Hepatic epoxide hydrase. Structure-activity relationships for substrates and inhibitors

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which—taken with great caution because of the still very limited data—the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

Microsomal epoxide hydrolase (EPHX1) is an evolutionarily highly conserved biotransformation enzyme for converting epoxides to diols. Notably, the enzyme is able to either detoxify or bioactivate a wide range of substrates. Mutations and polymorphic variants in the EPHX1 gene have been associated with susceptibility to several human diseases including cancer. This review summarizes the key knowledge concerning EPHX1 gene and protein structure, expression pattern and regulation, and substrate specificity. The relevance of EPHX1 for human pathology is especially discussed.

The 2014 Bernard B. Brodie award lecture-epoxide hydrolases: drug metabolism to therapeutics for chronic pain

Dr. Bernard Brodie's legacy is built on fundamental discoveries in pharmacology and drug metabolism that were then translated to the clinic to improve patient care. Similarly, the development of a novel class of therapeutics termed the soluble epoxide hydrolase (sEH) inhibitors was originally spurred by fundamental research exploring the biochemistry and physiology of the sEH. Here, we present an overview of the history and current state of research on epoxide hydrolases, specifically focusing on sEHs. In doing so, we start with the translational project studying the metabolism of the insect juvenile hormone mimic R-20458 [(E)-6,7-epoxy-1-(4-ethylphenoxy)-3,7-dimethyl-2-octene], which led to the identification of the mammalian sEH. Further investigation of this enzyme and its substrates, including the epoxyeicosatrienoic acids, led to insight into mechanisms of inflammation, chronic and neuropathic pain, angiogenesis, and other physiologic processes. This basic knowledge in turn led to the development of potent inhibitors of the sEH that are promising therapeutics for pain, hypertension, chronic obstructive pulmonary disorder, arthritis, and other disorders.

Basic properties and molecular mechanisms of exogenous chemical carcinogens

Exogenous chemical carcinogenesis is an extremely complex multifactorial process during which gene-environment interactions involving chronic exposure to exogenous chemical carcinogens (ECCs) and polymorphisms of cancer susceptibility genes add further complexity. We describe the properties and molecular mechanisms of ECCs that contribute to induce and generate cancer. A basic and specific property of many lipophilic organic ECCs including polycyclic aromatic hydrocarbons and polyhalogenated aromatic hydrocarbons is their ability to bioaccumulate in the adipose tissue from where they may be released in the blood circulation and target peripheral tissues for carcinogenesis. Many organic ECCs are procarcinogens and consequently need to be activated by the cytochrome P450 (CYP) system and/or other enzymes before they can adduct DNA and proteins. Because they contribute not only to the cocarcinogenic and promoting effects of many aromatic pollutants but also to their mutagenic effects, the aryl hydrocarbon receptor-activating and the inducible CYP systems are central to exogenous chemical carcinogenesis. Another basic property of ECCs is their ability to induce stable and bulky DNA adducts that cannot be simply repaired by the different repair systems. In addition, following ECC exposure, mutagenesis may also be caused indirectly by free-radical production and by epigenetic alterations. As a result of complex molecular interplays, direct and/or indirect mutagenesis may especially account for the carcinogenic effects of many exogenous metals and metalloids. Because of these molecular properties and action mechanisms, we conclude that ECCs could be major contributors to human cancer, with obviously great public health consequences.

Metabolism of carcinogens, possibilities for modulation

One of the structural elements which are widely occurring in very many chemical mutagens and carcinogens are aromatic and olefinic moieties. These can be transformed into epoxides by microsomal monooxygenases. Such epoxides may spontaneously react with nucleophilic centers in the cell and thereby covalently bind to DNA, RNA and protein. Such a reaction may lead to cytotoxicity, allergy, mutagenicity and/or carcinogenicity, depending on the properties of the epoxide in question. An important contributing factor is the presence of enzymes controlling the concentration of such epoxides. There are several microsomal monooxygenases which differ in activity and substrate specificity. With large substrates, some monooxygenases preferentially attack at one specific site different from that attacked by others. Some of these pathways lead to reactive products, others are detoxification pathways. Also important are the enzymes which metabolize epoxides, such as epoxide hydrolases and glutathione transferases. Such enzymes can act as inactivating and in some specific cases also as co-activating enzymes. Moreover, precursor-sequestering enzymes such as dihydrodiol dehydrogenase, glucuronosyl transferases and sulphotransferases are important for the control of reactive epoxides. These enzymes themselves are subject to control by many endogenous and exogenous factors. By virtue of their contribution to the control of carcinogenic metabolites such modulators can act as modifiers of tumorigenesis and can be used experimentally to study the role of the various individual enzymes.

The 3,4-oxide is responsible for the DNA binding of benzo[ghi]perylene, a polycyclic aromatic hydrocarbon without a "classic" bay-region

The polycyclic aromatic hydrocarbon (PAH) benzo[ghi]perylene (BghiP) lacks a "classic" bay-region and is therefore unable to form vicinal dihydrodiol epoxides thought to be responsible for the genotoxicity of carcinogenic PAHs like benzo[a]pyrene. The bacterial mutagenicity of BghiP increases considerably after inhibition of the microsomal epoxide hydrolase (mEH) indicating arene oxides as genotoxic metabolites. Two K-region epoxides of BghiP, 3,4-epoxy-3,4-dihydro-BghiP (3,4-oxide) and 3,4,11,12-bisepoxy-3,4,11,12-tetrahydro-BghiP (3,4,11,12-bisoxide) identified in microsomal incubations of BghiP are weak bacterial mutagens in strain TA98 of Salmonella typhimurium with 5.5 and 1.5 his+-revertant colonies/nmol, respectively. After microsomal activation of BghiP in the presence of calf thymus DNA three DNA adducts were detected using 32P-postlabeling. The total DNA binding of 2.1 fmol/microg DNA, representing 7 adducts in 10(7) nucleotides, was raised 3.6-fold when mEH was inhibited indicating arene oxides as DNA binding metabolites. Co-chromatography revealed the identity between the main adduct of metabolically activated BghiP and the main adduct of the 3,4-oxide. DNA adducts of BghiP originating from the 3,4,11,12-bisoxide were not found. Therefore, a K-region epoxide is proposed to be responsible for the genotoxicity of BghiP and possibly of other PAHs without a "classic" bay-region.

Gerry Brooks and epoxide hydrolases: four decades to a pharmaceutical

The pioneering work of Gerry Brooks on cyclodiene insecticides led to the discovery of a class of enzymes known as epoxide hydrolases. The results from four decades of work confirm Brooks' first observations that the microsomal epoxide hydrolase is important in foreign compound metabolism. Brooks and associates went on to be the first to carry out a systematic study of the inhibition of this enzyme. A second role for this enzyme family was in the degradation of insect juvenile hormone (JH). JH epoxide hydrolases have now been cloned and expressed from several species, and there is interest in developing inhibitors for them. Interestingly, the distantly related mammalian soluble epoxide hydrolase has emerged as a promising pharmacological target for treating hypertension, inflammatory disease and pain. Tight-binding transition-state inhibitors were developed with good ADME (absorption, distribution, metabolism and excretion). These compounds stabilize endogenous epoxides of fatty acids, including arachidonic acid, which have profound therapeutic effects. Now EHs from microorganisms and plants are used in green chemistry. From his seminal work, Dr Brooks opened the field of epoxide hydrolase research in many directions including xenobiotic metabolism, insect physiology and human health, as well as asymmetric organic synthesis.

Influence of foreign compounds on formation and disposition of reactive metabolites

Many toxic compounds are unreactive and need biotransformation in order to exert their toxic effects. Several enzymes control the formation or disposition of reactive metabolites. Especially well studied is the group of enzymes responsible for the control of reactive epoxides. Such epoxides may bind spontaneously to DNA, RNA and protein. These alterations of critical cellular macromolecules may disturb the normal biochemistry of the cell and lead to cytotoxic, allergenic, mutagenic and carcinogenic effects. Whether these effects will be manifested depends on the chemical reactivity as well as on other properties (geometry, lipophilicity) of the epoxide in question. Enzymes controlling the concentration of epoxides are another important contributing factor. Several microsomal monooxygenases exist. Some monooxygenases preferentially attack large substrates at single sites, specific for each enzyme. Some of these steps produce reactive metabolites; others are detoxification pathways. Enzymes that metabolize the epoxides represent a further determining factor. These enzymes include epoxide hydrolase (EC 3.3.2.3) and glutathione transferases (EC 2.5.1.18), which do not play a purely inactivating role but can, in some cases, act also as coactivating enzymes. Some of these enzymes have been shown to be influenced by foreign compounds. Acute effects by activation and inhibition of the enzymes as well as long-term effects by induction and repression have been observed. Since different foreign compounds differentially influence various enzymes, they can produce changes not only in overall metabolic activity but also changes in metabolite pattern and in selective toxicities.

Unusual regioselectivity and active site topology of human cytochrome P450 2J2

The oxidation of six derivatives of terfenadone by recombinant human CYP2J2 (CYP = cytochrome P450) was studied by high-performance liquid chromatography coupled to mass spectrometry (MS) using tandem MS techniques and by 1H NMR spectroscopy. CYP2J2 exhibited a surprising regioselectivity in favor of the hydroxylation of the substrate terminal chain at the weakly reactive homobenzylic position. In contrast, hydroxylation of the same substrates by CYP3A4 mainly occurred on the most chemically reactive sites of the substrates (N-oxidation and benzylic hydroxylation). A 3D homology model of CYP2J2 was constructed using recently published structures of CYP2A6, CYP2B4, CYP2C8, CYP2C9, and CYP2D6 as templates. In contrast with other CYP2 structures, it revealed an active site cavity with a severely restricted access of substrates to the heme through a narrow hydrophobic channel. Dynamic docking of terfenadone derivatives in the CYP2J2 active site allowed one to interpret the unexpected regioselectivity of the hydroxylation of these substrates by CYP2J2, which is mainly based on this restricted access to the iron. The structural features that have been found to be important for recognition of substrates or inhibitors by CYP2J2 were also interpreted on the basis of CYP2J2-substrate interactions in this model.

Molecular modelling of human microsomal epoxide hydrolase (EH) by homology with a fungal (Aspergillus niger) EH crystal structure of 1.8 A resolution: structure-activity relationships in epoxides inhibiting EH activity

Homology modelling of the human microsomal epoxide hydrolase (EH) enzyme based on the fungal (Aspergillus niger) EH crystallographic template is reported. The active site lies in a well-defined, essentially hydrophobic, pocket within the overall enzyme structure. Two tyrosine residues, that are conserved in all known mammalian EH sequences, are able to form hydrogen bonds (one per tyrosine residue) with the epoxide oxygen atom on the known EH substrate, styrene oxide. There is also a small hydrophobic cleft, within the active site region, where the phenyl group of styrene oxide can bind, but this appears to be restricted such that the presence of bulky side-chains will render poor substrate status to the incoming epoxide molecule. Quantitative structure-activity relationship (QSAR) studies on a series of low molecular weight epoxides provide useful results which appear to be generally consistent with the human microsomal EH model, and thus may be used predictively for assessing the EH substrate and/or inhibitor status of untested compounds.

Kinetics of propylene oxide metabolism in microsomes and cytosol of different organs from mouse, rat, and humans

Kinetics of the metabolic inactivation of 1,2-epoxypropane (propylene oxide; PO) catalyzed by glutathione S-transferase (GST) and by epoxide hydrolase (EH) were investigated at 37 degrees C in cytosol and microsomes of liver and lung of B6C3F1 mice, F344 rats, and humans and of respiratory and olfactory nasal mucosa of F344 rats. In all of these tissues, GST and EH activities were detected. GST activity for PO was found in cytosolic fractions exclusively. EH activity for PO could be determined only in microsomes, with the exception of human livers where some cytosolic activity also occurred, representing 1-3% of the corresponding GST activity. For GST, the ratio of the maximum metabolic rate (V(max)) to the apparent Michaelis constant (K(m)) could be quantified for all tissues. In liver and lung, these ratios ranged from 12 (human liver) to 106 microl/min/mg protein (mouse lung). Corresponding values for EH ranged from 4.4 (mouse liver) to 46 (human lung). The lowest V(max) value for EH was found in mouse lung (7.1 nmol/min/mg protein); the highest was found in human liver (80 nmol/min/mg protein). K(m) values for EH-mediated PO hydrolysis in liver and lung ranged from 0.83 (human lung) to 3.7 mmol/L (mouse liver). With respect to liver and lung, the highest V(max)/K(m) ratios were obtained for GST in mouse and for EH in human tissues. GST activities were higher in lung than in liver of mouse and human and were alike in both rat tissues. Species-specific EH activities in lung were similar to those in liver. In rat nasal mucosa, GST and EH activities were much higher than in rat liver.

Gas chromatography-electron capture determination of styrene-7,8-oxide enantiomers

The enantiomers of styrene-7,8-oxide (phenyloxirane, SO) were determined using a method based on base catalysed hydrolysis with sodium methoxide. The oxirane ring opening resulted in formation, without racemisation, of the enantiomeric pairs of the two regional isomers, 2-methoxy-1-phenylethanol and 2-methoxy-2-phenylethanol. The structure of these regional isomers was confirmed by gas chromatography-mass spectrometry (GC-MS) and proton nuclear magnetic resonance (1H-NMR). To improve sensitivity of determination, the formed methoxy alcohols were subsequently derivatised with pentafluoropropionic anhydride enabling electron capture detection. This derivatization proceeded also without racemisation and the formed pentafluoropropionyl derivatives were separated on two serially coupled columns, a non-chiral AT 1705 and a chiral CP Chirasil-Dex-CB. As internal standard 2S,3S-(-)-2-methyl-3-phenyloxirane was used. The limit of quantitation of the method was 0.2 microM. The repeatability of the method was assessed at two concentration levels (2.5 and 25 microM) and ranged from 6 to 9% for both enantiomers. The method was applied to the determination of the rate and enantioselectivity of the cytochrome P-450 dependent oxidation of styrene to SO enantiomers in human liver microsomes.

Metabolic activation of the proestrogens trans-stilbene and trans-stilbene oxide by rat liver microsomes

A liver microsome-mediated activation of the proestrogens trans-stilbene and trans-stilbene oxide was found in this study. trans-Stilbene and trans-stilbene oxide were negative in estrogen reporter assay using estrogen-responsive human breast cancer cell line MCF-7 and growth assay in rat pituitary tumor cell line MtT/E-2. However, these compounds exhibited estrogenic activity after incubation with liver microsomes of 3-methylcholanthrene-treated rats in the presence of NADPH. In contrast, cis-stilbene and cis-stilbene oxide did not show estrogenic activity after such incubation. When trans-stilbene was incubated with the liver microsomes of 3-methylcholanthrene-treated rats in the presence of NADPH, two metabolites were detected by HPLC. They were identified unequivocally as trans-4-hydroxystilbene and trans-4, 4'-dihydroxystilbene by mass and UV spectral comparison with authentic samples. The oxidase activity of the liver microsomes toward trans-stilbene was inhibited by SKF 525-A and alpha -naphthoflavone. Minor activity was observed when liver microsomes of untreated or phenobarbital-treated rats were used instead of those from 3-methylcholanthrene-treated rats. trans-4-Hydroxystilbene and trans-4,4'-dihydroxystilbene exhibited significant estrogenic activities. These results suggest that the estrogenic activities of trans-stilbene and trans-stilbene oxide were due to formation of hydroxylated metabolites.

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

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

Visualization of a covalent intermediate between microsomal epoxide hydrolase, but not cholesterol epoxide hydrolase, and their substrates

Mammalian soluble and microsomal epoxide hydrolases have been proposed to belong to the family of alpha/beta-hydrolase-fold enzymes. These enzymes hydrolyse their substrates by a catalytic triad, with the first step of the enzymatic reaction being the formation of a covalent enzyme-substrate ester. In the present paper, we describe the direct visualization of the ester formation between rat microsomal epoxide hydrolase and its substrate. Microsomal epoxide hydrolase was precipitated with acetone after brief incubation with [1-(14)C]epoxystearic acid. After denaturing SDS gel electrophoresis the protein-bound radioactivity was detected by fluorography. Pure epoxide hydrolase and crude microsomes showed a single radioactive signal of the expected molecular mass that could be suppressed by inclusion of the competitive inhibitor 1,1,1-trichloropropene oxide in the incubation mixture. In a similar manner, 4-fluorochalcone-oxide-sensitive binding of epoxystearic acid to rat soluble epoxide hydrolase could be demonstrated in rat liver cytosol. Under similar conditions, no covalent binding of [26-(14)C]cholesterol-5alpha,6alpha-epoxide to microsomal proteins or solubilized fractions tenfold enriched in cholesterol epoxide hydrolase activity could be observed. Our data provide definitive proof for the formation of an enzyme-substrate-ester intermediate formed in the course of epoxide hydrolysis by microsomal epoxide hydrolase, show no formation of a covalent intermediate between cholesterol epoxide hydrolase and its substrate under the same conditions as those under which an intermediate was shown for both microsomal and soluble epoxide hydrolases and therefore indicate that the cholesterol epoxide hydrolase apparently does not act by a similar mechanism and is probably not structurally related to microsomal and soluble epoxide hydrolases.

Preferential use of less toxic detoxification pathways by long-lived species

This study was undertaken to determine whether the detoxification pathways producing the least oxidative stress appear to be favored in longer-lived mammalian species. Firstly, we focused on the cytochrome P-450 monooxygenase system. Although this system is an important component of the defenses that protect living organisms against toxic chemicals, some reactions catalyzed by the cytochrome P-450 system result in the formation of products that are highly reactive as well as active oxygen species. Our results suggest that the lower amount of hepatic cytochrome P-450 content found in longer-lived species may have evolved to reduce the toxic side-effect of this detoxification system. Support of the idea that the cytochrome P-450 system is an important source of oxidative stress is the positive correlation between cytochrome P-450 content and the amount of oxidized proteins found in liver of different human individuals. Secondly, we have measured the specific activity of other detoxification enzymes as a function of life span. Instead of a direct comparison of detoxification capabilities of the species, the approach used in this study was: (1) to select those detoxification enzymes which utilize the same substrate but differ in toxicity of the intermediate compounds formed in the reaction, and (2) to measure the levels of these enzymes in the two pathways to determine which pathway is dominant for each species. Our results suggest that the detoxification pathways producing the least oxidative stress do appear to be favored in longer-lived species.

Localization of drug-metabolizing enzyme activities to blood-brain interfaces and circumventricular organs

The brain, with the exception of the choroid plexuses and circumventricular organs, is partially protected from the invasion of blood-borne chemicals by the specific morphological properties of the cerebral micro-vessels, namely, the tight junctions of the blood-brain barrier. Recently, several enzymes that are primarily involved in hepatic drug metabolism have been shown to exist in the brain, albeit at relatively low specific activities. In the present study, the hypothesis that these enzymes are located primarily at blood-brain interfaces, where they form an "enzymatic barrier," is tested. By using microdissection techniques or a gradient-centrifugation isolation procedure, the activities of seven drug-metabolizing enzymes in isolated microvessels, choroid plexuses, meningeal membranes, and tissue from three circumventricular organs (the neural lobe of the hypophysis, pineal gland, and median eminence) were assayed. With two exceptions, the activities of these enzymes were higher in the three circumventricular organs and cerebral microvessel than in the cortex. Very high membrane-bound epoxide hydrolase and UDP-glucuronosyltransferase activities (approaching those in liver) and somewhat high 7-benzoxyresorufin-O-dealkylase and NADPH-cytochrome P-450 reductase activities were determined in the choroid plexuses. The pia-arachnoid membranes, but not the dura matter, displayed drug-metabolizing enzyme activities, notably that of epoxide hydrolase. The drug-metabolizing enzymes located at these nonparenchymal sites may function to protect brain tissue from harmful compounds.

Subcellular localization of cytochrome P450, and activities of several enzymes responsible for drug metabolism in the human brain

We studied the subcellular distribution of cytochrome P450 and related monooxygenase activities in six regions of human brains removed at autopsy. The content of total cytochrome P450 was found to be at least nine times higher in the mitochondrial fraction than in the microsomes in all the regions studied. However, cytochrome P450-dependent enzymatic activities which are representative of different isoforms metabolizing exogenous molecules exhibited a microsomal prevalence, a situation previously observed in rat brain. The other drug-metabolizing enzymes catalysing functionalization and conjugation reactions, presented the following characteristics in human brain: (i) a low activity of NADPH-cytochrome P450 reductase, which also catalyses the reduction of some xenobiotics; (ii) a high specific activity of the membrane-bound epoxide hydrolase; (iii) among the enzymes catalysing conjugation reactions, 1-naphthol-UDP-glucuronosyltransferase activity was barely or not detectable, whereas the mean glutathione-S-transferase activity was 15 times higher than the activity measured in rat brain. The presence of several drug-metabolizing enzyme activities in human brain microvessels, and particularly the high activity of epoxide hydrolase, suggests a participation of these enzymes in the metabolic blood-brain barrier.

Biphenyl and fluorinated derivatives: liver enzyme-mediated mutagenicity detected in Salmonella typhimurium and Chinese hamster V79 cells

Hepatocarcinogenic polychlorinated and polybrominated biphenyls usually show negative results in in vitro mutagenicity assays. Problems in their testing result from their low water solubility and their slow rate of metabolism. We therefore investigated better soluble model compounds, namely biphenyl and its 3 possible monofluorinated derivatives. In the direct test, these compounds proved to be nonmutagenic in Salmonella typhimurium TA98 and TA100 (reversion to histidine prototrophy) and in Chinese hamster V79 cells (acquisition of resistance to 6-thioguanine). However, when the exposure was carried out in the presence of NADPH-fortified postmitochondrial fraction of liver homogenate from Aroclor 1254-treated rats, all 4 compounds showed mutagenic activity in V79 cells. 3-Fluorobiphenyl produced strong mutagenic effects in S. typhimurium TA100 as well, whereas the other biphenyls were inactive. In strain TA98, 3- and 4-fluorobiphenyl showed mutagenic activity. This mutagenicity was enhanced in the presence of 1,1,1-trichloropropene 2,3-oxide, an inhibitor of microsomal epoxide hydrolase, thus suggesting that epoxides may be active metabolites.

Interaction of valproic acid and some analogues with microsomal epoxide hydrolase

Valproic acid (VPA) and its analogues valpromide (VPM), valproyl-Coenzyme A (VP-CoA) and valproyl-ethylester (VPE) were examined as potential inhibitors of microsomal epoxide hydrolase (mEHb) using styrene-7,8-oxide (STO) and benzo(a)pyrene-4,5-oxide (BPO) as enzyme substrates. The effect of each potential inhibitor was examined using mEHb from rat liver, human livers (from a child, woman and man) and from human placenta. Of the compounds tested, only VPM (2 mM) expressed significant inhibition of mEHb activity with a maximum inhibition of 49%, 48%, 35% and 33% for liver microsomes from the child, woman, man and rat, respectively, using STO (2 mM) as substrate. Human placenta mEHb was inhibited 59% under the same conditions. The inhibition was found to be competitive, with closely related KI values of 0.11, 0.16, 0.28, 0.27 and 0.31 mM for mEHb obtained from rat liver, human placenta, child, female and male liver, respectively. VPA demonstrated only a slight inhibition (maximum 16%) of mEHb at high concentrations (10 mM), and VP-CoA was found to activate STO hydrolysis slightly at concentrations between 1 and 5 mM. VPE caused a moderate concentration-dependent activation of mEHb in all microsomal preparations examined. The inhibitory or activating properties of each compound were independent of the substrate and influenced slightly by the pH used in the incubation medium. The lack of inhibition of mEHb by VPA and its analogues other than VPM shows that neither masking of the carboxyl function of VPA nor the introduction of higher lipophilicity are sufficient to account for the inhibitory properties of VPM for mEHb. A molecular mechanism for the inhibition of mEHb by VPM is discussed.

Enantio- and regioselectivity in the epoxide-hydrolase-catalyzed ring opening of aliphatic oxiranes: Part II: Dialkyl- and trialkylsubstituted oxiranes

The extent of substrate enantioselectivity and regioselectivity of a series of aliphatic 2,3-dialkyl- and trialkylsubstituted oxiranes in their in vitro epoxide-hydrolase-catalyzed hydrolysis depends on the size of the alkyl residues and on the substitution pattern of the oxirane ring. The enzyme-catalyzed hydrolysis of cis-oxiranes, containing at least one methyl substituent, shows complete or nearly complete substrate enantioselectivity and regioselectivity with nucleophilic attack by water occurring with inversion of configuration at the methylsubstituted ring carbon atom of (S)-configuration. In the hydrolysis of the isomeric trans-oxiranes, both enantiomers are metabolized with a higher rate for the (2S;3S)-enantiomer. The conversion of trimethyloxirane occurs with high substrate enantioselectivity in favor of the (S)-enantiomer and with complete regioselectivity at the monomethylsubstituted ring carbon atom. The differentiation of the enantiotopic ring carbon atoms (product enantioselectivity) in the smallest aliphatic meso-oxirane, cis-2,3-dimethyloxirane, leads to (2R;3R)-butane-2,3-diol with ee = 86%. cis-2-Ethyl-3-propyloxirane, possessing alkyl residues larger than methyl, represents an extremely poor substrate in the epoxide-hydrolase-catalyzed hydrolysis process.

Enantio- and regioselectivity in the epoxide-hydrolase-catalyzed ring opening of simple aliphatic oxiranes: Part I: Monoalkylsubstituted oxiranes

The in vitro conversion of chiral aliphatic monoalkylsubstituted oxiranes into 1,2-diols catalyzed by epoxide hydrolase of rat liver microsomes occurs with substrate enantioselectivity and regioselectivity. Substrate enantioselectivity is generally low, and has the same sense, for methyloxirane, vinyloxirane, epichloro-, and epibromohydrin. In the hydrolysis of t-butyloxirane inhibitory effects are involved leading to a complex pattern of enantioselectivity. All investigated monosubstituted aliphatic oxiranes are hydrolyzed with high regioselectivity by nucleophilic attack of water at the unsubstituted ring carbon atom. The enantiomeric excess of the unreacted oxirane substrates and the diol metabolites formed were determined by complexation and inclusion gas chromatography.

Stable expression of cytochrome P450IIIA7 cDNA in human breast cancer cell line MCF-7 and its application to cytotoxicity testing

A mammalian cell expression plasmid containing cytochrome P450IIIA7 complementary DNA was constructed. Breast cancer cells (MCF-7) were transfected with the plasmid and neomycin-resistant selection marker plasmid. We established three cell lines, termed M13, M21, and M27, which expressed the cytochrome P450IIIA7 as examined by RNA blot and immunoblot analyses. These cell lines showed 8- to 10-fold higher sensitivity against aflatoxin B1 compared to parental MCF-7 cells, suggesting that cytochromes P450IIIA7 expressed in the cells were responsible for the production of the cytotoxic metabolite of aflatoxin B1.

Oxidative conversion of 6-fluorobenzo(c)phenanthrene to its K-region oxide by liver microsomes from 3-methylcholanthrene treated rats: reversal of stereoselectivity of cytochrome P-450c due to the influence of fluoro group

We have shown earlier that metabolism of carcinogenic 6-fluorobenzo(c)-phenanthrene by liver microsomes of 3-methylcholanthrene treated rats generate K-region oxide as the major metabolite, while no K-region oxide survives in benzo(c)-phenanthrene metabolism under identical conditions. To understand the influence of fluoro group on the generation of K-region oxide from this hydrocarbon, we have determined the enantiomeric composition and absolute configuration of the metabolic 6-fluorobenzo(c)phen-anthrene-7,8-oxide. Interestingly, the microsomal cytochrome P-450c forms predominantly the 5R,6S enantiomer from B(c)Ph, while it exhibits a reversal of stereoselectivity with 6-fluorobenzo(c)phenanthrene forming predominantly the 7S,8R enantiomer. We have attributed this observation to an unfavourable interaction of the fluoro group with the hydrophobic binding pocket of the isozyme.

Metabolic activation of 1,2,4-trichlorobenzene and pentachlorobenzene by rat liver microsomes: a major role for quinone metabolites

Microsomal metabolism of 1,2,4-[14C]trichlorobenzene (1,2,4-TrCB) and [14C]pentachlorobenzene (PeCB) was studied with special emphasis on the conversion-dependent covalent binding to protein and DNA. 1,2,4-TrCB was metabolized to 2,3,6- and 2,4,5-trichlorophenol, and to a lesser extent to 2,4,6- and 2,3,5-trichlorophenol, and trichlorohydroquinone. About 10% of all metabolites became covalently bound to protein in a rather nonselective way. For 1,2,4-TrCB and PeCB a strong correlation between secondary metabolism to hydroquinones and covalent binding was established. Protein binding was completely inhibited by the addition of ascorbic acid, indicating quinone metabolites as the sole reactive species formed. Both 1,2,4-TrCB and PeCB alkylated DNA, although to a much lesser extent than protein (0.5 and 0.3% of all metabolites, respectively). Nonquinone intermediates, presumably epoxides, were responsible for a minor portion of the observed DNA binding, since complete inhibition by ascorbic acid was not reached. The differential role of cytochrome P450 both in primary and in secondary metabolism was demonstrated by the use of microsomes from rats pretreated with different inducers. Dexamethasone (DEX) microsomes (cytochrome P450IIIA1) showed the highest activity toward these chlorinated benzenes (14 nmol/mg/5 min for 1,2,4-TrCB and 36 nmol/mg/10 min for PeCB, both with regard to the formation of phenols and to the formation of protein-bound metabolites. In addition, DEX microsomes preferentially formed 2,3,6-trichlorophenol, whereas other microsomal suspensions formed 2,4,5-trichlorophenol as the major isomer. The present study clearly demonstrates the high alkylating potency of secondary quinone metabolites derived from chlorinated benzenes and poses a need for reevaluation of the role of epoxides in the observed toxicity of these compounds.

Expression of arylhydrocarbon hydroxylase, epoxide hydrolases, glutathione S-transferase and UDP-glucuronosyltransferases in H5-6 hepatoma cells

1. The presence of arylhydrocarbon hydroxylase (cytochrome P-450 IA1 dependent), glutathione S-transferase, two distinct forms of epoxide hydrolases and UDP-glucuronosyltransferases was detected in H5-6 hepatoma cell homogenates using model substrates, selective inhibitors and specific antibodies. 2. The activity of arylhydrocarbon hydroxylase decreased strongly at the first days after plating and remained at a minimal value (1.5 pmol/min per mg) after 5 days of culture. 3. The hydratation of trans-stilbene oxide catalyzed by the soluble form of epoxide hydrolase was very low (11.0 pmol/min per mg), whereas the hepatoma cells contained appreciable amounts of the membrane-bound epoxide hydrolase and glutathione S-transferase measured with cis-stilbene oxide as substrate (maximal specific activity: 1.46 and 2.73 nmol/min per mg, respectively). 4. These cells also glucuronidated 1-naphthol efficiently (6 nmol/min per mg) and, at a lower extent, bilirubin (12 pmol/min per mg). 5. Addition of fenofibrate (70 microM) into the culture medium for 1-3 days failed to significantly stimulate the activity of cytosolic epoxide hydrolase. Only bilirubin glucuronidation increased 2-fold after 2 days of presence of the drug.

Metabolism of 3-hydroxychrysene by rat liver microsomal preparations

3-Hydroxychrysene, a metabolite of the polycyclic aromatic hydrocarbon (PAH) chrysene, was metabolised by rat liver microsomal preparations obtained from Arochlor 1254-pretreated rats. Eight major metabolites were isolated by high performance liquid chromatography and characterised by u.v. spectroscopy and a variety of mass spectrometric techniques. The metabolites were unambiguously identified as 9-hydroxy-trans-1,2-dihydroxy-1,2-dihydrochrysene and 9-hydroxy-r-1,t-2,t-3,c-4-tetrahydroxy-1,2,3,4-tetrahydrochrysene and tentatively identified as 3-hydroxy-trans-5,6-dihydroxy-5,6-dihydrochrysene (since chrysene is a symmetrical molecule the 3- and 9-positions are equivalent), 9-hydroxy-trans-3,4-dihydroxy-3,4-dihydrochrysene, 1,2,3-trihydroxy-1,2,3,4-tetrahydrochrysene, an oxidised phenol and two diphenols. These results indicate that 3-hydroxychrysene can be further metabolised via a number of different pathways including those involving the formation of phenol- and triol-epoxides.

Activation of 8-methoxypsoralen by cytochrome P-450. Enzyme kinetics of covalent binding and influence of inhibitors and inducers of drug metabolism

The kinetics of covalent binding of reactive metabolites of 8-methoxypsoralen (8-MOP) to protein were measured in incubations of liver microsomes of rats pretreated for 3 days with i.p. injections of 80 mg/kg/day of beta-naphthoflavone (BNF), phenobarbital (PB), 8-MOP, or vehicle. Covalent binding of radioactivity derived from [14C]8-MOP (labeled at the metabolically stable 4-position in the coumarin ring) required NADPH, obeyed classical Michaelis-Menten kinetics, and was inducible by both PB and BNF. Plots of V versus V/[S] were linear in liver microsomes of rats pretreated with vehicle, PB, or 8-MOP; respective values for Km were 26, 24 and 13 microM and for Vmax were 0.61, 1.70 and 0.50 nmol bound/min/mg protein. In microsomes of rats pretreated with BNF, high- and low-affinity components of covalent binding were observed with respective values for Km of 4.7 and 117 microM and for Vmax of 0.77 and 1.71 nmol bound/min/mg protein. Addition of glutathione and cysteine to the incubations decreased covalent binding by 33 and 67%, respectively, presumably by trapping reactive electrophilic metabolites. Inhibition of epoxide hydrolase with 1,1,1-trichloropropene-2,3-oxide did not affect covalent binding of reactive metabolites of 8-MOP. SKF-525A was a potent inhibitor of both the metabolism of 8-MOP and covalent binding in microsomes from rats pretreated with PB, but had only a slight effect in microsomes from rats pretreated with BNF. In contrast, alpha-naphthoflavone almost completely inhibited metabolism of 8-MOP and covalent binding in BNF-induced microsomes but had no effect in PB-induced microsomes. Apparent covalent binding was reduced by 39% in incubations with 8-MOP labeled with tritium in the metabolically labile methoxy group. Collectively, these results indicate that 8-MOP is biotransformed by two or more isozymes of cytochrome P-450 to reactive electrophiles capable of binding to tissue macromolecules.

The low reactivity of 5H-dibenzo[a, d]cycloheptene 10,11-oxide in microsomal epoxide hydrolase catalysed hydration

1. The rate of hydration of 5H-dibenzo[a, d]cycloheptene 10,11-oxide catalysed by rabbit liver microsomal epoxide hydrolase has been compared with that of the acyclic analogue cis-stilbene oxide. The latter was shown to be a much better substrate for the enzyme than the former epoxide. 2. The kinetic parameters for the hydration of 5H-dibenzo[a,d]cycloheptene 10,11-oxide and cis-stilbene oxide have been determined using a rabbit liver microsomal preparation at 37 degrees C and pH 7.4. The much slower hydration of the former is due to a much lower Vs and not to a higher Km relative to the latter epoxide. 3. 5H-Dibenzo[a,d]cycloheptene 10,11-oxide inhibited the hydration of cis-stilbene oxide by rabbit liver microsomal epoxide hydrolase.

Cytochrome P-450-catalyzed asymmetric epoxidation of simple prochiral and chiral aliphatic alkenes: species dependence and effect of enzyme induction on enantioselective oxirane formation

The enantioselectivity of the in vitro conversion of simple prochiral and chiral aliphatic alkenes into oxiranes by liver microsomes of untreated or induced (phenobarbital) rats, of untreated or induced (phenobarbital, benzo[a] pyrene) mice, and of humans was determined by complexation gas chromatography. The enantiomeric excess (ee) of the epoxides extends from 0 (trimethyloxirane) to 50% (ethyloxirane). The configuration (R or S) of the enantiomers formed in excess is consistent for homologous oxiranes but is species dependent and in some cases influenced by enzyme induction. Enantioselectivity differences of aliphatic alkene epoxidation by human liver microsomes of four individuals are negligible.

Antimutagenesis by shift in monooxygenase isoenzymes and induction of epoxide hydrolase

The widely occurring aromatic and olefinic structural elements can be transformed into epoxides by microsomal monooxygenases. These epoxides may react with nucleophilic centers in the cell and thereby covalently bind to DNA, RNA and protein. Such a reaction may lead to cytotoxicity, allergy, mutagenicity and/or carcinogenicity, depending on the properties of the epoxide in question. An important contributing factor is the presence and relative activity of enzymes controlling the concentration of such epoxides. There are several microsomal monooxygenases which differ in activity and substrate specificity. On individual substrates individual cytochromes P-450 often preferentially attack at one specific site different from that attacked by others. Some of these pathways lead to reactive products, others are detoxification pathways. Also important are the enzymes which metabolize epoxides, such as epoxide hydrolases and glutathione transferases. Such enzymes can act as inactivating and in some specific cases also as co-activating enzymes. Moreover, precursor-sequestering enzymes such as dihydrodiol dehydrogenase, glucuronosyl transferases and sulfotransferases are important for the control of reactive epoxides. These enzymes themselves are subject to control by many endogenous and exogenous factors. By virtue of their contribution to the control of mutagenic metabolites such modulators can exert antimutagenic activity. An especially interesting antimutagen, whose mechanism of antimutagenic action is modulation of mutagen-metabolizing enzymes, is trans-stilbene oxide. This agent selectively induces the synthesis of some specific cytochrome P-450 isoenzymes at the expense of others, so that the metabolism of benzo[a]pyrene is shifted from the route leading to the highly mutagenic 7,8-dihydrodiol 9,10-epoxides to the route leading to the much less mutagenic 4,5-epoxide. Moreover, the same agent potently induces microsomal epoxide hydrolase which inactivates the latter epoxide. The combined effects lead to a drastic antimutagenic effect, the molecular mechanism of which is given by these changes in mutagen-metabolizing enzymes.

An in vitro study of the microsomal metabolism and cellular toxicity of phenytoin, sorbinil and mianserin

1. The cytotoxicity of metabolites generated from phenytoin, sorbinil and mianserin by human and mouse liver microsomes was assessed by co-incubation with human mononuclear leucocytes as target cells. Cytotoxicity was determined by trypan blue dye exclusion. 2. Phenytoin and sorbinil were metabolised by NADPH-dependent murine microsomal enzymes to cytotoxic metabolites. Cytotoxicity produced by both drugs was significantly enhanced by the epoxide hydrolase inhibitor trichloropropane oxide (TCPO). No significant cytotoxicity was observed in the presence of human liver microsomes. 3. Mianserin was metabolised by both human and mouse liver microsomes to a cytotoxin. Cytotoxicity was greater in the presence of human liver microsomes (13.7 +/- 2.2%; mean +/- s.d. for four livers, compared with 6.0 +/- 2.4%, mean +/- s.d., n = 4, with mouse liver microsomes), and was unaffected by pretreatment with TCPO. 4. Stable metabolites were quantified by radiometric high performance liquid chromatography. Phenytoin and sorbinil were metabolised to 5-(p-hydroxyphenyl)-5-phenyl-hydantoin (0.3-0.5% of incubated radioactivity) and 2-hydroxysorbinil (0.4-2.7% of incubated radioactivity), respectively, by both human and mouse liver microsomes. 5. Mianserin was metabolised to 8-hydroxymianserin and desmethylmianserin by both human and mouse liver microsomes. Desmethylmianserin was the major product in incubations with human liver microsomes (32.3 +/- 12%, mean +/- s.d. for four livers), whereas 8-hydroxymianserin was the predominant metabolite generated by mouse liver microsomes (25.9 +/- 1.5%, mean +/- s.d., n = 4). 6. Generation of electrophilic metabolites was assessed by determination of the amount of radiolabelled material which became irreversibly bound to protein. Only mouse liver microsomes activated phenytoin to a chemically reactive metabolite, whereas both mouse and human liver microsomes generated reactive metabolites from sorbinil and mianserin. 7. These studies show that drug cytotoxicity can be mediated by low concentrations (circa microM) of metabolites generated by NADPH-dependent hepatic microsomal enzymes; however demonstration of cytotoxicity in vitro has not been established as a means of predicting in vivo toxicity.

Human liver cytosolic epoxide hydrolases

Human liver epoxide hydrolases were characterized by several criteria and a cytosolic cis-stilbene oxide hydrolase (cEHCSO) was purified to apparent homogeneity. Styrene oxide and five phenylmethyloxiranes were tested as substrates for human liver epoxide hydrolases. With microsomes activity was highest with trans-2-methylstyrene oxide, followed by styrene 7,8-oxide, cis-2-methylstyrene oxide, cis-1,2-dimethylstyrene oxide, trans-1,2-dimethylstyrene oxide and 2,2-dimethylstyrene oxide. With cytosol the same order was obtained for the first three substrates, whereas activity with 2,2-dimethylstyrene oxide was higher than with cis-1,2-dimethylstyrene oxide and no hydrolysis occurred with trans-1,2-dimethylstyrene oxide. Generally, activities were lower with cytosol than with microsomes. The isoelectric point for both microsomal styrene 7,8-oxide and cis-stilbene oxide hydrolyzing activity was 7.0, whereas cEHCSO had an isoelectric point of 9.2 and cytosolic trans-stilbene oxide hydrolase (cEHTSO) of 5.7. The cytosolic epoxide hydrolases could be separated by anion-exchange chromatography and gel filtration. The latter technique revealed a higher molecular mass for cEHCSO than for cEHTSO. Both cytosolic epoxide hydrolases showed higher activities at pH 7.4 than at pH 9.0, whereas the opposite was true for microsomal epoxide hydrolase. The effects of ethanol, methanol, tetrahydrofuran, acetonitrile, acetone and dimethylsulfoxide on microsomal epoxide hydrolase depended on the substrate tested, whereas both cytosolic enzymes were not at all, or only slightly, affected by these solvents. Effects of different enzyme modulators on microsomal epoxide hydrolase also depended on the substrates used. Trichloropropene oxide and styrene 7,8-oxide strongly inhibited cEHCSO whereas cEHTSO was moderately affected by these compounds. Immunochemical investigations revealed a close relationship between cEHCSO and rat liver microsomal, but not cytosolic, epoxide hydrolase. Interestingly, cEHTSO has no immunological relationship to rat microsomal, nor to rat cytosolic epoxide hydrolase. cEHTSO from human liver differed also from its counterpart in the rat in that it was only moderately affected by tetrahydrofuran, acetonitrile and trichloropropene oxide. Five steps were necessary to purify cEHCSO. The enzyme has a molecular mass (49 kDa) identical to that of rat liver microsomal epoxide hydrolase.

Computational approaches to the identification of suspect toxic molecules

We have presented computational approaches that can be used for the relatively rapid identification of suspect toxigens, including carcinogens, in two different classes of compounds: (a) halogenated olefins and epoxides, and (b) substituted dibenzo-p-dioxins. A common element in these approaches is the key role played by the molecular electrostatic potential. It is applied in two different ways, however; it is used to assess the reactivity of a specific site in the case of the epoxides, and for the dibenzo-p-dioxins the focus is on the overall pattern of negative regions above the molecular plane. While we are continuing to develop and refine both types of analysis, especially that related to the dibenzo-p-dioxins, the results obtained so far are encouraging, and indicate that these can be regarded as useful screening techniques for identifying compounds that require further and more exhaustive investigation.

1,2-Epoxycycloalkanes: substrates and inhibitors of microsomal and cytosolic epoxide hydrolases in mouse liver

Six different 1,2-epoxycycloalkanes, whose rings were constituted of 5 to 12 carbon atoms, were tested as possible inhibitors of epoxide-metabolizing enzymes and substrates for the microsomal and cytosolic epoxide hydrolases (mEH, cEH) in mouse liver. The geometric configurations and the relative steric hindrances of these epoxides were estimated from their ease of hydrolysis in acidic conditions to the corresponding diols, their abilities to react with nitrobenzylpyridine, and the chemical shifts of the groups associated with the oxirane rings measured by proton and 13C-NMR. The cyclopentene, -hexene, -heptene, -octene and -decene oxides adopted mainly a cis-configuration. By contrast, cyclododecene oxide presented a trans-configuration. Steric hindrance increased with the size of the ring and was particularly strong when cyclooctene, -decene and -dodecene oxides were considered. With the exception of cyclohexene oxide, all the compounds were weak inhibitors of EH and glutathione S-transferase (GST) activities. Cyclohexene oxide exhibited a selective inhibition of the mEH with an I50 of 4.0.10(-6) M. As the size of the ring increased, inhibitory potency was gradually lost. The cEH and the GST activities were less sensitive to the inhibitory effects of these epoxides (I50, 1 mM or above). A marked difference between the substrate selectivities of mEH and cEH for these epoxides was observed. The mEH hydrated all of the cyclic epoxides, although some of them at a very low rate; the best substrate was the cycloheptene oxide (2.3 nmol/min/mg protein). On the other hand, cyclodecene oxide was a substrate of cEH, but no diol formation was detected when cyclopentene, -hexene and -dodecene oxides were incubated with cytosolic enzyme.

Glutathione conjugation and bacterial mutagenicity of racemic and enantiomerically pure cis- and trans-methyl epoxycinnamates

This paper describes the ability of racemic, and enantiomerically pure cis- and trans-methyl epoxycinnamates (methyl 3-phenyl-2,3-epoxy-propanoates) to undergo glutathione conjugation and subsequent excretion as mercapturic acid and on the mutagenicities of these epoxy esters in the Ames assay. In incubation mixtures containing rat liver cytosol (9,000 g), the decrease of glutathione due to the epoxy esters occurred enzymatically. The highest glutathione depletion was found for the cis-epoxy cinnamic esters. Adult male rats administered a single i.p. dose of racemic trans- and cis-epoxy cinnamates (0.7 mmol/kg, n = 4) excreted thioethers in urine. Higher urinary thioether excretion was found after the cis-epoxy ester dosing. The structures of the thioether metabolites isolated from the urinary extracts were identified by TLC and confirmed by synthesis and mass spectrometry (FAB+). The thioethers appeared to be hydroxy mercapturic acids. The N-alkylating potential of the racemic epoxy esters was determined using 4-(p-nitrobenzyl)pyridine (= NBP). The trans-epoxy ester appeared to react much better with NBP than the cis-compound. Mutagenic effects of racemic trans-epoxy cinnamate as well as the enantiomerically pure trans-epoxy cinnamates were observed in the Ames test with S. typhimurium strains TA1535, TA1537, TA1538 and TA100 without metabolic activation. No mutagenic responses were detected using any of the epoxy cinnamates with S9 activation. By comparing the mutagenicity and the enzymatically catalyzed glutathione conjugation it follows that the activity of the respective enantiomeric methyl cinnamates goes in the opposite order. Glutathione conjugation plays a protective role in the detoxication in living organism of the potentially toxic methyl epoxy cinnamates.

Microsomal metabolism of picene

Picene, a polycyclic aromatic hydrocarbon (PAH) of environmental relevance has recently been predicted to be carcinogenic, based on quantum mechanical calculation, although in several animal studies no carcinogenicity could be detected. In order to find out if the metabolism of this PAH can provide an explanation for its lack of carcinogenicity, picene was incubated with the hepatic microsomal fraction of Sprague-Dawley rats, which had been pretreated with Aroclor 1254. Sixteen ethyl acetate-extractable metabolites could be separated by reversed-phase high-performance liquid chromatography. Comparison of the chromatographic behavior and the UV and mass spectral properties of the metabolites with those of synthetic derivatives of picene allowed the identification of trans-1,2-, -3,4-, -5,6-dihydrodiol as well as 2- and 4-phenol as microsomal metabolites of picene. At a substrate concentration of 2.7 microM and an amount of 68 micrograms microsomal protein per ml incubation volume, 4-picenol was the main microsomal metabolite with 32.2% of total metabolic conversion, followed by the 1,2-(bay-region)dihydrodiol with 16.7%, the 3,4-(M-region)dihydrodiol with 15.9%, 2-picenol with 9.1% and the 5,6-(K-region)dihydrodiol with 1.6%. In this respect the metabolism of picene is not significantly different from that of the carcinogenic PAH benzo[a]pyrene and dibenz[a,h]anthracene. The M-region dihydrodiols, potential precursors of electrophilically reactive dihydrodiol bay-region epoxides, are formed from all three PAHs at 11-16% of total metabolic conversion. From the 2.8- to 4.4-fold lower amounts of polar and water-soluble metabolites of picene as compared to dibenz[a,h]anthracene and benzo[a]pyrene it is deduced that dihydrodiol epoxides are generated from picene to a much smaller extent than from the two carcinogenic PAHs. The lacking carcinogenicity of picene could therefore result from the inability of microsomal enzymes to transform its M-region dihydrodiol to dihydrodiol bay-region epoxides in amounts necessary to initiate carcinogenesis.

Cytosolic epoxide hydrolase

Epoxide hydrolase activity is recovered in the high-speed supernatant fraction from the liver of all mammals so far examined, including man. For some as yet unexplained reason, the rat has a very low level of this activity, so that cytosolic epoxide hydrolase is generally studied in mice. This enzyme selectively hydrolyzes trans epoxides, thereby complementing the activity of microsomal epoxide hydrolase, for which cis epoxides are better substrates. Cytosolic epoxide hydrolase has been purified to homogeneity from the livers of mice, rabbits and humans. Certain of the physicochemical and enzymatic properties of the mouse enzyme have been thoroughly characterized. Neither the primary amino acid, cDNA nor gene sequences for this protein are yet known, but such characterization is presently in progress. Unlike microsomal epoxide hydrolase and most other enzymes involved in xenobiotic metabolism, cytosolic epoxide hydrolase is not induced by treatment of rodents with substances such as phenobarbital, 2-acetylaminofluorene, trans-stilbene oxide, or butylated hydroxyanisole. The only xenobiotics presently known to induce cytosolic epoxide hydrolase are substances which also cause peroxisome proliferation, e.g., clofibrate, nafenopin and phthalate esters. These and other observations indicate that this enzyme may actually be localized in peroxisomes in vivo and is recovered in the high-speed supernatant because of fragmentation of these fragile organelles during homogenization, i.e., recovery of this enzyme in the cytosolic fraction is an artefact. The functional significance of cytosolic epoxide hydrolase is still largely unknown. In addition to deactivating xenobiotic epoxides to which the organism is exposed directly or which are produced during xenobiotic metabolism, primarily by the cytochrome P-450 system, this enzyme may be involved in cellular defenses against oxidative stress.