with carcinogenicity. We have proposed that measurement of phenanthrene metabolites in human urine could be used to identify interindividual differences in metabolic activation and detoxification of PAH, and that these differences may be related to cancer susceptibility in smokers and other exposed individuals. Previously, we reported a method for quantitation of r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene (trans, anti-PheT) in human urine. trans, anti-PheT is the ultimate product of the diol epoxide metabolic activation pathway of phenanthrene. In this study, we have extended our carcinogen metabolite phenotyping approach by developing a method for quantitation of phenanthrols in human urine. PAH phenols such as phenanthrols are considered as detoxification products. After treatment of the urine by β-glucuronidase and arylsulfatase, a fraction enriched in phenanthrols was prepared by partitioning and solid phase extraction. The phenanthrols were silylated and analyzed by gas chromatography-positive ion chemical ionization-mass spectrometry with selected ion monitoring. [ring-13C6]3-phenanthrol was used as an internal standard. Accurate and reproducible quantitation of four phenanthrols, 1-phenanthrol (1-HOPhe), 2-HOPhe, 3-HOPhe, and 4-HOPhe, was readily achieved. In smokers, mean levels of 1-HOPhe (0.96 ± 1.2 pmol/mg creatinine) and 3-HOPhe (0.82 ± 0.62 pmol/mg creatinine) were greater than those of 2-HOPhe (0.47 ± 0.29 pmol/mg creatinine), and 4-HOPhe (0.11 ± 0.07 pmol/mg creatinine). There were no significant differences between the levels of any of the phenanthrols in smokers and nonsmokers. Total levels of the quantified phenanthrols were highly correlated with those of 3-HOPhe. Ratios of phenanthrene metabolites representing activation and detoxification were calculated as trans, anti-PheT divided by 3-HOPhe. There was a 7.5-fold spread of ratios in smokers, and a 12.3-fold spread in nonsmokers, suggesting that this may be a useful parameter for distinguishing individual metabolic responses to PAH exposure.

Metabolism of aromatic hydrocarbons by the filamentous fungus Cyclothyrium sp

The metabolism of biphenyl, naphthalene, anthracene, phenanthrene, pyrene and benzo[a]pyrene by Cyclothyrium sp. CBS 109850, a coelomycete isolated for the first time in Brazil from industrially polluted estuarine sediment, was studied. The metabolites were extracted and separated by high performance liquid chromatography (HPLC) and characterized by UV spectral analyses and mass, and proton nuclear magnetic resonance ((1)H NMR) spectrometry. Cyclothyrium sp. transformed biphenyl to 4-hydroxybiphenyl and anthracene to anthracene trans-1,2-dihydrodiol. This isolate metabolized 90% of [9-(14)C]phenanthrene, producing phenanthrene trans-9,10-dihydrodiol as a major metabolite, phenanthrene trans-3,4-dihydrodiol, 1-hydroxyphenanthrene, 3-hydroxyphenanthrene, 4-hydroxyphenanthrene, and a novel metabolite, 2-hydroxy-7-methoxyphenanthrene. Circular dichroism spectra analyses indicated that the major enantiomers of phenanthrene trans-9, 10-dihydrodiol, phenanthrene trans-3,4-dihydrodiol and pyrene trans-4,5-dihydrodiol, a pyrene metabolite produced previously by Cyclothyrium sp. CBS 109850, were predominantly in the (R,R) configuration, revealing a high stereoselectivity for initial monooxygenation and enzymatic hydration of phenanthrene and pyrene by Cyclothyrium sp. CBS109850. The results also show a high regioselectivity since the K-regions of phenanthrene and pyrene were the major sites of metabolism.

Comparative Metabolism of the Aza Polynuclear Aromatic Hydrocarbon Dibenz[a,h]acridine by Recombinant Human and Rat Cytochrome P450s

To assess the role of human and rat cytochrome P450s in the metabolism of aza-polynuclear aromatic hydrocarbons (aza-PAHs) and to examine the influence of heterocyclic nitrogen on the metabolism of these chemicals, we have investigated the biotransformation of dibenz[a,h]acridine (DB[a,h]ACR), an aza-PAH with two nonidentical bay regions, by recombinant human cytochromes P450 1A1, 1B1, and 3A4 and rat P450 1A1. Among the three P450s, 1A1 was the most effective in metabolizing DB[a,h]ACR followed by 1B1 and 3A4. The major DB[a,h]ACR metabolites produced by human P450 1A1 and 1B1 were the dihydrodiols with a bay region double bond, namely, DB[a,h]ACR-3,4-diol and DB[a,h]ACR-10,11-diol (putative proximate carcinogen). P450 1A1 produced a higher proportion of DB[a,h]ACR-10,11-diol (derived from the benzo ring adjacent to the nitrogen) (44.7%) than of DB[a,h]ACR-3,4-diol (derived from benzo ring away from the nitrogen) (23.8%). In contrast, 1B1 produced a much greater proportion of 3,4-diol (54.7%) than of 10,11-diol (6.4%). These data indicate that (i) human P450 1A1 and 1B1 differ dramatically with respect to the regiospecific metabolism of DB[a,h]ACR, (ii) human P450 1A1 is substantially more active than human P450 1B1 in the metabolic activation of the aza-PAH to its 10,11-diol, and (iii) the presence of nitrogen influences the relative extent to which the two benzo ring diols with a bay region double bond are formed by human P450s 1A1 and 1B1. In contrast to human P450s 1A1 and 1B1, rat P450 1A1 showed no regioselectivity in the metabolism of DB[a,h]ACR producing nearly equal proportions of 10,11-diol and 3,4-diol. Despite significant differences in their regioselectivity, human P450 1A1 and 1B1 and rat P450 1A1 showed similar stereoselectivity in the metabolism of DB[a,h]ACR to its diols having a bay region double bond, producing primarily the R,R enantiomers (>94%). The data of these studies indicate that human and rat P450 1A1 differ in their regioselectivity in the metabolism of DB[a,h]ACR to its two benzo ring diols with a bay region double bond and consequently in their ability to metabolically activate the parent aza-PAH. However, human and rat P450 1A1 do not differ with respect to their stereoselectivity in the metabolism of DB[a,h]ACR to the diols.

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

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

Metabolism of phenanthrene by house fly CYP6D1 and dog liver cytochrome P450

Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous class of environmental contaminants. The compound phenanthrene is a model PAH. A novel fluorometric method for measuring phenanthrene metabolism in vitro was developed and verified with direct measurement of [14C]phenanthrene using dog liver microsomes. The fluorometric assay and direct measurement of [14C]phenanthrene metabolism were used to show that CYP6D1, a house fly cytochrome P450, is the major house fly P450 involved in phenanthrene metabolism. Phenanthrene was metabolized by microsomes from the LPR strain of house fly that overexpresses CYP6D1, but metabolism was not observed in the CS strain that has a lower level of CYP6D1. Furthermore, the majority of phenanthrene metabolism was inhibited by a CYP6D1-specific antibody. This study increases the number of known substrates of CYP6D1 and identifies polyaromatic hydrocarbons as potential substrates of CYP6D1. The utility of CYP6D1 as an agent in bioremediation and the utility of the new fluorometric assay for understanding PAH metabolism in insects and mammals are discussed.

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.

Assessment of specificity of eight chemical inhibitors using cDNA-expressed cytochromes P450

1. The selectivity of eight chemical inhibitors has been extensively evaluated with 10 cDNA-expressed human cytochrome P450 isoforms (CYP). The results indicate that sulphaphenazole, quinidine and alpha-naphthoflavone are selective inhibitors of CYP2C9 (IC50 = 0.5-0.7 microM), CYP2D6 (0.3-0.4 microM) and CYP1A (0.05-5 microM) respectively on the basis of the IC50, which are much lower than those of other P450 isoforms (> 10-fold). 2. Ketoconazole exhibited potent inhibition of both CYP3A4-catalysed metabolism of phenanthrene, testosterone, diazepam (IC50 = 0.03-0.5 microM) and CYP1A1-catalysed deethylation of 7-ethoxycoumarin (0.33 microM). The selectivity of ketoconazole for other P450s was highly related to the concentration used. 3. Diethyldithiocarbamate, orphenadrine and furafylline were shown separately to be less selective inhibitors of CYP2E1, CYP2B6 and CYP1A isoforms by a broad range of IC50 that overlap those observed with other P450 isoforms. 4. Furafylline, quinidine and alpha-naphthoflavone activated CYP3A4-catalysed phenanthrene metabolism by 1.7-, 2- and 15-fold respectively. 5. The selectivity of orphenadrine and ketoconazole was further examined by using inhibitory monoclonal antibodies (MAb). Inhibitory MAb specific for the individual P450 isoforms may be of greater value than chemical inhibitors.

Comparative in vitro metabolism of indinavir in primates--a unique stereoselective hydroxylation in monkey

1. The in vitro metabolism of indinavir (CRIXIVAN, MK-0639, L-735,524), an HIV protease inhibitor, was evaluated using liver microsomes from cynomolgus monkey, rhesus monkey, chimpanzee and human. Indinavir exhibited marked species differences in metabolism. The overall rate of indinavir metabolism varied > 4-fold among primates (84 pmol/min/mg protein in cynomolgus monkey versus 20.4 pmol/min/mg protein in human) and followed the rank order: cynomolgus monkey > rhesus monkey > chimpanzee > human. 2. The cis-(indan)hydroxylated metabolite of indinavir was formed only in cynomolgus and rhesus monkey livers, whereas trans-(indan)hydroxylation and N-dealkylation were observed as the major metabolites in all primates tested. Inhibition studies with P450-selective inhibitors (ketoconazole, quinine, quinidine) and monoclonal antibodies (against CYP2D6 or CYP3A4) indicated that a cytochrome P450 isoform of the CYP2D subfamily is involved in the formation of the unique cis-(indan) hydroxylated metabolite in monkey, whereas all other oxidative metabolites, including the trans-(indan)hydroxylated metabolite, are formed by CYP3A isoform(s). 3. The present study has demonstrated that monkeys were unique in their abilities to form the stereoselective metabolite and were not appropriate surrogates for the qualitative prediction of indinavir metabolism in human.

Genetically engineered cells stably expressing cytochrome P450 and their application to mutagen assays

Genetically engineered cells transiently and stably expressing cytochrome P450 (P450), a key enzyme for biotransformation of a wide variety of compounds, have provided new tools for investigation of P450 functions such as P450-mediated metabolic activation of chemicals. This review will focus on the development of mammalian cell lines stably expressing P450s and application to toxicology testings. Stable expression systems have an advantage over transient ones in that a series of the process from metabolic activation of test compounds to the appearance of toxicological consequences occurs entirely in the same intact cells. Indeed, many cell lines stably expressing a single form of mammalian P450 have been established so far and applied to cytotoxic or genotoxic assays, the endpoints of which contained mutations at hprt and other gene loci, chromosomal aberrations, sister chromatid exchanges, micronuclei, morphological transformation, and 32P-postlabeling. Analyses of metabolites of toxic substances have also been carried out, using the intact cells or microsomal fractions prepared from the cells. The stable expression systems clearly indicate the form of P450 enzyme capable of activating a certain chemical. More recently, coexpression of P450 together with other components of microsomal electron transfer systems such as NADPH-cytochrome P450 reductase has been successfully performed to increase the metabolic capacity of the heterologously expressed P450. In addition, to reconstruct the entire metabolic activation system for certain heterocyclic amines, cell lines which simultaneously express a form of human P450 and a phase II enzyme, N-acetyltransferase, were established. These cells were highly sensitive to some carcinogenic heterocyclic amines. In genetic toxicology, such a coexpression system for two or more enzymes will provide useful materials which mimic in vivo activation systems.

Inhibitory monoclonal antibody to human cytochrome P450 2B6

The human cytochrome P450 2B6 metabolizes, among numerous other substrates, diazepam, 7-ethoxycoumarin, testosterone, and phenanthrene. A recombinant baculovirus containing the human 2B6 cDNA was constructed and used to express 2B6 in Sf9 insect cells. The 2B6 was present at 1.8 +/- 0.4% of the total cellular protein and was purified to a specific content of 13.3 nmol/mg protein. Mice were immunized with the purified 2B6, and a total of 811 hybridomas were obtained from the fusion of NS-1 myeloma cells and spleen cells of the immunized mice. Monoclonal antibodies (MAbs) from 24 of the hybrids exhibited immunobinding to 2B6 as determined by ELISA. One of the MAbs, 49-10-20, showed a strong immunoblotting activity and was highly inhibitory to 2B6 enzyme activity. MAb 49-10-20 inhibited cDNA-expressed 2B6-catalyzed metabolism of diazepam, phenanthrene, 7-ethoxycoumarin, and testosterone by 90-91%. MAb 49-10-20 showed extremely high specificity for 2B6 and did not bind to 17 other human and rodent P450s or inhibit the metabolism of phenanthrene catalyzed by human 1A2, 2A6, 2C8, 2C9, 2D6, 2E1, 3A4, and 3A5. MAb 49-10-20 was used to determine the contribution of 2B6 to the metabolism of phenanthrene and diazepam in human liver. In ten liver samples, MAb 49-10-20 inhibited phenanthrene metabolism variably by a wide range of 8-42% and diazepam demethylation by 1-23%. The degree of inhibition by the 2B6 specific MAb 49-10-20 defines the contribution of 2B6 to phenanthrene and diazepam metabolism in each human liver. This technique using inhibitory MAb 49-10-20 determines the contribution of 2B6 to the metabolism of its substrates in a human tissue containing multiple P450s. This study is a prototype for the use of specific and highly inhibitory MAbs to determine individual P450 function.

Regio- and stereoselectivity in the metabolism of benzo[c]phenanthrene mediated by genetically engineered V79 Chinese hamster cells expressing rat and human cytochromes P450

Regio- and stereoselective metabolism mediated by cytochrome P450 (CYP) and metabolite-dependent cytotoxicity of benzo[c]phenanthrene (B[c]Ph) and its trans-3,4-dihydrodiol, the metabolic precursor of the carcinogenic fjord-region B[c]Ph-3,4-dihydrodiol 1,2-epoxides (B[c]PhDE), were investigated with V79 Chinese hamster cells genetically engineered for three rat and six human CYP isoforms. The order of the capabilities of the CYP isoforms to metabolize B[c]Ph was as follows: h1A1>r1A1>r1A2>h1B1>h1A2>r2B1>>h2E1>h2A6>h3A4. Regardless of the species, all individual CYP isoforms preferentially catalyzed the oxidation of B[c]Ph at the 5,6-position (K-region) except human CYP1A1 and human CYP1A2, which oxidized both the 5,6- and the 3,4-position with similar efficiency. While human CYP1A1, rat CYP1A1 and rat CYP1A2 formed almost exclusively the (-)-B[c]Ph-3R,4R-dihydrodiol, human CYP1A2 produced both the (-)-3R,4R- and the (+)-3S,4S-dihydrodiol enantiomers in a ratio of 2:1. Stereoselective activation of B[c]Ph, the (±)-B[c]Ph-3,4-dihydrodiol and its (-)-3R,4R-enantiomer to the fjord-region (-)-anti-B[c]PhDE occurred upon incubation with rat CYP1A1 and rat CYP1A2 as indicated by the formation of two stereoisomeric tetraols, the hydrolysis products of the labile anti-B[c]PhDE. The formation of tetraols in the culture medium was accompanied by a concentration-dependent increase in cytotoxicity indicating that this effect was mediated by the fjord-region (-)-anti-B[c]PhDE formed as reactive intermediate. All human and rat CYP-expressing V79 cell lines investigated did not show any significant capacity to metabolize the (+)-3S,4S-dihydrodiol. The present study indicates that the human CYP isoforms 1A1 and 1B1 have complementary catalytic properties to activate B[c]Ph to its fjord-region B[c]PhDE, whereas other human isoforms play a minor role. Activation of B[c]Ph by human CYP1A1 and 1B1 is less efficient than by rat CYP1A1 or rat CYP1A2, but proceeds with similar stereoselectivity via the (-)-3R,4R-dihydrodiol to the strong carcinogen (-)-anti-B[c]PhDE with (R,S,S,R)-configuration.

Evaluation of atypical cytochrome P450 kinetics with two-substrate models: evidence that multiple substrates can simultaneously bind to cytochrome P450 active sites

Some cytochrome P450 catalyzed reactions show atypical kinetics, and these kinetic processes can be grouped into five categories: activation, autoactivation, partial inhibition, substrate inhibition, and biphasic saturation curves. A two-site model in which the enzyme can bind two substrate molecules simultaneously is presented which can be used to describe all of these observed kinetic properties. Sigmoidal kinetic characteristics were observed for carbamazepine metabolism by CYP3A4 and naphthalene metabolism by CYPs 2B6, 2C8, 2C9, and 3A5 as well as dapsone metabolism by CYP2C9. Naphthalene metabolism by CYP3A4 and naproxen metabolism by CYP2C9 demonstrated nonhyperbolic enzyme kinetics suggestive of a low Km, low Vmax component for the first substrate molecule and a high Km, high Vmax component for the second substrate molecule. 7, 8-Benzoflavone activation of phenanthrene metabolism by CYP3A4 and dapsone activation of flurbiprofen and naproxen metabolism by CYP2C9 were also observed. Furthermore, partial inhibition of 7, 8-benzoflavone metabolism by phenanthrene was observed. These results demonstrate that various P450 isoforms may exhibit atypical enzyme kinetics depending on the substrate(s) employed and that these results may be explained by a model which includes simultaneous binding of two substrate molecules in the active site.