Analysis of plasminogen activation by the plasmin–staphylokinase complex in plasma of α2-antiplasmin-deficient mice

Staphylokinase (SAK) expresses plasminogen activator (PA) activity by forming a complex with plasmin; this PA activity is inhibited by α2-antiplasmin (α2-AP) in plasma. However, SAK's activity is protected against inhibition by α2-AP in the presence of fibrin because the plasmin–SAK complex binds to fibrin. In the present study, the interaction between SAK and murine plasminogen was investigated in the plasma of α2-AP-deficient (α2-AP) mice or plasminogen-deficient (Plg) mice. Although the human plasmin–SAK complex was formed in equimolar mixtures of plasmin and SAK, the murine plasmin–SAK complex was not formed. Human plasminogen was activated by the human plasmin–SAK complex, although equimolar mixtures of murine plasmin and SAK did not activate murine plasminogen. These findings suggest that SAK does not react with murine plasmin. However, the murine plasminogen was activated by the human plasmin–SAK complex, although this activation was approximately 100-fold weaker than human plasminogen. Human and wild-type mouse plasminogens were not activated by the human plasmin–SAK complex in their plasma. In α2-AP mouse plasma, murine plasminogen was activated by the human plasmin–SAK complex. Human or murine plasminogen, which had been added to Plg mouse plasma, was not activated by the human plasmin–SAK complex. However, plasma clot lysis by the human plasmin–SAK complex was observed in both human and murine plasma. These findings indicate that(1) murine plasmin does not react with SAK, (2) human plasmin–SAK complex activates murine plasminogen, (3) this activation is inhibited by murine α2-AP, but (4) this activation is not inhibited by murine α2-AP in the presence of fibrin.