Cholesterol Mevalonic acid formation

Figure 20.18 The initial steps in the biosynthesis of cholesterol leading to mevalonic acid formation.

The antiinflammatory effects of statins likely result from their ability to inhibit the formation of mevalonic acid. Downstream products of this molecule include not only the end product, cholesterol, but also several isoprenoid intermediates that covalently modify ( pre-nylate ) certain key intracellular signaling molecules. Statin treatment reduces leukocyte adhesion, accumulation of macrophages, MMPs, tissue factor, and other proinflammatory mediators. By acting on the MHC class II transactivator (CIITA), statins also interfere with antigen presentation and subsequent T-cell activation. Statin treatment can also limit platelet activation in some assays as well. All these results support the concept that in addition to their favorable effect on the lipid profile, statins can also exert an array of antiinflammatory and immunomodulatory actions. 

In Box 10.12 we saw that nature employs a Claisen reaction between two molecules of acetyl-CoA to form acetoacetyl-CoA as the first step in the biosynthesis of mevalonic acid and subsequenfiy cholesterol. This was a direct analogy for the Claisen reaction between two molecules of ethyl acetate. In fact, in nature, the formation of acetoacetyl-CoA by this particular reaction using the enolate anion from acetyl-CoA is pretty rare. 

LDL is catabolized chiefly in hepatocytes and other cells by receptor-mediated endocytosis. Cholesteryl esters from LDL are hydrolyzed, yielding free cholesterol for the synthesis of cell membranes. Cells also obtain cholesterol by synthesis via a pathway involving the formation of mevalonic acid by HMG-CoA reductase. Production of this enzyme and of LDL receptors is transcriptionally regulated by the content of cholesterol in the cell. Normally, about 70% of LDL is removed from plasma by hepatocytes. Even more cholesterol is delivered to the liver via IDL and chylomicrons. Unlike other cells,... 

Similarly, the experiments of Matolcsy et al. (1974a 1974b 1975), in which several metabolite analogues of mevalonic acid and homomevalonic acid have been tested for anti-juvenile action, were unsuccessful. Support for the rationale that these metabolite analogues would block the formation of juvenile hormone at the mevalonate level was provided by the successful inhibition of cholesterol biosynthesis by mevalonate analogues in humans. Yet, a fluorinated mevalonate analogue, 4-fluoromethyl-4-hydroxy-tetrahydro-2H-pyran-2-one (FMev, 72) was found to act as a potent inhibitor of JH biosynthesis by Quistad and co-workers... 

The first recognized human metabolic defect in the biosynthesis of cholesterol and isoprenoids was mevalonic aciduria [10]. Mevalonic aciduria is an autosomal recessive disorder that is quite rare, with only 30 known patients (D. Haas, 2006). In normal individuals, a small amount of mevalonic acid diffuses from cells into the plasma at levels proportional to the rate of cellular cholesterol formation. Patients with the severe, classical form of mevalonic aciduria excrete 10,000-200,000 times the normal amount of mevalonic acid because they have severely reduced amounts of mevalonate kinase activity. Their clinical features include failure to thrive, anemia, gastroenteropathy, hepatosplenomegaly, psychomotor retardation, hypotonia, ataxia, cataracts, and dysmorphic features [10]. Surprisingly, patients with severe deficiencies in mevalonate kinase show normal plasma cholesterol levels and cultured mevalonic aciduria fibroblasts have a rate of cholesterol synthesis that is half that of normal cells. Close examination of cholesterogenic enzymes in mevalonic aciduria fibroblasts has revealed a 6-fold increase in HMG-CoA reductase activity, which is postulated to compensate for the low mevalonate kinase activity. Thus, mevalonate is overproduced. 

The incorporation of malonate into mevalonic acid and steroids has been investigated further. Experiments with normal and tumorous rats have demonstrated the previously unsuspected fact that the S-methyl group of methionine is incorporated into cholesterol and cholest-7-en-3jS-ol. Some of the enzymes involved in mevalonate synthesis have been isolated. Yeast acetoacetyl coenzyme A thiolase (EC 2.3.1.9) has a molecular weight of about 190 000 and 3-hydroxy-3-methyl glutaryl coenzyme A synthetase (EC 4.1.3.5) a molecular weight of 130 000. Rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) used only [4/ - H]NADPH in the formation of mevalonic acid with incorporation of two tritium atoms (at Hp and Hp) (see Scheme 1). 

Hydroxy-3-methylglutaryl-coenzyme A-reductase (HMG-CoA-reducta-se) catalyses the formation of mevalonic acid, an early intermediate in the biosynthesis of cholesterol. 

Several conditions can lead to the formation of abnormal plasma hpoproteins. A well-known possibility is the nephrotic syndrome where we have a plasmatic activator of HMG-CoA reductase and in addition difficulties in excreting mevalonic acid. This leads to a strongly increased production of cholesterol and hpoproteins. In renal insufficiency there is an increase of LP-B LP-C-III particles in the LDL density range. This means a dysequihbrium. 

Condensation of mevalolactone with 8-alanine furnished the amide CXXIV, which has been reported to inhibit cholesterol formation in vivo [311]. A derivative of the thiol analog of mevalonic acid (CXXV) has been found to reduce serum cholesterol levels in the hypercholesterolemic rat at high doses [312]. Cysteine bearing a 5-carbon residue, (CXXVI) has been isolated from cat urine and named fehnine [313]. It has been speculated that felinine may be a normal product of mammalian metabolism and participates in the transfer of Cg isoprene units in the biosynthesis of sterols [313]. Felinine has been synthesized by Trippett [314], Eggerer [315], and more recently by Schoeberl et al. [316]. 

DE Waard, a., and G. Popjak Studies on the biosynthesis of cholesterol. 9. Formation of phosphorylated derivatives of mevalonic acid in liver-enzyme preparations. Biochem. J. 73, 410 (1959). 

Theories on the in vivo synthesis of phytanic acid concentrate mainly on two possibilities. One of them deals with the possible formation of phytanic acid from isopren units or four molecules of mevalonic acid (or mevalonate), (Kahlke 1964 a, Kahlke and Richterich 1965). Instead of an end-to-end condensation of two molecules of famesyl pyrophosphate which results in the formation of squalene and finally cholesterol, a fourth active isoprenoid unit might be attached to famesyl pyrophosphate. From this intermediate several steps of hydrogenation and oxidation would be required for the formation of phytanic acid. This hypothesis now appears unlikely since Steinberg (1965) was unable to detect any activity in the phytanic acid fraction after administration of labeled mevalonate to a patient with HAP (case T.E. of Refsum). 

Cholesterol consists of four fused rings and an eight-membered hydrocarbon chain. It is synthesized from acetyl CoA. The first two reactions of the pathway are similar to that of ketogenesis, with the formation of HMG CoA. The rate limiting step is the synthesis of mevalonic acid, catalysed by HMG CoA reductase. It requires the reducing properties of 2NADPH and releases acetyl CoA. A five-carbon isoprene unit is then formed from mevalonic acid using ATP. A series of condensation reactions between isoprene units follows, which ends in the formation of squalene, a 30-carbon compound. Squalene is converted to lanosterol by hydroxylation then cyclization. The conversion of lanosterol to cholesterol is a multi-step process that involves many enzymes located in the endoplasmic reticulum. Thus, cholesterol synthesis occurs in the endoplasmic reticulum and the cytoplasm of all cells in the body. 

Lovastatin is rapidly metabolised, which is undesirable for any dmg likely to be chronically administered. In order to achieve once-daily dosing, steric hindrance around the ester carbonyl was increased by the introduction of an extra methyl group close to the ester bond in order to reduce the rate of esterase hydrolysis. This resulted in formation of simvastatin, a highly successful lipid-lowering agent. Simvastatin (like lovastatin) is inactive until metabolised in the liver to form its active metabolite mevinolinic acid (Fig. 8.48). Part of mevino-linic acid is stmcturally similar to the HMG portion of HMG-GoA, the substrate for HMG-CoA reductase, and hence competes with it for the active site of the enzyme. This reduces the amount of mevalonic acid which is produced. Mevalonic acid is a precursor of cholesterol. 

Formation of cholesterol and some of its derivatives. All of the carbon atoms of cholesterol are derived from acetyl-CoA by way of mevalonate in a pathway with 33 reaction steps. From cholesterol a wide variety of steroids and bile acids and bile salts are formed. Many of the reactions leading to cholesterol derivatives are organ-... 

Terpenoids do not necessarily contain exact multiples of five carbons and allowance has to be made for the loss or addition of one or more fragments and possible molecular rearrangements during biosynthesis. In reality the terpenoids are biosynthesized from acetate units derived from the primary metabolism of fatty acids, carbohydrates and some amino acids (see Fig. 2.10). Acetate has been shown to be the sole primary precursor of the terpenoid cholesterol. The major route for terpenoid biosynthesis, the mevalonate pathway, is summarized in Fig. 2.16. Acetyl-CoA is involved in the generation of the C6 mevalonate unit, a process that involves reduction by NADPH. Subsequent decarboxylation during phosphorylation (i.e. addition of phosphate) in the presence of ATP yields the fundamental isoprenoid unit, isopentenyl pyrophosphate (IPP), from which the terpenoids are synthesized by enzymatic condensation reactions. Recently, an alternative pathway has been discovered for the formation of IPP in various eubacteria and plants, which involves the condensation of glyceraldehyde 3-phosphate and pyruvate to form the intermediate 1-deoxy-D-xylulose 5-phosphate (Fig. 2.16 e.g. Eisenreich et al. 1998). We consider some of the more common examples of the main classes of terpenoids below. 

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