Summary
Factor VIII Leiden is a genetic variant of coagulation factor VIII which has been
detected in the plasma of a patient with mild haemophilia A. In this patient’s plasma
factor VIII procoagulant antigen was in 5-fold excess over factor VIII procoagulant
activity, indicating the presence of an abnormal factor VIII molecule. The variant
factor VIII was isolated from the patient’s plasma, and its functional properties
were studied in a factor X-activating system consisting of purified components. The
isolated factor VIII Leiden was normally activated by factor Xa and by thrombin, but
the activity of the factor Villa was about 3% of normal. The defect of factor Villa
Leiden was studied by comparison with normal factor Villa in kinetic experiments of
factor Xa formation. The results support the hypothesis that factor Villa Leiden has
a reduced affinity for phospholipid-bound factor IXa in the intrinsic factor X-activating
complex.
Keywords
Factor VIII - Genetic variant - Haemophilia A
+
- Factor X activation