Publications

• Epoxide hydrolases: biochemistry and molecular biology.

  Fretland A.J., Omiecinski C.J.

  Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous envi ... >> More

  Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous environments and chemically reactive. In the case of xenobiotics and certain endogenous substances, epoxide intermediates have been implicated as ultimate mutagenic and carcinogenic initiators Adams et al. (Chem. Biol. Interact. 95 (1995) 57-77) Guengrich (Properties and Metabolic roles 4 (1982) 5-30) Sayer et al. (J. Biol. Chem. 260 (1985) 1630-1640). Therefore, it is of vital importance for the biological organism to regulate levels of these reactive species. The epoxide hydrolases (E.C. 3.3.2. 3) belong to a sub-category of a broad group of hydrolytic enzymes that include esterases, proteases, dehalogenases, and lipases Beetham et al. (DNA Cell Biol. 14 (1995) 61-71). In particular, the epoxide hydrolases are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding dihydrodiol products. Simple epoxides are hydrated to their corresponding vicinal dihydrodiols, and arene oxides to trans-dihydrodiols. In general, this hydration leads to more stable and less reactive intermediates, however exceptions do exist. In mammalian species, there are at least five epoxide hydrolase forms, microsomal cholesterol 5,6-oxide hydrolase, hepoxilin A(3) hydrolase, leukotriene A(4) hydrolase, soluble, and microsomal epoxide hydrolase. Each of these enzymes is distinct chemically and immunologically. Table 1 illustrates some general properties for each of these classes of hydrolases. Fig. 1 provides an overview of selected model substrates for each class of epoxide hydrolase. << Less

  Chem Biol Interact 129:41-59(2000) [PubMed] [EuropePMC]

  This publication is cited by 4 other entries.

• Structure of an atypical epoxide hydrolase from Mycobacterium tuberculosis gives insights into its function.

  Johansson P., Unge T., Cronin A., Arand M., Bergfors T., Jones T.A., Mowbray S.L.

  Epoxide hydrolases are vital to many organisms by virtue of their roles in detoxification, metabolism and processing of signaling molecules. The Mycobacterium tuberculosis genome encodes an unusually large number of epoxide hydrolases, suggesting that they might be of particular importance to thes ... >> More

  Epoxide hydrolases are vital to many organisms by virtue of their roles in detoxification, metabolism and processing of signaling molecules. The Mycobacterium tuberculosis genome encodes an unusually large number of epoxide hydrolases, suggesting that they might be of particular importance to these bacteria. We report here the first structure of an epoxide hydrolase from M.tuberculosis, solved to a resolution of 2.5 A using single-wavelength anomalous dispersion (SAD) from a selenomethionine-substituted protein. The enzyme features a deep active-site pocket created by the packing of three helices onto a curved six-stranded beta-sheet. This structure is similar to a previously described limonene-1,2-epoxide hydrolase from Rhodococcus erythropolis and unlike the alpha/beta-hydrolase fold typical of mammalian epoxide hydrolases (EH). A number of changes in the mycobacterial enzyme create a wider and deeper substrate-binding pocket than is found in its Rhodococcus homologue. Interestingly, each structure contains a different type of endogenous ligand of unknown origin bound in its active site. As a consequence of its wider substrate-binding pocket, the mycobacterial EH is capable of hydrolyzing long or bulky lipophilic epoxides such as 10,11-epoxystearic acid and cholesterol 5,6-oxide at appreciable rates, suggesting that similar compound(s) will serve as its physiological substrate(s). << Less

  J. Mol. Biol. 351:1048-1056(2005) [PubMed] [EuropePMC]

  This publication is cited by 2 other entries.

• 7-Dehydrocholesterol 5,6 beta-oxide as a mechanism-based inhibitor of microsomal cholesterol oxide hydrolase.

  Nashed N.T., Michaud D.P., Levin W., Jerina D.M.

  7-Dehydrocholesterol 5,6 beta-oxide covalently modifies and inactivates the rat liver microsomal enzyme cholesterol oxide hydrolase. The covalent modification is presumed to occur at the active site of the enzyme since 5,6 alpha-iminocholestanol, a potent competitive inhibitor of the enzyme, block ... >> More

  7-Dehydrocholesterol 5,6 beta-oxide covalently modifies and inactivates the rat liver microsomal enzyme cholesterol oxide hydrolase. The covalent modification is presumed to occur at the active site of the enzyme since 5,6 alpha-iminocholestanol, a potent competitive inhibitor of the enzyme, blocks incorporation of 3-[3H]-7-dehydrocholesterol 5,6 beta-oxide into the protein. Kinetics of the inactivation were measured both by following the loss of catalytic activity and by monitoring incorporation of 3-[3H]-7-dehydrocholesterol 5,6 beta-oxide into microsomal protein. Both the loss of catalytic activity and the incorporation of label followed first order kinetics. Linear plots of the reciprocal of the pseudo-first order rate constants for the loss of catalytic activity and for the incorporation of radioactivity versus reciprocal of inhibitor concentrations indicated saturation kinetics. The kinetic parameter kinac is found to be (2.83 +/-0.43)10(-3) s-1 measured either by incorporation of tritium (300 mM potassium phosphate buffer, pH 8.0, 2.4 mg of microsomal protein/ml at 37 degrees C) or by the loss of catalytic activity (300 mM potassium phosphate buffer, pH 7.5, 0.99 mg of microsomal protein/ml at 37 degrees C). Unlike xenobiotic microsomal epoxide hydrolase (EC 3.3.2.3) which is not inactivated or inhibited by 7-dehydrocholesterol 5,6 beta-oxide, cholesterol oxide hydrolase appears to hydrolyze cholesterol oxides via a positively charged transition state. << Less

  J Biol Chem 261:2510-2513(1986) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• Existence of multiple forms of microsomal epoxide hydrolases with radically different substrate specificities.

  Oesch F., Timms C.W., Walker C.H., Guenthner T.M., Sparrow A., Watabe T., Wolf C.R.

  Evidence for the existence in rat and rabbit liver of two microsomal epoxide hydrolases with radically different substrate specificities was obtained, one with a broad specificity (EHb), whilst the other catalyzed the hydrolysis of cholesterol 5 alpha,6 alpha-oxide (EHch), a reaction taken as diag ... >> More

  Evidence for the existence in rat and rabbit liver of two microsomal epoxide hydrolases with radically different substrate specificities was obtained, one with a broad specificity (EHb), whilst the other catalyzed the hydrolysis of cholesterol 5 alpha,6 alpha-oxide (EHch), a reaction taken as diagnostic since it was not observed with pure fractions of EHb. The two enzymes were physically separated by immunoprecipitation using antibodies which had been raised against EHb purified to apparent homogeneity. The substrate specificity of the two enzymes is radically different and mutually complementary. Cholesterol 5 alpha,6 alpha-oxide has a trisubstituted oxirane ring. All epoxides of this nature tested to date were not, or very poor, substrates of EHb. The two enzymes can also effectively be discriminated by inhibitors, in that 5 alpha,6 alpha-imino-5 alpha-cholestane-3 beta-ol potently inhibits EHch but not EHb whilst 1,1,1-trichloropropene oxide has the opposite specificity. The cytosolic EH did not significantly contribute to the catalysis of the hydrolysis of cholesterol 5 alpha,6 alpha-oxide. << Less

  Carcinogenesis 5:7-9(1984) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• Epoxide hydrolases: their roles and interactions with lipid metabolism.

  Newman J.W., Morisseau C., Hammock B.D.

  The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of infl ... >> More

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

  Prog Lipid Res 44:1-51(2005) [PubMed] [EuropePMC]

  This publication is cited by 5 other entries.

• Catalytic properties and inhibition of hepatic cholesterol-epoxide hydrolase.

  Sevanian A., McLeod L.L.

  Hepatic cholesterol-epoxide hydrolase is a microsomal enzyme which appears to be catalytically distinct from the epoxide hydrolase responsible for the catabolism of a wide variety of aromatic and aliphatic epoxides. The diastereomeric forms of cholesterol epoxide, cholesterol 5 alpha,6 alpha-, and ... >> More

  Hepatic cholesterol-epoxide hydrolase is a microsomal enzyme which appears to be catalytically distinct from the epoxide hydrolase responsible for the catabolism of a wide variety of aromatic and aliphatic epoxides. The diastereomeric forms of cholesterol epoxide, cholesterol 5 alpha,6 alpha-, and cholesterol 5 beta,6 beta-epoxides are converted to cholestane-3 beta,5 alpha,6 beta-triol with equal facility. Kinetic analysis of cholesterol-epoxide hydrolase demonstrated that both diastereomers bind to a common catalytic site. Apparent Km values of 3.69 and 4.42 microM were derived for cholesterol 5 alpha,6 alpha- and cholesterol 5 beta,6 beta-epoxide, respectively. In addition, enzyme activity with both diastereomers was product-inhibited by cholestanetriol through a competitive mechanism with the apparent Ki for cholestanetriol being 10.8 and 6.8 microM against cholesterol alpha- and beta-epoxides, respectively. This inhibitory effect of cholestanetriol may account for the difference observed in the hydration rates for the cholesterol epoxide isomers when they are incubated together in the presence of liver microsomes. Inhibitors of epoxide hydrolase were studied, and three oxidation products were found to be particularly effective against cholesterol-epoxide hydrolase while producing no significant inhibition of styrene-epoxide hydrolase. These inhibitors were 7-ketocholesterol, 6-ketocholestanol, and 7-ketocholestanol, the latter displaying an apparent Ki lower than the Km for either cholesterol epoxide isomer. None of the xenobiotic epoxide hydrolase inhibitors or activators studied affected cholesterol-epoxide hydrolase activity. << Less

  J Biol Chem 261:54-59(1986) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.

• Distinct rat hepatic microsomal epoxide hydrolases catalyze the hydration of cholesterol 5,6 alpha-oxide and certain xenobiotic alkene and arene oxides.

  Levin W., Michaud D.P., Thomas P.E., Jerina D.M.

  Metabolism of cholesterol 5,6 alpha-oxide to the 5,6-glycol is catalyzed by a rat liver microsomal epoxide hydrolase that is distinct from the microsomal epoxide hydrolase that metabolizes a wide range of xenobiotic alkene and arene oxides. The two enzymes are antigenically distinct, and the purif ... >> More

  Metabolism of cholesterol 5,6 alpha-oxide to the 5,6-glycol is catalyzed by a rat liver microsomal epoxide hydrolase that is distinct from the microsomal epoxide hydrolase that metabolizes a wide range of xenobiotic alkene and arene oxides. The two enzymes are antigenically distinct, and the purified microsomal epoxide hydrolase that metabolizes xenobiotic oxides does not catalyze the hydration of cholesterol 5,6 alpha-oxide. In vivo treatment of rats with inducers of microsomal epoxide hydrolase does not enhance the activity of cholesterol 5,6 alpha-oxide hydrolase and, in some cases, actually depresses enzyme activity in the resultant microsomal preparations. Octene 1,2-oxide and benz[a]anthracene 5,6-oxide, both good substrates for xenobiotic epoxide hydrolase, are not competitive inhibitors of cholesterol oxide hydration by rat liver microsomes. The above results establish the existence of a liver microsomal epoxide hydrolase that is under different regulatory control and that appears to have a different substrate specificity than the well-characterized microsomal epoxide hydrolase involved in the metabolism of a widely diverse group of alkene and arene oxides. << Less

  Arch Biochem Biophys 220:485-494(1983) [PubMed] [EuropePMC]

  This publication is cited by 1 other entry.