Cholesterol synthesis squalene

Analysis of the details of the pathway was helped by the discovery by Nancy Bucher (1953) that cholesterol synthesis took place in cell-free post-mitochondrial supernatants. ATP, Mg2+ and NAD+ were required. Tchen and Bloch extended these findings to show that squalene could be formed anaerobically but the conversion of squalene to cholesterol was oxygen dependent, the oxygen of the intermediate lanosterol being derived from 8C>2 not H2180. It therefore became possible to focus either on the conversion of acetate to squalene or on the latter s cyclization to the sterol. 

Scheme 7.6 Stereoselective acetylation of the hydroxyl diethyl ester (9) to the (S)-acetate as an intermediate for the synthesis of a squalene synthase inhibitor ( ) of cholesterol synthesis.

Squalene takes part in metabolism as precursor for synthesis of steroids and structurally quite similar to (3-carotene, coenzyme qlO, vitamins Ki, E, and D. The squalene in skin and fat tissue comes from endogenous cholesterol synthesis as well as dietary resources in people who consume high amounts of olive and fish oil especially shark liver (Gershbein and Singh, 1969). Squalene is synthesized by squalene synthase which converts two units of farnesyl pyrophosphate, direct precursor for terpenes and steroids, into squalene. As a secosteroid, vitamin D biosynthesis is also regulated by squalene. Moreover, being precursor for each steroid family makes squalene a crucial component of the body. 

One of the best therapeutic approaches may be to prevent absorption of cholesterol from the intestines by inclusion of a higher fiber content in the diet.66 Supplementation with a cholesterol-binding resin may provide additional protection. Plant sterols also interfere with cholesterol absorption. Incorporation of esters of sitostanol into margarine provides an easy method of administration. Supplemental vitamin E may also be of value.q Another effective approach is to decrease the rate of cholesterol synthesis by administration of drugs that inhibit the synthesis of cholesterol. Inhibitors of HMG-CoA reductase,s hh (e.g., vaLostatin) iso-pentenyl-PP isomerase, squalene synthase (e.g.,... 

Squalene Final acyclic intermediate in cholesterol synthesis, acts as feedback inhibitor of cholesterol synthesis... 

Plant sterols inhibit the intestinal absorption of cholesterol and so have a useful hypocholesterolemic action. They also inhibit endogenous synthesis of cholesterol, by inhibiting and repressing the regulatory enzyme of cholesterol synthesis, hydroxymethylglutaryl (HMG)-CoA reductase. Other compounds synthesized from mevalonate also inhibit and repress HMG-CoA reductase and have a hypocholesterolemic action, including squalene (found in relatively large amounts in olive oil), ubiquinone (Section 14.6), and the tocotrienols (Section 4.1). 

Hydroxylation reactions play a very important role in the synthesis of cholesterol from squalene and in the conversion of cholesterol into steroid hormones and hile salts. All these hydroxylations require NADPH and O. The oxygen atom of... 

Cholesterol is formed in the liver (85%) and intestine (12%) - this constitutes 97% of the body s cholesterol synthesis of 3.2 mmol/day (= 1.25 g/day). Serum cholesterol is esterized to an extent of 70-80% with fatty acids (ca. 53% linolic acid, ca 23% oleic acid, ca 12% palmitic acid). The cholesterol pool (distributed in the liver, plasma and erythrocytes) is 5.16 mmol/day (= 2.0 g/day). Homocysteine stimulates the production of cholesterol in the liver cells as well as its subsequent secretion. Cholesterol may be removed from the pool by being channelled into the bile or, as VLDL and HDL particles, into the plasma. The key enzyme in the synthesis of cholesterol is hydroxy-methyl-glutaryl-CoA reductase (HGM-CoA reductase), which has a half-life of only 3 hours. Cholesterol is produced via the intermediate stages of mevalonate, squalene and lanosterol. Cholesterol esters are formed in the plasma by the linking of a lecithin fatty acid to free cholesterol (by means of LCAT) with the simultaneous release of lysolecithin. (s. figs. 3.8, 3.9) (s. tab. 3.8)... 

C. Reduction of HMG CoA to mevalonic acid is an early step in cholesterol synthesis. Inhibition of this step would lead to an increase in cellular levels of HMG CoA and a decrease in squalene, an intermediate beyond this step, and cholesterol. The decreased cholesterol levels in cells cause ACAT activity to decrease and synthesis of LDL receptors to increase. Because the receptors function (but at a less than normal rate), more receptors cause more LDL to be taken up from the blood. Consequently, blood cholesterol levels decrease, but blood triacylglyc-erol levels do not change much, since LDL does not contain much triacylglycerol. 

Relatively little is known about plant sterols. (Most of the research effort in steroid metabolism has been expended in the investigation of steroid-related human diseases.) It appears, however, that the initial phase of plant sterol synthesis is very similar to that of cholesterol synthesis with the following exception. In plants and algae the cyclization of squalene-2,3-epoxide leads to the synthesis of cycloartenol (Figure 12.30) instead of lanosterol. Many subsequent reactions in plant sterol pathways involve SAM-mediated methylation reactions. There appear to be two separate isoprenoid biosynthetic pathways in plant cells the ER/cyto-plasm pathway and a separate chloroplast pathway. The roles of these pathways in plant isoprenoid metabolism are still unclear. 

Cholesterol synthesis can be divided into three phases formation of HMG-CoA from acetyl-CoA, conversion of HMG-CoA to squalene, and conversion of squalene to cholesterol. Cholesterol is the precursor for all steroid hormones and the bile salts. Bile salts are used to emulsify dietary fat. They are the primary means by which the body can rid itself of cholesterol. 

Previous investigations from several laboratories have demonstrated that both microsomal membranes and the cytosolic fraction from rat hver are required for the biological synthesis of cholesterol [1-4]. Specifically, the following conversions have been reported to require both microsomes and cytosol acetate to cholesterol [4] squalene to cholesterol [1] squalene-2,3-oxide to lanosterol [3] lanosterol to cholesterol [1,5] A -cholestenol to cholesterol [6] lanosterol to dihydrolanosterol [7] various 4,4-dimethyl sterols to cholesterol [8] and 7-dehydrocholesterol to cholesterol [9,10]. 

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). 

Evidence in favor of peroxisomal involvement in cholesterol biosynthesis is the following. The molecular cloning of cDNAs encoding many of these enzymes revealed peroxisomal targeting sequences (W.J. Kovacs, 2003). The availability of specific antibodies allowed immunocytochemical localization to peroxisomes [4] (W.J. Kovacs, 2006). Fibroblasts from individuals with peroxisome biogenesis disorders showed reduced enzymatic activities of cholesterol biosynthetic enzymes, reduced rates of cholesterol synthesis, and lower cholesterol content [4]. Together these data suggest that peroxisomes may play a role in all steps in the cholesterol biosynthetic pathway, except the conversion of famesyl-PP to squalene. The latter reaction is catalyzed by squalene synthase, which is found. solely in the ER. 

Cholesterol is formed biosynthetically from isopentenyl pyrophosphate (active isoprene). The majority of cholesterol in the body derives from de novo biosynthesis in the liver [1,2]. Cholesterol synthetic pathway has been assumed to occur primarily in the cytoplasm and endoplasmic reticulum (ER). However, more recent evidences have suggested that the enzymes, except squalene synthase, squalene epoxidase and oxidosqualene cyclase, are partly localized in the peroxisomes, which are essential for normal cholesterol synthesis [11]. 

Hopanoids are pentacyclic molecules that are found in bacteria and in some plants. As an example, a typical bacterial hopanoid is shown below. Organisms that make hopanoids use a pathway similar to that for cholesterol synthesis. The biosynthetic pathway for hopane includes the formation of squalene, followed by more steps to form the final product itself, a C30 compound. Hopane is similar to lanosterol (text, p. 726), but lacks the hydroxyl group. 

Q Bloch had to flee as a Jew from the Nazi regime in 1936 via Switzerland to the USA. At Columbia University in New York, he was able to show together with David Rittenberg in 1942, using isotopically labelled acetate, that this was a precursor for the cholesterol synthesis in animals. The polyene cyclisation of the triterpene squalene produces lanosterol, which is degraded to cholesterol, the central intermediate of all human steroids. [350]... 

Several studies, in part contradictory, are available concerning the influence of nicotinic acid on cholesterol biosynthesis. Perry (1960) reported from work with rat liver slices a decreased incorporation of C-acetate into cholesterol with high concentrations of nicotinic acid in the medium Schade and Saltman (1959) had obtained similar results in rabbits fed with nicotinic acid. On the other hand, Merrill and Lemley-Stone (1957) found increased cholesterol synthesis in liver slices of rats fed nicotinic acid, while Duncan and Best (1960) reported that nicotinic acid has no effect on C-acetate incorporation. Parsons (1961 a) studied the effects of nicotinic acid and niacin on incorporation of C-acetate in man, and stated that considerably less conversion into serum cholesterol (free and esterifled) and into erythrocyte cholesterol occurred during nicotinic acid administration. The concept of inhibition of cholesterol synthesis is also held by Goldsmith (1962) the point of inhibition supposedly occurs before the formation of squalene, because sterol intermediates between squalene and cholesterol could not be detected in serum. 

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