Cholesterol Inhibition of Pentacyclic Triterpenoid Biosynthesis in Tetrahymena pyriformis*†

SYNOPSIS. Tetrahymena pyriformis synthesizes tetrahymanol and “diplopterol” from acetate, mevalonate or squalene. The formation of these pentacyclic triterpenoid alcohols is inhibited by the addition of cholesterol to the culture fluid of the ciliates. Cholesterol also inhibits the biosynthesis of squalene from acetate or mevalonic acid. The synthesis of other terpene derivatives from acetate and mevalonate continues in the presence of cholesterol, thus suggesting that a major block occurs after “isoprene” formation and before squalene formation. Further, inhibition of squalene conversion to the pentacyclic alcohols by cholesterol has been established.