ORGANIZATION AND ROLE OF PLATELET MEMBRANE PHOSPHOLIPIDS AS STUDIED WITH PURIFIED PHOSPHOLIPASES

The action of various purified phospholipases on intact cells is reviewed, with special emphasis on platelets. Their effects are strongly dependent on the substrate specificity of the enzymes. Moreover, phospholipases can be divided into two groups according to their ability to hydrolyse phospholipids in an intact cell. Comparative studies of their behaviour towards phospholipid monolayers maintained at different surface pressures have allowed to estimate the surface pressure of erythrocyte and platelet membrane as 31-34 dynes/cm. Non-lytic degradation of platelet phospholipids has been used to identify the phospholipids directly accessible on the platelet surface. An asymmetric distribution of phospholipids has been described for the membrane whose outer leaflet contains essentially choline-phospholipids, whereas the anionic procoagulant phospholipids (phosphatidylserine and phosphatidylinositol) are confined to the inner leaflet. Furthermore, as a consequence of phospholipid asymmetry, arachidonic acid is concentrated towards the inside of the platelets, since only 6% of the total cell content is present in the exterior half of the platelet membrane. The platelet aggregation induced by exogenous phospholipase C is accompanied by phospholipid modifications such as phosphatidic and lysophosphatidic acid production. Since these changes occur also with physiological aggregating agents, some evidence is given for the involvement of endogenous platelet phospholipase C and diglyceride lipase, whose exact roles remain to be determined. © 1979 Birkhäuser Verlag.