TRANSFER OF PHOSPHATIDYLCHOLINE, PHOSPHATIDYLETHANOLAMINE AND SPHINGOMYELIN FROM LOW-DENSITY-LIPOPROTEIN AND HIGH-DENSITY-LIPOPROTEIN TO HUMAN PLATELETS

Following a 1 h incubation of human platelets with low-density lipopro tein (LDL) labelled in the apoprotein fraction (I-125-apoB) or in phos pholipid fractions [C-14-labelled phosphatidylcholine (PC), phosphatid ylethanolamine (PE) or sphingomyelin (SM)], the percentage of total C- 14 associated with the cells was about 3-fold higher than the percenta ge of I-125. Differences in temperature sensitivity also indicated dif ferential interactions of phospholipids and apoprotein with platelets. In order to assess the amount of [C-14]phospholipid transferred from LDL or high-density lipoprotein (HDL) to the cells, the quantity of bo und lipoproteins was estimated by adding an excess of unlabelled lipop rotein, or by selectively degrading LDL- and HDL-associated [C-14]PC a nd [C-14]PE with phospholipase C. Incubation of platelets with LDL or HDL containing pyrenedecanoic acid-labelled PC or SM (py-PC, py-SM) in creased pyrene monomer fluorescence, indicating incorporation of the p hospholipids into platelets. With HDL as donor, incorporation of py-SM was greater than uptake of py-PC. Pretreating platelets with elastase dose-dependently inhibited uptake of py-SM and py-PC. Treatment of ce lls with phospholipase C indicated that the uptake of[C-14]PC by plate lets, and not the binding of lipoproteins to the cells, was partially inhibited by elastase. In conclusion, LDL and HDL rapidly deliver SM, PC and PE to platelets. Incorporation of LDL-derived phospholipids int o platelets is unlikely to be mediated by endocytosis of lipoprotein p articles. The uptake of the two choline-containing phospholipids appea rs to require the presence of specialized platelet membrane protein(s) .