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When eight strains of the oleaginous yeasts Candida curvata, Lipomyces starkeyi, Rhodosporidium toruloides and Trichosporon cutaneum were starved of carbon after having accumulated lipid up to 34% of their biomass, the lipid was readily converted to new biomass in all cases except the two strains of L. starkeyi. When C. curvata D was grown in a two-stage chemostat with the second stage as a carbon-starvation vessel (but containing NHZ) biosynthesis of new biomass reached 1.9 0.2 g per g lipid utilized. Experiments in a single-stage chemostat undergoing transition from lipid accumulation (nitrogen-limited medium) to carbon-starvation conditions showed that the lipid in C. curvata was rapidly mobilized. When the lipid was pre-labelled with 14C, a transitory pool of rapidly metabolizable non-lipid material appeared within 1.5 h of the initiation of starvation. Rates of lipid loss indicated that initiation of lipid degradation occurred immediately carbon was lost from the external medium.

Low rates of triacylglycerol (TAG) biosynthesis were observed in cell-free extracts of Candida curvata, but rates were increased up to 10-fold by adding either alpha- or beta-cyclodextrins. Spheroplasts, whole or gently disrupted, had higher rates of incorporation of both [U-14C]glycerol 3-phosphate or [1-14C]oleate into triacylglycerol and the intermediates of its biosynthesis: lysophosphatic acid, phosphatidic acid and diacylglycerol. Fatty acyl-CoA synthetase was highest with palmitate, oleate and linoleate but was some 6- to 8-fold lower with stearate. Stearate and stearoyl-CoA were poorly incorporated into lipids. Subcellular fractionation of the spheroplasts into mitochondrial, microsomal, lipid bodies and supernatant fractions diminished the rates of 14C incorporation of oleate into triacylglycerol. By comparing the relative specific activities for each activity in each fraction, the fatty acyl-CoA synthetase activity appeared mainly in the lipid bodies, and that for phosphatidic acid formation was mainly in the mitochondrion; other activities were too weak and too dispersed for accurate assessment of their location. Recombining all the subcellular fractions restored triacylglycerol synthesizing activity. Omitting any single fraction from the mixture did not result in restoration of triacylglycerol synthesizing activity. Starvation of the yeast, which leads to utilization of the endogenous lipid reserves, stimulated fatty acyl-CoA synthetase activity, but diminished phosphatidic acid and triacylglycerol biosynthesis indicating probable induction of beta-oxidation in the peroxisomes and repression of lipid biosynthesis.

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