Lipoprotein(a) enhances plasma clot lysis in vitro

Human plasma lipoprotein(a) (Lp(a)) is a macromolecular complex that contains a protein homologous to plasminogen, the precursor of plasmin. We confirmed recent reports that Lp(a) is not activated by streptokinase or tissue plasminogen activator (t-PA) to yield plasmin-like activity. In testing the hypothesis that Lp(a) can competitively inhibit plasma clot lysis mediated by plasmin, the present study shows that Lp(a) significantly enhanced plasma clot lysis mediated by streptokinase or t-PA. The enhancement, however, was not observed in a plasmin-mediated clot lysis using a purified fibrinogen system. The addition of alpha-2-antiplasmin (alpha 2-plasmin inhibitor) to this system inhibited fibrinolysis; however, no inhibition was observed in the presence of Lp(a). One potential explanation for the Lp(a)-enhanced plasma clot lysis is that Lp(a) neutralizes the activity of alpha 2-antiplasmin present in plasma, thereby restoring the activity of plasmin to lyse the clot.