Lysine side chains induce conformational changes in plasminogen (Pg) that regulate the process of fibrinolysis or blood clot dissolution. A lysine side-chain mimic, epsilon amino caproic acid (EACA), enhances the activation of Pg by urinary-type and tissue-type Pg activators but inhibits Pg activation induced by streptokinase (SK). Our studies of the mechanism of this inhibition revealed that EACA (IC50 10 μM) also potently blocked amidolytic activity by SK and Pg at doses nearly 10000-fold lower than that required to inhibit the amidolytic activity of plasmin. Different Pg fragments were used to assess the role of the kringles in mediating the inhibitory effects of EACA: mini-Pg which lacks kringles 1-4 of Glu-Pg and micro-Pg which lacks all kringles and contains only the catalytic domain. SK bound with similar affinities to Glu-Pg (K(A) = 2.3 x 109 M-1) and to mini-Pg (K(A) = 3.8 x 109 M-1) but with significantly lower affinity to micro-Pg (K(A) = 6 x 107 M-1). EACA potently inhibited the binding of Glu- Pg to SK (K(i) = 5.7 μM), but was less potent (K(i) = 81.1 μM) for inhibiting the binding of mini-Pg to SK and had no significant inhibitory effects on the binding of micro-Pg and SK. In assays simulating substrate binding, EACA also potently inhibited the binding of Glu-Pg to the SK-Glu-Pg activator complex, but had negligible effects on micro-Pg binding. Taken together, these studies indicate that EACA inhibits Pg activation by blocking activator complex formation and substrate binding, through a kringle- dependent mechanism. Thus, in addition to interactions between SK and the protease domain, interactions between SK and the kringle domain(s) play a key role in Pg activation.
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