Cycloheximide inhibits sterol biosynthesis in cell-free preparations of rat liver

Cycloheximide, frequently used as an inhibitor of protein synthesis in vivo and in vitro, has been found to cause a significant reduction of the synthesis of digitonin-precipitable sterols from acetate, but not from mevalonate, at a concentration of 1 mM in the 10,000 x g supernatant fraction of rat liver homogenate preparations. The results of studies of the metabolism of labeled leucine under the same conditions indicated that the effect of cycloheximide on sterol synthesis from acetate was not related to an effect of cycloheximide on protein synthesis. Preincubation of the 10,000 x g supernatant fraction of rat liver homogenates with cycloheximide (1 mM) caused a significant reduction in the levels of acetate thiokinase and hydroxymethylglutaryl-CoA synthase activities but not of acetoacetyl-CoA thiolase activity. Preincubation of the 100,000 x g supernatant fraction of rat liver homogenates with cycloheximide (1 mM or 0.3 mM) also caused a significant reduction of the levels of hydroxymethylglutaryl-CoA synthase activity. When cycloheximide (1 mM) was preincubated with the 100,000 x g supernatant fraction, a reduction in the level of acetate thiokinase activity was observed. Preincubation of rat liver microsomes with cycloheximide (1 mM and 3 mM) had no effect on the level of hydroxymethylglutaryl-CoA reductase activity. These results suggest that biological effects observed upon exposure of cells or tissues to high concentrations of cycloheximide may not be exclusively due to effects of the cycloheximide on the synthesis of protein.