On the Kinetics of the Reaction between Human Antiplasmin and Plasmin

The reaction between antiplasmin (A) and plasmin (P) proceeds in at least two steps: a very fast reversible second-order reaction followed by a slower irreversible first-order reaction, and may be represented by: . The two forms of plasmin with different affinities for lysine-Sepharose (plasmin I and plasmin II) react at different rates with antiplasmin. The k1 is (3.8±0.3)×107 M−1 s−1, for the reaction between plasmin I and antiplasmin, and (1.8±0.2) × 107 M−1 s−1 for plasmin II and antiplasmin. This reaction rate is one of the fastest so far described for protein-protein interactions. The k1 for trypsin and antiplasmin is (1.8±0.2) × 10−10 M for plasmin II with antiplasmin. The second step has a half-time of approximately 165 s for both types of plasmin, corresponding to a k2 of 4.2×10−3 s−1. Plasmin which has 6-aminohexanoic acid bound to its lysine binding site or the substrate d-valyl-l-lysyl-p-nitroanilide bound to its active site does not react, or reacts very slowly, with antiplasmin. From these findings we postulate that plasmin formed in the circulating blood is rapidly neutralized by antiplasmin, but plasmin bound to fibrin is not. This hypothesis may provide a molecular basis for the mechanism of thrombolysis in vivo.