Sterols synthesis

FIGURE 2 Examples of the variety of structures obtained in natural product screens. I, zaragozic acid A, is an inhibitor of mammalian and fungal sterol synthesis, obtained from fungi (48) II, L-696,474, is an inhibitor of the HIV protease, obtained from fungi (51) III, dehydrosoyasaponin I, is an agonist of the calcium-activated potassium channel, obtained from a medicinal plant (58) IV, tetrandrine, is an inhibitor of L-type calcium channels, obtained from a plant (78). 

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. 

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

In research for new fungicides and antimycotics, one has to look for a concept for pathogen-specific inhibitors. This means that inhibition of sterol synthesis should not take place at any common step the aim is to only inhibit pathogen-specific steps in biosynthesis. The reason for this is to minimize the risk of human toxicity. If one compares the biosynthesis of mammalian cholesterol to that of ergosterol, the main sterol of pathogenic fungi, it becomes obvious that there are at least four pathogen-specific steps to be inhibited. 

Another target, that at first seems to be unfavorable since it is principally common for all organisms, is the enzyme HMG-CoA-reduc-tase which is the regulatory enzyme in terpenoid biosynthesis. Results from trials with naturally produced inhibitors for that enzyme, such as Compactine and Mevinoline, indicate that these compounds are able to lower the cholesterol content in mammals, but not markedly depress sterol synthesis in fungi U3). 

Since the invasive form of Candida in vaginal mycosis is the pseudo-mycelium, we also looked at this morphologically specialized material. We observed that bifonazole causes an accumulation of dihydrolanosterol exclusively, whereas clotrimazole causes the normal accumulation of 24-methylenedihydrolanosterol, dihydrolanosterol, and lanosterol (Figure 18). However, after bifonazole application, the rate of total sterol synthesis is lowered by a factor of two, a result which will be discussed later. 

Kokke, W. C. M. C., Fenical, W., Bohlin, L., and Djerassi, C., Sterol synthesis by cultured zooxanthellae implications concerning sterol metabolism in the host-symbiont association in Caribbean gorgonians, Comp. Biochem. Physiol., 68B, 281, 1981. 

A20. Andersen, J. M., and Dietschy, J. M., Regulation of sterol synthesis in adrenal gland of the rat by both high and low density human plasma lipoproteins. Biochem. Biophys. Res. Common. 72, 880-885 (1976). 

Inhibition of mammalian sterol synthesis Dose- dependent inhibitory effect No inhibition No inhibition... 

Research groups in Japan [1] have screened over 8000 microbial extracts for their ability to produce an inhibitor of sterol synthesis in vitro. These studies led to the isolation of mevastatin from cultures of Penicillium citrinum. Mevastatin (Fig. 4.2) served as the lead-molecule for seven statins which currently are among the best-selling drugs. 

The synthesis of sterols from acetate, but not from MVA, was enhanced by incubation of leucocytes from a patient with heterozygous familial hypercholes-terolaemia in a medium containing lipid-free serum, compared with sterol synthesis in the same medium by leucocytes from normal individuals.27 This was held to be due to an increased induction of HMG-CoA reductase, perhaps caused by defective... 

HMG-CoA reductase is under negative-feedback control by the sterol synthesis end products cholesterol and bile salts. The important indicator of HMG-CoA reductase is the blood cholesterol concentration, which is mostly determined by the presence of LDL particle. 

FPP is necessary for the synthesis of both sterols and longer chain nonsterol isoprenoids. The first committed step in sterol synthesis is catalyzed by the enzyme squalene synthesis and involves the head-to-head condensation of two FPP molecules to form squalene [15]. This is followed by cyclization steps, leading to sterol synthesis. The addition of IPP to FPP via the enzyme GGPP synthase yields the 20-carbon GGPP [16]. FPP and GGPP are substrates in the prenylation reactions catalyzed by the enzymes farnesyl transferase (FTase) and geranylgeranyl transferase (GGTase) I and II [17-20]. 

Sinensky, M., Beck, L.A., Leonard, S., and Evans, R. (1990). Differential inhibitory effects of lovastatin on protein isoprenylation and sterol synthesis. J Biol Chem 265 19937-19941. 

Rahier, A. and Taton, M. (1997) Fungicides as tools in studying postsqualene sterol synthesis in plants. Pestic. Biochem. Physiol, 57,1-27. 

Nomura, T. Kitasaka, Y. Tokatsuto, S. Reid, J.B. Fukami, M. Yoko, T. Brassinosteroid/sterol synthesis and plant growth affected by Ika and Ikb mutation of pea. Plant Physiol. 1999, 119, 1517-1526. 

Triadimefon binds to hepatic cytochrome P450 and inhibits microsomal enzyme activities. It inhibits sterol demethylation and thus sterol synthesis. Fungi sensitive to triadimefon utilize ergosterol as the primary sterol, the production of which is inhibited. It is also thought that triadimefon may have actions similar to those caused by indirect-acting dopamine agonists. 

Patients with familial hypercholesterolaemia exhibit lower levels of plasma cholesterol after an operation for portacaval anastomosis, and it has now been shown in rats that such an operation causes an increase in HMG-CoA reductase and cholesterol 7a -hydroxylase activities. Many transplantable human and rodent hepatomas do not control the rate of sterol biosynthesis and HMG-CoA reductase levels in response to dietary cholesterol as normal liver cells do. However, certain hepatoma cells have now been found that, although lacking feedback regulation of choles-terologenesis in vivo, retain their regulatory ability in vitro It thus appears that malignant transformation is not necessarily linked to the loss of regulation by the cell of HMG-CoA reductase activity or sterol synthesis. 

The major site of steroid synthesis in animals has been considered to be the liver, but recently (1975) this has been disputed and detailed studies now show that sterol synthesis in guinea pig occurs more readily in ileum and lung than in liver under a variety of conditions" " and a brief study indicates similar sites of biosynthesis in swine." All tissues of guinea pig studied had an active feedback system controlling cholesterol biosynthesis and the results of feeding cholesterol and cholestyramine" showed that both the former and possibly bile acids suppress cholesterol synthesis in the liver to a far lesser extent than in the small intestine. 

Leukocytes from subjects with familial hypercholesterolemia responded to incubation in a lipid-depleted medium with a higher activation of sterol synthesis and an enhanced induction of HMG-CoA reductase compared to leukocytes from controls. This increase in sterol synthesis and HMG-CoA reductase was correlated with a loss of cholesterol into the medium. 

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