Inhibition of squalene epoxidase

A group of fungicides that inhibit squalene epoxidation has been developed primarily for use against pathogenic fungi in medicine. Epoxidation of squalene is catalyzed by squalene epoxidase (a flavoprotein) that starts the complicated cyclization of squalene. The squalene-2,3-epoxide formed by this enzyme is further metabolized to a protosterol cation intermediate, which is transformed to either cycloartenol in plants (cycloartenol synthase) or lanosterol (lanosterol synthase). Cycloartenol is the precursor to plant sterols, whereas lanosterol is the precursor of cholesterol and the other sterols in animals, and to ergosterol in plants. 

Terbinafine, which also has a complicated structure, is an example of a fungicide that inhibits this enzymatic step. It is used as a fungicide against systemic and dermal infections in humans. 

Several other substances toxic to fungi inhibit squalene epoxidase, the key enzyme in this complicated ring formation. 

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Naftiflne is highly active against dermatophytes but less active against yeasts. Its mechanism of action is based on selective inhibition of squalene epoxidase, a key enzyme for the synthesis of ergos-terol. Side effects include local irritation and erythema. Contact with mucous membranes should be avoided. 

Fig. 40.8. The squalene epoxidase inhibitors, allyl amines. Naftifine was the first drug shown to act by inhibition of squalene epoxidase, as does the much older thinocarbamate, tolnaftate.

G Petranyi, NS Ryder, A Stutz. Allylamine derivatives new class of synthetic antifungal agents inhibiting fungal squalene epoxidase. Science 224 1239-1241,1984. 

Inhibitors of squalene epoxidase are used in antifungal drugs to treat athlete s foot, jock itch, ringworm, and nail infections. The drug Tinactin (tolnaf-tate) inhibits squalene epoxidase, which blocks the synthesis of the steroids the fungus needs to make its cell membrane. The defective cell membrane kills the fungus. 

Squalene is converted into the first sterol, lanosterol, by the action of squalene epoxidase and oxidosqualene lanosterol cyclase. The catalytic mechanism for the cyclase s four cyclization reactions was revealed when the crystal stmcture of the human enzyme was obtained (R. Thoma, 2004). Oxidosqualene lanosterol cyclase is considered an attractive target for developing inhibitors of the cholesterol biosynthetic pathway because its inhibition leads to the production of 24,25-epoxycholesterol (M.W. Huff, 2005). This oxysterol is a potent ligand activator of the liver X receptor (LXR) and leads to expression of several genes that promote cellular cholesterol efflux, such as ABCAl, ABCG5, and ABCG8 (Section 4.1). Thus, inhibitors of oxidosqualene lanosterol cyclase could be therapeutically advantageous because they would reduce cholesterol synthesis and promote cholesterol efflux (M.W. Huff, 2005). 

Terbinqfine is a synthetic allylamine that is structurally similar to naftifine. It probably acts by inhibiting fungal squalene epoxidase and blocking ergosterol biosynthesis. Terbinafine is well absorbed, but bioavailability is only 40% because of first-pass hepatic metabolism. Proteins bind... 

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. 

The synthetic thiocarbamates, of which tolnaftate (Fig. 5.20J) is an example, also inhibit squalene epoxidase. Tolnaftate inhibits this enzyme from C. albicans, but is inactive against whole cells, presumably because of its inability to penetrate the cell wall. Tolnaftate is used topically in the treatment or prophylaxis of tinea. 

This synthetic allylamine derivative inhibits the enzyme squalene epoxidase at an early stage in fungal sterol biosynthesis. Acting as a structural analogue of squalene, naffidine causes the accumulation of this unsaturated hydrocarbon, and a decrease in ergosterol in the fungal cell membrane. 

It is a synthetic allylamine derivative, which exerts its antifungal effect by inhibiting squalene epoxidase leading to deficiency of ergosterol and corresponding accumulation of squalene which causes fungal cell death. 

Mechanism of Action. Terbinafine inhibits a specific enzyme (squalene epoxidase) that is responsible for sterol synthesis in the fungal cell membrane. This action impairs cell wall synthesis, with subsequent loss of cell membrane function and integrity. Inhibition of this enzyme causes squalene to accumulate in the fungal cell, which can also impair cell function and lead to death of the fungus. 

Figure 10 Selective growth inhibition ofS. pombe JY266 and RSE strains by terbinafine and tolnaftate (inhibitor of fungal squalene epoxidase) and by NB-598 (inhibitor of mammalian squalene epoxidase). Strain JY-266 possesses the fungal squalene epoxidase gene and transformant strain RSE possesses fungal and rat squalene epoxidase genes.

However, none of compounds A-E was found to be able to inhibit squalene epoxidase. On the contrary, compound G has been shown to be an excellent antioxidant in this reaction. It presents a potential cure for atherosclerosis and other diseases related to oxidation.497... 

Since ergosterol is used in the formation of the leishmanial cell membrane, inhibition of ergosterol biosynthesis has been considered as a useful target for chemotherapeutic attack. Allylamines (eg. terbinafine) and imidazole antifungals (eg. ketoconazole) have been found to interfere with different steps in the biosynthetic pathway of C28 sterols in leishmania and fungi. Allylamines inhibit the microsomal squalene 2,3-epoxidase and, therefore, inhibit the synthesis of squalene epoxide, the precursor of lanosterol. Imidazoles, on other hand, inhibit cytochrome P-450 dependent C-14 demethylation of lanosterol leading to decreased or no synthesis of ergosterol [30]. 

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