Incorporation of 32PO4 into phospholipids of blood platelets

Human or rabbit platelets in an artificial medium without phosphate were incubated with carrier-free [32P] orthophosphate for I h, washed and resuspended in Tyrode-albumin solution containing unlabelled phosphate. Aliquots of platelet suspension were subjected to lipid extraction at various times and the extracted phospholipids were separated by thin layer chromatography. The specific radioactivities of triphosphoinositide (TPI) and diphosphoinositide (DPI) were highest during the labelling period and then declined rapidly after the platelets were resuspended in the medium containing unlabelled phosphate. The percentage of total phospholipid 32P in TPI and DPI decreased from over 95% at 1 h to less than 50% at 12 h with either rabbit or human platelets. The specific radioactivity of monophosphoinositide (MPI) reached its greatest value 6-8 h after labelling. In the in vitro incubation studies, phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidylethanolamine (PE) were negligibly labelled in the initial hours but at 12 h (rabbit or human platelets) PC contained more than 30% of the radioactivity. PE and PS contained less than 10% of the total phospholipid-bound radioactivity at the end of the incubation period. The pattern of incorporation of [32P]orthophosphate into the inositol phospholipids, PC, PS and PE that had been found in the in vitro studies was confirmed by an in vivo study in which rabbit platelets labelled with 32P in vitro were infused into rabbits and harvested after 35 h. These findings indicate that the phosphates of all the phospholipids studied in these experiments turn over in vitro and in vivo but the phosphates of the inositol phospholipids turn over most rapidly.