Microsomal and cytosolic epoxide hydrolases in rhesus monkey liver, and in normal and neoplastic human liver

Clinical relevance of increased serum preneoplastic antigen in hepatitis C-related hepatocellular carcinoma

BACKGROUND

The prognosis of hepatocellular carcinoma (HCC) patients remains poor despite advances in treatment modalities and diagnosis. It is important to identify useful markers for the early detection of HCC in patients. Preneoplastic antigen (PNA), originally reported in a rat carcinogenesis model, is increased in the tissues and serum of HCC patients.

AIM

To determine the diagnostic value of PNA for discriminating HCC and to characterize PNA-positive HCC.

METHODS

Patients with hepatitis C virus (HCV)-related hepatic disorders were prospectively enrolled in this study, which included patients with hepatitis, with cirrhosis, and with HCC. A novel enzyme-linked immunosorbent assay was developed to measure serum PNA concentrations in patients.

RESULTS

Serum PNA concentrations were measured in 89 controls and 141 patients with HCV infections (50 hepatitis, 44 cirrhosis, and 47 HCC). Compared with control and non-HCC patients, PNA was increased in HCC. On receiver operating characteristic curve analysis, the sensitivity of PNA was similar to the HCC markers des-γ-carboxy-prothrombin (DCP) and α-fetoprotein (AFP), but the specificity of PNA was lower. There was no correlation between PNA and AFP and a significant but weak correlation between PNA and DCP in HCC patients. Importantly, the correlations with biochemical markers were completely different for PNA, AFP, and DCP; glutamyl transpeptidase was highly correlated with PNA, but not with AFP or DCP, and was significantly higher in PNA-high patients than in PNA-low patients with HCV-related HCC.

CONCLUSION

PNA may have the potential to diagnose a novel type of HCC in which glutamyl transpeptidase is positively expressed but AFP or DCP is weakly or negatively expressed.

Expression, purification, and characterisation of human soluble Epoxide Hydrolase (hsEH) and of its functional C-terminal domain

The human soluble Epoxide Hydrolase (hsEH) is an enzyme involved in the hydrolysis of endogenous anti-inflammatory and cardio-protective signalling mediators known as epoxyeicosatrienoic acids (EETs). EETs’ conversion into the corresponding diols by hsEH generates non-bioactive molecules, thereby the enzyme inhibition would be expected to enhance the EETs bioavailability, and their beneficial properties. Numerous inhibitors have been developed to target the enzyme, some of which are showing promising antihypertensive and anti-inflammatory properties in vivo. Thus far, the preparation of the recombinant enzyme for enzymatic and structural in vitro studies has been performed mainly using a baculovirus expression system. More recently, it was reported that the enzyme could be exogenously expressed and isolated from E. coli, although limited amounts of active protein were obtained. We herein describe two novel methods to yield pure recombinant enzyme. The first describes the expression and purification of the full-length enzyme from eukaryotic cells HEK293-F, whilst the second concerns the C-terminal domain of hsEH obtained from the cost-effective and rapid E. coli prokaryotic system. The two methods successfully generated satisfactory amounts of functional enzyme, with virtually identical enzymatic activity. Overall, the protocols described in this paper can be employed for the recombinant expression and purification of active hsEH, to be used in future biomedical investigations and for high-throughput screening of inhibitors for potential use in the treatment of cardiovascular disease.

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hsEH is a key regulator of cardiovascular homeostasis.

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A HEK293-F mammalian expression system for hsEH full-length (FL) was developed.

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An E. coli expression system for the hsEH C-terminal Domain (CTD) was established.

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Both proteins exhibited the same enzymatic specific activity in vitro.

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The CTD preparation provides benefits of easy operation, and high yield and purity.

Oxetane Substrates of Human Microsomal Epoxide Hydrolase

Oxetanyl building blocks are increasingly used in drug discovery because of the improved drug-like properties they confer on drug candidates, yet little is currently known about their biotransformation. A series of oxetane-containing analogs was studied and we provide the first direct evidence of oxetane hydrolysis by human recombinant microsomal epoxide hydrolase (mEH). Incubations with human liver fractions and hepatocytes were performed with and without inhibitors of cytochrome P450 (P450), mEH and soluble epoxide hydrolase (sEH). Reaction dependence on NADPH was investigated in subcellular fractions. A full kinetic characterization of oxetane hydrolysis is presented, in both human liver microsomes and human recombinant mEH. In human liver fractions and hepatocytes, hydrolysis by mEH was the only oxetane ring-opening metabolic route, with no contribution from sEH or from cytochrome P450-catalyzed oxidation. Minimally altering the structural elements in the immediate vicinity of the oxetane can greatly modulate the efficiency of hydrolytic ring cleavage. In particular, higher pK_a in the vicinity of the oxetane and an increased distance between the oxetane ring and the benzylic nitrogen improve reaction rate, which is further enhanced by the presence of methyl groups near or on the oxetane. This work defines oxetanes as the first nonepoxide class of substrates for human mEH, which was previously known to catalyze the hydrolytic ring opening of electrophilic and potentially toxic epoxide-containing drugs, drug metabolites, and exogenous organochemicals. These findings will be of value for the development of biologically active oxetanes and may be exploited for the biocatalytic generation of enantiomerically pure oxetanes and diols.

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.

Expression and characterization of an epoxide hydrolase from Anopheles gambiae with high activity on epoxy fatty acids

In insects, epoxide hydrolases (EHs) play critical roles in the metabolism of xenobiotic epoxides from the food resources and in the regulation of endogenous chemical mediators, such as juvenile hormones. Using the baculovirus expression system, we expressed and characterized an epoxide hydrolase from Anopheles gambiae (AgEH) that is distinct in evolutionary history from insect juvenile hormone epoxide hydrolases (JHEHs). We partially purified the enzyme by ion exchange chromatography and isoelectric focusing. The experimentally determined molecular weight and pI were estimated to be 35 kD and 6.3 respectively, different than the theoretical ones. The AgEH had the greatest activity on long chain epoxy fatty acids such as 14,15-epoxyeicosatrienoic acids (14,15-EET) and 9,10-epoxy-12Z-octadecenoic acids (9,10-EpOME or leukotoxin) among the substrates evaluated. Juvenile hormone III, a terpenoid insect growth regulator, was the next best substrate tested. The AgEH showed kinetics comparable to the mammalian soluble epoxide hydrolases, and the activity could be inhibited by AUDA [12-(3-adamantan-1-yl-ureido) dodecanoic acid], a urea-based inhibitor designed to inhibit the mammalian soluble epoxide hydrolases. The rabbit serum generated against the soluble epoxide hydrolase of Mus musculus can both cross-react with natural and denatured forms of the AgEH, suggesting immunologically they are similar. The study suggests there are mammalian sEH homologs in insects, and epoxy fatty acids may be important chemical mediators in insects.

The role of epoxide hydrolases in health and disease

Epoxide hydrolases (EH) are ubiquitously expressed in all living organisms and in almost all organs and tissues. They are mainly subdivided into microsomal and soluble EH and catalyze the hydration of epoxides, three-membered-cyclic ethers, to their corresponding dihydrodiols. Owning to the high chemical reactivity of xenobiotic epoxides, microsomal EH is considered protective enzyme against mutagenic and carcinogenic initiation. Nevertheless, several endogenously produced epoxides of fatty acids function as important regulatory mediators. By mediating the formation of cytotoxic dihydrodiol fatty acids on the expense of cytoprotective epoxides of fatty acids, soluble EH is considered to have cytotoxic activity. Indeed, the attenuation of microsomal EH, achieved by chemical inhibitors or preexists due to specific genetic polymorphisms, is linked to the aggravation of the toxicity of xenobiotics, as well as the risk of cancer and inflammatory diseases, whereas soluble EH inhibition has been emerged as a promising intervention against several diseases, most importantly cardiovascular, lung and metabolic diseases. However, there is reportedly a significant overlap in substrate selectivity between microsomal and soluble EH. In addition, microsomal and soluble EH were found to have the same catalytic triad and identical molecular mechanism. Consequently, the physiological functions of microsomal and soluble EH are also overlapped. Thus, studying the biological effects of microsomal or soluble EH alterations needs to include the effects on both the metabolism of reactive metabolites, as well as epoxides of fatty acids. This review focuses on the multifaceted role of EH in the metabolism of xenobiotic and endogenous epoxides and the impact of EH modulations.

Development of monoclonal antibodies to human microsomal epoxide hydrolase and analysis of "preneoplastic antigen"-like molecules

Microsomal epoxide hydrolase (mEH) is a drug metabolizing enzyme which resides on the endoplasmic reticulum (ER) membrane and catalyzes the hydration of reactive epoxide intermediates that are formed by cytochrome P450s. mEH is also thought to have a role in bile acid transport on the plasma membrane of hepatocytes. It is speculated that efficient execution of such multiple functions is secured by its orientation and association with cytochrome P450 enzymes on the ER membrane and formation of a multiple transport system on the plasma membrane. In certain disease status, mEH loses its association with the membrane and can be detected as distinct antigens in the cytosol of preneoplastic foci of liver (preneoplastic antigen), in the serum in association with hepatitis C virus infection (AN antigen), or in some brain tumors. To analyze the antigenic structures of mEH in physiological and pathological conditions, we developed monoclonal antibodies against different portions of mEH. Five different kinds of antibodies were obtained: three, anti-N-terminal portions; one anti-C-terminal; and one, anti-conformational epitope. By combining these antibodies, we developed antigen detection methods which are specific to either the membrane-bound form or the linearized form of mEH. These methods detected mEH in the culture medium released from a hepatocellular carcinoma cell line and a glioblastoma cell line, which was found to be a multimolecular complex with a unique antigenic structure different from that of the membrane-bound form of mEH. These antibodies and antigen detection methods may be useful to study pathological changes of mEH in various human diseases.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

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.

Properties of enzymes hydrating epoxides in human epidermis and liver

1. Cytosolic and microsomal epoxide hydrolyzing enzymes of human skin and liver were compared and found to be different. 2. Epidermal and hepatic cytosolic epoxide hydrolases were different in terms of substrate selectivity, pI, inhibitor sensitivity and affinity chromatographic properties. 3. Microsomal epoxide hydrolases had the same pIs but different substrate selectivities. 4. Cytosolic epoxide hydrolase from adults had higher specific activity than that from neonates or cultured epidermis, but lower activity than adult hepatic enzymes. 5. The sizes of cytosolic epoxide hydrolase from epidermis and liver were similar and lower than that from cultured fibroblasts. 6. Cytosolic epoxide hydrolase from all sources shared similar antigenic determinants.

Human and murine cytosolic epoxide hydrolase: physical and structural properties

1. Human and murine liver cytosolic epoxide hydrolase (CEH) had an apparent Mw of 59,000 by SDS-PAGE. 2. Peptide maps of CNBr, trypsin and Staphylococcus aureus V8 digests, as well as amino acid analysis, showed that human and murine CEH were similar. Uninduced and clofibrate induced murine CEH appeared qualitatively identical. 3. The CEHs shared antigenic determinants as determined by Western blotting. 4. Circular dichroism spectra indicate that human CEH had 39% alpha-helix. Uninduced and clofibrate induced murine CEH had 38 and 35% alpha-helix, respectively.

Rabbit microsomal epoxide hydrolase: isolation and characterization of the xenobiotic metabolizing enzyme cDNA

Many endogenous and xenobiotic chemicals are metabolized to epoxides which may be enzymatically hydrated, via microsomal epoxide hydrolase (mEH), to less reactive dihydrodiol derivatives. On the basis of the reported rabbit mEH amino acid sequence [F. S. Heinemann and J. Ozols (1984) J. Biol. Chem. 259, 797-804], we constructed a 35 base oligonucleotide which was used to screen rabbit liver cDNA libraries. Overlapping rabbit mEH clones were isolated and the full-length cDNA sequence of 1653 bp was determined. The rabbit nucleotide sequence has a high degree of similarity (greater than 75%) with cDNA sequences reported for rat and human mEH. Northern blot analyses with fragments of the rabbit cDNA demonstrate that mEH messenger RNA (mRNA) is expressed constitutively in the liver and induced following exposure to phenobarbital or polychlorinated biphenyls. Constitutive expression of mEH mRNA is also observed in rabbit kidney, testes, and lung. Using benzo[alpha]pyrene-4,5-oxide as substrate, mEH enzymatic activity is shown to correlate closely with tissue levels of mEH mRNA. Southern blot analyses of rabbit DNA suggest that the mEH gene exists as a single copy per haploid genome. The mEH amino acid sequences of the human and rat were compared to that of the deduced rabbit protein in order to analyze the degree of conservation and hydropathy profiles in these species. This comparison permitted the formulation of a computer-assisted model of mammalian mEH as it may relate to the microsomal membrane.

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.

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.

Purification of microsomal epoxide hydrolase from liver of rhesus monkey: partial separation of cis- and trans-stilbene oxide hydrolase

Solubilized rhesus monkey liver microsomes were used as the starting material for the purification of epoxide (cis-stilbene oxide) hydrolase. Successive chromatography over DEAE-Sephacel followed by CM-cellulose resulted in two peaks of activity, CM A and CM B. Passage of these two eluates over separate hydroxyapatite columns resulted in two peaks of activity from CM A, HA A1, and HA A2, and one peak from CM B and HA B, with respective recoveries of 1, 7, and 0.2% of cis-stilbene oxide hydrolase activities. A similar recovery was found for benzo[a]pyrene-4,5-oxide hydrolase, while trans-stilbene oxide hydrolase activity coeluted only in HA A2. Fraction HA A1 was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoblots of the three eluates and solubilized microsomes incubated with anti-HA A1 demonstrated a single band at 49 kDa in each fraction. The three eluates were differentially affected by the inhibitors of epoxide hydrolase, trichloropropene oxide and 4-phenylchalcone oxide, and addition of Lubrol PX and phospholipid. Immunoprecipitation of HA A2 resulted in coprecipitation of cis- and trans-stilbene oxide hydrolase activity. Upon immunoprecipitation of solubilized microsomes, all the cis-stilbene oxide and benzo[a]pyrene-4,5-oxide, but only 50-60% of trans-stilbene oxide hydrolase activity was precipitated. These studies support findings with other species that (i) an immunochemically distinct cytosolic-like epoxide hydrolase exists in microsomes, and (ii) microsomal epoxide hydrolase activity can be separated during ion-exchange chromatography giving proteins with similar molecular weights and immunochemical cross-reactivity. The precipitation of cis- and trans-stilbene oxide hydrolase activity in eluate HA A2 provides convincing evidence that these isozymes are not structurally identical.

Comparison of the sex and subcellular distributions, catalytic and immunochemical reactivities of hepatic epoxide hydrolases in seven mammalian species

Sex and species differences in hepatic epoxide hydrolase activities towards cis- and trans-stilbene oxide were examined in common laboratory animals, as well as in monkey and man. In general trans-stilbene oxide was found to be a good substrate for epoxide hydrolase activity in cytosolic fractions, whereas the cis isomer was selectively hydrated by the microsomal fraction (with the exception of man, where the cytosol also hydrated this isomer efficiently). The specific cytosolic epoxide hydrolase activity was highest in mouse, followed by hamster and rabbit. Epoxide hydrolase activity in the crude 'mitochondrial' fraction towards trans-stilbene oxide was also highest in mouse and low in all other species examined. Microsomal epoxide hydrolase activity was highest in monkey, followed by guinea pig, human and rabbit, which all had similar activities. Sex differences were generally small, but where significant, male animals had higher catalytic activities than females of the same species in most cases. Antibodies raised against microsomal epoxide hydrolase purified from rat liver reacted with microsomes from all species investigated, indicating structural conservation of this protein. Antibodies directed towards cytosolic epoxide hydrolase purified from mouse liver reacted only with liver cytosol from mouse and hamster and with the 'mitochondrial' fraction from mouse in immunodiffusion experiments. Immunoblotting also revealed reaction with rat liver cytosol. The cytosolic and 'mitochondrial' epoxide hydrolases in all three mouse strains and in both sexes for each strain were immunochemically identical. The anomalies in human liver epoxide hydrolase activities observed here indicate that no single common laboratory animal is a good model for man with regard to these activities.

Affinity purification of cytosolic epoxide hydrolase from human, rhesus monkey, baboon, rabbit, rat and mouse liver

An affinity purification system based on elution of cytosolic epoxide hydrolase from a methoxycitronellyl thiol ligand with 4-azidochalcone oxide was applied to a variety of samples including liver from human, monkey, baboon, rabbit, rat and mouse as well as mammary gland from mouse. Hepatic tissues yielded a major 58 kDa band on SDS-PAGE, but the system had to be modified slightly to remove a 33 kDa band for rat. All of the affinity purified hydrolases showed similar properties with regard to substrate selectivity, pH dependence and mobilities on SDS-PAGE.

Immunochemical characterization of human lung epoxide hydrolases

Immunochemical techniques were used to investigate the biochemical properties of human lung epoxide hydrolases. Two epoxide hydrolases with different immunoreactive properties were identified. These two epoxide hydrolases were found in both cytosolic and microsomal cell fractions. Immunotitration of enzyme activity showed that enzymes that catalyze the hydration of benzo(a)pyrene 4,5-oxide react with antiserum to rat microsomal epoxide hydrolase; those that hydrate trans-stilbene oxide do not. Immunotitration and Western blot experiments showed that microsomal and cytosolic benzo(a)pyrene 4,5-oxide hydrolases have significant structural homology. Immunohistochemical staining of human lung benzo(a)pyrene 4,5-oxide hydrolase showed that the enzyme is localized primarily in the bronchial epithelium. No cell type-specific localization was observed. An enzyme-linked immunosorbent assay was developed which allows direct quantitation of benzo(a)pyrene 4,5-oxide hydrolase protein. Levels of enzyme protein detected by this assay correlated well with enzyme levels determined by substrate conversion assays.

Characterization of multiple epoxide hydrolase activities in mouse liver nuclear envelope

A nuclear envelope-associated epoxide hydrolase in mouse liver that hydrates trans-stilbene oxide has been identified and characterized. This epoxide hydrolase is distinct from the enzyme in nuclear envelopes that hydrates benzo[a]pyrene 4,5-oxide and other arene oxides. This distinction was demonstrated by the criteria of pH optima, response to specific inhibitors in vitro, and precipitation by specific antibodies. The new epoxide hydrolase had a pH optimum of 6.8, was poorly inhibited by trichloropropene oxide, was potently inhibited by 4-phenylchalcone oxide, and did not bind to antiserum against benzo[a]pyrene 4,5-oxide hydrolase. This nuclear enzyme is similar in many of its properties to cytosolic and microsomal trans-stilbene oxide hydrolases and may be nuclear envelope-bound form of these other epoxide hydrolases. It differed from these other trans-stilbene oxide hydrolases in that its affinities for both trans-stilbene oxide (measured as apparent Km) and 4-phenylchalcone oxide (measured as I50) were 4- to 20-fold lower than those of either the cytosolic or microsomal forms.

Distribution and inducibility of cytosolic epoxide hydrolase in male Sprague-Dawley rats

Cytosolic epoxide hydrolase (cEH) activity has been determined in liver and various extrahepatic tissues of male Sprague-Dawley rats using trans-stilbene oxide (TSO) and trans-ethylstyrene oxide (TESO) as substrates. Large interindividual differences in the specific activity of cytosolic epoxide hydrolase in the liver from more than 80 individual rats were observed varying by a factor of 38. In a randomly selected group of five animals liver cEH varied by a factor of 3.9 and kidney cEH by a factor of 2.7, whereas liver microsomal epoxide hydrolase and lactate dehydrogenase showed only very low variations (1.4- and 1.1-fold, respectively). The individual relative activity of kidney cEH was related to that of the liver. Cytosolic epoxide hydrolase activity was present in all of six extrahepatic rat tissues investigated. Interestingly specific activities were very high in the heart and kidney (higher than in liver), followed by liver greater than brain greater than lung greater than testis greater than spleen. TSO and TESO hydrolases in subcellular fractions of rat liver were present at highest specific activities in the cytosolic and the heavy mitochondrial fraction. As indicated by the marker enzymes, catalase, urate oxidase and cytochrome oxidase, this organelle-bound epoxide hydrolase activity may be of peroxisomal and/or mitochondrial origin. In the microsomal fraction, TSO and TESO hydrolase activity is very low, whereas STO hydrolase activity is highest in this fraction and very low in cytosol. In kidney, subcellular distribution is similar to that observed in liver. None of the commonly used inducers of xenobiotic metabolizing enzymes caused significant changes in the specific activities of rat hepatic cEH (trans-stilbene oxide, alpha-pregnenolone carbonitrile, 3-methylcholanthrene, beta-naphthoflavone, isosafrole, butylated hydroxytoluene, 2,3,7,8-tetrachlorodibenzo-p-dioxin, dibenzo[a,h]anthracene, phenobarbitone). However, clofibrate, a hypolipidemic agent, very strongly induced rat liver cEH (about 5-fold), whereas microsomal epoxide hydrolase activity was not affected. Specific activity of kidney cEH was increased about 2-fold.

Epoxide hydrolysis in the cytosol of rat liver, kidney, and testis. Measurement in the presence of glutathione and the effect of dietary clofibrate

The hydrolysis of trans- and cis-stilbene oxide and benzo[a]pyrene-4,5-oxide was measured in cytosol and microsomes of liver, kidney, and testis of control and clofibrate-fed rats. Significant levels of nonprotein sulfhydryls were detected in cytosol from liver (4.6 mM) and testis (1.5 mM). Glutathione was moderately stable in these fractions and interfered with the partition assays as conjugates were retained in the aqueous phase along with diols. When the products were separated by thin-layer chromatography, significant amounts of glutathione-conjugates were found to have been formed in the cytosol of liver and testis. Overnight dialysis or preincubation of cytosol with 0.5 mM diethylmaleate eliminated conjugate formation without affecting diol production. In dialyzed cytosol from clofibrate-fed rats (0.5%, 14 days), the rates of hydrolysis of trans-stilbene oxide were 506, 171, and 96% of controls for liver, kidney, and testis, respectively, and 126% of controls in liver microsomes. Rates of hydrolysis of cis-stilbene oxide were 149, 172, and 96% of controls in microsomes and 154, 124, and 91% of controls in cytosols from livers, kidneys, and testis of clofibrate-fed rats respectively. Hydrolysis of benzo[a]pyrene-4,5-oxide was similar to that of cis-stilbene oxide. Conjugation of the cis-stilbene oxide with glutathione was detected in cytosols from all three tissues with lesser amounts in the microsomes from liver and kidneys. After clofibrate treatment, the rates of this activity were 200, 173, and 95% of controls in cytosol from liver, kidneys and testis, and 203 and 202% of controls in microsomes from liver and kidneys respectively. These results indicate that epoxide hydrolysis and conjugation in rat liver and kidney are responsive to clofibrate treatment and support other evidence which suggests that hydrolysis of cis- and trans-stilbene oxides in cytosol is catalyzed, in part, by distinct enzymes.

Properties of cytosolic epoxide hydrolase purified from the liver of untreated and clofibrate-treated mice. Characterization of optimal assay conditions, substrate specificity and effects of modulators on the catalytic activity

We have characterized certain catalytic properties of cytosolic epoxide hydrolases purified from untreated and clofibrate-treated mouse liver. The enzyme activity was found to be sensitive to oxygen, but nitrogen-saturated buffers containing dithiothreitol maintained high activity for at least 12 h at 0 degrees C. Linearity of the hydration of trans-stilbene oxide with time and protein was established, the pH optimum was broad (6.5 to 7.4) and the temperature optimum was close to 50 degrees C for both forms. The activity was independent of ionic strength, with the exception of the control form in the absence of dithiothreitol, where a lower activity was observed at low ionic strength. The activity decreased when ethanol was replaced by acetone or acetonitrile as solvent for the substrate. Tetrahydrofuran was found to be highly inhibitory, while dimethylsulfoxide had less pronounced effects. The apparent Km values were 4.9 microM, 73 microM and 1980 microM for the control form with trans-stilbene oxide, cis-stilbene oxide and styrene oxide as substrates, respectively. The Km values for the enzyme from clofibrate-treated mice were in the same range, although the V values were higher for all three substrates with this form. The highest turnover was found for trans-beta-propylstyrene oxide as substrate, followed by trans-beta-ethylstyrene oxide. Little or no activity was observed with benzo[a]pyrene 4,5-oxide or cholesterol 5,6 alpha-oxide. The enzymes were found to be sensitive to 5,5'-dithiobis(2-nitrobenzoic acid) and a phenylmercuric salt. alpha-Naphthoflavone, beta-naphthoflavone and chalcone derivatives also inhibited the activity, while none of the compounds known to activate microsomal epoxide hydrolase activated the cytosolic forms.

Assessment of rat liver microsomal epoxide hydrolase as a marker of hepatocarcinogenesis

The influence of eleven xenobiotics on the activity and amount of hepatic microsomal epoxide hydrolase was determined. Activity was assayed using three different substrates after rats were fed, throughout 3 weeks, diets containing one of six hepatocarcinogens, viz. 2-acetylaminofluorene, 3'-methyl-4-dimethylaminoazobenzene, 4'-fluoro-4-dimethylaminoazobenzene, thioacetamide, aflatoxin B1 and ethionine. Five hepatocarcinogens induced activity 4- to 10-fold; ethionine was relatively ineffective as an inducer. Two non-carcinogenic analogues of hepatocarcinogens, viz. fluorene and p-aminoazobenzene, caused no appreciable increase in enzyme activity, but phenobarbital, barbital and 1-naphthylisothiocyanate induced activity 2- to 3-fold. All eleven xenobiotics increased the amount of microsomal epoxide hydrolase 2- to 9-fold when examined immunochemically using either a radial diffusion assay or an enzyme-linked immunosorbent assay (ELISA). Serum glutamic oxaloacetic acid transaminase activity was not appreciably elevated by feeding ten of the xenobiotics, suggesting that inductions were not owing to toxicity. Using ELISA, microsomal epoxide hydrolase was detected in post-microsomal (PM) supernatant fractions from control rat liver, thus confirming an earlier report by Gill et al. [Carcinogenesis 3, 1307 (1982)]. The eleven xenobiotics induced the amount of ELISA-detectable antigen in PM supernatant fractions by 3- to 34-fold. Longer centrifugation of PM supernatant fractions yielded a pellet fraction that contained 92 +/- 1.2% of the ELISA-detectable antigen irrespective of the xenobiotic regimen. Relationships between xenobiotic induction of microsomal epoxide hydrolase activity and amount and hepatocarcinogenesis are discussed.

Effect of dietary clofibrate on epoxide hydrolase activity in tissues of mice

The effects of dietary clofibrate on the epoxide-metabolizing enzymes of mouse liver, kidney, lung and testis were evaluated using trans-stilbene oxide as a selective substrate for the cytosolic epoxide hydrolase, cis-stilbene oxide and benzo[a]pyrene 4,5-oxide as substrates for the microsomal form, and cis-stilbene oxide as a substrate for glutathione S-transferase activity. The hydration of trans-stilbene oxide was greatest in liver followed by kidney greater than lung greater than testis. Its hydrolysis was increased significantly in the cytosolic fraction of liver and kidney of clofibrate-treated mice and in the microsomes from the liver. Isoelectric focusing indicates that the same enzyme is responsible for hydrolysis of trans-stilbene oxide in normal and induced liver and kidney. Clofibrate induced glutathione S-transferase activity on cis-stilbene oxide only in the liver. Hydrolysis of both cis-stilbene oxide and benzo[a]pyrene 4,5-oxide was highest in testis followed by liver greater than lung greater than kidney. Hydration of cis-stilbene oxide was induced significantly in both liver and kidney by clofibrate but that of benzo[a]pyrene 4,5-oxide was induced only in the liver. These and other data based on ratios of hydration of benzo[a]pyrene 4,5-oxide to cis-stilbene oxide in tissues of normal and induced animals indicate that there are one or more novel epoxide hydrolase activities which cannot be accounted for by either the classical cytosolic or microsomal hydrolases. These effects are notable in the microsomes of kidney and especially in the cytosol of testis.

Epoxide metabolism in the liver of mice treated with clofibrate (ethyl-alpha-(p-chlorophenoxyisobutyrate)), a peroxisome proliferator

An increase in cytosolic epoxide hydrolase (cEH) activity occurs in the livers of mice treated with peroxisome proliferating-hypolipidemic-nongenotoxic carcinogens. As increases in activity of epoxide metabolizing enzymes may reflect the carcinogenic mechanism, a detailed comparison of the response of cEH, microsomal epoxide hydrolase (mEH), and cytosolic glutathione S-transferase (cGST) activities using the geometrical isomers trans- and cis-stilbene oxide as substrates has been performed in livers from mice treated with clofibrate (ethyl-alpha-(p-chlorophenoxyisobutyrate]. The maximal increase of cEH activity occurred at lower dietary doses of clofibrate (0.5%) and within a shorter time (5 days) than mEH and cGST (2%, 14 days) activity. After 14 days at 0.5% clofibrate, cEH, mEH, and cGST activities were 250, 175, and 165% and 290, 220, and 75% of control values in male and female mice, respectively. Withdrawal of clofibrate from the diet resulted in a reversion of activities to control values within 7 days. Clofibrate treatment shifted the apparent subcellular compartmentation of all three enzymatic activities with an increase in the ratio of soluble to particulate activity. In particular, the relative specific activity of all three enzymes decreased in the light mitochondrial (peroxisomal) cell fraction, and an increase of a mEH-like activity (benzo[a]pyrene-4,5-oxide and cis-stilbene oxide hydrolysis) in the cytosol occurred. Both the increase of cEH activity and the appearance of mEH-like activity in the cytosol are novel responses of epoxide metabolizing enzymes, which may be related to the novel cellular responses that follow clofibrate treatment, peroxisome proliferation, hypolipidemia, and nongenotoxic carcinogenesis.

Immunological similarity of epoxide hydrolase activity in the mitochondrial and cytosolic fractions of mouse liver

The light and heavy mitochondrial fractions of mouse liver have relatively high levels of epoxide hydrolase (EH) activity when monitored with trans-stilbene oxide as substrate. Using double diffusion analysis and immunoprecipitation experiments it was shown that EH activity in the mitochondrial fractions is immunologically similar to cytosolic EH, but immunologically dissimilar from microsomal EH. The EHs in the mitochondrial and cytosolic fractions also have a similar pI.