Immunocytochemical analysis of soluble epoxide hydrolase and catalase in mouse and rat hepatocytes demonstrates a peroxisomal localization before and after clofibrate treatment

The intracellular localization of soluble epoxide hydrolase and catalase was investigated in hepatocytes from untreated and clofibrate-treated male C57B1/6 mice and from untreated male Sprague-Dawley rats. Polyclonal rabbit antibodies directed against purified mouse liver cytosolic epoxide hydrolase and rat liver catalase were used and their specificity ascertained by Ouchterlony immunodiffusion and immunoblotting. The IgG fraction was purified and incubated with cryosections of isolated hepatocytes or liver tissue, priorly fixed in 4% paraformaldehyde, and protein-A gold conjugates were used to visualize the antigen-antibody reaction. The soluble form(s) of epoxide hydrolase was found to be localized in the matrix of peroxisomes in hepatocytes from normal and clofibrate-treated mice and normal rats. No significant reactivity was found against plasma membrane, nuclei, mitochondria, the Golgi apparatus, endoplasmic reticulum, lysosomes, or cytosol. Catalase was also localized to peroxisomes in all samples investigated. Accordingly, both the catalase and the epoxide hydrolase activities routinely recovered in the high-speed supernatant after subfractionation of rat and mouse liver tissue mostly seemed to be due to extensive matrix leakage from peroxisomes, and this phenomenon may also be found in other species. Rat hepatocytes contained less epoxide hydrolase than mouse hepatocytes, as judged by both immunocytochemical labeling and biochemical data. Clofibrate treatment of mice decreased the labeling density of epoxide hydrolase and catalase in hepatocytes peroxisomes, as expected, and more unlabeled peroxisomes were observed.     

Differential subcellular localization of endogenous and transfected soluble epoxide hydrolase in mammalian cells: evidence for isozyme variants

Endogenous, constitutive soluble epoxide hydrolase in mice 3T3 cells was localized via immunofluorescence microscopy exclusively in peroxisomes, whereas transiently expressed mouse soluble epoxide hydrolase (from clofibrate-treated liver) accumulated only in the cytosol of 3T3 and HeLa cells. When the C-terminal lie of mouse soluble epoxide hydrolase was mutated to generate a prototypic putative type 1 PTS (-SKI to -SKL), the enzyme targeted to peroxisomes. The possibility that soluble epoxide hydrolase-SKI was sorted slowly to peroxiosmes from the cytosol was examined by stably expressing rat soluble epoxide hydrolase-SKI appended to the green fluorescent protein. Green fluorescent protein soluble epoxide hydrolase-SKI was strictly cytosolic, indicating that -SKI was not a temporally inefficient putative type 1 PTS. Import of soluble epoxide hydrolase-SKI into peroxisomes in plant cells revealed that the context of -SKI on soluble epoxide hydrolase was targeting permissible. These results show that the C-terminal -SKI is a non-functional putative type 1 PTS on soluble epoxide hydrolase and suggest the existence of distinct cytosolic and peroxisomal targeting variants of soluble epoxide hydrolase in mouse and rat.     

In vitro translation of cytosolic and peroxisomal epoxide hydrolase and catalase on liver polyribosomes from untreated and clofibrate-treated C57B1/6 mice

Free and membrane-bound polyribosomes were isolated from the livers of untreated and clofibrate-treated male C57B1/6 mice. The in vitro translation products were investigated in a rabbit reticulocyte cell-free system by immunoprecipitation of cytosolic epoxide hydrolase, catalase and albumin. The soluble forms of epoxide hydrolase present in cytosol and in peroxisomes were found to be synthesized on free polyribosomes and could not be distinguished from each other, since only one primary translation product was found with the methods used. Clofibrate treatment was found to increase total protein synthesis, synthesis of soluble epoxide hydrolase and translational efficiency of the isolated polyribosomes.     

Microsomal and cytosolic epoxide hydrolases, the peroxisomal fatty acid beta-oxidation system and catalase. Activities, distribution and induction in rat liver parenchymal and non-parenchymal cells

A number of structurally unrelated hypolipidaemic agents and certain phthalate-ester plasticizers induce hepatomegaly and proliferation of peroxisomes in rodent liver, but there is relatively limited data regarding the specific effects of these drugs on liver non-parenchymal cells. In the present study, liver parenchymal, Kupffer and endothelial cells from untreated and fenofibrate-fed rats were isolated and the activities of two enzymes associated with peroxisomes (catalase and the peroxisomal fatty acid beta-oxidation system) as well as cytosolic and microsomal epoxide hydrolase were measured. Microsomal epoxide hydrolase, cytosolic epoxide hydrolase and catalase activities were 7-12-fold higher in parenchymal cells than in Kupffer or endothelial cells from untreated rats; the peroxisomal fatty acid beta-oxidation activity was only detected in parenchymal cells. Fenofibrate increased catalase, cytosolic epoxide hydrolase and peroxisomal fatty acid beta-oxidation activities in parenchymal cells by about 1.5-, 3.5- and 20-fold, respectively. The induction of catalase (2-3-fold) and cytosolic epoxide hydrolase (3-5-fold) was also observed in Kupffer and endothelial cells; furthermore, a low peroxisomal fatty acid beta-oxidation activity was detected in endothelial cells. Morphological examination by electron microscopy showed that peroxisomes were confined to liver parenchymal cells in untreated animals, but could also be observed in endothelial cells after administration of fenofibrate.     

Morphometric and cytochemical evaluation of clofibrate-induced peroxisomal proliferation in adult rat hepatocytes cultured on floating collagen gels

The present ultrastructural morphometric and cytochemical studies demonstrate clofibrate induced changes in peroxisomes in adult rat hepatocytes maintained for 14 days in primary culture on floating collagen gels. Catalase activity and the number and diameter of peroxisomes were reduced in hepatocytes cultured for between 2/3 and 7 days. However, hepatocytes cultured for 7-14 days had well-developed peroxisomes containing crystalloid nucleoids. The number of anucleoid peroxisomes in hepatocytes treated with 2 mM Na clofibrate increased with culture age, and by day 14 the number was 2.9 times greater than in freshly isolated hepatocytes. Catalase activity, as well as the number of nucleoid-containing peroxisomes were much greater in treated hepatocytes than in untreated controls, but decreased slightly with culture age. The diameter of peroxisomes was not reduced in the treated cells. These results suggest that the treatment with Na clofibrate is effective both for proliferation and maintenance of peroxisomes and for enhancing catalase activity. In treated hepatocytes, matrical plates were formed in peroxisomes from days 5 to 14 and the number of plate-containing peroxisomes increased with culture age.     

Hepatic enzymes, CoASH and long-chain acyl-CoA in subcellular fractions as affected by drugs inducing peroxisomes and smooth endoplasmic reticulum

1. The activities of acyl-CoA hydrolase, catalase, urate oxidase and peroxisomal palmitoyl-CoA oxidation as well as the protein content and the level of CoASH and long-chain acyl-CoA were measured in subcellular fractions of liver from rats fed diets containing phenobarbital (0.1% w/w) or clofibrate (0.3% w/w). 2. Whereas phenobarbital administration resulted in increased microsomal protein, the clofibrate-induced increase was almost entirely attributed to the mitochondrial fraction with minor contribution from the light mitochondrial fraction. 3. The specific activity of palmitoyl-CoA hydrolase in the microsomal fraction was only slightly affected while the mitochondrial enzyme was increased to a marked extent (3-4-fold) by clofibrate. 4. Phenobarbital administration mainly enhanced the microsomal palmitoyl-CoA hydrolase. 5. The increased long-chain acyl-CoA and CoASH level observed after clofibrate treatment was mainly associated with the mitochondrial, light mitochondrial and cytosolic fractions, while the slight increase in the levels of these compounds found after phenobarbital feeding was largely of microsomal origin. 6. The findings suggest that there is an intraperoxisomal CoASH and long-chain acyl-CoA pool. 7. The specific activity of palmitoyl-CoA hydrolase, catalase and peroxisomal palmitoyl-CoA oxidation was increased in the lipid-rich floating layer of the cytosol-fraction. 8. The changes distribution of the peroxisomal marker enzymes and microsomal palmitoyl-CoA hydrolase after treatment with hypolipidemic drugs may be related to the origin of peroxisomes.     

Increase in hepatic catalase and glycerol phosphate dehydrogenase activities on administration of clofibrate and clofenapate to the rat

1. Clofenapate (methyl 2-[4-(p-chlorophenyl)phenoxy]-2-methylpropionate) fed to the rat in the diet increased the content of mitochondrial protein in the liver by 50-60%. In this respect it resembled the related compound clofibrate (ethyl alpha-p-chlorophenoxyisobutyrate), which is widely used as an antihypercholesterolaemic drug. 2. Both compounds when fed to the rat enhanced the activity of alpha-glycerol phosphate dehydrogenase in the liver mitochondria manyfold, but were without effect on the enzyme in the soluble fraction. 3. On the other hand, the catalase activity in the supernatant fraction increased twofold after administration of the drugs. The mitochondrial catalase activity showed a consistent decrease. 4. It was unlikely that under the influence of the drug the increase in catalase activity took place in the mitochondrial particles and was leached into the cytosol during isolation. 5. The increase in catalase activity in the cytosol under the influence of the drug is best explained on the assumption that peroxisomes which contain this enzyme, and which are known to increase on administration of the drug, were broken during the process of cellular fractionation and released the enzyme into the cytosol. 6. All the above effects shown by both drugs were fully reversed when drugs were withdrawn from the diet. 7. Clofenapate was effective in bringing about the above changes when administered to the animal at one-hundredth the concentration of clofibrate.     

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.     

Comparison of the potencies of (+)- and (−)-2-ethylhexanoic acid in causing peroxisome proliferation and related biological effects in mouse liver

Male C57BL/6 mice were exposed to 1% (w/w) (+)- or (?)-2-ethylhexanoic acid or an equimolar mixture of these enantiomers in their diet for 4 or 10 days. A significant increase in liver weight and a 2- to 3-fold increase in the protein content of the mitochondrial fraction were seen in all cases. Peroxisomal palmitoyl-CoA oxidation was increased 2- to 3.5-fold after 4 days of treatment and 4- to 5-fold after 10 days, while the corresponding increases in peroxisomal lauroyl-CoA oxidase activity were 2- to 3-fold and 9- to 12-fold, respectively. Peroxisomal catalase activity was unchanged, whereas the microsomal and cytosolic activities were increased 2- to 3-fold and 6- to 16-fold, respectively. These treatments also induced microsomal ω-hydroxylation of lauric acid 7-fold and soluble epoxide hydrolase activity in the mitochondrial and cytosolic fractions, as well as microsomal epoxide hydrolase activity about 50–100%. The only significant differences observed between the effects of (+)-2-ethylhexanoic acid and its (?)-enantiomer were on peroxisomal palmitoyl-CoA oxidation and lauroyl-CoA oxidase activity after 4 days of treatment. In both these cases the (+)-enantiomer resulted in increases which were 50–75% greater than those seen with the (?)-form. © 1994 Wiley-Liss, Inc.     

Properties of peroxisomes and their induction by clofibrate in normal adult rat hepatocytes in primary culture

Alterations in peroxisomes and catalase activity and their responsiveness to clofibrate in adult rat hepatocytes in primary culture were investigated. The numbers of peroxisomes with and without crystalloid nucleotids per unit cytoplasmic area were preserved in cultured hepatocytes for 2 d after seeding at a level comparable to that of freshly isolated hepatocytes. At Day 3 in culture, the number of anucleoid peroxisomes was reduced in untreated hepatocytes, accompanied by more significant reduction in the number of nucleoid-containing peroxisomes, which decreased until Day 5. Peroxisome diameters were reduced in untreated hepatocytes at Day 2 and this decrease in the diameter was continued until Day 7. Catalase activity in untreated hepatocytes decreased markedly with culture age. The number of anucleoid peroxisomes was significantly greater in hepatocytes treated with 2 mM clofibrate in culture than in freshly isolated hepatocytes for 2 d or in untreated hepatocytes of the same culture age through 7 d. The number of nucleoid-containing peroxisomes in the treated cells began to decrease in 3 d, but was greater than that of untreated cells at Days 3 and 5. Peroxisomes with well-developed nucleoids were observed frequently in the treated cells even at Day 7. Peroxisome diameters were greater in the treated cells than in untreated cells at Days 3, 5, and 7. Catalase activity was always higher in the treated cells than in untreated cells. These results suggest that clofibrate is effective in inducing peroxisome proliferation as well as in maintaining the organelles in cultured hepatocytes.     

Carnitine octanoyltransferase of mouse liver peroxisomes: properties and effect of hypolipidemic drugs

Carnitine octanoyltransferase (COT) in 500g supernatant fluids from mouse liver has a specific activity at least twice that of carnitine acetyltransferase (CAT) or carnitine palmitoyltransferase (CPT). When mice are fed diets containing the lipid-lowering drugs, clofibrate or nafenopin, the specific activity of COT increases 4- and 11-fold, respectively. Liver homogenates from mice fed a control diet, and diets containing clofibrate, nafenopin, or Wy-14,643 were fractionated by sucrose gradient centrifugation, and the subcellular distribution of carnitine acyltransferases was determined. In the controls, peroxisomes contained about 70% of the total COT. The specific activity of COT in the peroxisomal peak was 12-fold greater than either CAT or CPT, and 20-fold greater than the COT activity in the mitochondrial fraction. Treatment with hypolipidemic drugs increased the specific activity of peroxisomal COT 2- to 3-fold and CAT 6- to 12-fold, while mitochondrial COT increased 5- to 11-fold and CAT 19- to 54-fold. COT was purified to homogeneity from livers of mice treated with Wy-14,643. It had an apparent Mr of 60,000 by Sephadex G-100 and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and a maximum activity for octanoyl-CoA with acetyl-CoA and palmitoyl-CoA having activities of 2 and 10%, respectively, when 100 microM acyl-CoA substrates were used. The Km's for 1-carnitine, octanoyl-CoA, palmitoyl-CoA, and acetyl-CoA were 130, 15, 69, and 155 microM, respectively, in the forward direction; and in the reverse direction were 110, 100, 104, and 783 microM for CoASH, octanoylcarnitine, palmitoylcarnitine, and acetylcarnitine, respectively. With Vmax conditions, acetyl-CoA and palmitoyl-CoA had activities of 8 and 26% of the activity for octanoyl-CoA, and acetylcarnitine and palmitoylcarnitine had activities of 7 and 22%, respectively, of the activity for octanoylcarnitine. It is concluded that COT is a separate enzyme present in large amounts in the matrix of mouse liver peroxisomes, with kinetic properties that greatly favor medium-chain acylcarnitine formation.     

Is the cytosolic catalase induced by peroxisome proliferators in mouse liver on its way to the peroxisomes?

Dietary treatment of male C57B1/6 mice with clofibrate, nafenopin or WY-14.643 resulted in a modest (at most 2-fold) increase in the total catalase activity in the whole homogenate and mitochondrial fraction prepared from the livers of these animals. On the other hand, the catalase activity recovered in the cytosolic fraction was increased 12- to 18-fold, i.e. 30-35% of the total catalase activity in the hepatic homogenate was present in the high-speed supernatant fraction after treatment with these peroxisome proliferators. A study of the time course of the changes in peroxisomal and cytosolic catalase activities demonstrated that the peroxisomal activity both increased upon initiation of exposure and decreased after termination of treatment several days after the increase and decrease, respectively, in the corresponding cytosolic activity. This finding suggests that the cytosolic catalase may be on its way to incorporation into peroxisomes.     

The regulation of cytosolic epoxide hydrolase in mice

The concentration of cytosolic epoxide hydrolase in untreated and clofibrate-treated mouse liver extracts was estimated by immunoblotting. Clofibrate treatment of mice was found to increase liver cytosolic epoxide hydrolase concentration by two fold, showing that the increase in cytosolic epoxide hydrolase in mouse liver after clofibrate treatment is primarily due to induction. The induced and uninduced cytosolic epoxide hydrolase, and epoxide hydrolase in the cytosolic and mitochondrial fractions were compared and found to be identical or very similar. Cytosolic epoxide hydrolases in kidney and liver were similar in molecular weight and antigenic properties.     

Hepatic levels of cytosolic, microsomal and 'mitochondrial' epoxide hydrolases and other drug-metabolizing enzymes after treatment of mice with various xenobiotics and endogenous compounds

This study was performed in order to study the response of epoxide hydrolases in different subcellular compartments of mouse liver to treatment with various compounds. Male C57BL/6 mice were treated with 31 different compounds--including traditional inducers of xenobiotic-metabolizing systems, liver carcinogens, stilbene derivatives, endogenous compounds and various other drugs and xenobiotics. The effects on liver somatic index; protein contents in 'mitochondria', microsomes and cytosol prepared from the liver; epoxide hydrolase activity towards trans- or cis-stilbene oxide in these three fractions; microsomal cytochrome P-450 content; cytosolic and 'mitochondrial' glutathione transferase activity and cytosolic DT-diaphorase activity were then determined. Cytosolic epoxide hydrolase activity was induced by chlorinated paraffins, di(2-ethylhexyl)phthalate and clofibrate and depressed by alpha-naphthylisothiocyanate, 3-methylcholanthrene, benzil and quercitin. Radial immunodiffusion revealed similar changes in the amount of enzyme protein present, except for two cases, where the increase in amount was larger; and the enzyme seems to be inhibited by benzil. Microsomal epoxide hydrolase activity was induced by these same compounds and several others as well, including dibenzoylmethane, butylated hydroxyanisole and polychlorinated biphenyls. 'Mitochondrial' epoxide hydrolase activity towards trans-stilbene oxide was not affected by those compounds which induced the cytosolic enzyme, but increased about two-fold after treatment with 2-acetylaminofluorene, DL-ethionine, aflatoxin B1 and phenobarbital. There does not seem to be any co-regulation of different forms of epoxide hydrolase in mouse liver. In general small effects were observed on liver weight and protein contents in the different subcellular fractions. Polychlorinated biphenyls were the most potent of the 8 compounds which induced cytochrome P-450, while butylated hydroxyanisole induced cytosolic glutathione transferase activity to the highest extent. 'Mitochondrial' glutathione transferase activity was most induced by certain of the stilbene derivatives. The most potent inducers of DT-diaphorase activity were 3-methylcholanthrene, polychlorinated biphenyls and dinitrotoluene.     

Properties of peroxisomes and their induction by clofibrate in normal adult rat hepatocytes in primary culture

Summary Alterations in peroxisomes and catalase activity and their responsiveness to clofibrate in adult rat hepatocytes in primary culture were investigated. The numbers of peroxisomes with and without crystalloid nucleoids per unit cytoplasmic area were preserved in cultured hepatocytes for 2 d after seeding at a level comparable to that of freshly isolated hepatocytes. At Day 3 in culture, the number of anucleoid peroxisomes was reduced in untreated hepatocytes, accompanied by more significant reduction in the number of nucleoid-containing peroxisomes, which decreased until Day 5. Peroxisome diameters were reduced in untreated hepatocytes at Day 2 and this decrease in the diameter was continued until Day 7. Catalase activity in untreated hepatocytes decreased markedly with culture age. The number of anucleoid peroxisomes was significantly greater in hepatocytes treated with 2mM clofibrate in culture than in freshly isolated hepatocytes for 2 d or in untreated hepatocytes of the same culture age through 7 d. The number of nucleoid-containing peroxisomes in the treated cells began to decrease in 3 d, but was greater than that of untreated cells at Days 3 and 5. Peroxisomes with well-developed nucleoids were observed frequently in the treated cells even at Day 7. Peroxisome diameters were greater in the treated cells than in untreated cells at Days 3, 5, and 7. Catalase activity was always higher in the treated cells than in untreated cells. These results suggest that clofibrate is effective in inducing peroxisome proliferation as well as in maintaining the organelles in cultured hepatocytes. This work was supported in part by Grants-in-Aid for Scientific Research from Ministry of Education, Science and Culture, Japan, 448143, 50168, 501069, and 577196, and by a Grant-in-Aid from Hokkaido Geriatrics Research Institute.     

Alterations in the integrity of peroxisomal membranes in livers of mice treated with peroxisome proliferators

Catalase leakage from its particulate compartment within the light mitochondrial fraction of liver was used as an index of the integrity of peroxisomes in untreated mice and in mice treated with the peroxisome proliferators clofibrate(ethyl-p-chlorophenoxyisobutyrate), Wy-14,643(4-chloro-6[2,3-xylidino)-2-pyrimidinylthio]acetic acid) and DEHP(di-(2-ethylhexyl)phthalate).Catalase leakage represented about 2% of the total catalase activity when fractions from untreated mice were incubated at 4°C, increasing to about 5% during 60 min incubation at 37°C. In fractions from livers of mice treated with peroxisome proliferators, catalase leakage was significantly higher, being 7–11% at 4°C and increasing to approximately 20% after 60 min incubation at 37°C. The pattern of release was similar for all proliferators. Parallel data were obtained for catalase latency in these fractions, i.e. following 60 min incubation at 37°C, free (non-latent) catalase activity was 18% in control mice and 65, 67, and 83% in fractions from clofibrate-, Wy-14,643- and DEHP-treated mice, respectively. Differences in catalase leakage from peroxisomes in fractions from untreated mice and clofibrate-treated mice were also apparent following treatments designed to effect membrane permeabilization, as in freeze-thawing, osmotic rupture, and extraction with Triton X-100 and lysophosphatidylcholine.These data are consistent with a significant alteration in the integrity of the membranes of peroxisomes in livers of mice which have been treated with peroxisome proliferators, and furthermore indicate a commonality of effect of these agents.     

Cardiac and hepatic acyl-CoA and acylcarnitine hydrolase activities in clofibrate-fed rabbits

Catalase activity in the heart of male rabbits was 21% of that found in the liver; clofibrate feeding (0.3% w/w for 10 days) resulted in an 80% increase in both cardiac and hepatic catalase activities. Fatty acyl-CoA oxidase activity in control heart was 11% of that found in control liver; this peroxisomal activity did not increase subsequent to clofibrate feeding. Only acyl-CoA hydrolase activity in the cardiac supernatant was elevated by clofibrate feeding. Acylcarnitine hydrolase activity was increased significantly in the homogenate, extract and supernatant of both heart and liver from the clofibrate-fed rabbit. Clofibrate feeding increased CoASH and carnitine tissue levels in heart and liver.     

Effects of ethanol and clofibrate on expression of cytochrome P-450 enzymes and epoxide hydrolase in cultures and cocultures of rat hepatocytes

Cultured and cocultured rat hepatocytes were used to study the effects of ethanol and clofibrate on cytochrome P-450 (P-450) enzymes and epoxide hydrolase. We showed that in the presence of ethanol, clofibrate or both compounds, rat hepatocytes were able to express, after 3 days of pure culture, quantitatively and qualitatively reasonable levels of most cytochrome P-450 enzymes and epoxide hydrolase, compared to freshly isolated hepatocytes. However, ethanol induced the P-450IA subfamily, and clofibrate the P-450-IVA subfamily. In cocultures, after 6 days, most P-450 enzymes were still expressed while P-450IIC11 was completely lost. Ethanol and clofibrate had the same effect as in pure culture. These results show, by modifying culture medium conditions and cell-cell interactions, that it is possible to maintain reasonable xenobiotic-metabolizing-enzyme expression; however, these conditions have to be improved in order to preserve better P-450 expression. The mechanism of these effects and the inducibility of these systems remain to be elucidated by a study at molecular level.     

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.     

Comparison of crude and affinity purified cytosolic epoxide hydrolases from hepatic tissue of control and clofibrate-fed mice

An affinity purification procedure was developed for the cytosolic epoxide hydrolase based upon the selective binding of the enzyme to immobilized methoxycitronellyl thiol. Several elution systems were examined, but the most successful system employed selective elution with a chalcone oxide. This affinity system allowed the purification of the cytosolic epoxide hydrolase activity from livers of both control and clofibrate-fed mice. A variety of biochemical techniques including pH dependence, substrate preference, kinetics, inhibition, amino acid analysis, peptide mapping, Western blotting, analytical isoelectric focusing, and gel permeation chromatography failed to distinguish between the enzymes purified from control and clofibrate-fed animals. The quantitative removal of the cytosolic epoxide hydrolase acting on trans-stilbene oxide from 100,000g supernatants, allowed analysis of remaining activities acting differentially on cis-stilbene oxide and benzo[a]pyrene 4,5-oxide. Such analysis indicated the existence of a novel epoxide hydrolase activity in the cytosol of mouse liver preparations.