Regulation by α2-antiplasmin and fibrin of the activation of plasminogen with recombinant staphylokinase in plasma

Abstract

The effects of α2-antiplasmin and fibrin on the activation of plasminogen by recombinant staphylokinase (STAR) were studied in an effort to elucidate further the molecular basis of the fibrin-specificity of this fibrinolytic agent. In purified systems consisting of 1.5 μmol/L intact or low-Mr plasminogen and 3 μmol/L α2-antiplasmin 37°C and in the absence of fibrin, STAR did not induce plasminogen activation and plasmin-α2-antiplasmin complex (PAP) formation. Addition of a purified fibrin clot (30% vol at a concentration of 3 mg/mL) to mixtures containing intact plasminogen caused approximately 40% plasminogen activation within 2 hours, whereas in mixtures containing low-Mr plasminogen, no activation was observed. In contrast, 10 nmol/L streptokinase (SK) induced 74% to 100% plasminogen activation within 2 hours in mixtures containing either intact or low-Mr plasminogen, in both the absence and the presence of fibrin. In citrated human plasma in the absence of fibrin, 30 nmol/L STAR did not induce measurable plasminogen activation and PAP formation (<1.5% within 2 hours), whereas addition of a plasma clot (12% vol) resulted in complete clot lysis and conversion of 19% ± 8% of the plasminogen to PAP within 2 hours. Addition of a second plasma clot produced 23% ± 2% additional plasminogen activation. Equipotent concentrations for plasma clot lysis of SK (100 nmol/L) induced 54% ± 11 % plasminogen activation in the absence and 49% ± 16% in the presence of fibrin. Addition of 50 mmol/L 6-aminohexanoic acid (6-AHA) abolished the effect of fibrin on plasminogen activation with STAR, but not on activation with SK. In α2-antiplasmin-depleted human plasma in the absence of fibrin, 30 nmol/L STAR did not induce fibrinogen breakdown (>90% residual fibrinogen after 6 hours), whereas 30 nmol/L preformed plasmin-STAR complex induced extensive fibrinogen degradation (70% within 20 minutes). Thus, in the absence of fibrin, α2-antiplasmin inhibits the activation of plasminogen by STAR, by preventing generation of active plasmin-STAR complex. Fibrin stimulates plasminogen activation by STAR via mechanisms involving the lysine-binding sites of plasminogen, probably by facilitating the generation of plasmin-STAR complex and by delaying its inhibition at the clot surface. © 1993 by The American Society of Hematology.