Inhibition of sterol biosynthesis by ergosterol and cholesterol in Saccharomyces cerevisiae

When accumulation of squalene was used as a measure of the flow of carbon into the sterol pathway in whole cells of semi-anaerobic Saccharomyces cerevisiae, both ergosterol and cholesterol were found to be inhibitory. However, at equivalent concentrations in the medium ergosterol was substantially the more potent inhibitor. Marked differences found in the absorption and esterification of the two sterols failed to account for the observed difference in their capacities to act as feedback agents. Cholesterol was much more effectively absorbed as well as esterified, but, when the abilities of the two sterols to lower the squalene level were calculated on the basis of free sterol in the cells, ergosterol remained more effective by a factor of four.