Microsomal epoxide hydrolase activities

N. R. Kitteringham, C. Davis, N. Howard, M. Pirmohamed, B. K. Park, Interindividual and Interspecies Variation in Hepatic Microsomal Epoxide Hydrolase Activity Studies with cw-Stilbene Oxide, Carbamazepide 10,11-Epoxide and Naphthalene , J. Pharmacol. Exp. Ther. 1996, 278, 1018 - 1027. 

Pentachloroethane given to rats by gavage during a two-year study caused chronic, diffuse kidney inflammation and renal papilla mineralization. A single dose also reduced hepatic cytochrome P450 content and microsomal epoxide hydrolase activities. Inhalation exposure of rabbits to pentachloroethane decreased their total antibody titres (lARC, 1986). 

DePierre, J. W., Seidegard, J., Morgenstern, R., Balk, L., Meijer, J., Astrom, A., Norelius, I., and Ernster, L. (1984) Induction of cytosolic glutathione transferase and microsomal epoxide hydrolase activities in extrahepatic organs of the rat by phenobarbi-tal, 3-methylcholanthrene and trans-stilbene oxide. Xenobiotica 14, 295-301. 

Microsomal epoxide hydrolase activities are listed in Table IX. There was significantly higher (0.05 level) hepatic microsomal epoxide hydrolase activity in pectin-,Metamuoil-, and cellulose-treated rats, as compared to control-, lignin- and Lab Chow-fed animals. There was no significant difference in intestinal microsomal epoxide hydrolase activities among any of the treatment groups. 

Lewis DF, Lake BG, Bird MG. Molecular modelling of human microsomal epoxide hydrolase (EH) by homology with a fungal (Aspergillus niger) EH crystal structure of 1.8 A resolution structure-activity relationships in epoxides inhibiting EH activity. Toxicol In Vitro 2005 19 517-22. 

Table 10.1. Relative Activity of Rat Liver Microsomal Epoxide Hydrolase toward Some ...

Table 10.2. Epoxide Hydrolase Activity in Human Liver Microsomes [119]... 

N. Jourenkova-Mironova, K. Mitrunen, C. Bouchardy, P. Dayer, S. Benhamou, A. Hir-vonen, High-Activity Microsomal Epoxide Hydrolase Genotypes and the Risk of Oral, Pharynx, and Larynx Cancers , Cancer Res. 2000, 60, 534 - 536. 

T. Friedberg, R. Holler, B. Lollmann, M. Arand, F. Oesch, The Catalytic Activity of the Endoplasmic Reticulum-Resident Protein Microsomal Epoxide Hydrolase towards Carcinogens Is Retained on Inversion of Its Membrane Topology , Biochem. J. 1996, 319, 131 - 136. 

Enzyme catalysed hydrolysis of racemic epoxides is interesting from a practical point of view. This reaction is catalysed by epoxide hydrolases (EHs, EC 3.3.2.3) (Archelas and Furstoss, 1998). Mammalian EHs are the most widely studied and they are divided into five groups among which the soluble (cytosolic) epoxide hydrolases (sEH) and microsomal epoxide hydrolases (mEH) are best charactelised. The mechanism of sEH from rat starts with a nucleophilic attack by Asp333 on a carbon of the epoxide (usually the least hindered one) to form a glycol monoester intermediate which is stabilised by an oxyanion hole. A water molecule activated by His523 releases the 1,2-diol product. An... 

Friedberg, T, Lollmann, B., Becker. R., Holler, R. Oesch, F. (1994) The microsomal epoxide hydrolase has a single membrane signal anchor sequence which dispensable for the catalitic activity of this protein. Biochem. J., 303, 967-972... 

The intrinsic rates of formation of monoepoxides in human, rat and mouse liver microsomes are roughly similar, when epoxide hydrolase is inhibited, whereas the amount of monoepoxides at the end of incubation is two and even 15 times higher in mouse liver microsomes than in rat and human liver microsomes, respectively (Bogaards et al., 1996). Thus, differences in epoxide hydrolase activity between different species may be of importance for toxicological outcomes. 

Conversion of arene oxides to dihydrodiols by mouse liver cytosol epoxide hydrolase and microsomal epoxide hydrolase has been compared, and it is found that the former is less active than the latter.200... 

Hydration of epoxides catalyzed by epoxide hydrolase is involved in both detoxication and intoxication reactions. With high concentrations of styrene oxide as a substrate, the relative activity of hepatic microsomal epoxide hydrolase in several animal species is rhesus monkey > human = guinea pig > rabbit > rat > mouse. With some substrates, such as epoxidized lipids, the cytosolic hydrolase may be much more important than the microsomal enzyme. 

The assay method described by Eaton and Stapleton (1989), measures the activities of both cytosolic glutathione 5-transferase and microsomal epoxide hydrolase toward benzo[a]pyrene-4,5-oxide as a substrate. These enzymes are important in the biotransformation of many epoxide xenobiotics, including potentially carcinogenic arene oxides. 

Table 9.5 Epoxide hydrolase activity in liver microsomes from various species of animals...

There are two distinct types of epoxide hydrolases, both widely distributed in mammalian tissues. One type is localized primarily in the endoplasmic reticulum, the second in the cytosol. The microsomal and cytosolic enzymes have different properties, including substrate selectivities. Several inducers of xenobiotic metabolizing enzymes, including pheno-barbital, planar PCB congeners, and ra s-stilbene oxide, selectively increase (induce) microsomal, but not cytosolic, epoxide hydrolase activity. 

Dermal treatment of healthy volunteers with 10% coal tar for 4 days produced an 18-fold induction of CYP1A1 mRNA levels in coal-tar-treated skin (Li et al. 1995). In vitro incubation of DNA with coal tar fume concentrates in the presence of mouse and yeast microsomes expressing various cytochrome P450 isoforms or the aryl hydrocarbon hydroxylase receptor (AHR) demonstrated that coal tar fume condensates require metabolic activation to produce DNA adducts (Genevois et al. 1998). Both the AHR and CYP1A were involved in the metabolism of coal tar fume condensate. It was also shown that the reactive metabolites formed by CYP1A are substrates for microsomal epoxide hydrolase. 

When racemic aryl glycidyl ethers were subjected to aminolysis in aqueous buffer catalyzed by hepatic microsomal epoxide hydrolase from rat, the corresponding (S)-configurated amino-alcohols were obtained in 51-88% ee 131. On the other hand, when azide was employed as nucleophile for the asymmetric opening of 2-methyl-1,2-epoxyheptane in the presence of an immobilized crude enzyme preparation derived from Rhodococcus sp., which contains an epoxide hydrolase activity, the reaction revealed a complex picture 1321. The (S)-epoxide from the racemate was hydrolyzed (as in the absence of azide), and the less readily accepted (i )-enantiomer was transformed into the corresponding azido-alcohol (ee >60%). Although at present only speculations can be made about the actual mechanism of both the aminolysis and azidolysis reaction, in both cases it was proven that the reaction was catalyzed by a protein and that no reaction was observed in the absence of biocatalyst... 

Fig. 2 Activation of Nrf-2 by redox changes. Nrf-2 is an important transcription factor that binds to the antioxidant response element (ARE), a DNA sequence in the promoter of antioxidant enzymes such as glutamylcysteine ligase (GCL), heme oxygenase (HO-1), and microsomal epoxide hydrolase. Under normal conditions, Nrf-2 is found in the cytoplasm attached to Keapl. Keap 1 contains critical thiols essential in binding and retaining Nrf-2 in the cytoplasm. NAPQI can directly, through covalent binding, or indirectly, through GSH depletion and/or increase ROS, cause the oxidation of thiols in Keapl. The oxidation of Keapl causes Nrf-2 to detach and translocate to the nucleus. Nfr-2 in the nucleus binds to the ARE and transactivates antioxidant genes such as GCL to counter the redox disturbance caused by NAPQI...

Epoxide Hydrolase. Epoxide hydrolase (EH) activity has been studied in numerous tissues with many different substrates (71-72). Hepatic microsomal EH has been shown to be induced by polycyclic aromatic hydrocarbons, but is insensitive to cytochrome P-450 type inducers (73). Epoxide hydrolase has been related to the activation of polycyclic aromatic hydrocarbons into reactive carcinogens (70). Epoxide hydrolase activity has been determined... 

Moghaddam et al. (13) reported that linoleic acid and arachidonic acid can be metabolized to their dihydroxy-THFA (tetrahydrofuran-diols) in vitro by microsomal cytochrome P450 epoxidations, followed by microsomal epoxide hydrolase. In their metabolic pathways, saturated dihydroxy-THFA are produced because 9,10(12,13)-dihydroxy-12,13(9,10)-epoxy-octadecanoate converted from linoleic acid methyl ester are cyclized (13). These saturated dihydroxy-THFA exhibit cytotoxic activity and mitogenic activity for breast cancer and... 

Preparations of purified phenobarbital-induced cytochrome P-450 (P-450 PB-Bj and P-450 LMj from rat and rabbit liver, respectively [377,399]), metabolised arachidonic acid by epoxidation of each of the four double bonds [397]. Using such preparations with negligible epoxide hydrolase activity, the four epoxides could be isolated [397,400]. The monooxygenase metabolism of arachidonic acid is summarised in Fig. 10. In the presence of soluble or microsomal epoxide hydrolase, the corresponding 1,2-diols are rapidly formed. Liver and renal cortical microsomes may further metabohse the 1,2-diols by wl- and w2-oxidation to yield a family of tri hydroxy compounds and other metabolites, with polarity similar to those of many prostaglandins [396,401]. 

Utilizing steroid substrates, MEH was able to hydrolyze not only epoxides, but also the corresponding heteroatom derivatives such as aziridines to form trans-, 2-aminoalcohols albeit at slower rates (Scheme 2.90) [583]. The thiirane, however, was inert towards enzymatic hydrolysis. The enzyme responsible for this activity was assumed to be the same microsomal epoxide hydrolase, but this assumption was not proven. 

So far five different types of epoxide hydrolases have been characterized in mammals, and they have been grouped depending on their enzymatic activity and biochemical characteristics. These groups include the soluble epoxide hydrolases (sEHs, also referred to as cytosolic epoxide hydrolases) [50], microsomal epoxide hydrolases (mEHs) [34], leukotriene A4 hydrolase (LTA44H) [51], cholesterol epoxide hydrolase [52], and hepoxilin hydrolase [53]. Several reviews have focused on these enzymes of medical importance [34,54]. 

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