Factor X bound to the surface of activated human platelets is preferentially activated by platelet-bound factor IXa

Factor X is a zymogen in the blood coagulation system which is activated by the serine protease, factor IXa, in a reaction that is promoted by the presence of stimulated platelets. We have shown previously that platelets possess a binding site for factor IXa, the occupancy of which is correlated with the rate of factor X activation (Ahmad et al., 1989b,c). Similarly, we have described a different binding site on the surface of activated platelets to which the substrate for this reaction, factor X, can bind (see the accompanying paper). This "zymogen binding site" is of moderate affinity and is relatively nonspecific; apparently shared to some degree by factor X and other vitamin K-dependent proteins, most notably prothrombin. We have found that prothrombin fragment 1 not only is able to displace factor X from this platelet binding site but also possesses the ability to inhibit the platelet-dependent activation of factor X. We have developed two mathematical models for the activation of factor X by platelet-bound factor IXa. The first model assumes that factor X is activated in a manner that is totally unrelated to the presumptive zymogen binding site, whereas the second model requires factor X to first bind to this site before it may interact with platelet-bound factor IXa and become activated. Within the context of each of these models, we have evaluated three mechanisms by which prothrombin fragment 1 may inhibit factor X activation. The data presented herein are most consistent with the precept that platelet-bound factor X is activated by platelet-bound factor IXa (kcat approximately 0.0011 s-1) in an explicitly two-dimensional reaction (Km.2D approximately 230 molecules per platelet). Prothrombin fragment 1 is believed to disrupt this reaction by competing with factor X for the zymogen binding site (Ki approximately 470 nM) and, to a lesser degree, by displacing factor IXa from its binding site (Ki approximately 7 microM). These findings suggest that platelet-bound zymogen factor X represents a kinetically important pool of substrate that is preferentially activated on the surface of activated platelets.