Cytochrome P450 catalyzed oxidation of monochlorobenzene, 1,2- and 1,4-dichlorobenzene in rat, mouse, and human liver microsomes

Exposure to p-dichlorobenzene and serum α-Klotho levels among US participants in their middle and late adulthood

P-dichlorobenzene (p-DCB) is a volatile compound commonly used as pest repellent and air deodorant in the home and public buildings, leading to a widespread exposure in indoor environments. There has been an increasing concern about its metabolic and endocrine effects. In this study, we explored the relation between p-DCB exposure and serum levels of soluble α-Klotho, an anti-aging hormone, in US adults. A nationally representative subsample of 1485 adults 40-79 ages in the 2013-2016 National Health and Nutrition Examination Survey was analyzed for the association between p-DCB exposure, measured as urinary concentrations of 2,5-dichlorophenol (2,5-DCP), the major metabolite of p-DCB, and serum α-Klotho levels using multiple general linear models, adjusting for potential confounders. Age- and sex-specific analyses were further conducted. The weighted geometric mean of urinary 2,5-DCP was 2.43 μg/L and the weighted mean of serum α-Klotho was 831.97 pg/mL in the study participants during 2013-2016. After adjusting for potential confounders and urinary creatinine, urinary 2,5-DCP was significantly associated with decreased serum levels of α-Klotho (regression coefficient β = -9.88; p = 0.0133) in the total study population. When age- and sex-specific analyses being conducted, a significantly inverse association was found in older adults aged 60-79 years (β = -20.40; p = 0.0001) and in males (β = -13.81; p = 0.0097), but not in the middle ages (40-59 years) and in females. The strongest association was observed in older (60-79 years) male participants, with a 25.43 pg/mL reduction of α-Klotho levels per 1-unit increase of 2,5-DCP concentrations (p = 0.0008). This is the first study demonstrating a relation between p-DCB exposure, measured as 2,5-DCP, and decreased α-Klotho levels in older males. Additional studies would further explore these interactions and elucidate the pathogenesis of the potential effects of p-DCB exposure on aging.

Fungal Unspecific Peroxygenases Oxidize the Majority of Organic EPA Priority Pollutants

Unspecific peroxygenases (UPOs) are secreted fungal enzymes with promiscuity for oxygen transfer and oxidation reactions. Functionally, they represent hybrids of P450 monooxygenases and heme peroxidases; phylogenetically they belong to the family of heme-thiolate peroxidases. Two UPOs from the basidiomycetous fungi Agrocybe aegerita (AaeUPO) and Marasmius rotula (MroUPO) converted 35 out of 40 compounds listed as EPA priority pollutants, including chlorinated benzenes and their derivatives, halogenated biphenyl ethers, nitroaromatic compounds, polycyclic aromatic hydrocarbons (PAHs) and phthalic acid derivatives. These oxygenations and oxidations resulted in diverse products and—if at all—were limited for three reasons: (i) steric hindrance caused by multiple substitutions or bulkiness of the compound as such (e.g., hexachlorobenzene or large PAHs), (ii) strong inactivation of aromatic rings (e.g., nitrobenzene), and (iii) low water solubility (e.g., complex arenes). The general outcome of our study is that UPOs can be considered as extracellular counterparts of intracellular monooxygenases, both with respect to catalyzed reactions and catalytic versatility. Therefore, they should be taken into consideration as a relevant biocatalytic detoxification and biodegradation tool used by fungi when confronted with toxins, xenobiotics and pollutants in their natural environments.

Characterization of in vitro metabolites of TM-2, a potential antitumor drug, in rat, dog and human liver microsomes using liquid chromatography/tandem mass spectrometry

RATIONALE

TM-2 (13-(N-Boc-3-i-butylisoserinoyl-4,10-β-diacetoxy-2-α-benzoyloxy-5-β,20-epoxy-1,13-α-dihydroxy-9-oxo-19-norcyclopropa[g]tax-11-ene) is a novel semi-synthetic taxane derivative. Our previous study demonstrated that it is a promising taxane derivative. The in vitro comparative metabolic profile of a drug between animals and humans is a key issue that should be investigated at early stages of drug development to better select drug candidates. In this study, the in vitro metabolic pathways of TM-2 in rat, dog and human liver microsomes were established and compared.

METHODS

TM-2 was incubated with liver microsomes in the presence of NADPH. Two different types of mass spectrometers - a hybrid linear trap quadrupole orbitrap (LC/LTQ-Orbitrap) mass spectrometer and a triple-quadrupole tandem mass spectrometer (LC/QqQ) were employed to acquire structural information of TM-2 metabolites. Accurate mass measurement using LC/LTQ-Orbitrap was used to determine the accurate mass data and elemental compositions of metabolites thereby confirming the proposed structures of the metabolites. For the chemical inhibition study, selective P450 inhibitors were added to incubations to initially characterize the cytochrome P450 (CYP) enzymes involved in the metabolism of TM-2.

RESULTS

A total of 12 components (M1-M12) were detected and identified as the metabolites of TM-2 in vitro. M1-M5 were formed by hydroxylation of the taxane ring or the lateral chain. Hydroxylated products can be further oxidized to the dihydroxylated metabolites M6-M10. M11 was a trihydroxylated metabolite. M12 was tentatively identified as a carboxylic acid derivative. The metabolism of TM-2 is much the same in all three species with some differences. The chemical inhibition study initially demonstrated that the formation of M2, the major metabolite of TM-2, is mainly mediated by CYP3A4.

CONCLUSIONS

Hydroxylation is the major biotransformation of the TM-2 pathway in vitro. CYP3A4 may play a dominant role in the formation of M2 in liver microsomes. The knowledge of the metabolic pathways of TM-2 is important to support further research of TM-2.

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.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Chlorinated benzenes cause concomitantly oxidative stress and induction of apoptotic markers in lung epithelial cells (A549) at nonacute toxic concentrations

In industrialized countries, people spend more time indoors and are therefore increasingly exposed to volatile organic compounds that are emitted at working places and from consumer products, paintings, and furniture, with chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) being representatives of the halogenated arenes. To unravel the molecular effects of low concentrations typical for indoor and occupational exposure, we exposed human lung epithelial cells to CB and DCB and analyzed the effects on the proteome level by 2-D DIGE, where 860 protein spots were detected. A set of 25 and 30 proteins were found to be significantly altered due to exposure to environmentally relevant concentrations of 10(-2) g/m(3) of CB or 10(-3) g/m(3) of DCB (2.2 and 0.17 ppm), respectively. The most enriched pathways were cell death signaling, oxidative stress response, protein quality control, and metabolism. The involvement of oxidative stress was validated by ROS measurement. Among the regulated proteins, 28, for example, voltage-dependent anion-selective channel protein 2, PDCD6IP protein, heat shock protein beta-1, proliferating cell nuclear antigen, nucleophosmin, seryl-tRNA synthetase, prohibitin, and protein arginine N-methyltransferase 1, could be correlated with the molecular pathway of cell death signaling. Caspase 3 activation by cleavage was confirmed for both CB and DCB by immunoblotting. Treatment with CB or DCB also caused differential protein phosphorylation, for example, at the proteins HNRNP C1/C2, serine-threonine receptor associated protein, and transaldolase 1. Compared to previous results, where cells were exposed to styrene, for the chlorinated aromatic substances besides oxidative stress, apoptosis was found as the predominant cellular response mechanism.

Metabolism of new-generation taxanes in human, pig, minipig and rat liver microsomes

The novel taxanes SB-T-1102, SB-T-1214 and SB-T-1216 are up to 1000-fold more cytotoxic for resistant tumour cells than clinically used paclitaxel and docetaxel, and the current study has examined the metabolism of these new taxanes in human, rat, pig and minipig liver microsomes. Metabolites were characterized by high-performance liquid chromatography (HPLC)/tandem mass spectrometry (MS/MS) analysis. Metabolic pathways derived from their structures were confirmed by investigating subsequent metabolism of purified metabolites. SB-T-1102, SB-T-1214 and SB-T-1216 were metabolized to 14, 10 and 11 products, respectively. In contrast to docetaxel, side-chain hydroxylation did not occur at their tert-butyl group, but on the isobutyl (SB-T-1102) or isobutenyl (SB-T-1214 and SB-T-1216) chains. Species differences in their metabolism were observed. For example, human and untreated rat microsomes hydroxylated SB-T-1216 preferentially at the side-chain, whereas pig and minipig microsomes preferentially metabolized more at the taxane core. The increased formation of secondary and tertiary metabolites in rat microsomes with high expression of CYP3A1/2 compared with uninduced rats confirmed the role of CYP3A in taxane metabolism. All major products were formed by human cDNA-expressed CYP3A4 and none by CYP1A2, 1B1, 2A6, 2C9 and 2E1, indicating the principal role of CYP3A orthologues in SB-T metabolism. The knowledge of metabolic pathways of the examined agents and of their rates of formation is important due to possible metabolic inactivation of these three novel drugs with a great potential for the therapy of taxane-resistant tumours. The relatively slow metabolism of SB-T-1102 could be favourable for its antitumour efficiency in vivo.

Gene expression response in target organ and whole blood varies as a function of target organ injury phenotype

This report details the standardized experimental design and the different data streams that were collected (histopathology, clinical chemistry, hematology and gene expression from the target tissue (liver) and a bio-available tissue (blood)) after treatment with eight known hepatotoxicants (at multiple time points and doses with multiple biological replicates). The results of the study demonstrate the classification of histopathological differences, likely reflecting differences in mechanisms of cell-specific toxicity, using either liver tissue or blood transcriptomic data.

Quantification of urinary conjugates of bisphenol A, 2,5-dichlorophenol, and 2-hydroxy-4-methoxybenzophenone in humans by online solid phase extraction-high performance liquid chromatography-tandem mass spectrometry

Urinary concentrations of phenols or their metabolites have been used as biomarkers to assess the prevalence of exposure to these compounds in the general population. Total urinary concentrations, which include both free and conjugated (glucuronide and sulfated) forms of the compounds, are usually reported. From a toxicologic standpoint, the relative concentrations of the free species compared with their conjugated analogs can be important because conjugation may reduce the potential biologic activity of the phenols. In this study, we determined the percentage of glucuronide and sulfate conjugates of three phenolic compounds, bisphenol A (BPA), 2,5-dichlorophenol (2,5-DCP), and 2-hydroxy-4-methoxybenzophenone (benzophenone-3, BP-3) in 30 urine samples collected between 2000 and 2004 from a demographically diverse group of anonymous adult volunteers. We used a sensitive on-line solid phase extraction-isotope dilution-high performance liquid chromatography-tandem mass spectrometry method. These three phenols were detected frequently in the urine samples tested. Only small percentages of the compounds (9.5% for BPA, and 3% for 2,5-DCP and BP-3) were excreted in their free form. The percentage of the sulfate conjugate was about twice that of the free compound. The glucuronide conjugate was the major metabolite, representing 69.5% (BPA), 89% (2,5-DCP), and 84.6% (BP-3) of the total amount excreted in urine. These results are in agreement with those reported before which suggested that BPA-glucuronide was an important BPA urinary metabolite in humans. To our knowledge, this is the first study describing the distribution of urinary conjugates of BP-3 and 2,5-DCP in humans.

DIFFERENT IN VITRO METABOLISM OF PACLITAXEL AND DOCETAXEL IN HUMANS, RATS, PIGS, AND MINIPIGS

We investigated cytochrome P450 (P450)-catalyzed metabolism of the important cancer drugs paclitaxel and docetaxel in rat, pig, minipig, and human liver microsomes and cDNA-expressed P450 enzymes. In rat microsomes, paclitaxel was metabolized mainly to C3'-hydroxypaclitaxel (C3'-OHP) and to a lesser extent to C2-hydroxypaclitaxel (C2-OHP), di-hydroxypaclitaxel (di-OHP), and another unknown monohydroxylated paclitaxel. In pig and minipig microsomes, this unknown hydroxypaclitaxel was the main metabolite, whereas C3'-OHP was a minor product. In minipigs, C2-OHP was the next minor product. In human liver microsomes, 6 alpha-hydroxypaclitaxel (6 alpha-OHP) was the main metabolite, followed by C3'-OHP and C2-OHP. Among different cDNA-expressed human P450 enzymes (CYP1A2, 1B1, 2A6, 2C9, 2E1, and 3A4), only CYP3A4 enzyme formed C3'-OHP and C2-OHP. Docetaxel was metabolized in pig, minipig, rat, and human liver microsomes mainly to hydroxydocetaxel (OHDTX), whereas CYP3A-induced rat microsomes produced primarily diastereomeric hydroxyoxazolidinones. Human liver microsomes from 10 different individuals formed OHDTX at different rates correlated with CYP3A4 content. Troleandomycin as a selective inhibitor of CYP3A inhibited the formation of C3'-OHP, C2-OHP, and di-OHP, as well as the unknown OHP produced in rat, minipig, and pig microsomes. In human liver microsomes, troleandomycin inhibited C3'-OHP and C2-OHP formation, and a suitable inhibitor of human CYP2C8, fisetin, strongly inhibited the formation of 6 alpha-OHP, known to be catalyzed by human CYP2C8. In conclusion, the metabolism of docetaxel is the same in all four species, but metabolism of paclitaxel is different, and 6 alpha-OHP remains a uniquely human metabolite. Pigs and minipigs compared with each other formed the same metabolites of paclitaxel.

Development of a physiologically based pharmacokinetic model for chlorobenzene in F-344 rats

A physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and elimination of chlorobenzene in rats was developed. Partition coefficients were experimentally determined in rat tissues and blood samples using an in vitro vial equilibration technique. These solubility ratios were in agreement with previous reports. The in vivo metabolism of chlorobenzene was evaluated using groups of three F344 male rats exposed to initial chlorobenzene concentrations ranging from 82 to 6750 ppm in a closed, recirculating gas uptake system. An optimal fit of the family of uptake curves was obtained by adjusting Michaelis-Menten metabolic constants, K(m) (affinity) and Vmax (capacity), using the PBPK model. At the highest chamber concentration, the uptake curve could not be modeled without the addition of a first-order (Kfo) metabolic pathway. Pretreatment with pyrazole, an inhibitor of oxidative microsomal metabolism, had no impact on the slope of the uptake curve. The completed PBPK model was evaluated against real-time exhaled breath data collected from rats receiving either an intraperitoneal (i.p.) injection or oral gavage dose of chlorobenzene in corn oil. Exhaled breath profiles were evaluated and absorption rates were determined. Development of the chlorobenzene PBPK model in rats is the first step toward future extrapolations to apply to humans.

In vitro metabolism of zolmitriptan in rat cytochromes induced with β-naphthoflavone and the interaction between six drugs and zolmitriptan

Zolmitriptan is a novel and highly selective 5-HT(1B/1D) receptor agonist used as an acute oral treatment for migraine. There are few reports regarding the in vitro metabolism of zolmitriptan. Previous studies indicated zolmitriptan was metabolized via CYP1A2 in human hepatic microsomes. In order to study the enzyme kinetics and drug interaction, the metabolism of zolmitriptan and possible drug-drug interactions were investigated in rat hepatic microsomes induced with different inducers. An active metabolite, N-demethylzolmitriptan, was detected and another minor, inactive metabolite that was reported in human hepatic microsomes was not detected in this study. The enzyme kinetics for the formation of N-demethylzolmitriptan from zolmitriptan in rat liver microsomes pretreated with BNF were 96+/-22 microM (K(m)), 11+/-3 pmol min(-1)mg protein(-1) (V(max)), and 0.12+/-0.02 microl min(-1)mg protein(-1) (CL(int)). Fluvoxamine and diphenytriazol inhibited zolmitriptan N-demethylase activity catalyzed by CYP1A2 (K(i)=3.8+/-0.3 and 3.2+/-0.1 microM, respectively). Diazepam and propranolol elicited a slight inhibitory effect on the metabolism of zolmitriptan (K(i)=70+/-11 and 90+/-18 microM, respectively). Cimetidine and moclobemide produced no significant effect on the metabolism of zolmitriptan. Fluvoxamine yielded a k(inactivation) value of 0.16 min(-1), and K(i) of 57 microM. The results suggest that rat hepatic microsomes are a reasonable model to study the metabolism of zolmitriptan, although there is a difference in the amount of minor, inactive metabolites between human hepatic microsomes and rat liver microsomes. The results of the inhibition experiments provided information for the interactions between zolmitriptan and drugs co-administrated in clinic, and it is helpful to explain the drug-drug interactions of clinical relevance on enzyme level. This study aso demonstrated that fluvoxamine may be a mechanism-based inactivator of CYP1A2.

Paclitaxel metabolism in rat and human liver microsomes is inhibited by phenolic antioxidants

Paclitaxel is an important, recently introduced anti-neoplastic drug. Paclitaxel metabolites are virtually inactive in comparison with the parent drug. The study investigated whether phenolic antioxidants could inhibit metabolic inactivation sufficiently to increase paclitaxel effects. Cytochrome p450 (CYP)-catalysed metabolism of paclitaxel was investigated in rat and human liver microsomes. In rat microsomes, paclitaxel was metabolised mainly to C3'-hydroxypaclitaxel (C3'-OHP), less to C2-hydroxypaclitaxel (C2-OHP), di-hydroxypaclitaxel (di-OHP) and another monohydroxylated paclitaxel. In human liver microsomes, 6alpha-hydroxypaclitaxel (6alpha-OHP), formed by CYP2C8, was the main metabolite, while C3'-OHP, C2-OHP and another product different from di-OHP were minor metabolites, formed by CYP3A4. In individual human livers 6alpha-OHP was formed at 1.8-fold to 13-fold higher rates than C3'-OHP. Kinetic parameters (K(m) and V(max)) of production of various metabolites in rat and human liver microsomes revealed differences between species as well as human individual differences. Nine phenolic antioxidants ((+)-catechin, (-)-epicatechin, fisetin, gallic acid, morin, myricetin, naringenin, quercetin and resveratrol) were tested for inhibition of paclitaxel metabolism. In rat microsomes, resveratrol was more inhibitory than fisetin; the other phenolic antioxidants were without effect. In human microsomes, the inhibiting potency decreased in the order fisetin >quercetin >morin >resveratrol, while the other phenolic antioxidants were not inhibitory; the formation of 6alpha-OHP (CYP2C8) was generally more inhibited than that of C3'-OHP. The inhibition was mostly mixed-type. The results suggest that oral administration of some phenolic substances might increase paclitaxel blood concentrations during chemotherapy.

p-Nitrophenol and glutathione response in medaka (Oryzias latipes) exposed to MX, a drinking water carcinogen

When chlorine is introduced into public drinking water for disinfection, it can react with organic compounds in surface waters to form toxic by-products such as 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX). We investigated the effect of exposure to MX on cytochrome P450 2E1 (CYP2E1)-like activity and total glutathione (GSH) in the liver of the small fish model, medaka (Oryzias latipes). The multi-site carcinogen methylazoxymethanol acetate (MAMAc) was the positive control compound. Both medaka liver microsome preparations and S-9 fractions catalyzed the hydroxylation of p-nitrophenol (PNP), suggesting CYP2E1-like activity in the medaka. Male medaka exposed for 96 h to the CYP2E1 inducers ethanol and acetone under fasted conditions showed significant increases in PNP-hydroxylation activity. Furthermore, total reduced hepatic GSH was reduced in fish fasted for 96 h, indicating that normal feeding is a factor in maintaining xenobiotic defenses. Exposure to MX and MAMAc induced significant increases in hepatic CYP2E1-like activity, however MX exposure did not alter hepatic GSH levels. These data strengthen the role of the medaka as a suitable species for examining cytochrome P450 and GSH detoxification processes and the role these systems play in chemical carcinogenesis.