Aspects of lipid metabolism in oleaginous yeasts
1. Phosphoketolase Studies Pentulose 5-phosphate phosphoketolase (Pu5PPPK) was shown to be induced in nine yeasts grown on xylose. The enzyme was active with both xylulose 5-phosphate (Xu5P) and ribulose 5-phosphate (Ru5P) as assay substrate. Partial purification of the enzyme led to a decrease in activity for Ru5P, but did not distinguish between the possibilities that the enzyme may have a dual substrate specificity and that Ru5P 3-epimerase may be closely associated with the phosphoketolase. Lipid levels and ATP:citrate lyase activities were similar in oleaginous yeasts grown on xylose and glucose. It was concluded that Pu5PPK does not affect the process of lipid accumulation in these yeasts. 2. Lipid turnover studies Yeasts of the genera Candida, Trichosporon and Rhodosporidium were able to degrade their storage lipid for cell proliferation under carbon starvation, conditions. Yeasts of the genus Lipomyces were incapable of this. All yeasts examined accumulated glycogen simultaneously with lipid and degraded it during carbon starvation conditions. Lipid turnover was rapid and was accompanied by peroxisome formation and repression of the enzyme ATP:citrate lyase. Other key enzymes in lipid metabolism were also affected. Similar changes occurred when the yeasts were grown on an exogenous lipid source. 3. Triacylglycerol biosynthesis Triacylglycerol biosynthesis activities were demonstrated in whole sphaeroplasts from Candida curvata D and Lipomyces starkeyi CBS 1809. Biosynthesis was shown to proceed via the α-glycerol 3-phosphate pathway. The sub-cellular location of biosynthetic activity was not identified but electron microscopy studies indicated that the endoplasmic reticulum may be the location. Fatty acyl-CoA synthetases were shown to be active with linoleic, oleic and palmitic acids but not with stearic acid. Activity of this enzyme was enhanced during carbon starvation conditions. 4. Lipid globule studies Globules were isolated from three species of oleaginous yeasts and examined by electron microscopy. They had a rough laminar surface and were 1 .3-5µm in diameter. Their major component was triacylglycerol.