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‐Mr 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).

The Mechanism of a Bacterial Plasminogen Activator Intermediate between Streptokinase and Staphylokinase

“…The rate constant k 1 of the reaction between b-plasmin and ␣ 2 -antiplasmin was measured as described (6). Briefly b-plasmin (14 nM) was added to cuvettes containing S2251 (500 nM), SUPA (0 -500 nM), and the change in absorbance at 405 nm recorded for 90 s prior to the addition of human ␣ 2 -antiplasmin (Calbiochem; 90 nM).…”

“…The residual enzyme activity was measured by the first derivative (dA/dt) of the curve before 60% of the enzyme was inactivated. The apparent rate constant (k 1, app ) of the inhibition of the enzyme activity by ␣ 2 -antiplasmin in the presence of various concentrations of SUPA was calculated from the classical second-order rate equation as described (6).…”

“…SK binds with significantly greater affinity to plasmin than SAK (5). The SAK⅐plasmin complex, but not the SK⅐plasmin complex is inactivated by ␣ 2 -antiplasmin, the serine protease inhibitor of plasmin (2,6). Among all of the Pg activators, SK has the unique ability to bind and nonproteolytically generate an active site in the Pg zymogen (i.e.…”

Catabolic Pathways for Streptokinase, Plasmin, and Streptokinase Activator Complex in Mice

“…In human patients, however, the major fraction of an injected streptokinase dose would be expected to immediately form activator complex (3,11). How the activator complex is eliminated from the circulation is unclear especially in consideration of its large size (>120,000) and numerous studies that detail its lack of reactivity with the major plasma protease inhibitors (3,12,13).…”

“…This complex functions as a plasminogen activator. The association reaction is extremely rapid and the two proteins do not readily dissociate under neutral, nondenaturing conditions (3,4).…”