Epoxide hydrolases EC 3323

Epoxide hydrolases (EHs)38 catalyze the hydrolytic cleavage of oxirane rings. Because of the higher electron attracting force of the oxygen atom compared with the two carbon atoms of oxirane rings, epoxides possess two electrophilic carbon atoms, the electrophilic reactivity of which is enhanced by the tension of the three-membered oxirane ring. Depending on the influences of the rest of the molecule this can make epoxides highly cytotoxic, genotoxic, and carcinogenic. Therefore, the EH-catalyzed hydrolytic opening of oxirane rings is normally a detoxification reaction. Some important, and predictable, exceptions are discussed below.

Several mammalian EHs are specialized for endogenous substrates, while two mammalian EHs preferentially metabolize xenobiotic epoxides.38 These are the microsomal epoxide hydrolase (mEH)39,40 and the soluble epoxide hydrolase (sEH).41,42 The two enzymes are a/b hydrolase fold proteins, thus, in spite of a very low similarity of their amino acid sequences (<15%), they share a common three-dimensional structure43,44 that displays a catalytic triad in its catalytic center: an aspartate as the catalytic nucleophile and a histidine/glutamate pair (mEH) or a histidine/ aspartate pair (sEH) as the water-activating charge relay system.45 The oxirane ring of the substrate is attacked by the aspartate to form an enzyme-substrate ester that is hydrolyzed by a water molecule activated by proton abstraction through the histidine/acidic amino acid pair. This releases the product, a vicinal (trans-, if applicable) diol, and restores free enzyme. While the second step of detoxification (hydrolysis of the enzyme-substrate ester to give the diol product) is slow, it is of prime importance that the first step (covalent binding of the epoxide to the enzyme) is very fast. This leads to a practically instantaneous removal of the toxic epoxide from the system as long as the enzyme is present in excess (i.e., as long as the slow regeneration of the free enzyme is unnecessary for the removal of further epoxide).46 mEH, which is a major constituent of the endoplasmic reticulum, is present in large amounts. This generates for epoxides that are mEH substrates a practical threshold concentration below which genotoxic damage is negligible,47 an unusual and remarkable situation for genotoxic carcinogens.

mEH38 is highly expressed in the liver and several steroidogenic organs and at more moderate levels in many tissues of man and other mammals. mEH is induced by phenobarbital, trans-stilbene oxide, Aroclor 1254, and by several other xenobiotics including many antioxidants such as ethoxyquin. mEH metabolizes many structurally diverse epoxides. Structural requirements for a mEH substrate are sufficient lipophilicity and lack of trans-substitution at the oxirane ring.39 mEH substrates include epoxides metabolically formed from drugs, such as carbamazepine and phenytoin, from occupational compounds such as styrene, and from environmental compounds such as polycyclic aromatic hydrocarbons.38 With respect to toxification/detoxification it is important to note that epoxide hydrolysis - normally a detoxification reaction — can be involved in overall toxification pathways in some predictable cases. The mEH-catalyzed hydrolysis of pre-bay epoxides metabolically formed from angular PAHs (e.g., the benzo[a]pyrene-7,8-epoxide) prevents its isomerization to the corresponding phenols, products of comparatively low toxicity that are easily conjugated and then excreted. Instead, the pre-bay dihydrodiol (e.g., the benzo[a]pyrene-7,8-dihydrodiol) is formed, which can be further metabolized by a variety of enzymes to the highly electrophilically reactive, mutagenic and carcinogenic dihydrodiol bay region epoxides (e.g., benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide), which are not (or very poor) substrates of EHs (see Figure 3). Thus, depending on which CYPs are predominantly present (and consequently which metabolic pathways of the large PAHs are predominantly operative) higher amounts of mEH can substantially increase the mutagenicity of angular PAHs such as benzo[a]pyrene, i.e., mEHs can paradoxically but predictably contribute to toxification.15,16

sEH42 is predominantly expressed in liver, kidney, heart, brain, and in low amounts in several other organs. It has an unusual and toxicologically important dual localization in the cytosol and in the matrix of peroxisomes, the latter performing several metabolic functions including long chain fatty acid degradation. Several peroxisomal enzymes produce hydrogen peroxide. The fact that sEH is present in peroxisomes and is induced by peroxisome proliferators (which induce peroxisomal beta-oxidation and thereby hydrogen peroxide generation), suggests a protective role of sEH against hydrogen peroxide-induced oxidative damage. Fatty acid epoxides, products of lipid peroxidation, are in fact good substrates of sEH. Some fatty acid epoxides derived from arachidonic acid or linolenic acid are involved in signal transduction. Thus, sEH is likely to have an important regulatory function and sEH inducers or inhibitors are likely to render sEH an important toxification enzyme by disturbance of such regulatory functions. In addition, the sEH catalyzed metabolism of leukotoxin leads to the considerably more toxic 9,10-dihydroxyoctadec-12-enoic acid. This toxification suggests a central role of sEH in the pathogenesis of multiple organ failure, in particular the adult respiratory distress syndrome (ARDS), which develops as a consequence of a leukotoxin overproduction by leukocytes.42

The substrate specificity of sEH is complementary to that of mEH. It metabolizes many trans-substituted epoxides, such as trans-stilbene oxide and trans-ethyl styrene oxide, which are not hydrolyzed by mEH. Conversely, in contrast to mEH, sEH does not hydrolyze or poorly hydrolyzes epoxides derived from most polycyclic aromatic hydrocarbons.38 For extrapolations of metabolism-related toxicities between species it is important to note that sEH expression displays enormous interspecies differences. The rat has a particularly low level of sEH in the liver (only 0.01% of the total soluble hepatic protein). In contrast, the enzyme is highly abundant in mouse liver (0.3% of the soluble hepatic protein), and is about three times more active than rat sEH. In humans, sEH represents about 0.1% of the soluble liver proteins and possesses a specific activity similar to that of the rat.38

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