ABSTRACT
The major sterols to be discussed in this chapter are ergosterol and cholesterol, which are essential constituents of cell membranes of yeasts and fungi, and of mammals respectively. Both sterols are synthesized in a similar fashion start-
ing from acetyl-coenzyme A (Fig. 6.1). The synthesis takes place in different parts of the cell: acetate to mevalonate is carried out in the microsomal fraction while mevalonate to squalene is a cytoplasmic function; squalene conversion to ergosterol and cholesterol is performed in the microsomes and this is the first part of the pathway to require oxygen. The pathways to ergosterol and cholesterol are common up to lanosterol. Furthermore, the removal of the methyl group attached to the carbon atom at position 14 of lanosterol is also common, although in yeasts and fungi it does not necessarily occur next in sequence (Fig. 6.3). Not all the enzymes have been purified, but it appears that, although yeast and mammalian enzymes catalyse the same interconversions, they are not necessarily identical, and thus lend themselves to selective inhibition.
