Plasminogen Activation by Streptokinase via a Unique Mechanism

• Published in: The Journal of biological chemistry 1998-01, Vol.273 (5), p.3110-3116
• Main Authors: Young, K C, Shi, G Y, Wu, D H, Chang, L C, Chang, B I, Ou, C P, Wu, H L
• Format: Article
• Language: English
• Subjects: Aminocaproic Acid - pharmacology; Binding Sites; Binding, Competitive; Humans; Kringles; Plasminogen Activators - pharmacology; Recombinant Proteins - pharmacology; Streptokinase - pharmacology
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.273.5.3110

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 × 10 −7 and 2.44 × 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 × 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 NH 2 -terminal domain of SK to human plasmin.