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.     

Irreversible protein binding of norethisterone (norethindrone) epoxide.

14,15-3H-Norethisterone-4 beta, 5 beta-epoxide, a metabolite of norethisterone, was incubated with several proteins and nucleic acids. After 30 min incubation 0.19 nmol of the epoxide were irreversibly bound per mg albumin which contains free sulfhydryl groups; proteins without SH-groups, such as concanavalin A, gamma-globulin, DNA and RNA, did not irreversibly bind norethisterone epoxide. A superoxide (O2) generating enzyme system comprised of xanthine oxidase and hypoxanthine was capable of catalyzing the irreversible binding of the parent compound, norethisterone, to albumin, indicating that an oxidation product was formed which reacted with the protein. When norethisterone epoxide was incubated for 60 min with hepatic microsomes of rats in absence of NADPH, about 2.0 nmol of the epoxide were irreversibly incorporated per mg microsomal protein. This binding was increased to 5.2 nmol by addition of a NADPH regenerating system. Addition of glutathione and cytosol decreased only the NADPH-dependent protein binding; phenobarbital pretreatment of rats induced this NADPH-dependent binding of norethisterone epoxide to microsomal protein by a factor of 2. In presence of NADPH, binding of the epoxide to microsomal protein depended on substrate concentration used. The results indicate that norethisterone epoxide is able to chemically react with proteins. In addition, hepatic microsomal enzymes convert the epoxide to another metabolite which also can react with proteins.     

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 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.     

Strain differences in rat liver (UDP-glucuronyltransferase activity towards androsterone.

Male Donryu, Wistar King rats showed discontinuous variations in hepatic microsomal UDP-glucuronyltransferase activities towards androsterone, but not towards testosterone, bilirubin, phenolphthalein and 4-nitrophenol. Fresh microsomal fraction with a low transferase activity towards androsterone formed 0.049--0.080 nmole of glucuronide/min per mg of protein, whereas fresh microsomal fraction with a high transferase activity towards androsterone formed 0.335--0.557 nmol of glucuronide/min per mg of protein. The microsomal fraction with low enzyme activity towards androsterone was not stimulated by treatment with Triton X-100 or freezing and thawing. In contrast, male Long Evans and Sprague-Dawley rats did not exhibit such diversity.     

Elevation of hepatic epoxide hydratase activity by ethoxyquin is due to increased synthesis of the enzyme

Feeding of the antioxidant ethoxyquin to rats leads to an increase of epoxide hydratase activity in liver microsomes. The apparent half life of the increase is 3–4 days. Elevation of epoxide hydratase activity is also obtained by intraperitoneal treatment of mice with ethoxyquin. This elevation is prevented by concomitant treatment with cycloheximide. When radiolabelled leucine is incorporated into microsomal protein by liver cell fractions from either ethoxyquin-fed or untreated rats, gel electrophoresis reveals that ethoxyquin feeding increases incorporation into epoxide hydratase. These results suggest that the elevation of epoxide hydratase activity by ethoxyquin is due to increased biosynthesis of the enzyme, i.e. enzyme induction.     

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).     

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.     

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.     

Drug metabolism components in liver microsomes from Hypostomus punctatus, a Brazilian benthic fish (Cascudo)

1. The major components of hepatic drug biotransformation system were identified in a Brazilian freshwater benthic fish. 2. Cytochrome P-450 difference spectra were obtained adding 0.02 mM phenazine ethosulphate and 2 mM ascorbate to microsomal suspensions. Basal levels of P-450 were high (0.9 nmol/mg of microsomal protein) and were not induced by 3-MC. 3. Microsomal NADPH-cytochrome C reductase activity was determined in presence of 1.3 x 10(-4) M NADPH, 3.3 x 10(-5) M cytochrome C, 1.0 x 10(-4) M EDTA, 66 micrograms of microsomal protein per ml in a 0.3 M Tris-HCl buffer, pH 8.6. Basal levels of NADPH-cytochrome C were 152.7 nmoles/min/mg of microsomal protein.     

Characterisation of tolbutamide hydroxylase activity in the common brushtail possum, (Trichosurus vulpecula) and koala (Phascolarctos cinereus): inhibition by the eucalyptus terpene 1,8-cineole

Plant constituents such as terpenes are major constituents of the essential oil in Eucalyptus sp. 1,8-Cineole and p-cymene (Terpenes present in high amounts in Eucalyptus leaves) are potential substrates for the CYP family of enzymes. We have investigated tolbutamide hydroxylase as a probe substrate reaction in both koala and terpene pretreated and control brushtail possum liver microsomes and examined inhibition of this reaction by Eucalyptus terpenes. The specific activity determined for tolbutamide hydroxylase in the terpene treated brushtails was significantly higher than that for the control animals (1865+/-334 nmol/mg microsomal protein per min versus 895+/-27 nmol/mg microsomal protein per min). The activity determined in koala microsomes was 8159+/-370 nmol/mg microsomal protein per min. Vmax values and Km values for the terpene treated possum, control, possum and koala were 1932-2225 nmol/mg microsomal protein per min and 0.80 0.81 mM; 1406-1484 nmol/mg microsomal protein per min and 0.87-0.92 mM and 5895-6403 nmol/mg microsomal protein per min and 0.067-0.071 mM, respectively. Terpenes were examined as potential inhibitors of tolbutamide hydroxylase activity. 1,8-Cineole was found to be a competitive inhibitor for the enzyme responsible for tolbutamide hydroxylation (Ki 15 microM) in the possum. In koala liver microsomes stimulation of tolbutamide hydroxylase activity was observed when concentrations of cineole were increased. Therefore, although inhibition was observed, the type of inhibition could not be determined.     

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.     

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.     

Monooxygenase activity of rat liver microsomes immobilized by entrapment in a crosslinked prepolymerized polyacrylamide hydrazide

Rat liver microsomes were immobilized by entrapment in a chemically crosslinked synthetic gel obtained by crosslinking prepolymerized polyacrylamide-hydrazide with glyoxal. Approximately 88% of the microsomal fraction was entrapped in the gel. The specific rate of O-demethylation of p-nitroanisole was used to assay the microsomal cytochrome P-450 activity of the immobilized microsomal preparations. The gel entrapped microsomes showed monooxygenase activity at 37 degrees C of Vmax = 2.3 nmol p-nitrophenol/min per nmol cytochrome P-450, similar to that of microsomes in suspension. The Km value for the p-nitroanisole-immobilized microsomal cytochrome P-450 system (1.2 X 10(-5) M) was rather close to that of microsomes in suspension (0.8 X 10(-5) M). Under the experimental conditions used the pH activity curve of the immobilized preparation was shifted towards more alkaline values by approx. 0.5 pH unit in comparison with microsomes in suspension. The rate of cytochrome c reduction by the immobilized microsomal system (11.7 nmol/min per mg protein) at 25 degrees C was considerably lower than that of the control (microsomes in suspension, 78 nmol/min per mg protein). Enzyme activity in both preparations showed the same temperature dependence at the temperature range of 10 to 37 degrees C. The immobilized microsomal monooxygenase system could be operated continuously for several hours at 37 degrees C provided that adequate amounts of an NADPH-generating system were added periodically. Under similar conditions a control microsomal suspension lost its enzymic activity within 90 min.     

Purification of human liver microsomal epoxide hydrase. Differences in the properties of the human and rat enzymes.

Human liver microsomal epoxide hydrase has been highly purified to a specific activity (570 to 620 nmol/min/mg of protein) comparable to that of the rat enzyme using styrene oxide as substrate. Like the purified rat liver microsomal epoxide hydrase, the human enzyme has a minimum molecular weight of 49,000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and exhibits broad substrate specificity toward a variety of alkene and arene oxides. Despite these similarities, the human and rat enzymes are different proteins as judged by their immunochemical properties as well as their relative catalytic activities toward certain substrates.     

Liver and mammary arylhydrocarbon hydroxylase and epoxide hydratase in lactating rats fed polybrominated biphenyls

Female rats were fed polybrominated biphenyls (PBBs) (50 ppm) from day 8 of gestation through day 14 postpartum. Hepatic and mammary liver to body weight ratios, microsomal protein, arylhydrocarbon (benzo(a)pyrene) hydroxylase (AHH) activity and epoxide hydratase (EH) activities were measured. Exposure to PBBs significantly increased liver to body weight ratio, hepatic microsomal protein and hepatic AHH and EH activities. Mammary AHH activity was increased and EH activity was decreased after PBBs. These data demonstrate that AHH and EH are present in mammary tissue and can be altered by exposure to PBBs.     

Acyl-Coenzyme A synthetase and fatty acid oxidation in rat liver peroxisomes.

Rat liver peroxisomes oxidized palmitate in the presence of ATP, CoA and NAD+, and the rate of palmitate oxidation exceeded that of palmitoyl-CoA oxidation. Acyl-CoA synthetase [acid: CoA ligase (AMP-forming); EC 6.2.1.3] was found in peroxisomes. The substrate specificity of the peroxisomal synthetase towards fatty acids with various carbon chain lengths was similar to that of the microsomal enzyme. The peroxisomal synthetase activity toward palmitate (40--100 nmol/min per mg protein) was higher than the rate of palmitate oxidation by the peroxisomal system (0.7--1.7 nmol/min per mg protein). The data show that peroxisomes activate long chain fatty acids and oxidize their acyl-CoA derivatives.     

Enzymatic Mechanisms Involved in Phenanthrene Degradation by the White Rot Fungus Pleurotus ostreatus

The enzymatic mechanisms involved in the degradation of phenanthrene by the white rot fungus Pleurotus ostreatus were examined. Phase I metabolism (cytochrome P-450 monooxygenase and epoxide hydrolase) and phase II conjugation (glutathione S-transferase, aryl sulfotransferase, UDP-glucuronosyltransferase, and UDP-glucosyltransferase) enzyme activities were determined for mycelial extracts of P. ostreatus. Cytochrome P-450 was detected in both cytosolic and microsomal fractions at 0.16 and 0.38 nmol min(sup-1) mg of protein(sup1), respectively. Both fractions oxidized [9,10-(sup14)C]phenanthrene to phenanthrene trans-9,10-dihydrodiol. The cytochrome P-450 inhibitors 1-aminobenzotriazole (0.1 mM), SKF-525A (proadifen, 0.1 mM), and carbon monoxide inhibited the cytosolic and microsomal P-450s differently. Cytosolic and microsomal epoxide hydrolase activities, with phenanthrene 9,10-oxide as the substrate, were similar, with specific activities of 0.50 and 0.41 nmol min(sup-1) mg of protein(sup-1), respectively. The epoxide hydrolase inhibitor cyclohexene oxide (5 mM) significantly inhibited the formation of phenanthrene trans-9,10-dihydrodiol in both fractions. The phase II enzyme 1-chloro-2,4-dinitrobenzene glutathione S-transferase was detected in the cytosolic fraction (4.16 nmol min(sup-1) mg of protein(sup-1)), whereas aryl adenosine-3(prm1)-phosphate-5(prm1)-phosphosulfate sulfotransferase (aryl PAPS sulfotransferase) UDP-glucuronosyltransferase, and UDP-glucosyltransferase had microsomal activities of 2.14, 4.25, and 4.21 nmol min(sup-1) mg of protein(sup-1), respectively, with low activity in the cytosolic fraction. However, when P. ostreatus culture broth incubated with phenanthrene was screened for phase II metabolites, no sulfate, glutathione, glucoside, or glucuronide conjugates of phenanthrene metabolites were detected. These experiments indicate the involvement of cytochrome P-450 monooxygenase and epoxide hydrolase in the initial phase I oxidation of phenanthrene to form phenanthrene trans-9,10-dihydrodiol. Laccase and manganese-independent peroxidase were not involved in the initial oxidation of phenanthrene. Although P. ostreatus had phase II xenobiotic metabolizing enzymes, conjugation reactions were not important for the elimination of hydroxylated phenanthrene.     

Development of cholinephosphotransferase in guinea pig lung mitochondria and microsomes

Development of mitochondrial and microsomal choline phosphotransferase in the fetal guinea pig lung was investigated. The activity in fetal mitochondria was more than twice of that in fetal microsomes. However, in adult lung, the enzyme was distributed mostly in microsomes. In fetal lung, both the mitochondrial and microsomal enzyme activity was greatest at approx. 81% of the total gestation period (55 days). The specific activity in the microsomal fraction then declined until term, but increased again in the 24-h newborn from 1.0 to 2.3 nmol/min per mg protein. The activity in the mitochondrial fraction declined after 61 days (2.8 nmol/min per mg) to a minimal level at term (0.6 nmol/min per mg). Although the enzyme activity decreased from day 55 (1.2 nmol/min per mg), the amount of phosphatidylcholine gradually increased between day 55 and term.     

Development of glycerophosphate acytransferase in guinea pig lung mitochondria and microsomes

Development of mitochondrial and microsomal glycerophosphate acyltransferase in the fetal guinea pig lung was investigated. Mitochondrial and microsomal enzyme activity gradually increased from 45 days to 55 days of gestation. The specific activity in the microsomal fraction (8.2 nmol/min per mg protein) then declined until term, but increased again in the 24-h newborn from 2.5 to 6.1 nmol/min per mg protein. Glycerophosphate acyltransferase activity in the mitochondrial fraction declined after 55 days (3.5 nmol/min per mg) to a minimum level at 60 days (1.8 nmol/min per mg), but increased again in the 24-h newborn (4.0 nmol/min per mg). The specific activity of both mitochondrial and microsomal enzyme declined after 24 h after birth until adult levels were attained. Glycerophosphate acyltransferase activity in mitochondria and microsomes from adult lung was 0.8 and 2.0 nmol/min per mg, respectively. Microsomal enzyme activity was consistently inhibited (over 95%) throughout gestation and adulthood by exposure to any one of several proteinases: trypsin, chymotrypsin, papain, bromelain, pronase and nagarse. Although mitochondrial enzyme activity was also inhibited by these proteinases, there was a continuous increase in proteinase-resistant glycerophosphate acyltransferase activity between 45 days of gestation and term. In contrast, adult mitochondrial enzyme activity was stimulated by all the proteinases studied. These results suggest that early in gestation, glycerophosphate acyltransferase lies more exposed on the cytoplasmic side of the mitochondrial outer membrane and as gestation progresses it becomes embedded into the phospholipid bilayer.