Biological synthesis of lanosterol and agnosterol.

There is considerable experimental evidence in support of the view that, in the biogenesis of cholesterol, squalene is an intermediate (1, 2). It has been suggested (3) that the cyclization of the aliphatic triterpenoid hydrocarbon to the tetracyclic ring system of the steroids occurs in a manner which would also rationalize the structure of lanosterol, a naturally occurring 4,4’, 14-trimethylcholestane derivative (4). In view of its structural relationship to squalene on the one hand and to cholesterol on the other, it has become of interest to investigate the possible role of lanosterol, or of related structures having 30 carbon atoms, as intermediates in the biosynthesis of cholesterol. That the synthesis of lanosterol from acetate takes place in mammalian tissues has now been demonstrated. Moreover, as will be shown in the accompanying paper (5), lanosterol serves as a precursor of cholesterol. At the start of this work, since pure lanosterol was not available to us, tentative information on the biogenesis of C& sterols was obtained by the use of natural “isocholesterol” as carrier. The “isocholesterol” of wool fat, the most abundant source of lanosterol, is known (6) to consist of the four components (Fig. l), lanosterol, dihydrolanosterol, agnosterol, and dihydroagnosterol, in variable proportions, with lanosterol as the major constituent. From this mixture pure lanosterol can be isolated only by very tedious procedures (6,7). For the demonstration of lanosterol biosynthesis, rat liver homogenates prepared according to the method of Bucher (8) were incubated with sodium acetat,e-l-Cl4 and the unsaponifiable fractions combined with small amounts of “isocholesterol” as carrier. Careful chromatography of the resulting mixture demonst,rated that a small but significant percentage of the total Cl4 in the unsaponifiable material remained associated with the “isocholesterol” fraction. When rat liver homogenates were incubated under similar conditions but with prior addition of suspensions of “isocholesterol,” the incorporation of acetate carbon into this fraction was mark-