Evidence for a leukotriene A4 hydrolase in Xenopus laevis skin exudate

Leukotriene A4 hydrolase is a cytosolic metalloenzyme of the arachidonic acid biosynthetic pathway responsible for leukotriene A4 conversion into leukotriene B4. In addition to its epoxide hydrolase properties, this enzyme exhibits an aminopeptidase activity which was used as an assay to monitor the purification of a novel form of leukotriene A4 hydrolase from Xenopus laevis skin exudate. This 70 kDa, secreted, form of leukotriene A4 hydrolase was identified by immunochemical cross-reactivity with anti-human leukotriene A4 hydrolase antibodies and by its capacity to convert leukotriene A4 into leukotriene B4. Moreover this enzyme produced a second metabolite which could be the leukotriene B4 isomer 5S,12R-dihydroxy-6,10-trans-8,14-4-cis-eicosatetraenoic acid, previously shown by Strömberg et al. (Eur.J. Biochem. 238 (1996) 599-605) to be formed by incubation of the leukotriene A4 with amphibian tissue extracts. Partial amino acid sequencing of peptides generated by endolysin C fragmentation of the purified enzyme confirmed the presence, in X. laevis skin secretions, of a related but distinct form of leukotriene A4 hydrolase which is likely to be responsible for the production of these eicosanoid metabolites of leukotriene A4.

Impact of inherited polymorphisms in glutathione S-transferase M1, microsomal epoxide hydrolase, cytochrome P450 enzymes on DNA, and blood protein adducts of benzo(a)pyrene-diolepoxide

The benzo(a)pyrene (BaP) metabolite benzo(a)pyrenediolepoxide (BPDE) is strongly implicated as a causative agent of lung cancer. To assess the risk of exposure to BaP, we made a combined analysis of levels of BPDE adducts to hemoglobin (Hb), serum albumin (SA), and lymphocyte DNA in 44 patients with incident lung cancer, as a prototype of a population mainly exposed to tobacco-derived BaP. We also investigated whether genetic polymorphisms of cytochrome P450IA1 (CYPIA1), microsomal epoxide hydrolase (mEH), and glutathione S-transferase M1 (GSTM1), which are involved in BaP metabolism, can be determinants of adduct formation. BPDE-Hb, BPDE-SA, and BPDE-DNA adducts were quantified as BaP tetrols released from hydrolysis of macromolecules and measured by high-resolution gas chromatography-negative ion chemical ionization-mass spectrometry to achieve high specificity and sensitivity. Individuals with detectable Hb adducts were positive for SA adducts but not vice versa, suggesting that BPDE-Hb adducts are less informative indicators of BaP exposure. Using PCR methods on DNA, we characterized GSTM1 deletion, CYPIA1 MspI and exon 7 valine variants, and mEH polymorphisms at amino acid positions 113 (EH3) and 139 (EH4). Levels of BPDE adducts were no different among CYPIA1, mEH, and GSTM1 genotypes. However, individuals with measurable BPDE-SA adducts were CYPIA1 variant carriers more frequently (P = 0.03). There was a slightly higher percentage of DNA detectable adducts in subjects with CYPIA1 exon 7 valine polymorphism. When subjects were classified by both polymorphisms on the mEH gene, those with two slow alleles (EH3 homozygous mutated) and no fast alleles (EH4 homozygous wild type) had a lower frequency of BPDE-SA adducts and no DNA adducts (P = 0.06). These results are based on a small number of observations thus far, but this exploratory study suggests that CYPIA1 and mEH variants might have an impact on BPDE exposure markers such as BPDE-SA adducts. Chemical specificity in adduct measurements is important to identify the biomarkers that reflect BaP exposure more accurately.

Differential leukotriene constrictor responses in human atherosclerotic coronary arteries

BACKGROUND

Leukotrienes are a class of biologically active lipids that have potent effects on the heart. To assess their role in coronary artery disease, we compared the contractile responses of leukotriene C4 (LTC4) and leukotriene D4 (LTD4) and their binding activity in both atherosclerotic and nonatherosclerotic human coronary arteries. We also studied expression of the enzymes that control their formation to understand how the 5-lipoxygenase (5-LO) pathway is activated in the coronary arteries.

METHODS AND RESULTS

The capacity of leukotrienes to affect coronary vessel tone and the influence of atherosclerosis was tested in organ baths. Leukotriene receptors were examined by autoradiography, and antibody binding to the various enzymes responsible for their formation was assessed by use of immunocytochemistry. Nonatherosclerotic coronary artery ring segments were unresponsive to LTC4 and LTD4. In contrast, LTC4 and LTD4 induced concentration-dependent contractions in atherosclerotic coronary arteries. Specific [3H]-LTC4 but not LTD4 binding to atherosclerotic coronary artery was evident, with no evidence of specific binding of [3H]-leukotrienes to nonatherosclerotic coronary artery. High-resolution autoradiography identified specific [3H]-LTC4 binding sites to smooth muscle cell and to regions of intimal proliferation and plaque. Cells showing positive antibody binding to 5-LO, FLAP (5-lipoxygenase activating protein), and leukotriene A4 hydrolase were also present in the coronary arteries and had a similar distribution to macrophages.

CONCLUSIONS

Atherosclerosis is associated with a specific leukotriene receptor(s) capable of inducing hyperreactivity of human epicardial coronary arteries in response to LTC4 and LTD4.

Quantification of multiple human cytochrome P450 mRNA molecules using competitive reverse transcriptase-PCR

We developed a quantitative competitive reverse transcriptase-polymerase chain reaction (QC RT-PCR) assay to measure mRNA levels of seven human cytochrome P450 (P450, CYP) genes and microsomal epoxide hydrolase (EH) simultaneously. This assay employs an exogenous recombinant RNA (rcRNA) molecule as an internal standard that shares PCR primer and hybridization probe sequences with CYP1A1, CYP1A2, CYP2A6/7, CYP2D6, CYP2E1, CYP2F1, CYP3A4/5/7, and EH mRNA. Because each rcRNA molecule contains several primer sequences, an entire battery of genes that exhibit differential responsiveness to various classes of xenobiotics may be measured simultaneously from one population of cDNA molecules. In this study, we demonstrated the precision and power of the assay using small amounts of human liver total RNA. We also report for the first time quantitative profiles of P450 and EH mRNA abundance in eight human livers. Cytochrome P450 2E1 mRNA maintained the highest abundance (average 6.67 x 10(7) molecules/microg of total RNA) and least variation (13 fold) in all livers examined. Cytochrome P450 1A2, CYP2A6/7, CYP2D6, CYP3A4/5, and EH mRNAs were approximately one order of magnitude less abundant than CYP2E1 transcripts, with CYP2D6 levels exhibiting the greatest variation (220 fold) between individuals. This QC RT-PCR assay should prove valuable for measuring basal and induced mRNAs in different cell types in vitro, as well as in biomonitoring applications where individuals are exposed or hypersusceptible to certain xenobiotic-initiated toxicities.

Cyclosporin A down-regulates the LTA4 hydrolase level in human keratinocyte cultures

Leukotriene A4 hydrolase is a key enzyme in the biosynthesis of leukotriene B4, a potent pro-inflammatory compound. The purpose of this study was to determine the capacity of antiinflammatory and anti-proliferative compounds to regulate the levels and activity of leukotriene A4 hydrolase in cultured human keratinocytes. The content of leukotriene A4 hydrolase was determined by Western blot analysis, and the activity of leukotriene A4 hydrolase was expressed as the leukotriene B4 formation after incubation of keratinocyte cultures with leukotriene A4. Leukotriene B4 was measured by revered-phase high performance liquid chromatography. Preincubation for 10 min of the cultured keratinocytes with the leukotriene A4 hydrolase inhibitor RP 64699 (0.1-10 microM) caused a significant dose-dependent inhibition of leukotriene B4 formation (IC50 = 0.7 microM). Cyclosporin A (0.1 micrograms/ml and 1.0 micrograms/ml) had no direct effect on leukotriene A4 hydrolase activity, but after incubation for 72 h there was a decrease in the mean leukotriene B4 formation per culture dish (35% and 48%, respectively). The decreased leukotriene B4 formation was caused mainly by a decrease in the mean leukotriene A4 hydrolase content per mg protein (30.1% at 0.1 micrograms/ml cyclosporin A and 40.0% at 1.0 micrograms/ml cyclosporin A), although keratinocyte proliferation was also slightly decreased. Incubations with 1.25-dihydroxyvitamin D3 (10(-7)-10(-10) M), all-trans retinoic acid (10(-6)-10(-10) M), eicosartienoic acid (10(-6)-10(-8) M), dexamethasone (10(-5)-10(-7) M), interferon-gamma (10 and 100 units/ml) or methotrexate (0.1-10 micrograms/ml) had no effect on either the leukotriene B4 formation or the amount of leukotriene A4 hydrolase in keratinocyte cultures. These results show that cyclosporin A, in contrast to other anti-inflammatory and anti-proliferative compounds, inhibits the level of leukotriene A4 hydrolase in keratinocyte cultures. Since similar cyclosporin A concentrations are obtained during treatment of psoriasis with cyclosporin A, the effect on leukotriene A4 hydrolase may play a role in the anti-inflammatory action of cyclosporin A.

Epoxide hydrolase in human and rat peroxisomes: implication for disorders of peroxisomal biogenesis

To understand the basis of excretion of excessive amounts of epoxydicarboxylic fatty acids (EDFA) in urine of patients with disorders of peroxisomal biogenesis (Pitt, J. J., and A. Poulos. 1993. Clin. Chim. Acta. 223: 23-29), the activity of epoxide hydrolase (EH) was measured in cultured skin fibroblasts from control subjects and patients with peroxisomal disorders. EH activity was approximately 40% lower in fibroblasts that lack intact peroxisomes (Zellweger syndrome), whereas the activity in other peroxisomal disorders (X-adrenoleukodystrophy and rhizomelic chondrodysplasia punctata) with intact peroxisomes was similar to control. To identify the specific enzyme/organelle that represents the decrease in EH activity in Zellweger cells, we have analyzed this activity in different subcellular organelles from control and Zellweger skin fibroblasts. EH activity was enriched in peroxisomes from control fibroblast. EH activity in isolated mitochondria, microsomes, or cytosol from Zellweger fibroblast was similar to that of control fibroblast. These observations indicate that deficient activity of EH in cells from Zellweger patients is due to lack of peroxisomal EH activity. The peroxisomal EH is differentially induced to a higher degree by ciprofibrate, a hypolipidemic agent and peroxisome proliferator, than EH activity in other organelles and cytoplasm. The high specific activity of EH in peroxisomes and differential induction of EH activity in peroxisomes as compared to other organelles, and the excretion of EDFA in patients who lack peroxisomes suggests that peroxisomal EH may be responsible for the detoxification of EDFA, and that this enzyme in peroxisomes may be a different protein than the EH found in other organelles.

Improved radiolabeled substrates for soluble epoxide hydrolase

Two rapid assays for the soluble epoxide hydrolase (sEH) are described. First, a sensitive radiometric assay based on thin-layer chromatography of [(14)C]-cis-9,10-epoxystearic acid and its corresponding diol ((14)C]-9,10-dihydroxystearic acid) is described. The cis fatty acid oxide exhibits higher specific activity of hydration with sEH from mouse, rat, human, and potato compared to trans-stilbene oxide (TSO). The K(m) and V(max) obtained for [(14)C]-cis-9,10-epoxystearic acid with mouse sEH are 11.0 microM and 3460 nmol/min/mg protein, respectively. [(14)C]-cis-9,10- Epoxystearic acid might more closely mimic the structures of natural substrates for sEH. Second, [2-(3)H]-trans-1,3-diphenyl-propene oxide ([(3)H]-tDPPO) and [2-(3)H]-cis-1,3-diphenylpropene oxide ([(3)H]-cDPPO) were synthesized and rapid radiometric assays for epoxide hydrolases (EHs) were developed by differential partitioning of the epoxide into iso-octane and its corresponding diol into aqueous phase containing methanol. It was shown that sEHs from mouse, rat, human, and potato rapidly hydrolyze [(3)H]-tDPPO and in comparison to TSO have 20-,49-,28-, and 7-fold higher rates, respectively. Mouse sEH hydrates [(3)H]-tDPPO at 26,200 nmol/min/mg protein, and a K(m)p4 of 2.80 microM is observed.

Fluorometric microassay to quantify microsomal epoxide hydrolase in 96-well plates

The microfluorometric assay, suitable for measuring microsomal epoxide hydrolase activity in cultured cells, is based on the conversion of benzo[a]pyrene-7,8-epoxide to the corresponding trans-benzo[a]pyrene-7,8-dihydrodiol, a compound that fluoresces at 403 nm when excited at 365 nm. Activity is determined by incubating S9 fractions obtained from microcultures with this epoxide and monitoring the fluorescence with a microplate reader. Under the assay conditions selected, the photodecomposition of the reaction product was minimized and the linearity of the reaction was extended. The major advantages of this method are: (1) high sensitivity with a detection limit of 5 pmol/well of trans-benzo[a]pyrene-7,8-dihydrodiol formed, which is comparable to the most sensitive radioactive methods; (2) minimal sample requirement (1-5 micrograms liver microsomes; 10-50 micrograms S9 fraction from cultured cells); (3) reduced consumption of hazardous reagents; and (4) a considerable reduction in assay time and facility for simultaneous determination of enzyme activity in multiple samples.

Distribution of microsomal epoxide hydrolase and glutathione S-transferase in the rat olfactory mucosa: relevance to distribution of lesions caused by systemically-administered olfactory toxicants

This study represents part an of ongoing effort to understand the mechanism underlying the distribution of the olfactory mucosal lesion resulting from the systemic administration of compounds such as 2,6-dichlorobenzonitrile (dichlobenil) and beta,beta'-iminodipropionitrile (IDPN). Immunohistochemistry was performed to localize the microsomal form of epoxide hydrolase (mEH) and glutathione S-transferase (GST) isozymes alpha, mu and pi in the rodent olfactory mucosa. GST-pi was found in abundance in the Bowman's glands of the mucosa lining the dorsal medial meatus (DMM) of the nasal cavity and in the nuclei of basal and sustentacular cells of the dorsal and lateral nasal cavity. Liver and olfactory mucosal levels of mEH are equivalent by Western blot analysis. mEH appeared to be localized in the apical cytoplasm of sustentacular cells in all regions of the olfactory mucosa except for the epithelium lining the DMM. These observations, coupled with the known profile of metabolites for dichlobenil, suggest that systemically-administered compounds causing site-specific lesions in the epithelium lining the DMM of the nasal cavity may do so by the in situ production of reactive epoxide metabolites which are then poorly capable of being detoxified. Thus, the distribution of metabolic enzymes, rather than the absolute level of an enzyme in a tissue, may dictate lesion distribution in the case of toxicants which are bioactivated in target tissues.

Localization of microsomal epoxide hydrolase in normal and neoplastic human kidney

Microsomal epoxide hydrolase is a xenobiotic metabolizing enzyme that catalyzes the conversion of toxic and carcinogenic epoxides to less toxic dihydrodiols. The cellular localization and distribution of microsomal epoxide hydrolase were investigated for the first time in normal and neoplastic human kidney. Light microscopic immunohistochemical studies using an alkaline phosphatase-anti-alkaline phosphatase technique showed that in normal kidney there was a wide distribution of epoxide hydrolase immunoreactivity. The main localization of epoxide hydrolase immunoreactivity was to the proximal and distal tubule epithelial cells. Strong epoxide hydrolase immunoreactivity was also identified in epithelium of the collecting ducts. In addition, epoxide hydrolase immunoreactivity was present in vascular endothelial cells, including endothelial cells lining glomerular capillaries. Epoxide hydrolase immunoreactivity was identified in all the renal tumors, and in each tumor immunoreactivity for epoxide hydrolase was localized to tumor cells. Immunoblotting of both normal kidney and tumor microsomes confirmed the presence of a single protein band of molecular weight 49 KD corresponding to the molecular weight of human hepatic microsomal epoxide hydrolase.

Expression of cytochrome P450 in rat pleural mesothelial cells in secondary cultures

Cultured rat pleural mesothelial cells (RPMC) isolated from male Sprague-Dawley rats have been shown to metabolize polycyclic aromatic hydrocarbons to more oxygenated metabolites. This capacity, which is maintained with passages, suggested the presence of monooxygenase enzymes. In order to clarify the enzymatic pathway, we investigated the expression of cytochromes P450 (CYP) in cultured RPMC by Western and Northern blot analyses. Cells were cultured in Ham's F10 medium supplemented with 10% fetal calf serum. The CYP expression was studied from passage 9 to 16 on different cell strains treated for 48 hours with P450 inducers. CYP1A1 apoprotein expression was very low in untreated cells, but was markedly induced after treatment with 1 microM 3-methylcholanthrene or 22 microM beta-naphthoflavone. CYP1A1 mRNA was not detected in untreated cells and appeared after 3-methylcholanthrene treatment. CYP2E1 apoprotein was constitutively expressed in cultured RPMC, and markedly increased by 170 mM ethanol, and 0.1 microM or 1 microM dexamethasone treatments. Unexpectedly, whereas the amount CYP2E1 mRNA was not modified by ethanol treatment, dexamethasone has a marked inductive effect on CYP2E1 mRNA level. The CYP expression pattern was found similar in RPMC issued from different rats, and not dependent on passage number. The CYP expression and the detection of NADPH-P450 reductase, and of epoxide hydrolase, ascertained that RPMC contain the overall enzymatic pathway required for the biotransformation and activation of procarcinogen compounds, such as polycyclic aromatic hydrocarbons and nitrosamines. Both expression and regulation properties are maintained in long-term cultures of RPMC.

Cytochrome P450 expression in oesophageal cancer

The cytochrome P450 superfamily of enzymes play a central part in the metabolism of carcinogens and anti-cancer drugs. The expression, cellular localisation, and distribution of different forms of P450 and the functionally associated enzymes epoxide hydrolase and glutathione S-transferases have been investigated in oesophageal cancer and non-neoplastic oesophageal tissue using immunohistochemistry. Expression of the different enzymes was confined to epithelial cells in both non-neoplastic samples and tumour samples except the CYP3A was also identified in mast cells and glutathione S-transferase pi was present in chronic inflammatory cells. CYP1A was present in a small percentage of non-neoplastic samples but both CYP2C and CYP3A were absent. Epoxide hydrolase was present in half of the non-neoplastic samples and the different classes of glutathione S-transferase were present in a low number of samples. In carcinomas CYP1A, CYP3A, epoxide hydrolase, and glutathione S-transferase pi were expressed in at least 60% of samples. The expression of glutathione S-transferases alpha and mu were significantly less in adenocarcinoma compared with squamous carcinoma.

Leukotriene A4 hydrolase in human skin

The biochemical properties and immunohistochemical localization of leukotriene (LT) A4 hydrolase were investigated in human skin. The activity of LTA4 hydrolase, which catalyzes the hydrolysis of LTA4 to LTB4, the most chemotactic compound known, was detected in the 100,000 x g supernatant of homogenates of human epidermis and a transformed epidermal cell line (HSC-1). No significant LTA4 hydrolase activity was detected in human whole skin or dermis. The enzymatic properties of LTA4 hydrolase isolated from human keratinocytes and peripheral leukocytes were similar. Their activities were inhibited by bestatin and captopril, and they were completely absorbed by anti-human LTA4 hydrolase antibody. By immunoblotting analysis using this antibody, LTA4 hydrolase was detected as a 70-kDa protein in human epidermis and HSC-1 and was found to be similar to the enzyme detected in peripheral mononuclear leukocytes. In human dermis, LTA4 hydrolase was barely detected by Western blotting. On the other hand, LTA4 hydrolase was demonstrated in the cytoplasm of keratinocytes in the epidermis, and in fibroblasts, infiltrating and endothelial cells in the dermis of normal human skin by immunohistochemical analysis using the immunoperoxidase method. These results suggest that LTB4 can be generated from LTA4 by LTA4 hydrolase in keratinocytes as well as fibroblasts, infiltrating and endothelial cells in the dermis of human skin.

Spectrophotometric substrates for cytosolic epoxide hydrolase

In this study, we demonstrate the utility of a broad class of spectrophotometric substrates for the assay of cytosolic epoxide hydrolase purified from murine liver. These substrates, epoxy esters or carbonates, cyclize spontaneously upon or during hydrolysis of the epoxide functionality. The alcohol released by cyclization may then be assayed directly or by coupling to a second reaction. The alcohol produced, or its secondary reaction products, can be selected to give an absorption in the visible or near-uv range of the spectrum. This allows the synthesis of a wide variety of useful spectrophotometric substrates. 4-Nitrophenyl (2S,3S)-2,3-epoxy-3-phenylpropyl carbonate, at pH 6.4 and 25 degrees C, had a Vmax of 22 mumol min-1 mg-1 and a Km of 16 microM when assayed with a conventional spectrophotometer. When assayed under the same conditions with a 96-well plate reader, the measured Vmax was 15 mumol min-1 mg-1 and the Km was 6.6 microM. Some of these compounds were also found to be substrates for glutathione S-transferase, microsomal epoxide hydrolase, and porcine liver carboxylesterase. Indeed, 4-nitrophenyl 3,4-epoxy-3-phenylbutanoate was a 3.4-fold better substrate for porcine liver carboxylesterase than 4-nitrophenyl acetate when initial rates of hydrolysis were measured under the same conditions.

Xenobiotic metabolising enzyme expression in colonic neoplasia

The cytochrome P450, epoxide hydrolase, and glutathione S-transferase enzyme families play an important part in the metabolism of many carcinogens and anti-cancer drugs. The expression of two forms of cytochrome P450 (P450 1A and P450 3A), epoxide hydrolase and of the alpha, mu, and pi forms of glutathione S-transferase in normal colon, colonic adenomas, and adenocarcinoma of the colon were studied by immunohistochemistry. This allowed the precise cellular site and distribution of each enzyme to be determined. Expression of all the xenobiotic metabolising enzymes studied was almost wholly confined to the epithelial cells, whether in normal, adenoma or carcinoma samples, except that cytochrome P450 3A was also identified in mast cells and glutathione S-transferase pi was also present in chronic inflammatory cells. Cytochrome P450 was present in only a small proportion of normal colon samples, whereas epoxide hydrolase and glutathione S-transferase mu were identified in about half, and glutathione S-transferase alpha and pi in most normal samples. By contrast all the enzyme forms studied were expressed in virtually all adenomas and in over half the carcinomas. These results suggest that cytochrome P450 1A and cytochrome P450 3A are more specific markers of colonic neoplasia than epoxide hydrolase or glutathione S-transferases alpha, mu, and pi.

Differential effects of human recombinant interleukin-1 beta and dexamethasone on hepatic drug-metabolizing enzymes in male and female rats

Interleukin-1 beta (IL-1) is one of the major inflammation mediators, commonly reported to be an inhibitor of hepatic drug metabolism. We studied the effect of IL-1 treatment on various drug-metabolizing enzymes in male and female rats. IL-1 induced both cytochrome P450 (P450) 3A1 activity and protein in females, but in males, IL-1 repressed P450 3A2 activity, without decreasing the protein. P450 1A1 was impaired in males, but was retained after dexamethasone pretreatment. IL-1 did not change P450 2B1/2 activity and protein, but counteracted their induction by dexamethasone. Uridine diphospho-glucuronosyltransferase (UGT) 1A2 (bilirubin) activity and its induction by dexamethasone were not affected by IL-1 treatment. Both P450 2C11 and epoxide hydrolase activities were repressed by IL-1 treatment, and both activities were impaired after dexamethasone treatment. These results clearly demonstrate that IL-1 acts at different steps of protein synthesis and gene expression. The effect of IL-1 on P450 was isoform-dependent, indicating that IL-1 can act on pretranscriptional events. The discrepancy between the variations of the activities and the protein of P450 3A2 suggests a post-translational regulation. For P450 2C11, 3A1, and for microsomal epoxide hydrolase, but not for P450 1A1 and 2B1/2, IL-1 mimics the glucocorticoid effects. These differential effects can affect the kinetics and the bioavailability of drugs used in pathologies in which IL-1 is increased.

Na(+)-dependent bile acid transport by hepatocytes is mediated by a protein similar to microsomal epoxide hydrolase

A protein mediating hepatocyte sodium-dependent bile acid transport across the sinusoidal plasma membrane has been purified by immunoprecipitation with monoclonal antibody (MAb) 25D-1, which specifically recognizes this protein on the surface of intact hepatocytes (Ananthanarayanan et al. J. Biol. Chem. 263: 8338-8343, 1988). The function of this protein was further established by proteoliposome reconstitution (von Dippe et al. J. Biol. Chem. 265: 14812-14816, 1990). NH2-terminal amino acid sequence analysis and amino acid composition revealed this protein to be closely related to the enzyme microsomal epoxide hydrolase (mEH). Both proteins exhibited the same elution times on a reverse-phase high-pressure liquid chromatography column, comigrated with an apparent molecular weight of 49,000 as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and possessed identical isoelectric points of 8.2. The MAb was capable of immunoprecipitating chromatographically purified mEH, as well as a protein derived from the sinusoidal plasma membrane that exhibited mEH activity comparable to that of the protein isolated from the endoplasmic reticulum. The subtilisin fragmentation patterns derived from chromatographically purified mEH and the MAb-precipitated plasma membrane protein were also identical. Hydropathy profile analysis of the amino acid sequence of mEH suggested the presence of four transmembrane domains. The results of these studies indicate that a protein that is involved in mediating sodium-dependent bile acid transport is closely related to mEH.

The expression of cytochrome P-450, epoxide hydrolase, and glutathione S-transferase in hepatocellular carcinoma

BACKGROUND

Cytochrome P-450, epoxide hydrolase, and glutathione S-transferase (GST) all play a key role in the metabolism of chemical carcinogens, mutagens, and various anti-cancer drugs. All these functionally associated enzymes might be involved in both the development of hepatocellular carcinoma and in determining the anti-cancer drug sensitivity of such tumors.

METHODS

The expression of two forms of cytochrome P-450 (P-450 IA and P-450 IIIA), microsomal epoxide hydrolase, and three classes of cytosolic GST (alpha, mu, and pi) have been studied immunohistochemically in human hepatocellular carcinoma.

RESULTS

The hepatocellular carcinomas were characterized by a consistently high expression of epoxide hydrolase and variable expression of the cytochromes P-450 and GST. Cytochrome P-450 IA and IIIA stained in 64.5% and 41.9% of the 31 hepatocellular carcinomas studied, respectively. Epoxide hydrolase was present in all tumors, and GST types alpha, pi, and mu were identified in 48.4%, 38.7%, and 74.2% of the hepatocellular carcinomas, respectively.

CONCLUSIONS

This study showed that the expression of xenobiotic-metabolizing enzymes in hepatocellular carcinoma is complex and the presence of different xenobiotic enzymes in hepatocellular carcinoma may contribute to the intrinsic drug resistance of these tumors.

Chronic physical activity: hepatic hypertrophy and increased total biotransformation enzyme activity

Does chronic voluntary physical activity alter hepatic or intestinal capacities for xenobiotic biotransformation? This question was investigated by comparing biotransformation enzyme activities in liver and small intestine of active and sedentary rats. Male rats allowed unlimited access to a running wheel and fed ad lib. for 6 weeks were weight-matched to sedentary controls; the active rats ate 22% more food than the sedentary rats (P less than 0.05). Active rats ran 2.8 +/- 0.6 miles/day. Liver weights were higher in the active rats (11.2 +/- 0.2 vs 9.8 +/- 0.2 g; P less than 0.05), as were total liver protein, and liver microsomal and cytosolic protein (P less than 0.05). As a result of liver hypertrophy, the active rats showed higher total liver activity of several biotransformation enzymes, including 2-naphthol sulfotransferase, styrene oxide hydrolase, benzphetamine N-demethylase, ethacrynic acid glutathione S-transferase and morphine UDP-glucuronosyltransferase (P less than 0.05). In contrast, there was no detectable difference in total liver N-acetyltransferase activity toward p-aminobenzoic acid, 2-naphthylamine, and 2-amino-fluorene as well as, relative hepatic enzyme activity (expressed per g liver or per mg protein) and total and relative intestinal enzyme activity. We conclude that chronic voluntary physical activity, accompanied by an increased food intake, results in liver hypertrophy and potentially increases total hepatic capacity to biotransform certain xenobiotic chemicals.

Cigarette smoke inhibits cytosolic but not microsomal epoxide hydrolase of human lung

The effect of cigarette smoke exposure on the activity of cytosolic and microsomal epoxide hydrolase (EH) has been investigated in human lung. Patients were classified as 'recent smokers' (n = 9) or 'non-recent smokers' (n = 10) according to whether they were or were not still smoking 1 month before surgery. Cytosolic EH was measured with [3H]trans-stilbene oxide as a substrate, whereas microsomal EH was measured with [7-3H]styrene oxide as a substrate. Microsomal EH activity did not differ between recent smokers (2.51 +/- 0.93 nmol min-1 mg-1) and non-recent smokers (2.74 +/- 1.10 nmol min-1 mg-1), whereas cytosolic EH activity was significantly lower in recent smokers (6.46 +/- 1.79 pmol min-1 mg-1) than in non-recent smokers (8.41 +/- 2.09 pmol min-1 mg-1, P less than 0.05). Cytosolic EH activity was correlated with the number of days that had passed since the cessation of smoking (r = 0.58, P less than 0.05) and the effect was dose-dependent, since the enzyme activity was inversely correlated with the number of cigarettes smoked per day (r = 0.85, P less than 0.01). This suggests that recent smoking exposure inhibits the activity of cytosolic EH but not microsomal EH, and that the inhibition increases with the number of cigarettes smoked per day. The contribution of cytosolic enzymes to xenobiotic metabolism may be remarkable in extrahepatic tissues. The inhibition of cytosolic EH by tobacco smoke may reduce the inactivation of carcinogenic epoxides in human lung tissues and so may increase a person's susceptibility to lung cancer.

Effects of monoterpenoids on in vivo DMBA-DNA adduct formation and on phase I hepatic metabolizing enzymes

We have previously demonstrated the anticarcinogenic effects of monocyclic monoterpenes such as limonene when given during the initiation phase of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary cancer in Wistar-Furth (WF) rats. Here we investigated the possible mechanisms for this chemoprevention activity including limonene's effects on DMBA-DNA adduct formation and hepatic metabolism of DMBA. Twenty-four hours after carcinogen administration, there were approximately 50% of the total DMBA-DNA adducts found in control animals formed in the liver, spleen, kidney and lung of limonene-fed animals. While circulating levels of DMBA and/or its metabolites were not different in control and limonene-fed rats, there was a 2.3-fold increase in DMBA and/or DMBA-derived metabolites in the urine of the limonene-fed animals. Studies of the effects of limonene and sobrerol, a hydroxylated monocyclic monoterpenoid with increased chemoprevention activity, on phase I metabolizing enzymes revealed that these terpenoids modulated cytochrome P450 (CYP) and epoxide hydratase (EH) activity. The 5% limonene diet increased total CYP to the same extent as phenobarbital (PB) treatment when compared to control, while 1% sobrerol (isoeffective in chemoprevention to 5% limonene) did not. However, both 5% limonene and 1% sobrerol diets greatly increased the levels of microsomal EH protein and associated hydrating activities towards benzo[a]pyrene 4,5-oxide when compared to control and PB treatment. These changes also modified the rate and regioselectivity of in vitro microsomal DMBA metabolism when compared to PB treatment or control. Identification of the specific isoforms of CYP induced by these terpenoids was performed using antibodies to CYP isozymes in Western blot analysis and inhibition studies of microsomal DMBA metabolism. Five per cent limonene was more effective than 1% sobrerol at increasing the levels of members of the CYP2B and 2C families but was equally effective at increasing EH. Furthermore, both terpenoid diets caused increased formation of the proximate carcinogen, DMBA 3,4-dihydrodiol. While these terpene-induced changes in hepatic CYP and EH do not explain the anticarcinogenic mechanism of these chemopreventive agents, or the ability of limonene systemically to reduce DMBA-DNA binding, they do reveal novel and selective induction mechanisms of hepatic enzymes.

Eicosanoid forming enzyme mRNA in human tissues. Analysis by quantitative polymerase chain reaction

The key enzymes in the formation of eicosanoids, including leukocyte 5-lipoxygenase (5LX), platelet 12-lipoxygenase (12LX), reticulocyte 15-lipoxygenase (15LX), prostaglandin G/H synthase cyclooxygenase, and leukotriene A4 (LTA) hydrolase have been studied extensively in recent years. Little is known, however, about the regulation of these enzymes at the gene level. We have developed a quantitative polymerase chain reaction (PCR) assay to quantify the mRNAs for these five enzymes, as well as for cytoplasmic beta-actin (bACT) mRNA. Human erythroleukemia (HEL) cells, which display megakaryocytic/erythroid characteristics, were selected as a source of RNA to characterize the assay. These cells expressed mRNA for bACT, LTA, cyclooxygenase, and 12LX (in decreasing order). mRNA for 5LX and 15LX was undetectable. Bronchoalveolar lavage fluid cells obtained from asthmatic patients, primarily alveolar macrophages, contained mRNA for bACT, LTA, 5LX, cyclooxygenase, and 15LX (in decreasing order). Treatment of HEL cells with phorbol 12-myristate 13-acetate or steroid administration to asthmatic patients apparently selectively regulated certain of these target genes. The utility of this assay in quantifying mRNA for the various target genes in blood cells, including platelets from patients with chronic myelogenous leukemia, has also been demonstrated. Studies on the regulation of genes for enzymes involved in the leukotriene and prostaglandin biosynthetic pathways, especially when only small tissue samples are available, will be facilitated with this approach.

Studies on the intracellular distributions of soluble epoxide hydrolase and of catalase by digitonin-permeabilization of hepatocytes isolated from control and clofibrate-treated mice

Digitonin permeabilization of hepatocytes from control and clofibrate-treated (0.5% by mass, 10 days) male C57bl/6 mice was used to study the intracellular distributions of soluble ('cytosolic') epoxide hydrolase and of catalase. The following conclusions were drawn. (1) About 60% of the total soluble epoxide hydrolase activity in control mouse hepatocytes is situated in the cytosol. (2) The rest is not mitochondrial, but probably peroxisomal. (3) Of the total catalase activity in control mouse hepatocytes, 5-10% is found in the cytosol. (4) Treatment of mice with clofibrate increases the total hepatocyte activity of soluble epoxide hydrolase 4-fold, but does not influence the relative distribution of this enzyme between cytosol and peroxisomes. (5) The total catalase activity is increased 3.5-fold by clofibrate treatment and 15-35% of this activity is shifted from the peroxisomes to the cytosol.