Lanosterol, cholesterol from

Step 5—Formation of Cholesterol The formation of cholesterol from lanosterol takes place in the membranes of the endoplasmic reticulum and involves changes in the steroid nucleus and side chain (Figure 26-3). The methyl groups on C,4 and C4 are removed to form 14-desmethyl lanosterol and then zymosterol. The double bond at 03—C9 is subsequently moved to Cj-Cg in two steps, forming desmosterol. Finally, the double bond of the side chain is reduced, producing cholesterol. The exact order in which the steps described actually take place is not known with certainty. 

The synthesis of all isoprenoids starts with acetyl-CoA, which in a series of six different enzyme reactions is converted into isopentenyl-diphosphate (-PP), the basic C-5 isoprene unit that is used for the synthesis of all subsequent isoprenoids (Fig. 5.1.1). At the level of farnesyl-PP the pathway divides into several branches that are involved in the production of the various isoprenoid end products. One of the major branches involves the cholesterol biosynthetic part of the pathway, of which squalene is the first committed intermediate in the production of sterols. Following cycliza-tion of squalene, lanosterol is produced. To eventually produce cholesterol from la-... 

Fig. 34. 3. Structures of selected sterols. Sources animal - lanosterol, cholesterol and ergosterol (also microbial) plant - all others. (From Warner, K., Su, C, and White, P.J. "Role of Antioxidants and Polymerization Inhibitors in Protecting Frying Oils" in Frying Technology and Practices, M.K. Gupta, K. Warner, and P.J. White (Eds.), pp. 37-49, AOCS Press, Champaign, IL 2004. With permission.)...

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)... 

Fig. 8 is a schematic diagram of a cell which shows the known sites in which sterol carrier proteins are involved in cholesterol biosynthesis, utilization and intracellular transfer. SCP, participates in the conversion of squalene to lanosterol and SCP2 participates in the conversion of lanosterol to cholesterol, the conversion of cholesterol to cholesterol ester by ACAT, and probably also in the conversion of cholesterol to 7a-hydroxycholesterol. SCPj transfers cholesterol from cytoplasmic lipid inclusion droplets to mitochondria in the adrenal and SCPj also translocates cholesterol from the outer to the inner mitochondrial membrane. 

Lanosterol is converted into cholesterol in a series of nineteen enzyme reactions [18]. The production of cholesterol from lanosterol involves the reduction of the double bond at C-24, demethylations of ge/w-dimethyl at C-4 and a tertiary methyl at C-l4, and isomerization of the double bond from C-8 to C-l. Two major pathways involving the same enzymes have been proposed [16-18]. Cholesterol biosynthetic pathway is outlined in... 

Discuss the cycUzation of squalene and the formation of cholesterol from lanosterol. Note the role of O2 in the formation of cholesterol. 

Within 2 years of the publication of the Whitmore paper, Robinson proposed the formation of the steroids (including cholesterol) from squalene (a C30 polyunsaturated polyisoprene molecule) via an incredible series of intermediates and rearrangements. Later, following the elucidation of the structure of lanosterol, R. B. Woodward and K. Bloch made a brilliant proposal that at once rationalized the biosynthetic origin of both lanosterol and cholesterol and implicated lanosterol as an intermediate in cholesterol bios)mthesis. Their mechanism involved the concerted (bonds made and broken simultaneously) cydization of four rings, as well as four rearrangements... 

This classification has a number of weaknesses. Some bacteria, for example, accept the zoosterol cholesterol from the host animals and use it as a component of membranes, some types of prokaryotic organisms even synthesise sterols de novo, some eubacteria (species of the genus Methylobacterium and Methylosphaera) synthesise 4-methylsterols and 4,4-dimethylsterols (including lanosterol). In many species of bacteria the so-called hopanoids, pentacychc... 

By incubation or perfusion of placental tissue Math acetate, this precursor is transformed into squalene, lanosterol, and cholesterol (Levitz et al., 1962, 1964 Van Leusden and Villee, 1965 C. A. Villee, 1967, 1969). On the other hand, placental perfusion with mevalonate results in the formation of squalene and lanosterol, but not of cholesterol (Tjcvitz et al., 1962). However, the in vi(7 o conversion of both acetate and mevalonate to cholesterol was found by Zelen-ski and Villee (1966) using a preparation of minced human term placenta. Tliese authors suggest that the formation of cholesterol from these precursors is through different metabolic pathways. 

In the biosynthesis of cholesterol from acetate and its multiples, hydroxylases are not thought to participate until the conversion of the lengthy squalene chain to lanosterol. This transformation is a magnificent and classic example of a reaction which was predicted on theoretical grounds and then proven through extensive laboratory investigation. Two excellent reviews on the pathways in the biosynthesis of cholesterol have appeared in recent literature, those of Cornforth et al. (1959) and Popjak and Cornforth (1960), and the reader is referred to these for more detailed information. 

The path from squalene (114) to the corresponding oxide and thence to lanosterol [79-63-0] (126), C qH qO, cholesterol [57-88-5] (127), and cycloartenol [469-38-5] (128) (Fig. 6) has been demonstrated in nonphotosynthetic organisms. It has not yet been demonstrated that there is an obligatory path paralleling the one known for generation of plant sterols despite the obvious stmctural relationships of, for example, cycloartenol (128), C qH qO, to cyclobuxine-D (129), C25H42N2O. The latter, obtained from the leaves of Buxus sempervirens E., has apparentiy found use medicinally for many disorders, from skin and venereal diseases to treatment of malaria and tuberculosis. In addition to cyclobuxine-D [2241-90-9] (129) from the Buxaceae, steroidal alkaloids are also found in the Solanaceae, Apocynaceae, and LiUaceae. 

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). 

From lanosterol, the pathway for steroid biosynthesis continues on to yield cholesterol. Cholesterol then becomes a branch point, serving as the common precursor from which all other steroids are derived. 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

Lanolin is a wax secreted by the sebaceous glands of sheep it is obtained from wool and it has been used as a lubricant and as an ingredient in pharmaceutical preparations. It contains esters of long chain alkanoic acids, both linear and branched, and of hydroxya-cids, cholesterol and lanosterol [2,83]. 

Cho SY, Kim JH, Paik YK (1998) Cholesterol biosynthesis from lanosterol differential inhibition of sterol delta 8-isomerase and other lanosterol-converting enzymes by tamoxifen. Mol Cells 8(2) 233-239... 

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. 

The route for the cyclization was easier to determine than the identification of the very reactive isoprene unit, and was understood in outline by 1960. Studies of labeled compounds detected within 10 min. of 14C-acetate addition to intestinal preparations showed label in squalene, lanosterol, and a further, unidentified ring compound, all with higher specific activities than cholesterol. By 75 min cholesterol was the main labeled compound. Clayton and Bloch then confirmed that lanosterol, previously known from sheep s wool, was converted to cholesterol with the extra three (methyl) carbon atoms being lost as carbon dioxide. 

Perhaps the most spectacular of the natural carbocation rearrangements is the concerted sequence of 1,2-methyl and 1,2-hydride Wagner-Meerwein shifts that occurs during the formation oflanosterol from squalene. Lanosterol is then the precursor of the steroid cholesterol in animals. 

By far the most impressive example of electrophilic addition in natural prodnct formation is in the biosynthesis of steroids. The snbstrate sqnalene oxide is cyclized to lanosterol in a process catalysed by a single enzyme. Lanosterol is then converted into the primary animal-steroid cholesterol. Sqnalene oxide comes from sqnalene, which is itself formed throngh a combination of two molecules of farnesyl diphosphate. 

Formation of cholesterol. Squalene, a linear isoprenoid, is cyclized, with O2 being consumed, to form lanosterol, a C30 sterol. Three methyl groups are cleaved from this in the subsequent reaction steps, to yield the end product cholesterol. Some of these reactions are catalyzed by cytochrome P450 systems (see p. 318). 

The all-tra 5 -squalene (C30H50), discovered in shark liver oil in the 1920s, is a triterpene, but one in which the isoprene rule at violated in one point. Rather than a head-to-tail arrangement of six units of isoprene, there appear to be farnesyl units that have been connected tail to tail. Almost aU steroids are biosynthesized from cholesterol. Cholesterol is biosynthesized from squalene, which is first converted to lanosterol. The conversion of squalene to the steroid skeleton is an oxirane, squalene-2,3-oxide, which is transformed by enzymes into lanosterol, a steroid alcohol naturally found in wool fat. The whole process is highly stereoselective. 

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