Ethoxyquin feeding to rats increases liver microsome-catalyzed formation of benzo(a)pyrene diol epoxide — DNA adduct

The ability of rat liver microsomes to catalyze the formation of benzo(a)pyrene 7,8-diol-9,10-epoxide — DNA nucleoside adduct was increased threefold by feeding 0.5% ethoxyquin to the animals. Microsomal epoxide hydratase activity was enhanced i parallel by a factor of 3 while aryl hydrocarbon hydroxylase activity was not induced. Liver microsomes from rat pretreated with 3-methylcholanthrene produced an increased proportion of diol epoxide — DNA adduct when ethoxyquin had been fed to the animals. The main chromatographic peak formed by microsomes from 3-methylcholanthrene treated rats which contains DNA adducts of secondary benzo(a)pyrene phenol metabolites is reduced when the animals had received ethoxyquin.     

The effects of gonadectomy on the hepatic activities of aryl hydrocarbon hydroxylase, epoxide hydratase, and glutathione S-transferase in Wistar rats pretreated with oral methadone . HCl.

Methadone . HCl given in the drinking water for 4 weeks increased microsomal epoxide hydratase activity in the liver of adult male Wistar rats, with no change in aryl hydrocarbon hydroxylase activity. In contrast, in female rats it raised aryl hydrocarbon hydroxylase with no change in epoxide hydratase activity. Gonadectomy altered the effect of methadone on epoxide hydratase, but not on aryl hydrocarbon hydroxylase activity, in both sexes. In ovariectomized rats, but not in controls, methadone nearly doubled the epoxide hydratase activity, whereas in male rats castration decreased the inductive effect of methadone. Gonadectomy had a significant effect on the results of methadone treatment with respect to glutathione S-transferase activity in female rats. A sex difference was noted in the control levels of aryl hydrocarbon hydroxylase and glutathione S-transferase, but not of epoxide hydratase activity. The glutathione S-transferase and aryl hydrocarbon hydroxylase activities were decreased in castrated male rats, whereas epoxide hydratase activity was unaltered. It is concluded that sex hormones play an important role in the induction of epoxide hydratase and glutathione S-transferase by methadone, but not of aryl hydrocarbon hydroxylase, at this particular dosage regime.     

Differential control of rat microsomal "aryl hydrocarbon" monooxygenase and epoxide hydratase.

A growing body of evidence implicates epoxide metabolites of mutagenic and carcinogenic polycyclic hydrocarbons as either the only species, or one of the contributing species responsible for these adverse effects. Selective induction of epoxide hydratase(s) catalyzing the transformation of epoxides to electrophilically unreactive dihydrodiols, under conditions not leading to increases in monooxygenase(s) responsible for epoxide formation would, therefore, be of interest. All inducers of rat hepatic epoxide hydratase (determined with [7-3H]styrene oxide as substrate) which have been discovered also induced monooxygenase (determined with benzo(a)pyrene as substrate) suggesting a possible common biosynthetic control of these enzymes. The enzyme levels observed in different sexes and at different stages of the ontogenetic development, possibly dependent on endogenous inducers, strengthened this view. No sex difference is epoxide hydratase activity was observed in young rats (1 to 5 days old) while epoxide hydratase levels were about 3-fold higher in adult males than in females, which was remarkably similar to the behavior of monooxygenase. Moreover, the prenatal development of epoxide hydratase and monooxygenase appeared to be similar--although the low enzyme levels precluded accurate determinations of the latter. Although different types of known monooxygenase inducers all led to epoxide hydratase induction in adult rat liver, their effect of epoxide hydratase and monooxygenase could be dissociated by transplacental treatment. Dissociation was clearest with inducers of the polycyclic hydrocarbon type which led to great induction of monooxygenase while epoxide hydratase remained unchanged. The increases in monooxygenase activity were very different when determined by two methods based on different principles, demonstrating that at least two monooxygenases are involved in oxidative metabolism of benzo(a)pyrene, and that the control of epoxide hydratase is not under common control with either of them.     

Immunohistochemical localization of epoxide hydratase in rat liver

An antibody produced against epoxide hydratase (EC 4.2.1.63) which had been purified to apparent homogeneity from hepatic microsomes of phenobarbital pretreated rats was employed in an unlabeled antibody peroxidase-antiperoxidase technique to localize the enzyme at the light microscopic level in the livers of untreated rats. Immunohistochemical staining for epoxide hydratase was detected in parenchymal cells throughout the liver lobule. Cells within the centrilobular regions, however, were observed to be stained more intensely than were those within the midzonal and periportal regions of the lobule. The results of this immunohistochemical study thus demonstrate that epoxide hydratase does not exhibit a uniform pattern of distribution within the liver lobule in untreated rats.     

Purification and specificity of a human microsomal epoxide hydratase

Epoxide hydratase was solubilized from human liver microsomal fractions and purified to an extent where the specific activity was 40-fold greater than that of the liver homogenate. Combination of homogenate and purified preparation showed that the increase in activity was not due to the removal of an inhibitor. Monosubstituted oxiranes with a lipophilic substituent larger than an ethyl group (isopropyl, t-butyl, n-hexyl, phenyl) readily interacted as substrates or inhibitors with this purified human epoxide hydratase, whereas those with a small substituent (methyl, ethyl, vinyl) were inactive, probably reflecting greater affinity of the former epoxides owing to lipophilic binding sites near the active site of the enzyme. In a series of oxiranes having a lipophilic substituent of sufficient size (styrene oxides), monosubstituted as well as 1,1- and cis-1,2-disubstituted oxiranes readily served as substrates or inhibitors of the enzyme, but not the trans-1,2-disubstituted, tri- or tetra-substituted oxiranes. trans-Substitution at the oxirane ring apparently prevents access of the oxirane ring to the active site by steric hindrance. Epoxide hydratase was also solubilized from microsomal fractions of rat and guinea-pig liver and purified by the same procedure. Structural requirements for effective interaction of substrates, inhibitors and activators were qualitatively identical for epoxide hydratase from the three sources. However, several quantitative differences were observed. Thus human hepatic epoxide hydratase seems to be very similar to, although not identical with, the enzyme from guinea pig or rat. Studies with epoxide hydratase from the latter two species therefore appear to be significant with respect to man. In addition, knowledge of structural requirements for epoxides to serve as substrates for human epoxide hydratase may prove useful for drug design. Compounds which need aromatic or olefinic moieties for their desired effect would not be expected to lead to accumulation of epoxides if their structure was such as to allow for a metabolically produced epoxide to be rapidly consumed by epoxide hydratase.     

Alterations in the enzyme activity and polypeptide composition of rat hepatic endoplasmic reticulum during acute exposure to 2-acetylaminofluorene

Studies have been made of the morphology, enzyme activity and protein composition of liver endoplasmic reticulum in rats exposed to acute doses of the carcinogen, 2-acetylaminofluorene (2-AAF). Electron microscopic examination revealed numerous ultrastructural changes in the hepatocyte; most consistent alterations were the disorganisation of endoplasmic reticulum system with apparent increase of smooth endoplasmic reticulum. Administration of 2-AAF to rats immediately depressed microsomal glucose-6-phosphatase activity and eventually induced epoxide hydratase activity 6–7-fold over control activity. The induction was time-dependent and maximal rates of induction were observed at dosages greater than 40 mg/kg body wt. The treatment also induced cytochrome b₅ content, NADH and NADPH cytochrome c reductase activities (1.0–1.5-fold). Only very small changes in the total content of cytochrome P-450 were noted. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of microsomal proteins from 2-AAF pretreated animals showed time-dependent induction of two polypeptides which differed slightly in migration, in the region of M_r = 48 000; the faster-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal proteins from 2-AAF exposed rats showed a reproducible deletion of a protein with molecular weight in the region of 67 000. The basis for the alterations in the protein composition of endoplasmic reticulum in response to 2-AAF treatment is discussed.     

Variations in induction of drug-metabolizing enzymes by trans-Stilbene oxide in rodent species

trans-Stilbene oxide (400 mg/kg) produced a 500% increase in the microsomal in the microsomal epoxide hydratase activity in rat and mouse with little change in the soluble enzyme activity. However, in guinea pig, the soluble epoxide hydratase activity increased by about 33% with only a small increase (47.6%) in the microsomal enzyme activity. The soluble glutathione S-transferase activities were also induced in both rat and mouse, with little change in that of the guinea pig. Increasing dosage of trans-stilbene oxide from 400 mg/kg to 1000 mg/kg had little effect on the above enzyme activities. That the guinea pig was not relatively refractory to all inducing agents was shown by the fact phenobarbital (100 mg/kg) and 3-methylcholanthrene (25 mg/kg) produced relatively similar increases in the activities of aniline hydroxylase and P-aminopyrineP-demethylase in rat, mouse and guinea pig. However, these inducers produced only a 15–20% stimulation in the soluble glutathione S-transferase and microsomal epoxide hydratase activities in guinea pig, when compared to a 50–80% increase in rat and mouse, suggesting a general resistance to induction by the phase II enzymes in guinea liver. In all animal models, the inducer markedly increased th emicrosomal total phospholipid content, although the sphingomyelin content itself was decreased. In both rat and mouse, the microsomal cholesterol content was significantly decreased while that in guinea pig was unaffected. Possible factors responsible for the observed species differences are discussed.     

A rapid assay for epoxide hydratase activity with benzo(a)pyrene 4,5-(K-region-)oxide as substrate

A rapid radiometric assay for epoxide hydratase activity has been developed using the highly mutagenic [³H]benzo(a)pyrene 4,5-(K-region-)oxide as substrate. By addition of dimethylsulfoxide after the incubation, conditions were found where the unreacted substrate could be separated from the product benzo(a)pyrene-4,5-dihydrodiol(trans) simply by extraction into petroleum ether. The product is then extracted into ethyl acetate and, radioactivity is measured by scintillation spectrometry. This assay allows a rapid measurement of epoxide hydratase activity with an epoxide derived from a carcinogenic polycyclic hydrocarbon as substrate and is at the same time sensitive enough for accurate determination of epoxide hydratase activity in preparations with extremely low enzyme levels such as rat skin homogenate (8–14 pmol of product/mg of protein/min).     

Enzymic hydration of benzene oxide: Assay and properties

A radiometric assay for epoxide hydratase using [¹⁴C]benzene oxide as substrate has been developed. The reaction product trans-1,2-[¹⁴C]dihydroxy-1,2-dihydrobenzene (benzene dihydrodiol) was separated from the other components by simple extraction of the unreacted substrate and phenol (a rearrangement product) into a mixture of light petroleum and diethyl ether followed by extraction of the benzene dihydrodiol into ethyl acetate. The product was then estimated by scintillation counting. Using this assay the enzymic hydration of benzene oxide and the possible existence of a microsomal epoxide hydratase with a greater specificity toward benzene oxide were reinvestigated. The sequence of activities of microsomes from various organs was liver > kidney > lung > skin, the pH optimum of enzymic benzene oxide hydration was about pH 9.0, which is similar to that of styrene oxide hydration and both activities were equally stable when liver microsomal fractions were stored. The effect of low molecular weight inhibitors upon the hydration of styrene and benzene oxide by liver microsomes was similar in some cases and dissimilar in others. However, all the dissimilarities could be explained without recourse to the hypothesis of the existence of a separate benzene oxide hydratase. During enzyme purification studies the activity toward benzene oxide was inhibited by the detergent used (cutscum) but was recovered when the detergent was removed. Solubilization without significant loss of activity was successful using sodium cholate. This allowed immunoprecipitation studies, which were performed using monospecific antiserum raised against homogeneous epoxide hydratase. The dose-response curves of the extent of precipitation of activity with increasing amounts of added antiserum were indistinguishable for benzene oxide and styrene oxide as substrate. At high antiserum concentrations precipitation was complete with both substrates. The findings, taken together, indicate the presence in rat liver microsomes of a single epoxide hydratase catalyzing the hydration of both styrene and benzene oxide or the presence of enzymes so closely related that these cannot be distinguished by any of the criteria tested.     

Alcohol dehydrogenase-coupled spectrophotometric assay of epoxide hydratase activity

A coupled assay was devised for the assay of liver microsomal epoxide hydratase using the ability of alcohol dehydrogenase to transfer electrons from diols to NAD⁺: epoxide hydratase activity was continuously monitored at 340 nm. Rates of hydrolysis of octene-1,2-oxide and styrene-7,8-oxide measured utilizing this assay were similar to those determined using gas-liquid chromatography and radiometric thin-layer chromatography, respectively. The assay was used to examine the ability of rat liver microsomes and highly purified rat liver microsomal epoxide hydratase fractions to hydrolyze 15 other epoxides.     

Synthesis and insertion, both in vivo and in vitro, of rat-liver cytochrome P-450 and epoxide hydratase into Xenopus laevis membranes

We described whole cell and cell-free systems capable of inserting into membranes cytochrome P-450 and epoxide hydratase made under the direction of rat liver RNA. The systems have been used to study the pathways followed by newly made secretory and integral membrane proteins. The cell-free system contains Xenopus laevis embryo membranes, and demonstrates competition for a common receptor between cytochrome P-450 and epoxide hydratase, and normal secretory proteins: evidence is provided for differential membrane receptor affinity. Thus, synthesis of secretory and membrane proteins appears to involve a common initial pathway. Microinjection of rat liver RNA into whole oocytes suggests that membrane insertion is neither cell type nor species specific, because functional rat liver enzymes are found inserted in the endoplasmic reticulum of the frog cell. Nonetheless, insertion is highly selective since albumin and several other proteins made under the direction of the injected liver RNA are sequestered within membrane vesicles and are then secreted by the oocyte, whilst epoxide hydratase and cytochrome P-450 are inserted into membranes but are not secreted.     

Immunochemical localization of epoxide hydratase in rat liver: effects of 2-acetylaminofluorene.

The distribution of epoxide hydratase was studied immunohistologically in paraffin sections of p-benzoquinone-fixed livers obtained from normal and 2-acetylaminofluorene-treated rats. In controls the enzyme was localized preferentially in the centrilobular hepatocytes. After the administration of 2-acetyl-aminofluorene, the staining was evenly distributed within the lobules suggesting the possibility that this hepatocarcinogen preferentially induced epoxide hydratase in perilobular parenchymal cells. Nonhepatocytic cells were considerably less extensively stained than hepatocytes.     

Cloning of epoxide hydratase complementary DNA

Tightly membrane-bound polysomes were isolated from livers of rats administered trans-stilbene oxide. Epoxide hydratase mRNA was enriched from these polysomes using immunochemical techniques and oligo(dT)-cellulose chromatography. This resulted in an increase in message concentration over that found in noninduced membrane-bound cDNA, synthesized from enriched mRNA, was inserted into the ampicillin resistance gene of pBR322 using oligo(dG)-oligo(dC) tailing. Clones containing sequences complementary to epoxide hydratase mRNA were selected by differential colony hybridization using [32P]cDNA synthesized from immunoenriched mRNA and [32P]cDNA synthesized from nonenriched mRNA. Plasmids from four clones, which only annealed with the enriched probe, were isolated and shown to specifically hybridize with epoxide hydratase mRNA by hybrid selection-translation. A composite restriction endonuclease map of the plasmid inserts was constructed which spanned 1310 base pairs and represented approximately 80% of the message sequence. The 3'-5' orientation of this map relative to the epoxide hydratase mRNA was also determined.     

A simple method for purification of epoxide hydratase from rat liver.

Rat liver epoxide hydratase was purified 460-fold to homogeneity by detergent solubilization and ion-exchange chromatography. The enzyme obtained in high yield (36%) exhibited a specific activity of 479nmol of styrene glycol formed/min per mg of protein, with styrene oxide as substrate. Only one polypeptide-staining band, mol.wt. 49500, was visible after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis.     

Characteristics of dehydroepiandrosterone as a peroxisome proliferator.

Treatment of rats with dehydroepiandrosterone (300 mg/kg body weight, per os, 14 days) caused a remarkable increase in the number of peroxisomes and peroxisomal beta-oxidation activity in the liver. The activities of carnitine acetyltransferase, microsomal laurate 12-hydroxylation, cytosolic palmitoyl-CoA hydrolase, malic enzyme and some other enzymes were also increased. The increases in these enzyme activities were all greater in male rats than in female rats. Immunoblot analysis revealed remarkable induction of acyl-CoA oxidase and enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional enzyme in the liver and to a smaller extent in the kidney, whereas no significant induction of these enzymes was found in the heart. The increase in the hepatic peroxisomal beta-oxidation activity reached a maximal level at day 5 of the treatment of dehydroepiandrosterone and the increased activity rapidly returned to the normal level on discontinuation of the treatment. The increase in the activity was also dose-dependent, which was saturable at a dose of more than 200 mg/kg body weight. All these features in enzyme induction caused by dehydroepiandrosterone correlate well with those observed in the treatment of clofibric acid, a peroxisome proliferator. Co-treatment of dehydroepiandrosterone and clofibric acid showed no synergism in the enhancement of peroxisomal beta-oxidation activity, suggesting the involvement of a common process in the mechanism by which these compounds induce the enzymes. These results indicate that dehydroepiandrosterone is a typical peroxisome proliferator. Since dehydroepiandrosterone is a naturally occurring C19 steroid in mammals, the structure of which is novel compared with those of peroxisome proliferators known so far, this compound could provide particular information in the understanding of the mechanisms underlying the induction of peroxisome proliferation.     

The effects of metyrapone, chalcone epoxide, benzil, clotrimazole and related compounds on the activity of microsomal epoxide hydrolase in situ, in purified form and in reconstituted systems towards different substrates

The influence of metyrapone, chalcone epoxide, benzil and clotrimazole on the activity of microsomal epoxide hydrolase towards styrene oxide, benzo[a]pyrene 4,5-oxide, estroxide and androstene oxide was investigated. The studies were performed using liver microsomes from rats, rabbits, mice and humans; epoxide hydrolase purified from rat liver microsomes to apparent homogeneity; and the purified enzyme incorporated into liposomes composed of egg-yolk phosphatidylcholine or total rat liver microsomal lipids. All four effectors were found to activate the hydrolysis of styrene oxide by epoxide hydrolase in situ in rat liver microsomal membranes, in agreement with earlier findings. Epoxide hydrolase activity towards styrene oxide in liver microsomes from mouse, rabbit and man was also increased by all four effectors. The most striking effect was a 680% activation by clotrimazole in rat liver microsomes. However, none of the effectors activated microsomal epoxide hydrolase more than 50% when benzo[a]pyrene 4,5-oxide, estroxide or androstene oxide was used as substrate. Indeed, clotrimazole was found to inhibit microsomal epoxide hydrolase activity towards estroxide 30-50% and towards androstene oxide 60-90%. The effects of these four compounds were found to be virtually identical in the preparations from rats, rabbits, mice and humans. The effects of metyrapone, chalcone epoxide, benzil and clotrimazole on purified epoxide hydrolase were qualitatively the same as those on epoxide hydrolase in intact microsomes, but much smaller in magnitude. These effects were increased in magnitude only slightly by incorporation of the purified enzyme into liposomes made from egg-yolk phosphatidylcholine. However, when incorporation into liposomes composed of total microsomal lipids was performed, the effects seen were essentially of the same magnitude as with intact microsomes. When the extent of activation was plotted against effector concentration, three different patterns were found with different effectors. Activation of epoxide hydrolase activity towards styrene oxide by clotrimazole was found to be uncompetitive with the substrate and highly structure specific. On the other hand, inhibition of epoxide hydrolase activity towards androstene oxide by clotrimazole was found to be competitive in microsomes. It is concluded that the marked effects of these four modulators on microsomal epoxide hydrolase activity are due to an interaction with the enzyme protein itself, but that the presence of total microsomal phospholipids allows the maximal expression leading to similar degrees of modulation as those observed in intact microsomes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Xenobiotica-metabolizing enzymes in Drosophila melanogaster: activities of epoxide hydratase and glutathione S-transferase compared with similar activities in rat liver.

Activities of epoxide hydratase and glutathione (GSH) S-transferase were investigated in subcellular fractions of Drosophila melanogaster, and these activities were compared with analogous enzymic activities in extracts from rat liver. Microsomes of Drosophila were active in the hydratation of styrene oxide catalyzed by epoxide hydratase. The post-microsomal supernatant of Drosophila catalyzed the conjugation of GSH with 1-chloro-2,4-dinitrobenzene. However, GSH S-transferase activity with styrene oxide as the electrophilic substrate was not measurable. The respective specific activities of epoxide hydratase (per mg microsomal protein) and GSH S-transferase (per mg cytosolic protein) were factors of 5- and 10-fold lower than the corresponding activities in rat liver. However, when expressed per gram body weight, activities of both epoxide hydratase and GSH S-transferase were 3 times higher for Drosophila enzymes. The apparent Km values for the two Drosophila enzymes were higher, whereas the apparent Km values were lower, than the values found for the rat-liver enzymes. Among 3 different Drosophila strains (a wild-type, a white eye-color carrying mutant strain and a DDT-resistant strain), preliminary experiments showed no differences as far as these two enzymic activities were concerned. It is concluded that the results obtained in genetic toxicology testing with Drosophila are probably relevant to effects to be expected in mammalian systems with compounds requiring metabolic processes involving the enzymes investigated here.     

Metabolism of [14C]diethylstilboestrol epoxide by rat liver in vitro.

1. The trans-epoxide of diethylstilboestrol and its pinacolone were synthesized chemically and the pinacolone shown to be formed from the epoxide by a non-enzymic process. 2. [14C]Diethylstiboestrol epoxide was converted by rat liver microsomal fraction into 4'-hydroxypropiophenone by a new type of cleavage reaction involving mono-oxygenase. Conditions for the formation of this metabolite and also water-soluble products were investigated together with the effect of inhibitors. A sex-difference in the conversion of diethylstilboestrol epoxide into 4'-hydroxypropiophenone and to polar and water-soluble products was observed. 3. Diethylstilboestrol epoxide was found to be a relatively stable compound that did not form a glutathione conjugate readily without further microsomal activation. A purified preparation of epoxide hydratase did not enhance its rate of conversion into the pinacolone. 4. Diethylstilboestrol epoxide was found to have about one-tenth the oestrogenic activity of diethylstilboestrol as measured by the increase in uterine weight or the induction of peroxidase in immature rat uteri. It was inactive as a mutagen when tested for its ability to inhibit bacteriophage phi X174 DNA viral replication. 5. The possible role of diethylstilboestrol epoxide as an intermediate in the metabolism of diethylstilboestrol and in mediating the harmful effects of this synthetic estrogen is discussed.     

A new enzyme immunoassay of microsomal rat liver epoxide hydrolase

Antiserum against purified rat liver microsomal epoxide hydrolase was produced in the rabbit. We developed an enzyme-linked immunosorbent assay which is reliable with regard to its analytical criteria. The concentration of epoxide hydrolase was measured in liver microsomes of control rats and animals treated with F 1379 (250 mg/kg/day) for 5, 7, 14, and 21 days. This hypolipidemic drug was able to induce strong epoxide hydrolase activity and enhance protein concentration. The gradual increase in epoxide hydrolase concentration paralleled the increase of epoxide hydrolase activity, with stabilization occurring after the 14th until the 21st day of treatment.