The blockage of the high-affinity lysine binding sites of plasminogen by EACA significantly inhibits prourokinase-induced plasminogen activation

Prourokinase-induced plasminogen activation is complex and involves three distinct reactions: (1) plasminogen activation by the intrinsic activity of prourokinase; (2) prourokinase activation by plasmin; (3) plasminogen activation by urokinase. To further understand some of the mechanisms involved, the effects of epsilon-aminocaproic acid (EACA), a lysine analogue, on these reactions were studied. At a low range of concentrations (10-50 microM), EACA significantly inhibited prourokinase-induced (Glu-/Lys-) plasminogen activation, prourokinase activation by Lys-plasmin, and (Glu-/Lys-) plasminogen activation by urokinase. However, no inhibition of plasminogen activation by Ala158-prourokinase (a plasmin-resistant mutant) occurred. Therefore, the overall inhibition of EACA on prourokinase-induced plasminogen activation was mainly due to inhibition of reactions 2 and 3, by blocking the high-affinity lysine binding interaction between plasmin and prourokinase, as well as between plasminogen and urokinase. These findings were consistent with kinetic studies which suggested that binding of kringle 1-4 of plasmin to the N-terminal region of prourokinase significantly promotes prourokinase activation, and that binding of kringle 1-4 of plasminogen to the C-terminal lysine158 of urokinase significantly promotes plasminogen activation. In conclusion, EACA was found to inhibit, rather than promote, prourokinase-induced plasminogen activation due to its blocking of the high-affinity lysine binding sites on plasmin(ogen).