Inhibitors of sterol synthesis. Studies of in vitro effects of C27 15-oxygenated sterols on sterol synthesis in cell free homogenates of rat liver.

5a-Cholest-8(14)-en-3@-01-15-one, a potent hypocholesterolemic agent (Schroepfer, G. J., Jr., Monger, D., Taylor, A. S., Chamberlain, J. S., Parish, E. J., Kisic, A., and Kandutsch A. A. (1977) Biochem Biophys. Res. Commun. 78, 1227-1233), is a potent inhibitor of sterol synthesis in cultured mammalian cells and causes a reduction in the levels of 3-hydroxy-3-methylglutaric acid (HMG)-CoA reductase activity in the same cells (Schroepfer, G. J., Jr., Parish, E. J., Chen, H. W., and Kandutsch A. A. (1977) J. Biol, Chem 252,8975-8980). Preincubation of 5a-cholest-8(14)-en-3/3-01-15-0ne, at concentration of M, with the 10,000 x g supernatant fraction of rat liver homogenate preparations had no effect on the activities of acetate thiokinase, cytosolic acetoacetyl-CoA thiolase and HMG-CoA synthase. Identical findings on the latter two enzymes were also made in 48,000 X g supernatant fraction of rat liver homogenate preparations. Similarly, direct addition of the A*~‘4~-15-ketosterol M) to rat liver microsomes, under a variety of conditions had no effect of the level of HMG-CoA reductase activity. 5a-Cholest-8(14)-en3fi-ol-15-one and 5a-cholesta-6,8(14)-dien-3/3-ol-15-one (at M) had no effect on the distribution of radioactivity in nonsaponifiable lipids derived from labeled acetate and mevalonate, respectively, in 10,000 X g supernatant fraction of rat liver homogenate preparations. These findings are in contrast to those made recently with another 15-oxygenated sterol (14~1-ethyl5a-cholest-7-ene-3@,15a-diol) which caused, at lo-‘ M, a marked accumulation of labeled lanosterol and 24,25dihydrolanosterol in cell-free homogenate preparations of rat liver (Raulston, D. L., Pajewski, T. N., Miller, L. R., Philipp, B. W., Shapiro, D. J., and Schroepfer, G. J., Jr. (1980) Biochem Znternat. 1,113-119). 5a-Cholest8(14)-en-3@-01-15-one M) caused slight (12-15%) inhibition of the synthesis of digitonin-precipitable sterols from labeled acetate, but not from mevalonate, upon incubation with cell-free homogenate preparations of rat liver. The effects of a number of other C2, 15-oxygenated sterols on the synthesis of digitonin-precipitable sterols from labeled acetate and/or mevalonate have also been investigated.