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Plasmin (P) is very rapidly (rate constant 2 × 107 M-1s-1) inactivated by α-2-antiplasmin (A), and blocking of the lysine binding sites (LBS) of plasmin reduces this rate 50 fold (Eur. J. Blochen. 84, 573-578, 1978). Using purified reactants and the plasmin substrate D Val-Leu-Lys-pNA (S 2251) the influence of streptokinase (SK) on the reaction between P and A was examined. Addition of SK to P reduces the reaction rate with A in a sigmoidal fashion. Assuming the formation of a reversible bimolecular P:SK complex which is un-reactive towards A then the dissociation constant Kd of this complex is less than 10-9 M. The LBS of P does not play a role in this interaction since similar Kd values are found with modified P, lacking LBS and in the presence of 0.3 mM 6-aminohexanoic acid which blocks the LBS. The P:SK complex is only very slowly inhibited by A with a rate constant < 5 x 102 M-1s-1.It is concluded that the α-2-antiplasmin is less reactive towards the plasmin-strepto-kinase complex due to a modification of the active centre of plasmin rather than an effect upon the lysine binding sites.

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