Effect of phospholipid on factor VIII coagulant activity and coagulant antigen

Incubation of purified factor VIII/von Willebrand factor complex or plasma with certain phospholipid preparation induced a two to threefold increase in apparent factor VIII activity, while a slight decrease of factor VIII coagulant antigen was observed. Between six or eightfold activity increases occurred when 10-3 units/ml of thrombin was present during the incubation. When the phospholipid was added first and the thrombin added 30 minutes later a twofold increase in apparent factor VIII activity occurred during the first part and this activity was then increased threefold further after the addition of thrombin. In separate experiments using gel filtration on Sepharose 2B, association of factor VIII activity to phospholipid vesicles could be demonstrated. This phospholipid bound factor VIII could be activated by thrombin. The relationship between these observations and the formation of factor X activating complex is discussed.