Regulation by alpha 2-antiplasmin and fibrin of the activation of plasminogen with recombinant staphylokinase in plasma.

The effects of alpha 2-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 mumol/L intact or low-M(r) plasminogen and 3 mumol/L alpha 2- antiplasmin, at 37 degrees C and in the absence of fibrin, STAR did not induce plasminogen activation and plasmin-alpha 2-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-M(r) 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-M(r) 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 ( 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, alpha 2- 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.