Loss of prothrombin and of factor Xa-factor Va interactions upon inactivation of factor Va by activated protein C.

Activated factor V (factor Va) is composed of two nonidentical subunits which can be dissociated on chelation of the bound Ca2+ with EDTA. The isolated subunits can be recombined in the presence of Ca2+ to form factor Va. The factor Va heavy chain (Mr = 94,000) binds to prothrombin in a specific and Ca2+-independent fashion. Following inactivation of either factor Va or the factor Va heavy chain by limited proteolysis with activated protein C, factor Va no longer binds to the immobilized prothrombin. Factor Va also binds specifically to (p-amidinophenyl)-methanesulfonyl-factor Xa-Affi-Gel 15. However, neither isolated subunit binds to this column. Factor Va inactivated by activated protein C is no longer retained by the factor Xa column. This data suggests that both subunits are required for optimal factor Va-factor Xa interaction and that inactivation of factor Va with activated protein C reduces the affinity of factor Va for both prothrombin and factor Xa.