Epoxide hydrolase benzo pyrene-4,5-oxide

Dehydrogenation A-Demethylation Hydroxylation Epoxidation Sulfoxidation Oxidations Acetaminophen, benzidine, DES, epinephrine Dimethylaniline, benzphetamine, aminocarb Benzo[a]pyrene, 2-aminofluorene, phenylbutazone 7,8-Dihydrobenzo[a]pyrene Methylphenylsulfide FANFT, ANFT, bilirubin Esterases and Amidases Paraoxon, dimethoate, phenyl acetate Epoxide Hydrolase Benzo(a)pyrene epoxide, styrene oxide DDT-Dehydrochlorinase p,p- DDT Glutathione Reductase Disulfiram... 

An example of the second type of chiral effect in metabolism is afforded by benzofa]-pyrene, also discussed in more detail in chapter 7. This carcinogenic polycyclic hydrocarbon is metabolized stereos elec lively by a particular cytochrome P-450 isozyme, CYP1A1, to the (+)-7R,8S oxide (chap. 7, Fig. 5.2), which in turn is metabolized by epoxide hydrolase to the (—)-7R,8S dihydrodiol. This metabolite is further metabolized to (- -)-benzo[aIpyrene, 7R,8S dihydrodiol, 9S,10R epoxide in which the hydroxyl group and epoxide are trans and which is more mutagenic than other enantiomers. The (—)-7R,8S dihydrodiol of benzo[aIpyrene is 10 times more tumorigenic than the (+)-7R,8S enantiomer. It was reported that in this case the configuration was more important for tumorigenicity than the chemical reactivity. 

Although a number of the epoxides and diol epoxides are mutagenic, the 7,8-dihydrodiol 9,10-oxide, shown in Figure 7.2, is believed to be the ultimate carcinogen. It should be noted that there are several possible diastereoisomers of this metabolite, but as the action of epoxide hydrolase yields a trans dihydrodiol and the epoxide ring produced by the cytochrome P-450 may be cis or trans, there are two diastereoisomers produced metabolically. Thus, benzo(a)pyrene... 

The stereospecific metabolism of racemic benzo [a] pyrene 4,5-, 7,8-, and 9,10-oxides (28, 366, and 367, respectively) by rat liver microsomes or highly purified epoxide hydrolase has been examined by Thakker et al.22 The optical purity of the metabolically formed benzo [a] pyrene 7,8- and 9,10-dihydrodiols is relatively low (8 and 22%, respectively). The metabolically formed benzo [a] pyrene-4,5-dihydrodiol is highly enriched in the (—) enantiomer (78% optical purity). The low optical purity of benzo [a] pyrene-7,8-dihydrodiol is attributed to the ability of epoxide hydrolase to act upon position 8 of both optical isomers of 366 with almost equal ease. 

The oxidation of naphthalene was one of the earliest examples of an epoxide as an intermediate in aromatic hydroxylation. As shown in Figure 7.3, the epoxide can rearrange nonenzymatically to yield predominantly 1-naphthol, or interact with the enzyme epoxide hydrolase to yield the dihydrodiol, or interact with glutathione S-transferase to yield the glutathione conjugate, which is ultimately metabolized to a mercapturic acid. These reactions are also of importance in the metabolism of other xenobiotics that contain an aromatic nucleus, such as the insecticide carbaryl and the carcinogen benzo(a)pyrene. 

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. 

A number of K-region arene oxides have been detected as intermediates in the metabolism of the corresponding PAHs in liver systems for example, phenanthrene, benz[a]anthracene, pyrene, benzo [a]pyrene, and dibenz(a,h)anthracene. These K-region arene-oxide metabolites were generally only detected by trapping the radiolabeled intermediate. The arene-oxide metabolite 102 obtained from a-naphthoflavone was found to be sufficiently stable with respect to isomerization and resistant to attack by epoxide hydrolase so that it could be isolated and identified spectroscopically. ... 

Figure 18. Stereochemical course of the microsomal epoxide-hydrolase catalyzed hydration of (-) )- and (—)-benzo[a]pyrene 4,5-oxide. About 99% of the attack by water occurs at the (5S)-carbon of the (—)-(4R,5S)-oxide and about 85% at the (4S)-carbon of the (-l-)-(4S,5R)-oxide. ...

Varying degrees of substrate enantioselectivity have been described for microsomal epoxide hydrolases. For example, for benzo(a]pyrene-4,5-oxide a 40-fold difference between the rates of hydration for the +) and (-) enantiomers was observed in vitro (Armstrong et al., 1980). Mono-substituted epoxides (such as 1,2-epoxyhexane and its geometrical isomer. 

R-configured carbon atom vide supra), microsomal epoxide hydrolase attacks at R- as well as S-configuxed carbon atoms. With some arene oxides, both enantiomers are attacked at the same carbon, irrespective of configuration (e.g., benzo[c]phenanthrene). Other arene oxides (e.g., benzo[a]-pyrene 4,5-oxide) undergo the primary reaction, predominantly at S-con-figured carbon (Yang, 1988). 

Figure 7 Conversion of benzo(a)pyrene 7,8-oxide to benzo(a)pyrene trans-7,8-dihydrodiol by microsomal epoxide hydrolase.

Figure 7.9 Another environmental carcinogen is benzo[a]pyrene (BaP). One site of metabolic attack is the terminal ring which is oxidized to form a 7,8-arene oxide. Epoxide hydrolase opens this to form a frans-7,8-dihydrodiol.The remaining C=C bond in the ring may also undergo oxidation to form an epoxide, with the oxidation directed to the same face as the allylic alcohol by coordination of the metabolizing enzyme. This epoxide is now on the opposite face as the benzylic hydroxyl group and is named BaP-frans-7,8-diol-anf/-9,10-epoxide.This is especially stable because it is adjacent to the sheltered bay region of the molecule. It can be opened by epoxide hydrolase to form a detoxified tetraol. Alternatively, DNA can open the epoxide to form a covalent adduct that is responsible for tumor initiation.

One of the most harmful of all the polycyclic aromatic compounds is benzo[a]pyrene, 12.13, which is a constituent of tobacco (and in greater quantities in marijuana) smoke, and was established to be the precarcinogenic compound in the cancers of the scrotum suffered by chimney sweeps in nineteenth-century Britain. In vivo, it is catalytically oxidized in the presence of various cytochromes and a hydrolase enzyme to 12.14, which is both carcinogenic and mutagenic (the epoxide group is susceptible to ring-opening reactions by nucleophiles such as the bases of DNA and proteins). 

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