Cholesterol epoxidation

The most important oxirane, from an anthropocentric viewpoint, is probably squalene oxide (72), a precursor of lanosterol (73) and thus of the maligned but essential cholesterol (74 Scheme 87) 78MI50501). The cyclization of (72) to (73) represents nucleophilic tr-attack on oxirane carbon cf. Section 5.05.3.4.3(t)()), and the process has also been extensively investigated in vitro (68ACR1). Oxiranes are even more ubiquitous in steroid biosynthesis than had been thought, for a cholesterol epoxide has been shown to be a product of mammalian steroid biosynthesis <81JA6974). 

Cholesterol epoxide hydrolase, which is expressed in the endoplasmic reticulum and catalyzes the trans-addition of H20 to cholesterol 5,6a-ox-ide and cholesterol 5,6/3-oxide, as well as to a number of other steroid 5,6-oxides. The products are the corresponding vicinal diols [54][55],... 

From the above, it is clear that the epoxide hydrolases of greatest significance in drug and xenobiotic metabolism are the microsomal and soluble ones. Their catalytic mechanism, which we now examine, is different from that of cholesterol epoxide hydrolase and LTA4 hydrolase (e.g., [57][58]). 

F. Muller, M. Arand, H. Frank, A. Seidel, W. Hinz, L. Winkler, K. Hanel, E. Blee, J. K. Beetham, B. D. Hammock, F. Oesch, Visualization of a Covalent Intermediate Between Microsomal Epoxide Hydrolase, but not Cholesterol Epoxide Hydrolase, and Their Substrates , Eur. J. Biochem. 1997, 245, 490 - 496. 

Figure 18.3. Scheme for the formation of cholesterol epoxides. Compounds are as follows (1) CHOL (2) a-epoxide (3) ft-epoxide (4) triol. For abbreviations, see Table 18.1. 

Several oxysterol classes present in oxLDL appear to be cytotoxic toward fibroblasts, ECs, and vascular smooth muscle cells, especially 7-hydroperoxycholes-terol (7-OOH-chol), 7P- and 7a-hydroxycholesterol (7-OH-chol), 7-ketocholesterol (7-keto-chol), and cholesterol epoxides (epoxy-chol). 7p-OOH-chol, a precursor of hydroxyl- and keto-oxysterols, was reported to be the most toxic. During LDL oxidation 7P-OOH-chol was produced in three to five times higher quantities than 7a-OOH-chol, other oxysterols and even hydroxy-nonenal, which is one of the most abundant lipid oxidation products. Cytotoxicity of oxysterols was connected to increased cellular oxidative stress. Some studies suggest that oxysterols are even involved in oxidative stress induction. Animal models indicate that dietary oxysterols can significantly decrease glutathione levels and increase expression of glutathione peroxidase and superoxide dismutase. In apolipoprotein-deficient mice, the NADPH-oxidase activity was induced by 7-keto-chol, 7p-OH-chol, and Sp,6P-epoxy-chol. The increased activity of NADPH oxidase yields more superoxide anions that amplify oxidative stress. 

So far five different types of epoxide hydrolases have been characterized in mammals, and they have been grouped depending on their enzymatic activity and biochemical characteristics. These groups include the soluble epoxide hydrolases (sEHs, also referred to as cytosolic epoxide hydrolases) [50], microsomal epoxide hydrolases (mEHs) [34], leukotriene A4 hydrolase (LTA44H) [51], cholesterol epoxide hydrolase [52], and hepoxilin hydrolase [53]. Several reviews have focused on these enzymes of medical importance [34,54]. 

FIGURE 26 10 The biosyn thetic conversion of squa lene to cholesterol proceeds through lanosterol Lano sterol IS formed by enzyme catalyzed cyclization of the 2 3 epoxide of squalene... 

Like the a2ole derivatives, it inhibits the biosynthesis of ergosterol. However, naftifine [65472-88-0] does not inhibit the cytochrome P-450 dependent C-14-demethylase, but the epoxidation of squalene. Squalene epoxidase cataly2es the first step in the conversion of squalene via lanosterol to ergosterol in yeasts and fungi or to cholesterol in mammalian cells. The squalene epoxidase in C. albicans is 150 times more sensitive to naftifine, C2 H2 N, than the en2yme in rat fiver (15). Naftifine is available as a 1% cream. 

Squalene epoxidase, a key enzyme in the biosynthesis of cholesterol (9), epoxidizes one face of one of the three different olefins in squalene (7) to give squalene epoxide (8), which then cyclizes eventually to give cholesterol (9) (Scheme 1). The AD of squalene (7)... 

Studies conducted by Barenghi eta.1. (1990) and Lodge etal. (1993) independently have demonstrated the facile, multicomponent analysis of a wide range of PUFA-derived peroxidation products (e.g. conjugated dienes, epoxides and oxysterols) in samples of oxidized LDL by high-field H-NMR spectroscopy. Figure 1.9 shows the applications of this technique to the detection of cholesterol oxidation products (7-ketocholesterol and the 5a, 6a and 5/3,60-epoxides) in isolated samples of plasma LDL pretreated with added coppcr(Il) or an admixture of this metal ion with H2O2, an experiment conducted in the authors laboratories. 

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