Thrombin activity of fibrin thrombi and soluble plasmic derivatives

Fibrin was prepared from purified fibrinogen, plasma, and pathologic arterial thrombi and assayed for thrombin activity. Activity was detected on fibrin from each of these sources when assayed by three techniques: the rate of release of FPA from fibrinogen, a clotting time assay, and the rate of hydrolysis of the chromogenic substrate S-2238. Of the labeled thrombin initially associated with fibrin during clot formation in vitro, all but 10% to 15% could be removed easily by manual compression or by incubation in buffer. Radiolabeled thrombin that remained bound to clots of purified fibrinogen retained full functional activity, whereas that bound to plasma clots expressed only 4% of expected activity. Plasmic lysis of fibrin from clots of purified fibrinogen released bound thrombin quantitatively into solution in active form. The solubilized thrombin retained association with specific macromolecular fibrin derivatives as demonstrated by sedimentation analysis, electrophoretic co-migration, and partitioning in agarose gel. Plasmic lysates of fibrin prepared in vitro from plasma or pathologic arterial clots also expressed thrombin activity, in an amount similar to the fibrin from which they were prepared. Our studies demonstrate the presence of functionally active thrombin on fibrin prepared from in vitro clots and in vivo thrombi as well as in association with soluble plasmic derivatives of these substrates. This activity may constitute a prothrombotic influence and may contribute to the elevated FPA-levels seen in patients with thrombotic disease.