Inhibition of plasminogen activation by monoclonal antibodies to the kringle 5-B chain segment of human plasminogen.
Three murine monoclonal antibodies (designated alpha Pg-28, alpha Pg-96, and alpha Pg-247) against human plasminogen were prepared. All three antibodies bound plasminogen and the elastase-digestion product of plasminogen consisting of residues Val442-Asn790 (miniplasminogen). The epitopes recognized by each antibody were distinct. Antibodies alpha Pg-96 and alpha Pg-247 blocked tissue plasminogen activator-dependent lysis in a fibrin plate assay while antibody alpha Pg-28 had no effect. Antibody alpha Pg-96 blocked urokinase- and tissue plasminogen activator-catalyzed, and streptokinase-mediated, plasminogen activation. Antibodies alpha Pg-28 and alpha Pg-247 partially inhibited tissue plasminogen activator-catalyzed plasminogen activation. Antibodies alpha Pg-28 and alpha Pg-247 also inhibited streptokinase-mediated plasminogen activation, but not urokinase-catalyzed activation. Antibody alpha Pg-247 inhibited plasmin catalysis of substrate S-2251 by decreasing the VMAX and increasing the KM of plasmin for the synthetic substrate S-2251 four-fold. The other antibodies had no significant effect on plasmin activity. This differential inhibition of plasminogen activation suggests that activation by urokinase, tissue plasminogen activator, and streptokinase possibly involve distinct regions of miniplasminogen structure.