Epoxide metabolism

Romer, S. et al.. Genetic engineering of a zeaxanthin-rich potato by antisense inactivation and co-suppression of carotenoid epoxidation, Metabol. Eng. 4, 263, 2002. 

Boyd JM, A Ellsworth, SA Ensign (2006) Characterization of 2-bromoethanesulfonate as a selective inhibitor of the coenzyme M-dependent pathway and enzymes of bacterial aliphatic epoxide metabolism. J Bacteriol 188 8062-8069. 

Krum JG, SA Ensign (2001) Evidence that a linear megaplasmid encodes enzymes of aliphatic alkene and epoxide metabolism and coenzyme M (2-mercaptoethanesulfonate) biosynthesis in Xanthobacter strain Py2. J Bacterial 183 2172-2177. 

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 al., unpublished results).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-... 

This epoxidation of AFB has been associated with aldrin epoxidase (AE) activity in trout (30). As with other epoxide carcinogens, OAFB may be a substrate 7or epoxide metabolizing enzyme systems such as epoxide hydrase (EH) (EC4.2.1.63) and glutathione-S-epoxide transferase (GTr) (EC4.4.1.7) found in mammals and fish (31, 32, 33, 34). AFB also undergoes a variety of other reactions, generally to less toxic metabolites depending on the species of animal involved (35, 36). The primary AFB metabolite in rainbow trout has been shown to be a reduced form of AFB, aflatoxicol (AFL) (24). 

Analogs were prepared through total chemical synthesis in which the unstable epoxide was replaced with a cyclopropyl group (4). This afforded compounds that were not subject to epoxide metabolizing enzymes, epoxide hydrolase, and glutathione transferase. We called these compounds PBT (PaceBioAcTive compounds). The PBT competed with the native compounds in binding to neutrophil... 

James, M.O., J.R. Fonts, and J.R. Bend. 1974. In vitro epoxide metabolism in some marine species. Bulletin Mt. Desert Island Biological Laboratory 14 41-46. 

Both compounds discussed in this section, oxidosqualene and epoxystyrene, are intermediates in metabolic pathways. That is to say that the epoxide is not found in the final product but rather serves to impart a specific reactivity to the molecule that is vital for the subsequent reaction step. The epoxide in oxidosqualene can be viewed as a masked cation that is required in order to initiate a series of C-C bond-... 

Epoxides are often encountered in nature, both as intermediates in key biosynthetic pathways and as secondary metabolites. The selective epoxidation of squa-lene, resulting in 2,3-squalene oxide, for example, is the prelude to the remarkable olefin oligomerization cascade that creates the steroid nucleus [7]. Tetrahydrodiols, the ultimate products of metabolism of polycyclic aromatic hydrocarbons, bind to the nucleic acids of mammalian cells and are implicated in carcinogenesis [8], In organic synthesis, epoxides are invaluable building blocks for introduction of diverse functionality into the hydrocarbon backbone in a 1,2-fashion. It is therefore not surprising that chemistry of epoxides has received much attention [9]. 

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

FIGURE 2.6 The procarcinogen benzo[a]pyrene oriented in the CYPlAl active site (stereo view) via n- n stacking between aromatic rings on the substrate and those of the complementary amino acid side chains, such that 7,8-epoxidation can occur. The substrate is shown with pale lines in the upper structures. The position of metabolism is indicated by an arrow in the lower structure (after Lewis 1996). 

The microsomal fraction consists mainly of vesicles (microsomes) derived from the endoplasmic reticulum (smooth and rough). It contains cytochrome P450 and NADPH/cytochrome P450 reductase (collectively the microsomal monooxygenase system), carboxylesterases, A-esterases, epoxide hydrolases, glucuronyl transferases, and other enzymes that metabolize xenobiotics. The 105,000 g supernatant contains soluble enzymes such as glutathione-5-trans-ferases, sulfotransferases, and certain esterases. The 11,000 g supernatant contains all of the types of enzyme listed earlier. 

Apart from monooxygenases, other enzymes concerned wih xenobiotic metabolism may also be induced. Some examples are given in Table 2.5. Induction of glucuronyl transferases is a common response and is associated with phenobarbital-type induction of CYP family 2. Glutathione transferase induction is also associated with this. A variety of compounds, including epoxides such as stilbene oxide and... 

Emphasis is given to the critical role of metabolism, both detoxication and activation, in determining toxicity. The principal enzymes involved are described, including monooxygenases, esterases, epoxide hydrolases, glutathione-5 -transferases, and glucuronyl transferases. Attention is given to the influence of enzyme induction and enzyme inhibition on toxicity. 

As mentioned earlier (Figure 5.5), aldrin and heptachlor are rapidly metabolized to their respective epoxides (i.e., dieldrin and heptachlor epoxide) by most vertebrate species. These two stable toxic compounds are the most important residues of the three insecticides found in terrestrial or aquatic food chains. In soils and sediments, aldrin and heptachlor are epoxidized relatively slowly and, in contrast to the situation in biota, may reach significant levels (note, however, the difference between aldrin and dieldrin half-lives in soil shown in Table 5.8). The important point is that, after entering the food chain, they are quickly converted to their epoxides, which become the dominant residues. 

In the examples given, there is good evidence for the formation of an unstable epoxide intermediate in the production of monohydroxymetabolites. However, there is an ongoing debate about the possible operation of other mechanisms of primary oxidative attack that do not involve epoxide formation, for example, in the production of 2 OH 3,3, 4,4 -TCB (Figure 6.3). As mentioned earlier, P450s of gene family 1 (CYP 1) tend to be specific for planar substrates, including coplanar PCBs they do not appear to be involved in the metabolism of nonplanar PCBs. On the other hand. 

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