Formation of intracellular fat droplets: interrelation of newly synthesized phosphatidylcholine and triglyceride in milk.

The lipogenic system of fresh goat milk was used to investigate the relationship of lipid synthesis to formation of fat droplets. The skim milk phase was incubated with [1-carbon-14] palmitate and incorporation of activity into the glycerolipids was assayed. In two representative experiments, 44 and 56% of the label was incorporated into the lipids. Most of this activity (78 to 84%) was in triglycerides. Of 12 and 10% activity in the phospholipids, about 60% was contained in phosphatidylcholine with substantially smaller amounts in the other phospholipids. The synthesis of triglyceride and phosphatidylcholine was further investigated in relation to distribution of labeled lipids in a cream layer and a sedimenting fluff (membrane) fraction derived by centrifuging the skim milks following incubation. The correlation coefficient for the amounts of activity in phosphatidylcholine and triglyceride in the cream layer was .97. The data are consistent with synthesis of phosphatidylcholine and triglyceride to meet the requirements of surface and volume expansion, respectively, in formation of fat droplets. Our findings suggest the interesting working hypothesis that phosphatidylcholine synthesis regulates development of fat droplets.