The interaction of human plasmin with human α2-antiplasmin was measured in the presence and absence of lysine-binding ligands using the corresponding active site fluorescence changes. The stopped-flow method allows for direct determination of reliable values of the second order rate constant for the fast association step of plasmin and α 2-antiplasmin in the absence of another interacting compound, e.g. a plasmin substrate. At pH 7.4,25°C, k 1= 2.2 X 107 M −1 s −1 was obtained. Substantial reductions in k 1 were seen in the presence of trans-4-(aminomethyl)cyclohexane-1-carboxylic acid at concentrations corresponding to lysine-binding site interactions at kringle 4 of plasmin; at saturation the rate constant is reduced 20-fold, whereas the effect of saturation of kringle 1 is only a 2-fold reduction. It is thus found that the interaction of α 2-antiplasmin with the lysine-binding site of kringle 1 is of little importance compared with that of kringle 4 in regulating the inhibition reaction of plasmin with α 2-antiplasmin. Similar results were recently obtained for the bovine plasmin-bovine α 2-antiplasmin reaction (Christensen et al. (1995) Biochem. J. 305, 97–102).
