The mechanism of human plasminogen (HPlg) activation by streptokinase (SK)-type activator was investigated with recombinant truncated SK peptides. An enzyme-substrate intermediate of HPlg·SK·HPlg ternary complex was demonstrated by a sandwich-binding experiment. Formation of the ternary complex was saturable, HPlg-specific, and inhibited by 6-aminocaproic acid. Three interaction sites between SK and HPlg were demonstrated. SK-(220-414) bound to HPlg with two binding sites: one to the micro-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, respectively. SK-(16-251) bound to a single site on the kringle 1-5 region 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 kringle 1-5 domain. Combination of SK-(220-414) and SK- (16-251), but not either 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- 414). 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.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology