Squalene - Lanosterol

The first recognizable steroid ring system is lanosterol it is formed first by the epoxidation of the double bond of squalene that was originally derived from a DMAPP through famesyl pyrophosphate, and 

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From squalene, lanosterol, the first cyclic precursor, is created by a remarkable set of enzyme-catalyzed addition reactions and rearrangements that create four fused rings and seven stereocenters. 

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. 

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-... 

Like all other classes of steroid hormones, the androgens are synthesized from acetyl coenzyme A via mevalonic acid, isopentenyl pyrophosphate, farnesyl pyrophosphate, squalene. lanosterol. and cholesterol. Enzyme... 

The sterols of starfish, which are typically A7-compounds, are thought to be derived from dietary A5-compounds, and it has been shown177 that two species of starfish can convert cholest-5-en-3/3-ol and sitosterol into their A7-analogues. However, de novo synthesis of squalene, lanosterol, and 5a-cholest-7-en-3j8-ol from [2-14C]MVA has been demonstrated178 in various starfish species, but it is of particular interest that the C26, C28, and C29 sterols were not labelled. The direct conversion of cholest-5-en-3/3-ol into 5 -cholest-7-en-3/3-ol in Calliphora eryth-rocephala has been demonstrated.179... 

Further work in the phylum Echinodermata shows a variable ability to biosynthesize steroids. In the class Holothuroidea and Echinoidea, the representatives examined could synthesize squalene but not triterpenoids or sterols from acetate. However, several examples from the class Asteroidea were able to synthesize squalene, lanosterol, and other steroids. In the later stages of steroid metabolism it was shown that cholesterol was converted into cholest-7-enol via cholestanol. 

Evidence has been accumulated that the biogenesis of plant steroids proceeds by the same scheme as in animals this sequence was proved up to squalene and recently squalene has been shown to be a precursor of, 8-sitosterol evidence was also adduced that this conversion includes lanosterol as an intermediate (233). Goutarel assumes that the sequence squalene, lanosterol, zymosterol, and desmosterol may be implied also in the biogenetic pathway of Apocynaceae alkaloids (205). It may be assumed that biogenetic intermediates from sterol or triterpene precursors of Apocynaceae or Buxaceae alkaloids, respectively, are 3-and/or 20-ketones which may give rise to mono- and diamines. 

Holotoxin Ai inhibits the RNA biosynthesis in Candida albicans and Saccharomyces carlsbergensis, as indicated by the decrease in incorporation of C-uridine to the acid-insoluble fraction of the cells. Similar results were obtained for glycoside fractions of 14 species of Pacific sea cucumbers [132]. Apparently the inhibition of RNA biosynthesis in Saccharomyces carlsbergensis is related to nucleotide loss from yeast cells after treatment with glycosides. Holotoxin Ai also inhibits biosyntheses of squalene, lanosterol and ergosterol in S. carlsbergensis [133]. Mitosis is arrested and DNA synthesis inhibited in onion root bulbs by crude holothurin [134]. 

The stereoisomeric mixture of -hydroxyalkyl selenides resulting from the reaction of the a-selenoalkyllithium and the carbonyl compound has been often cleanly and easily separated into its constituents by liquid chromatography on silica gel (Schemes 124,133,134, and 170 172).200.206.222,226,229,258 59 jj,jg has, therefore, allowed the synthesis of each of the two stereoisomers of various di- and tri-substituted alkenes (Schemes 124,170 and 171 Scheme 172, a) and epoxides (Scheme 124 Scheme 172, b), which are otherwise obtained as intractable mixtures of stereoisomers through the conventional phosphorus or sulfur ylide methods. Last but not least, 2-lithio-2-methylselenopropane can be used as the precursor of various compounds bearing gem dimethyl substituted carbons, such as squalene, oxido-squalene, lanosterol and cholesterol. Use of commercially available perdeuterated or Ci or — 2 acetone allows the straightforward synthesis of the corresponding labelled compounds... 

In addition to cholesterol and 5a-cholest-7-en-3y8-ol, many C2g and C29 conventional sterols are present in Echinodermata. These sterols are probably derived from the diet. However, echinoderms are able to synthesize sterols. Thus, [ C]mevalonic acid was incorporated into squalene, lanosterol and desmosterol by the sea urchin. Echinus esculentus [87]. The ability of ophiurpids to synthesize sterol from [ C]acetate has also been demonstrated [88]. Sterol biosynthesis by a holothuroid was first investigated by Numura [89], and sterol biosynthesis from [ C]acetate in sea cucumbers has also been reported [90]. 

In the biogenesis of steroids, the enzyme-catalyzed polycyclization of squalene (225) produces the tetracyclic substance lanosterol (2 ) which is eventually converted into cholesterol (227). Eschenmoser, Stork, and their co-workers (80-82) have proposed that the squalene-lanosterol conversion can be rationalized on the basis of stereoelectronic effects. The stereochemical course of this biological cyclization (83, 84) can be illustrated by considering the transformation of squalene oxide (228) (an intermediate in the biosynthesis of cholesterol (83, 84)) into dammaradienol 229. This transfor-... 

Squalene monooxygenase (2,3-epoxydizing) 2 2,3-oxidosqualene cyclase (e.g., 2,3-oxido-squalene lanosterol cyclase and 2,3-oxidosqualene cycloartenol cyclase)... 

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. 

Squalene oxydocyclase Rat liver Squalene Lanosterol Tchen and Bloch (1956)... 

Something of the order of the removal of the methyl groups is known. The group at C-14 is eliminated first (Fig. 2). Information bearing on this point has been published by Schneider et al. (1957) and Gautschi and Bloch (1958). Intestinal tissue from rats injected virith C -acetate, killed 5 to 10 minutes later, were found to contain labeled squalene, lanosterol, and a compound later identified as 4,4-dimethyl-J -cholestadien-3j3-ol. This material was also synthetically prepared and shown to be rapidly converted to cholesterol. 

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