It has previously been demonstrated that thrombin binds to fibrin during clot formation. We have studied the nature of this association and the fate of bound thrombin during fibrin degradation by plasmin. Approximately 10% of human thrombin used to clot human fibrinogen bound to the fibrin and could not be removed by washing in buffers of physiologic pH and ionic strength. Plasmic digestion released thrombin into the lysate where it retained enzymatic activity, as measured by chromogenic and clotting assays. The fibrin was degraded to a group of unique degradation complexes of molecular weight between 230,000 and greater than 800,000. The active thrombin was present in the lysate both as free enzyme and bound to certain of these complexes, primarily those larger than 230,000 daltons. At low initial thrombin concentrations, most of the active enzyme released during plasmic digestion was bound to fibrin derivatives, while at higher initial thrombin concentrations, most was released as free enzyme. This suggests that there are at least two types and/or sites of association between thrombin and fibrin. The presence of thrombin on fibrin clots and bound to soluble fibrin derivatives may have pathophysiologic importance in the propagation of thrombi and in the development of hypercoagulable states.
The effect of ionic strength on enzymatic activity of thrombin