The interaction between factor VIII clotting antigen and phospholipids in genetic variants of hemophilia and von Willebrand's disease.

The interaction of factor VIII with phospholipids was investigated in 11 patients with mild and moderate hemophilia A, 7 patients with von Willebrand's disease and in 10 healthy people as controls. The addition of phospholipid vesicles containing phosphatidylserine and phosphatidylethanolamine to normal plasma and that of patients with von Willebrand's disease resulted in the loss of almost two thirds of the factor VIII clotting antigen (VIII:CAg) measurable by IRMA. Defective interaction of phospholipids with VIII:CAg was noted in some genetic variants of mild and moderate hemophilia A. Thus four of the five families tested showed decreased binding of VIII:CAg to phospholipids. One of the families tested belonged to a genetic variant with much more VIII:CAg than VIII:C, and it was in members of this family that the binding capacity was most reduced. The most probable explanations for the defective interaction with phospholipids is that molecular defects of VIII:CAg result in either decreased binding to phospholipids or might lead to a stronger binding between VIII:CAg and the von Willebrand factor (VIIIR:Ag) in the factor VIII complex and thereby preventing the normal separation of the complex.