Epoxide hydratases

Walker, C.H., Bentley, P, and Oesch, F. (1978). Phylogenetic distribution of epoxide hydratase in different vertebrate species, strains, and tissues using three substrates. Biochemica et Biophysica Acta 539, 427 34. 

Acrylonitrile is also metabolized to CO which is eliminated through the lungs. Carbon dioxide is produced when acrylonitrile is metabolized to ethylene oxide and degraded to oxidation products and cyanide via the epoxide hydratase pathways (Farooqui and Ahmed 1982 Young et al. 1977). 

The glutathione 5-transferase pathway is sometimes in biochemical competition with the epoxide hydratase pathway, in that both deactivate intermediates of the MMFO. Epoxide hydratase is a microsomal enzyme that acts specifically to deactivate epoxide intermediates, by the addition of water across the C—O bond to form a diol. As a very broad generality, the glutathione 5-transferase pathway tends to be more prominent in rodents, while the epoxide hydratase pathway tends to be more dominant in nonrodents. 

Epoxide hydratase activity, with JH-benzo(a)pyrene 4,5-oxide as substrate, was assayed by the thin-layer chromatographic procedure of Jerina et al. (15). The protein content of microsomal and whole homogenate preparations was determined according to Lowry et al. (16), using bovine serum albumin as the standard, and microsomal cytochrome P-450 content was assayed by the method of Omura and Sato (17) on an Aminco DW-2A spectrophotometer. 

The elution profile of cytochrome P-448 (absorption at 418 nm) and epoxide hydratase activity from a sodium cholate-solubi-lized hepatic microsomal preparation (from DBA-treated male skates) applied to a DEAE-cellulose column and eluted with Buffer II is shown in Fig. 3. The void volume of the column contained significant amounts of epoxide hydratase activity. Fractions 40-70 (Fig. 3) were combined, and concentrated. The carbon monoxide difference spectrum, which had an absorption maximum at 448 nm in the induced state, is shown in Fig. 4. This form of the cytochrome (i.e.,... 

Figure 3. Elution profile of partially purified Cytochrome P-448 and epoxide hydratase activity of solubilized hepatic microsomes from DBA-treated male skates from a DEAE-cellulose column with Buffer II. Epoxide hydratase activity was determined with BP-4,5-oxide as the substrate (see Materials and Methods).

When all of the material absorbing at 418 nm (associated with the cytochrome P-448 fractions) was eluted from the DEAE-cellulose column (which in some experiments required more than 1 liter of Buffer II), elution was continued with a linear KC1 gradient (0-0.5 M) in Buffer II, as shown in Fig. 5. A different form(s) of cytochrome P-450 (fractions 130-155), having maximal absorption near 451 nm in the carbon monoxide ligated and reduced form (Fig. 6), was obtained although only 2- to 3-fold purification, relative to microsomes, was achieved. This form of cytochrome P-450 was extensively contaminated with epoxide hydratase activity. However, by combining fractions 130-150 (Fig. 5), it was possible to obtain cytochrome P-451 essentially free of cytochrome b5. The relative content of cytochrome P-448 and cytochrome P-451 Tn the DEAE-column eluates ranged from 1 1.1 to 1 1.6 in several different experiments. 

Epoxide hydrolases have also occasionally been termed epoxide hydratases or epoxide hydrases ... 

As well as detoxication via reaction with GSH, the reactive 3,4-epoxide can be removed by hydration to form the dihydrodiol, a reaction that is catalyzed by epoxide hydrolase (also known as epoxide hydratase). This enzyme is induced by pretreatment of animals with the polycyclic hydrocarbon 3-methylcholanthrene, as can be seen from the increased excretion of 4-bromophenyldihydrodiol (Table 7.5). This induction of a detoxication pathway offers a partial explanation for the decreased hepatotoxicity of bromobenzene observed in such animals. A further explanation, also apparent from the urinary metabolites, is the induction of the form of cytochrome P-450 that catalyzes the formation of the 2,3-epoxide. This potentially reactive metabolite readily rearranges to 2-bromophenol, and hence there is increased excretion of 2-bromophenol in these pretreated animals (Table 7.5). 

As discussed in Section 13.5, benzene-1,2-oxide is an intermediate in the biochemical oxidation of benzene. It is probably responsible for the toxicity of benzene. It is hydrolyzed by the action of epoxide hydratase to the dihydrodiol shown below ... 

Other enzymes associated with xenobiotic metabolism are also altered by dietary protein levels. Epoxide hydratase can hydrolyze various epoxides and appears to be important in decreasing their toxicity and carcinogenicity, although it is involved in the metabolic activation of certain carcinogens. Low dietary protein depresses epoxide hydratase activity in our dietary model (8). This may indicate both a decreased ability to detoxify epoxides and a decrease in metabolic activation of specific carcinogens, such as benzo(a)pyrene. Woodcock and Wood (19) have reported that dietary protein deficiency increases the activity of uridine diphosphate glucuronic acid (UDP6) transferase activity. This indicates that... 

There emerges a very complex picture of three hormones synthesized and secreted at variable rates, competing for carrier binding proteins, presumed receptor proteins, epoxide hydratase and carboxyl esterase enzymes (35,36). It is possible experimentally to measure tEe timing of critical periods for larval determination and to measure total levels of JH at these critical periods although both measurements involve extreme difficulty. Approaches to this were described recently by G.B. Staal (3 7) using third instar larvae of the tobacco hornworm moth, Manduca sexta, which were allatectomized and raised on JH impregnated diets as an experimentally reproducible method of JH therapy. 

Epoxide rings of certain alkene and arene compounds are hydrated enzymatically by epoxide hydrolases to form the corresponding iram-dihydrodiols (Figure 10.11). The epoxide hydrolases are a family of enzymes known to exist both in the endoplasmic reticulum and in the cytosol. In earlier studies they were named epoxide hydratase, epoxide hydrase, or epoxide hydrolase. Epoxide hydrolase, however, has been recommended by the International Union of Biochemists Nomenclature Committee and is now in general use. 

Oesch, E (1979). Epoxide Hydratase, In Progress in Drug Metabolism, Part 3 (J. W. Bridges and L. E Chasseaud, eds,), Wiley, Chichester, pp. 253-302. 

Another important group of hydrolytic enzymes are the epoxide hydrolases, also known as epoxide hydrase or epoxide hydratase, most commonly found in liver tissue. Epoxide hydrolases catalyze the hydration of arene oxides and aliphatic epoxides to their corresponding /raKt-dihydrodiols or diols, respectively, by activating a water molecule to attack one of the carbons of the arene oxide or epoxide [41]. Although one of the major metabolites of carbamazepine is the stable epoxide, this metabolite also undergoes hydrolysis to form the trans-dial metabolite (Fig. 3). Likewise, the anticonvulsants phenytoin and mephenytoin form arene oxides, which then form trans-dihy-drodiol that undergo further oxidation to form the catechols by the enzyme dihydrodiol dehydrogenase (Fig. 11) [42,43]. 

Epoxide hydratase/ hydrolase Hydrase Most tissues Soluble/microsomal... 

M. Bucket, M. Golan, H. U. Schmassmann, H. R. Glatt, P. Stasiecki, F. Oesch, The epoxide hydratase inducer fra/is-stilbene oxide shifts the metabolic epoxidation of benzo(a)pyrene from the bay- to the k-region and reduces its mutagenicity. Mol. Pharmacol. 16 (1979) 656. 

As well as detoxication via reaction with glutathione, the reactive 3,4-epoxide can be removed by hydration to form the dihydrodiol, a reaction which is catalysed by epoxide hydrolase (also known as epoxide hydratase). This enzyme is induced by pretreatment of animals with the polycyclic hydrocarbon... 

Epoxide hydrolase. Epoxide hydratase. Arene-oxide hydratase. 3.3.2.3 An epoxide + H(2)0 = a glycol. 

The most widely accepted pathway of PAH activation to yield DNA adducts involves the formation of anti- or syn-diol epoxides in the bay region [12, 13], Their formation is mediated by the sequential reaction of cytochrome P450 and epoxide hydratase, and is best described using the example of B[a]P. 

Benson, A. M., Cha, Y.-N., Bueding, E., Heine, H. S., and Talalay, P., Elevation of extrahepatic glutathione S-transferase and epoxide hydratase activities by 2(3)-tert-butyl-4-hydroxyanisole. Cancer Res. 39, 2971-2977 (1979). 

Kinetic Behavior of Microsomal Styrene Monooxygenase and Styrene Epoxide Hydratase in Different Animal Species" Experientia (1977), 708-709. 

The following values for the pharmacokinetic parameters were scaled up from vitro determinations of styrene monooxygenase and epoxide hydratase activities in mouse liver (15). 

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