Epoxide hydrolases oxide

Furstoss et al. have reported their studies on the use of an epoxide hydrolase with four styrene oxide derivatives (Figure 5.26) [39]. The (R)-diol (43) was obtained in 91% ee at 100% conversion from racemic (42), demonstrating an enantioconvergent... 

In the rabbit, the nonplanar PCB 2,2, 5,5 -tetrachlorobiphenyl (2,2, 5,5 -TCB) is converted into the 3, 4 -epoxide by monooxygenase attack on the meta-para position, and rearrangement yields two monohydroxymetabolites with substitution in the meta and para positions (Sundstrom et al. 1976). The epoxide is also transformed into a dihydrodiol by epoxide hydrolase attack (see Chapter 2, Section 2.3.2.4). This latter conversion is inhibited by 3,3,3-trichloropropene-l,2-oxide (TCPO), thus providing strong confirmatory evidence for the formation of an unstable epoxide in the primary oxidative attack (Forgue et al. 1980). 

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 al., unpublished results).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-... 

As with several other functional groups considered earlier, epoxides are most commonly found in alkaloid metabolites rather than the original compound. These epoxides may arise via oxidation of alkenes or aromatic hydrocarbons. Epoxide hydrolase catalyzes hydrolysis of epoxides to the more hydrophilic diol. As seen in Scheme 6, this is usually a stereospecific reaction that always yields a... 

Epoxides/arene oxides have varying degrees of chemical reactivity and can be detoxified by hydrolysis to dihydrodiols as shown in Figure 6.8. This can occur either nonenzy-matically, if the epoxide is very reactive, or it can be catalyzed enzymatically by epoxide hydrolase (EH). 

This chapter begins, thus, with a short introduction to the chemical reactivity of epoxides. We continue with a description of the epoxides hydrolases and their biochemistry, and devote most of its length to a systematic discussion of the substrates hydrated by these enzymes. Arene oxides and diol epoxides will be presented first, followed by a large variety of alkene and cy-cloalkene oxides. 

The microsomal epoxide hydrolases (microsomal EH, mEH), predominantly found in the endoplasmic reticulum, regio- and stereoselectively catalyze the hydration of both alkene and arene oxides, including oxides of polycyclic aromatic hydrocarbons. These enzymes have been purified to homogeneity from various species and tissues [22] [41 - 46], The human microsomal EH contains 455 amino acids (Mr 52.5 kDa) and is the product of the EPHX1 gene [47] (also known as HYL1 [48]). 

The human cytosolic epoxide hydrolase (cytosolic EH, cEH, also known as soluble EH) has 554 amino acids (Mr 62.3 kDa) and is the product of the EPHX2 gene [49]. Its specific substrate is trans-stilbene oxide, and it appears... 

Cholesterol epoxide hydrolase, which is expressed in the endoplasmic reticulum and catalyzes the trans-addition of H20 to cholesterol 5,6a-ox-ide and cholesterol 5,6/3-oxide, as well as to a number of other steroid 5,6-oxides. The products are the corresponding vicinal diols [54][55],... 

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

Together with glutathione conjugation, hydration is a major pathway in the inactivation and detoxification of arene oxides. Exceptions to this rule will be treated when discussing polycyclic aromatic hydrocarbons. Arene oxides are good substrates for microsomal EH, as evidenced in Table 10.1, where hydration of selected arene oxides, alkene oxides, and cy-cloalkene oxides by purified rat liver epoxide hydrolase is compared. The hy- ... 

Fig. 10.8. Simplified and partial metabolic scheme of benzene. Cytochrome P450 mediated oxidation (Reaction a) yields benzene oxide (10.1), which produces phenol (10.14) by isomerization (Reaction c) and 1,2-dihydro-1,2-dihydroxybenzene (10.13) by epoxide hydrolase catalyzed hydration (Reaction d). Direct formation of phenol also occurs (Reaction b). Phenol...

Like for benzene, the cytotoxicity of naphthalene is not due to the epoxide but to the quinone metabolites, namely 1,2-naphthoquinone and 1,4-naphthoquinone [85], As shown in Table 10.1, naphthalene 1,2-oxide (10.2) is a better substrate than benzene oxide for epoxide hydrolase. Its rapid isomerization to naphthalen-l-ol, facile enzymatic hydration to the dihydrodiol and lack of reactivity toward nucleophiles such as glutathione may explain its absence of direct toxicity [85],... 

Turning to enzymatic hydration, we see from the data in Table 10.1 that phenanthrene 9,10-oxide Fig. 10.10, 10.29) is an excellent substrate for epoxide hydrolase. Comparison of enzymatic hydration of the three isomeric phenanthrene oxides shows that the Vmax with the 9,10-oxide is greater than with the 1,2- or the 3,4-oxide the affinity was higher as well, as assessed by the tenfold lower Km value [90]. Furthermore, phenanthrene 9,10-oxide has a plane of symmetry and is, thus, an achiral molecule, but hydration gives rise to a chiral metabolite with high product enantioselectivity. Indeed, nucleophilic attack by epoxide hydrolase occurs at C(9) with inversion of configuration i.e., from below the oxirane ring as shown in Fig. 10.10) to yield the C-H9.S, 10.S )-9,10-dihydro-9,10-diol (10.30) [91],... 

The data in Table 10.1 suggest that the reactivity of epoxide hydrolase toward alkene oxides is highly variable and appears to depend, among other things, on the size of the substrate (compare epoxybutane to epoxyoctane), steric features (compare epoxyoctane to cycloalkene oxides), and electronic factors (see the chlorinated epoxides). In fact, comprehensive structure-metabolism relationships have not been reported for substrates of EH, in contrast to some narrow relationships that are valid for closely related series of substrates. A group of arene oxides, along with two alkene oxides to be discussed below (epoxyoctane and styrene oxide), are compared as substrates of human liver EH in Table 10.2 [119]. Clearly, the two alkene oxides are among the better substrates for the human enzyme, as they are for the rat enzyme (Table 10.1). 

T. M. Guenthner, D. Cai, R. Wallin, Co-Purification of Microsomal Epoxide Hydrolase with the Warfarin-Sensitive Vitamin Kx Oxide Reductase of the Vitamin K Cycle , Biochem. Pharmacol. 1998, 55, 169 - 175. 

W. Levin, D. P. Michaud, P. E. Thomas, D. M. Jerina, Distinct Rat Hepatic Microsomal Epoxide Hydrolases Catalyze the Hydration of Cholesterol 5,6a-Oxide and Certain Xenobiotic Alkene and Arene Oxides , Arch. Biochem. Biophys. 1983, 220, 485 - 494. 

C. Morisseau, G. Du, J. W. Newman, B. D. Hammock, Mechanism of Mammalian Soluble Epoxide Hydrolase by Chalcone Oxide Derivatives , Arch. Biochem. Biophys. 1998, 356, 214 - 228 C. Morisseau, M. H. Goodrow, D. Dowdy, J. Zheng, J. F. Greene, J. R. Sanborn, B. D. Hammock, Potent Urea and Carbamate Inhibitors of Soluble Epoxide Hydrolases , Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 8849 - 8854. 

P. J. van Bladeren, J. M. Sayer, D. E. Ryan, P. E. Thomas, W. Levin, D. M. Jerina, Differential Stereoselectivity of Cytochrome P450b and P450c in the Formation of Naphthalene and Anthracene 1,2-Oxides. The Role of Epoxide Hydrolase in Determining the Enantiomer Composition of the 1,2-Dihydrodiols Formed ,. /. Biol. Chem. 1985, 260, 10226- 10235. 

T. Watabe, K. Akamatsu, Enzymatic Hydrolysis of Mono-n-alkyl Substituted Ethylene Oxides and Their Inhibitory Effects on Hepatic Microsomal Epoxide Hydrolase , Chem. Pharm. Bull. 1974, 22, 2155 - 2158. 

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. 

G. Bellucci, C. Chiappe, G. Ingrosso, Kinetics and Stereochemistry of the Microsomal Epoxide Hydrolase-Catalyzed Hydrolysis of cw-Stilbene Oxides , Chirality 1994, 6, 577 - 582. 

G. Bellucci, C. Chiappe, F. Marioni, Enantioselectivity of the Enzymatic Hydrolysis of Cyclohexene Oxide and ( )-l-Methylcyclohexene Oxide A Comparison between Microsomal and Cytosolic Epoxide Hydrolases , J. Chem. Soc., Perkin Trans. 1 1989, 2369 -2373. 

G. Bellucci, C. Chiappe, F. Marioni, C. Simonelli, The Low Reactivity of 5//-Di bcnzo-[c(,rf]cyclohcplcne 10,11-Oxide in Microsomal Epoxide Hydrolase Catalyzed Hydration , Xenobiotica 1989, 19, 279 - 285 G. Bellucci, G. Berti, C. Chiappe, F. Fabri, F. Marioni, Product Enantioselectivity in the Microsomal Epoxide Hydrolase Catalyzed Hydrolysis of 10,11 -Di hydro-10,11 -epoxy-5//-dibenzo[c ,rf]cyclohcplcnc , J. Org. Chem. 1989, 54, 968 - 970. 

Optically pure trans- and czs-linalool oxides, constituents of several plants and fruits, are among the main aroma components of oolong and black tea. These compounds were prepared from 2,3-epoxylinalyl acetate (9) (Scheme 17) [102]. The key step consist of a separation of the diastereomeric mixture of 9 by employing an epoxide hydrolase preparation derived from Rhodococcus sp. NCIMB 11216, yielding the product diol and remaining epoxide in excellent diastereomeric excess (de>98%). Further follow-up chemistry gave both linalool... 

---