When sterol biosynthesis in oxygen-deprived wild type Saccharomyces cerevisiae was prevented by the presence of 2,3-iminosqualene, an inhibitor of 2,3-oxidosqualene cyclase, an absolute requirement for a sterol with a 24 beta-methyl group was found. Neither the configuration nor the size of the alkyl group at C-24 could be altered. For instance, while 24 beta-methylcholesterol (22-dihydrobrassicasterol) permitted good growth, contrary to earlier work without the inhibitor no growth at all resulted from the presence of cholesterol or its 24 alpha-methyl-, 24 alpha-ethyl-, or 24 beta-ethyl derivatives (campesterol, sitosterol, and clionasterol, respectively). The only sterol lacking a 24 beta-methyl group which allowed growth was desmosterol (24-dehydro-cholesterol), but desmosterol was metabolized to 24 beta-methylcholesterol by C1-transfer and reduction. When cholesterol supported growth in the absence of the inhibitor, small amounts of endogenously synthesized 24 beta-methylsterols (ergosterol and 22-dihydroergosterol) were identified. This previously unrecognized absolute specificity for both chirality and bulk at C-24 suggests the involvement of protein binding in at least one of the roles which sterol plays in this single-celled eukaryote.
Cholesterol Kinetics