Prothrombin activation on the activated platelet surface optimizes expression of procoagulant activity.

Effective hemostasis relies on the timely formation of α-thrombin via prothrombinase, a Ca(2+)-dependent complex of factors Va and Xa assembled on the activated platelet surface, which cleaves prothrombin at Arg271 and Arg320. Whereas initial cleavage at Arg271 generates the inactive intermediate prethrombin-2, initial cleavage at Arg320 generates the enzymatically active intermediate meizothrombin. To determine which of these intermediates is formed when prothrombin is processed on the activated platelet surface, the cleavage of prothrombin, and prothrombin mutants lacking either one of the cleavage sites, was monitored on the surface of either thrombin- or collagen-activated platelets. Regardless of the agonist used, prothrombin was initially cleaved at Arg271 generating prethrombin-2, with α-thrombin formation quickly after via cleavage at Arg320. The pathway used was independent of the source of factor Va (plasma- or platelet-derived) and was unaffected by soluble components of the platelet releasate. When both cleavage sites are presented within the same substrate molecule, Arg271 effectively competes against Arg320 (with an apparent IC(50) = 0.3μM), such that more than 90% to 95% of the initial cleavage occurs at Arg271. We hypothesize that use of the prethrombin-2 pathway serves to optimize the procoagulant activity expressed by activated platelets, by limiting the anticoagulant functions of the alternate intermediate, meizothrombin.