STEROL BIOSYNTHESIS BY STRAINS OFSACCHAROMYCES CEREVISIAEIN THE PRESENCE AND ABSENCE OF DISSOLVED OXYGEN

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Abstract
The content of sterols in Saccharomyces cerevisiae which has been harvested after anaerobic growth and then added to a complex nutrient medium, rises rapidly from ca. 1 mg/g dry yeast to ca. 10 mg in the presence of dissolved oxygen. A range of sterols, present principally as sterol esters, is formed during this period. The concentration of free sterols does not rise above 3 mg/g and esters are thought to form a reserve sterol pool. Cyclization of squalene to lanosterol in the presence of oxygen seems not to be markedly affected by oxygen concentration in contrast to demethylation and desaturation reactions on the pathway to ergosterol. When oxygen concentration falls to zero, further metabolism of preformed sterols continues, with the accumulation of episterol and ergosterol and reduction in the concentration of zymosterol and 24(28)-dehydroergosterol. During anaerobic growth a marked hydrolysis of sterol esters occurs and free sterols eventually predominate.