Summary
The role of factor VIII in the activation of human factor X by factor IXa, Ca2+ and phospholipid has been investigated. Factor VIII stimulated the factor Xa formation
after activation by factor Xa or thrombin; the activity of thrombin-activated factor
VIII was about 4-fold that of factor Xa-activated factor VIII. The isolated procoagulant
moiety of the factor VIII complex behaved identically to the complete complex, whereas
the von Willebrand factor moiety did not participate in the factor Xa formation. Thrombin-activated
factor VIII complex (factor Villa) was used to study the effect of factor Villa in
kinetic experiments. The results revealed a complex kinetic behaviour, including substrate
inhibition and non-linearity of the reaction rate with the enzyme concentration. Using
previously obtained insight into the kinetics of factor X activation in the absence
of factor VIII, the results were found to support the hypothesis that factor Villa
participates in the factor Xa formation in a complex with phospholipid-bound factor
IXa; the formation of the factor VUIa-factor IXa complex then increases the catalytic
efficiency of the factor IXa by 500-fold.
Keywords
Factor VIII - Factor IX - Factor X - Intrinsic pathway Summary