The mechanism of a bacterial plasminogen activator intermediate between streptokinase and staphylokinase

The therapeutic properties of plasminogen activators are dictated by their mechanism of action. Unlike staphylokinase, a single domain protein, streptokinase, a 3-domain (alpha, beta, and gamma) molecule, nonproteolytically activates human (h)-plasminogen and protects plasmin from inactivation by alpha (2)-antiplasmin. Because a streptokinase-like mechanism was hypothesized to require the streptokinase gamma -domain, we examined the mechanism of action of a novel two-domain (alpha,beta) Streptococcus uberis plasminogen activator (SUPA), Under conditions that quench trace plasmin, SUPA nonproteolytically generated an active site in bovine (b)-plasminogen, SUPA also competitively inhibited the inactivation of plasmin by alpha (2)-antiplasmin, Still, the lag phase in active site generation and plasminogen activation by SUPA was at least 8-fold longer than that of streptokinase. Recombinant streptokinase gamma -domain bound to the b-plasminogen SUPA complex and significantly reduced these lag phases. The SUPA-b.plasmin complex activated b-plasminogen with kinetic parameters comparable to those of streptokinase for h-plasminogen. The SUPA-b.plasmin complex also activated h-plasminogen but with a lower k(cat) (25-fold) and k(cat)/K-m (7.9-fold) than SK. We conclude that a gamma -domain is not required for a streptokinase-like activation of b-plasminogen, However, the streptokinase gamma -domain enhances the rates of active site formation in b-plasminogen and this enhancing effect may be required for efficient activation of plasminogen from other species.