Effects of fibrin and alpha2-antiplasmin on plasminogen activation by staphylokinase.

Staphylokinase obtains plasminogen activating activity by forming a complex with plasminogen. Although the enzymatic activity of staphylokinase is enhanced by fibrin, how fibrin enhances enzymatic activity has not been determined yet. The effects of fibrin, or fibrinogen fragments, on the activation of plasminogen by staphylokinase was investigated using CNBr-digested fibrinogen fragments (FCB-2 and FCB-5) and plasmin-degraded cross-linked fibrin fragments ((DD)E complex, DD fragments and E fragments). Kinetic analysis of the activity of staphylokinase revealed that its plasminogen activating activity, which was expressed as kcat/Km, was enhanced by FCB-2 (10-fold) and FCB-5 (5-fold). These fibrin fragments caused 38-, 30-, and 8.5-fold increases in activity for the DD fragment, (DD)E complex and E fragment, respectively. Although alpha2-antiplasmin inhibited the activation of plasminogen by staphylokinase, FCB-2 abolished its inhibitory effects, and the plasminogen activating activity of staphylokinase was restored. The inhibitory effects of alpha2-antiplasmin on the activation of mini-plasminogen by staphylokinase were less than for Glu- or Lys-plasminogen, and the inhibitory effect of alpha2-antiplasmin was not altered by fibrin or EACA. These findings indicate that the staphylokinase/plasmin(ogen) complex reacts with fibrin even in the presence of alpha2-antiplasmin, and efficient plasminogen activation takes place on the surface of fibrin.