Further Characterization Of SK-Potentiator Of Plasminogen

Streptokinase (SK) forms a complex with human plasminogen (plg) or plasmin, and the resulting complex (SK-activator) functions to convert plg to plasmin. We have indicated that human plasma contains a factor (SK-potentiator) which potentiates the capacity of SK to activate human plg. SK-potentiator has a molecular weight of 240,000, and composed of β and γ-chains of fibrinogen, α-chain being degraded. SK-potentiator crossreacts with anti-FDP-Y fragment. Immunodiffusion shows that SK-potentiator has an antigenic determinant in common with both FDP-Y and fibrinogen, and the other determinant not in common with fibrinogen but FDP-Y. Early FgDP potentiates SK-activator activity as much as SK-potentiator, but further degraded FgDP potentiates less than fibrinogen which still enhances SK-activator activity. The addition of thrombin to FgDP or SK-potentiator enhances SK-activator activity more than SK-potentiator. Thus removal of fibrinopeptides from FgDP or SK-potentiator results in better potentiator activity. When tranexamic acid (l mM) was added to the mixture of Glu-plg and UK, the activation of Glu-plg was enhanced, but tranexamic acid (l mM) added to SK-activator caused a decrease in SK-activator activity. The addition of fibrinogen or SK-potentiator to the mixture of tranexamic acid and SK-activator prevented the decrease of SK-activator activity to some extent, which may indicate that SK-potentiator competes with tranexamic acid for lysine binding sites (LBS) of plg and SK-potentiator forms a complex with SK-activator in spite of the presence of tranexamic acid. It is proposed that SK-potentiator binds with LBS of plg part of SK-activator and SK combines with light chain part of plg, the resulting SK-plg-potentiator complex being the better activator than SK-plg or SK-plasmin complex.