To evaluate the mechanism responsible for the generation of factor VIla in vivo, we measured the levels of this enzyme after administering purified factor IX concentrates to patients with hemophilia B. Their factor VIla levels were initially very-low and gradually increased to normal, but there were no significant changes in the generation of factor Xa or thrombin. The administration of 10 μg/kg body weight of recombinant factor Vila to patients with factor VII deficiency increased the circulating levels 35-fold, but this only resulted in normalization of the activation of factor IX and factor X. Our data indicate that factor IXa is primarily responsible for the basal levels of free factor Vila in vivo, and that changes in free factor Vila in the blood do not necessarily lead to alterations in factor X activation.

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