Assessment of the Relative Contribution of Different Protease Inhibitors to the Inhibition of Plasmin In Vivo

Summary

It has been shown that the most important inhibitor of plasmin is α₂-antiplasmin, however, other protease inhibitors are able to inhibit this proteolytic enzyme as well. The contribution of the various protease inhibitors to the inhibition of plasmin in vivo has never been quantitatively assessed.

To assess the relative contribution of the different protease inhibitors on the inhibition of plasmin we developed a series of sensitive immunoassays for the detection of complexes between plasmin and the protease inhibitors α₂-antiplasmin, α₂-macroglobulin, antithrombin III, α₁antitrypsin and C1-inhibitor, utilizing monoclonal antibodies that are specifically directed against complexed protease inhibitors and a monoclonal antibody against plasmin.

It was confirmed that α₂-antiplasmin is the most important inhibitor of plasmin in vivo, however, complexes of plasmin with α₂-macroglobulin, antithrombin III, α₁antitrypsin- and C1-inhibitor were also detected. Particularly during activation of fibrinolysis complexes between plasmin and inhibitors other than α₂-antiplasmin were detected. It was observed that during different situations the inhibition profile of plasmin was not constant e.g. in patients with diffuse intravascular coagulation plasma levels of plasmin-α₁-antitrypsin and plasmin-C1-inhibitor were increased whereas in plasma from patients who were treated with thrombolytic agents complexes of plasmin with α₂-macroglobulin and with antithrombin III were significantly elevated.

In conclusion, we confirmed the important role of α₂-antiplasmin in the inhibition of plasmin, however, in situations in which fibrinolysis is activated other protease inhibitors also account for the inhibition of plasmin in vivo. Further investigations to assess the role of the various protease inhibitors in the fibrinolytic system can be assisted by the assays described in this study.

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