Streptokinase may be less effective at saving lives in patients with heart attacks because it explosively generates plasmin in the bloodstream at sites distant from fibrin clots. We hypothesized that this rapid plasmin generation is due to SK's singular capacity to nonproteolytically generate the active protease SK·Pg*, and we examined whether the kringle domains regulate this process. An SK mutant lacking Ile-1 (ΔIle1-SK) does not form SK·Pg*, although it will form complexes with plasmin that can activate plasminogen. When compared to SK, ΔIle1-SK diminished the generation of plasmin in plasma by more than 30-fold, demonstrating that the formation of SK·Pg* plays an important role in SK activity in the blood. The rate of SK·Pg* formation (measured by an active site titrant) was much slower in Glu-Pg, which contains five kringle domains, than in Pg forms containing one kringle (mini-Pg) or no kringles (micro-Pg). In a similar manner, Streptococcus uberis Pg activator (SUPA), an SK-like molecule, generated SUPA·Pg* much slower with bovine Pg than bovine micro-Pg. The velocity of SK·Pg* formation was regulated by agents that influence the conformation of Pg through interactions with the kringle domains. Chloride ions, which maintain the compact Pg conformation, hindered SK·Pg* formation. In contrast, ε-aminocaproic acid, fibrin, and fibrinogen, which induce an extended Pg conformation, accelerated the formation of SK·Pg*. In summary, the explosive generation of plasmin in blood or plasma, which diminishes SK's therapeutic effects, is attributable to the formation of SK·Pg*, and this process is governed by kringle domains.
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