Location of phosphatidylethanolamine and phosphatidylserine in the human platelet plasma membrane

The location of phospholipids in the human platelet plasma membrane was probed with 2, 4, 6-trinitrobenzenesulfonate (TNBS). TNBS does not penetrate inintact cells and can label phosphatidylethanolamine (PE) and phosphatidylserine (PS). In tact platelets, PE is not accessible to TNBS during the initial 15 min. However, 6.9% PE reacts with TNBS after 30 min and 17.9% PE is labeled after 90 min. In intact platelets, PS is not labeled even after 2 h. In contrast, in phospholipids extracted from platelets 71% PE and 26.5% PS react with TNBS within 5 min. This indicates that PS is inaccessible and PE is relatively inaccessible to TNBS in intact platelets. After incubation of platelets with thrombin, there is increased labeling of PE but no labeling of PS. The incubation of platelets with thrombin (0.05 U/ml) for 5 min results in 16.2% increase of PE labeling during subsequent 30-min incubation with TNBS. PS does not appear to be a component of the functional surface of platelets. However, exposure of PE may have a critical role in platelet hemostatic function. The implication of the study is that there is asymmetry of phopholipids in the platelet plasma membrane which has considerable physiological significance.