EXCHANGE OF PHOSPHOLIPIDS BETWEEN CULTURED CHICK EMBRYO FIBROBLASTS AND THEIR GROWTH MEDIUM

The exchange of phospholipids between chick embryo fibroblasts and their growth medium was studied. It was found that the phospholipids of secondary chick fibroblasts, labeled with 32PO4 or 14C-choline, were continuously released into the growth medium. The release was dependent on the serum concentration of the medium; the greater the serum concentration, the greater was the rate of release. At 4 °C, there was very little release of phospholipid even in the presence of serum. Measurement of the appearance of DNA in the medium indicated that there was little death or disintegration of cells and, therefore, that phospholipids were released from living cells. The released phospholipids appeared to be associated with proteins, and these lipid-protein complexes could be fractionated by gel filtration into two fractions, having estimated molecular weights of 250,000 Daltons and greater than 5,000,000 Daltons. It is suggested that some of the released phospholipids become associated with serum proteins through an exchange process, but also that there is a release and exchange of phospholipids which are associated with cellular proteins. When medium containing released, labeled phospholipids was added to unlabeled cells, the labeled phospholipids were taken up by the cells and an equilibrium between cells and medium was established. © 1969.