THE ACTIVATION OF RAT PLATELETS INCREASES THE EXPOSURE OF POLYUNSATURATED FATTY-ACID ENRICHED PHOSPHOLIPIDS ON THE EXTERNAL LEAFLET OF THE PLASMA-MEMBRANE

Rat platelets have been hydrogenated in the presence of colloidal palladium adsorbed on the surface of the non water-soluble polymer polyvinylpolypyrrolidone. This non-permeating catalyst restricts hydrogenation of the fatty acyl double bonds of phospholipids only in the outer half of the plasma membrane. The pattern of hydrogenation of the molecular species present on the external side of the membrane is determined using desorption-chemical soft ionization-mass spectrometry (DCI-MS) before and after cell activation by the calcium ionophore A23187. The accessibility to the catalyst of the polyunsatured molecular species within each phospholipid class is compared for resting and activated cells. The abundance of polyunsaturated species of phosphatidyl-ethanolamine and -serine in the inner half of the resting biomembrane is confirmed in rat platelets. Phosphatidylcholine is especially rich in disaturated species in this membrane. The induced exposure of the polyunsaturated species of diacyl- and ether-phosphatidylethanolamine, and of phosphatidylserine on the external side of the membrane appears after activation by the calcium ionophore. A detailed quantitative analysis within a phospholipid class shows an unequal scrambling for diacyl-, alkyl-, alkenyl-phosphatidylethanolamine, and a variable involvement in the transmembrane redistribution following cell activation of the various molecular species as a function of the acyl moities.