PLASMINOGEN ACTIVATION BY STREPTOKINASE VIA A UNIQUE MECHANISM

The mechanism of human plasminogen (HPlg) activation by streptokinase (SK)-type activator was investigated with recombinant truncated SR pep tides. An enzyme substrate intermediate of HPlg SK HPlg ternary comple x was demonstrated by a sandwich-binding experiment. Formation of the ternary complex was saturable, HPlg-specific, and inhibited by 6-amino caproic acid. Three interaction sites between SK and HPlg were demonst rated. SK-(220-414) bound to HPlg with two binding sites: one to the m icro-HPlg region, the catalytic domain of HPlg, and one to the kringle 1-5 region, with K-d values of 1.50 x 10(-7) and 2.44 x 10(-6) M, res pectively. SK-(16-251) bound to a single site on the kringle 1-5 regio n of HPlg with a K-d of 4.09 x 10(-7) M. SK-(220-414) and SK-(16-251) competed for binding on the same or nearby location on the human kring le 1-5 domain. Combination of SK-(220-414) and SK-(16-251), but not ei ther peptide alone, could effectively activate HPlg. In addition, SK-( 16-251) dose-dependently enhanced the activation of HPlg by SK-(16-414 ), while the HPlg activation by SK-(16-414) was inhibited by SK-(220-4 14). We conclude that the HPlg that binds to the COOH-terminal domains of SK functions as an enzyme to catalyze the conversion of substrate HPlg that binds to the NH2-terminal domain of SK to human plasmin.