The human plasmin-derived light (B) chain X streptokinase complex: a second-generation thrombolytic agent.

Specific assay methods for the human plasmin-derived light (B) chain X streptokinase (B X SK) complex, in terms of both streptokinase (SK) and urokinase (UK) International Units, are described. The kinetic properties of various SK activator complexes with plasminogen, Val442-plasmin, and the plasmin-derived light (B) chain were compared to SK in terms of their catalytic efficiencies and Lineweaver-Burk plots. Similar kinetic data, and Lineweaver-Burk plots, are described for both highly purified high-molecular weight UK and low-molecular weight UK, including different clinical UK preparations. The B X SK complex has the highest catalytic efficiency of all the activator species studied. The Lineweaver-Burk plots of each of the various activator species are "fingerprints" of the enzymatic character of the activator. The B X SK complex is more like UK than SK, as an activator, in activating non-human plasminogen species. The biological half-life of the B X SK complex, in a dog model, was determined to be about 4 hr which is longer than the biological half-life(s) of SK in the same animal model, namely 0.6 hr (47%) and 2.8 hr (53%). This new second-generation activator complex may prove to be a useful thrombolytic agent in the treatment of thromboembolic diseases.