Epoxide-dihydrodiol pathway

The Epoxide-Dihydrodiol Pathway of Benzene and Phenyl-Containing Drugs... 

The Epoxide-Dihydrodiol Pathway of Bicyclic and Tricyclic Aromatic Hydrocarbons... 

Besides the drugs discussed above, other phenyl-containing xenobiotics are also substrates of the epoxide-dihydrodiol pathway. Thus, a number of isomeric dichlorobiphenyls (10.23, m = 2, n = 0, or m = n = 1) were metabolized by rat liver microsomes to stable monophenols and dihydrodiols [84], Like for benzene, the second step in this pathway can be assumed to be a de-... 

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.

In contrast, the pathway best known to yield adduct-forming metabolites (the ultimate carcinogens) is the formation of dihydrodiol epoxides, usually referred to as diol epoxides . This pathway involves three steps a) formation of an M-region epoxide, b) its hydration to the M-region dihydrodiol, and c) epoxidation of the latter at the vicinal C=C bond bordering the bay or fjord region. 

The polycyclic aromatic hydrocarbon carcinogens, which are very ubiquitous, are metabolized by the microsomal mixed-function oxidase system of target tissues to a variety of metabolites such as phenols, quinones, epoxides, dihydrodiols and dihydrodiol-epoxides ( ). The mqjor pathway of activation of benzo(a)pyrene (BP) leads to the formation of dihydrodiol-epoxide of BP which interacts predominantly with the 2-amino of guanine of DNA. The dihydrodiol-epoxide of BP appears to be the major ultimate electrophilic, mutagenic, and carcinogenic metabolite of BP ( ). Nevertheless, other metabolites such as certain phenols, epoxides and quinones may contribute to the overall carcinogenic activity of BP. In addition, a free radical mechanism may also be partly involved in its carcinogenic activity. 

A potentially powerful probe for sorting out the contribution of hydroperoxide-dependent and mixed-function oxidase-dependent polycyclic hydrocarbon oxidation is stereochemistry. Figure 9 summarizes the stereochemical differences in epoxidation of ( )-BP-7,8-dihydrodiol by hydroperoxide-dependent and mixed-function oxidase-dependent pathways (31,55,56). The (-)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (+)-anti-diol epoxide by both pathways whereas the (+)-enantiomer of BP-7,8-dihydrodiol is converted primarily to the (-)-anti-diol epoxide by hydroperoxide-dependent oxidation and to the (+)-syn-diol epoxide by mixed-function oxidases. The stereochemical course of oxidation by cytochrome P-450 isoenzymes was first elucidated for the methycholanthrene-inducible form but we have detected the same stereochemical profile using rat liver microsomes from control, phenobarbital-, or methyl-cholanthrene-induced animals (32). The only difference between the microsomal preparations is the rate of oxidation. 

Conversion of epoxides (arene oxides) into phenols is spontaneous. The conversion of epoxides into dihydrodiols is catalyzed by EH (EC 4.2.1.63). Hydroxyl containing PAHs can act as substrates for conjugases (C) (UDP glucuronsyl transferase (EC 2.4.1.17) and phenol sulphotransferase (EC 2.8.2.1)). This pathway usually leads to inactive excretable products. Epoxides are scavenged by GSH and the reaction is catalyzed by GSHt (EC 2.5.1.18). When GSH is depleted and/or the other pathways are saturated, epoxides of dihydrodiols (particularly 7,8-diol-9,10-epoxides in the case of BP) and phenol metabolites react with cellular macromolecules such as DNA, RNA, and protein. If repair mechanisms are exceeded the detrimental effects of PAH may result. 

Detailed kinetic studies comparing the chemical reactivity ofK-region vs. non-K-region arene oxides have yielded important information. In aqueous solution, the non-K-region epoxides of phenanthrene (the 1,2-oxide and 3,4-oxides) yielded exclusively phenols (the 1-phenol and 4-phenol, respectively, as major products) in an acid-catalyzed reaction, as do epoxides of lower arenes (Fig. 10.1). In contrast, the K-region epoxide (i.e., phenanthrene 9,10-oxide 10.29) gave at pH < 7 the 9-phenol and the 9,10-dihydro-9,10-diol (predominantly trans) in a ratio of ca. 3 1. Under these conditions, the formation of this dihydrodiol was found to result from trapping of the carbonium ion by H20 (Fig. 10.11, Pathway a). At pH > 9, the product formed was nearly ex-... 

The formation and reactivity of dihydrodiol epoxides is now illustrated for benzo[a]pyrene (BaP, 10.34, Fig. 10.13), one of the most extensively investigated PAHs and a highly active carcinogen. However, far from being exclusive to BaP, this toxification pathway is known to occur for a number of PAHs containing a bay or fjord region. 

In the pH range of 5 - 10, H20-catalyzed hydrolysis is the predominant mechanism (see Fig. 10.11, Pathway b), resulting in the formation of the (8R,9R)-dihydrodiol (10.133, Fig. 10.30). Thus, aflatoxin B1 exo-8,9-epoxide is possibly the most reactive oxirane of biological relevance. Such an extreme reactivity is mostly due to the electronic influence of 0(7), as also influenced by stereolectronic factors, i.e., the difference between the exo- and endo-epoxides. The structural and mechanistic analogies with the dihydro-diol epoxides of polycyclic aromatic hydrocarbons (Sect. 10.4.4) are worth noting. 

A further remarkable finding in the hydrolysis of aflatoxin B1 exo-8,9-epoxide is the relative instability of the dihydrodiol, which under basic conditions exists in equilibrium with an aflatoxin dialdehyde, more precisely a furofuran-ring-opened oxy anionic a-hydroxy dialdehyde (10.134, Fig. 10.30). The dihydrodiol is the predominant or exclusive species at pH < 7, whereas this is true for the dialdehyde at pH >9, the pK value of the equilibrium being 8.2 [204], The dialdehyde is known to form Schiff bases with primary amino groups leading to protein adducts. However, the slow rate of dialdehyde formation at physiological pH and its reduction by rat and human aldo-keto reductases cast doubts on the toxicological relevance of this pathway [206]. 

Biological systems possess a number of mechanisms for protection against toxic foreign compounds, some of which have already been mentioned. Thus, metabolic transformation to more polar metabolites, which are readily excreted, is one method of detoxication. For example, conjugation of paracetamol with glucuronic acid and sulfate facilitates elimination of the drug from the body and diverts the compound away from potentially toxic pathways (see chap. 7). Alternatively, a reactive metabolite may be converted into a stable metabolite. For example, reactive epoxides can be metabolized by epoxide hydrolase to stable dihydrodiols. 

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