Reexamination of the extent of the activation of Lys78 plasminogen by tissue plasminogen activator in the presence of polymerized fibrin.

The rate of the conversion of Lys78 plasminogen (Lys78-plg) to Lys78 plasmin by tissue plasminogen activator (tPA) was directly quantitated by SDS-PAGE. The apparent second-order rate constant (Kapp) was (0.27 +/- 0.07) x 10(3)/M/s for single-chain tPA and Lys78-plg, and was enhanced approximately 5-fold by fibrinogen and 17-fold by polymerized fibrin. Kapp was (1.57 +/- 0.46) x 10(3)/M/s for two-chain tPA and Lys78-plg, and was enhanced approximately 2.6-fold by fibrinogen and 3.6-fold by fibrin. The factor of the enhancement by polymerized fibrin was far less than in previous reports in which chromogenic substrate was commonly employed.