Tetrahymanol biosynthesis by the protozoan Tetrahymena pyriformis was progressively inhibited by the inclusion of cholesterol in the growth medium. Studies with labelled precursors of tetrahymanol have established that there are two major sites of inhibition in whole cells. The inhibition at the first site, between acetate and mevalonate, occurred rapidly after addition of cholesterol. The activity of 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34), a predominantly cytosolic enzyme in this organism, was not inhibited in cholesterol-grown cells nor by addition of cholesterol directly to the assay medium. The second major site of inhibition in whole cells is between mevalonate and squalene and this is accompanied by inhibition of the enzyme that converts farnesyl-pyrophosphate into squalene (squalene synthetase). Squalene cyclase is partially inhibited. The conversion of mevalonate into tetrahymanol in vitro was not inhibited by the addition of cholesterol to the assay medium. Tetrahymanol added to the culture medium is taken up by the cells but does not inhibit endogenous biosynthesis. It is suggested that cholesterol inhibits the later stages of tetrahymanol biosynthesis by causing a change in membrane structure and function which alters the activity of membrane-bound enzymes.
