Inhibition of Plasminogen Activation by Monoclonal Antibodies to the Kringle 5-B Chain Segment of Human Plasminogen

Abstract

Three murine monoclonal antibodies (designated αPg-28, αPg-96, and αPg-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 αPg-96 and αPg-247 blocked tissue plasminogen activator-dependent lysis in a fibrin plate assay while antibody αPg-28 had no effect. Antibody αPg-96 blocked urokinase- and tissue plasminogen activator-catalyzed, and streptokinase-mediated, plasminogen activation. Antibodies αPg-28 and αPg-247 partially inhibited tissue plasminogen activator-catalyzed plasminogen activation. Antibodies αPg-28 and αPg-247 also inhibited streptokinase-mediated plasminogen activation, but not urokinasecatalyzed activation. Antibody αPg-247 inhibited plasmin catalysis of substrate S-2251 by decreasing the V_MAX and increasing the K_M 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.