Saccharomyces cerevisiae NCYC 366, was grown anaerobically in batch culture, in the presence of one of a number of different sterols. Lipids were extracted from the cells and the sterol compositions analysed in order to find the degree of enrichment of the particular sterol provided in the medium. The free sterol content was found to range from 67% in the case of 7-dehydrocholesterol to 93% in cells grown in the presence of stigmasterol. The total lipid contents and the contents and compositions of other lipids were also analysed in order to find if any of the sterols in any way influenced lipid metabolism in the yeast. Lipids examined included free sterols, esterified sterols, phospholipids, triacylglycerols, squalene, and fatty-acyl compositions of phospholipids and neutral lipids. Variations among cells enriched with different sterols were only slight, the greatest range being in the sterol ester contents, where cells enriched in cholesterol had a content some three to ten times greater than other cells. An interesting result was that, in all cells, the fatty-acyl residues in the phospholipids consisted of 72 - 79% unsaturated residues, whereas there was less than 4% unsaturated residues in the neutral lipid. This finding is discussed in relation to the possible role of lipid containing vesicles in phospholipid synthesis. In order to check that the sterols were in fact being incorporated into the plasma membranes, these organelles were specifically labelled, isolated and their free sterol compositions measured. These showed that 70% of the sterol present was chemically identical with that supplemented to the medium. Sphaeroplasts of cells enriched in specific sterols were prepared, and their behaviour following suspension in hypotonic solutions of buffered sorbitol was measured. Two groups of responses were observed. Those sphaeroplasts enriched with a sterol possessing a double bond at C-22 remained relatively stable at 0.9 - 0.7M sorbitol before becoming susceptible to osmotic lysis, whereas those enriched with sterols without a double bond at C-22 lysed rapidly in sorbitol solutions below 1.0M. The function of sterol structure and membrane stability, particularly with regard to stretching is discussed, and the data obtained from osmotic lysis compared to some preliminary data on phospholipid-sterol interactions as a monolayer at an air-water interface.
|Date of Award||1975|