Anticoagulation by a thrombin precursor

A prothrombin mutant that is activated to yield stable meizothrombin produces dominant inhibition of clot formation in a mouse model of carotid artery injury.

Studies of clot formation in vivo that draw from established details of coagulation biochemistry have the potential for yielding unexpected insights into the regulation of coagulation or suggesting novel strategies for its inhibition. In this vein, Shim and colleagues (page 415) report studies directed at evaluating the potential contribution of meizothrombin to thrombus formation in a mouse carotid artery injury model.

Thrombin catalyzes a spectrum of cleavage reactions relevant to both the procoagulant and anticoagulant pathways of blood coagulation. These features establish thrombin as a key effector and regulator of clot formation. Two cleavages are responsible for the proteolytic conversion of prothrombin to thrombin. As a result of bond cleavage preference, meizothrombin is produced as the principal intermediate when prothrombin activation is catalyzed by the prothrombinase complex. This intermediate is a serine proteinase that is covalently tethered to the fragment 1.2 domain. Meizothrombin therefore represents a transiently produced variant of thrombin that retains membrane binding properties with important functional sites in the proteinase domain modulated by the fragment 1.2 domain. Human or bovine meizothrombin derivatives exhibit greatly reduced activity toward fibrinogen and platelets in comparison to thrombin but retain the ability to activate factor V or to bind thrombomodulin and catalyze protein C activation at an enhanced rate. The in vitro work suggests a significant shift in the activity spectrum of meizothrombin in favor of the anticoagulant reactions.

In their study, Shim and colleagues have prepared a recombinant mouse prothrombin derivative (aaII) with cleavage sites mutated to yield only meizothrombin following activation by prothrombinase. The resulting stable meizothrombin is shown to possess properties comparable to those described for the human and bovine derivatives. They also report the results obtained in a FeCl₃-induced carotid artery injury model using prothrombin^+/- mice infused with either wild-type II or aaII to complement prothrombin levels in these animals. The interesting finding is that infusion of aaII leads to a delay instead of shortening in carotid artery occlusion time in an apparently dose-dependent manner. Dominant inhibition of clot formation by aaII, which was infused into these animals, implicates a contribution from the anticoagulant properties of meizothrombin produced in this biologically relevant setting. Competitive alternate substrate effects between endogenous prothrombin and aaII for cleavage by prothrombinase could also contribute to this effect. Unanswered is whether meizothrombin transiently produced during normal prothrombin activation in vivo significantly participates as a negative effector of clot formation. However, the work suggests that infusion of such zymogen derivatives may yield a novel and potentially viable strategy for triggering the anticoagulant pathway through the selective activation of protein C in the vicinity of sites of prothrombin activation and ongoing thrombus formation.FIG1