Kinetics of the Reactions between Streptokinase, Plasmin and alpha2-Antiplasmin

Abstract

Streptokinase reacts very rapidly with human plasmin (rate constant 5.4 × 10⁷ M⁻¹ s⁻¹) forming a 1:1 stoichiometric complex which has a dissociation constant of 5 × 10⁻¹¹ M. This plasmin‐streptokinase complex is 10⁵ times less reactive towards α₂‐antiplasmin than plasmin, the inhibition rate constant being 1.4 × 10² M⁻¹ s⁻¹. The loss of reactivity of the streptokinase‐plasmin complex towards α₂‐antiplasmin is independent of the lysine binding sites in plasmin since low‐M_r plasmin, which lacks these sites, and plasmin in which the sites have been blocked by 6‐amino‐hexanoic acid, are both equally unreactive towards α₂‐antiplasmin on reaction with streptokinase. The plasmin‐streptokinase complex binds to Sepharose‐lysine and Sepharose‐fibrin monomer in the same fashion as free plasmin, showing that the lysine binding sites are fully exposed in the complex. Bovine plasmin is rapidly inhibited by human α₂‐antiplasmin (k₁= 1.6 × 10⁶ M⁻¹ s⁻¹) and similarly loses reactivity towards the inhibitor on complex formation with streptokinase (50% binding at 0.4 μM streptokinase).