Microsomal cytochrome epoxide hydrolase

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Parkinson, A., Thomas, P.E., Ryan, D.E. etal. (1983) Differential time course of induction of rat liver microsomal cytochrome P 450 isozymes and epoxide hydrolase by Aroclor 1254. Archives of Biochemistry and Biophysics, 225, 203-215. 

Figure 1. The major pathways in the metabolism of BaP to BaP epoxides, dihydrodiol, and 7,8-dihydrodiol-9,10-epoxides. The absolute configurations are as shown. The position of trans-addition of water is shown by an arrow. The optical purity of the 4,5-epoxide formed in BaP metabolism is dependent on the cytochrome P-450 isozymes present in the microsomal enzyme system. EH epoxide hydrolase.

It was recently reported that. >97% of BaP 4,5-epoxide metabolically formed from the metabolism of BaP in a reconstituted enzyme system containing purified cytochrome P-450c (P-448) is the 4S,5R enantiomer (24). The epoxide was determined by formation, separation and quantification of the diastereomeric trans-addition products of glutathione. Recently a BaP 4,5-epoxide was isolated from a metabolite mixture obtained from the metabolism of BaP by liver microsomes from 3-methylcholanthrene-treated Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene oxide, and was found to contain a 4S,5R/4R,5S enantiomer ratio of 94 6 (Chiu et. al., unpublished results). However, the content of the 4S,5R enantiomer was <60% when liver microsomes from untreated and phenobarbital-treated rats were used as the enzyme sources. Because BaP 4R,5S-epoxide is also hydrated predominantly to 4R,5R-dihydro-... 

The 8,9- and 10,11-dihydrodiols formed in the metabolism of BA and DMBA respectively are all highly enriched (>90%) in R,R enantiomers (Table III). Labeling experiments using molecular oxygen-18 in the in vitro metabolism of the respective parent compounds and subsequent mass spectral analyses of dihydrodiol metabolites and their acid-catalyzed dehydration products indicated that microsomal epoxide hydrolase-catalyzed hydration reactions occurred exclusively at the nonbenzylic carbons of the metabolically formed epoxide intermediates (unpublished results). These findings indicate that the 8,9- and 10,11-epoxide intermediates, formed in the metabolism of BA and DMBA respectively, contain predominantly the 8R,9S and 10S,11R enantiomer, respectively. These stereoselective epoxidation reactions are relatively insensitive to the cytochrome P-450 isozyme contents of different rat liver microsomal preparations (Table III). 

Farin EM, Omiecinski CJ. 1993. Regiospecific expression of cytochrome P-450s and microsomal epoxide hydrolase in human brain tissue. J Toxicol Environ Health 40 317-335. 

Parkinson A, safe SH, Robertson LW, et al. 1983. Immunochemical quantitation of cytochrome P-450 isozymes and epoxide hydrolase in liver microsomes from polychlorinated or polybrominated biphenyl-treated rats. J Biol Chem 258(9) 5967-5976. 

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). 

Snyder R, Chepiga T, Yang CS, et al. 1993a. Benzene metabolism by reconstituted cytochromes P450, 2B1, and 2E1 and its modulation by cytochrome b5, microsomal epoxide hydrolase, and glutathione transferases evidence for an important role of microsomal epoxide hydrolase in the formation of hydroquinone. Toxicol Appl Pharmacol 122(2) 172-181. 

Guengerich FP, Turvy CG (1991) Comparison of levels of several human microsomal cytochrome P-450 enzymes and epoxide hydrolase in normal and disease states using immunochemical analysis of surgical liver samples. /... 

Wolf, C.R., Moll, E., Friedberg, T., Oesch, F., Buchmann, A., Knhl-mann, W.D. Kunz, H.W. Characterization, localization and regulation of a novel phenobarbital-inducible form of cytochrome P 450, compared with three further P450-isoenzymes, NADPH P450-reductase glutathione transferases and microsomal epoxide hydrolase. Carcinogenesis 1984 5 993-1001... 

Thomas PE, Reik LM, Ryan DE, Levin W. 1981. Regulation of three forms of cytochrome P450 and epoxide hydrolase in rat liver microsomes Effects of age, sex, and induction. J. Biol. Chem. 256 1044-52... 

Willey JC, Coy EL, Frampton MW, Torres A, Apostolakos MJ, et al. 1997. Quantitative RT-PCR measurement of cytochromes P450 1A1, 1B1, and 2B7, microsomal epoxide hydrolase, and NADPH oxidoreductase expression in lung cells of smokers and nonsmokers. Am. J. Respir. Cell Mol. Biol. 17 114-24... 

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. 

Adams, J.D., H. Yagi, W. Levin, and D.M. Jerina (1995). Stereo-selectivity and regio-selectivity in the metabolism of 7,8-dihydrobenzo[a]pyrene by cytochrome P450, epoxide hydrolase and hepatic microsomes from 3-methylcholanthrene-treated rats. Chem. Biol. Interact. 95, 57-77. 

Metabolism of the c/s and trans isomers of chlordane by humans and laboratory animals appears to be qualitatively similar (Kutz et al. 1976, 1979), although monkeys may be less efficient than rats (Khasawinah 1989), and rats may metabolize frans-nonachlor more efficiently than humans do (Tashiro and Matsumura 1978). Metabolism appears to be largely oxidative, involving hepatic microsomal cytochrome P-450 (Kawano et al. 1989). Epoxide hydrolase is probably the predominant enzyme involved in further degradation of oxychlordane, but the process appears to be slow in animals and humans. In addition, reductive dehalogenation, probably resulting in the formation of reactive free radical intermediates, may be important in the toxicity of chlordane (Brimfield and Street 1981 Kawano et al. 1989). 

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]. 

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