Interaction of immobilized human plasminogen and plasmin with streptokinase

The complex of immobilized human plasminogen (iHPlg) and streptokinase (SK) has a lower catalytic activity in hydrolysis of plasmin-specific substrate S-2251 with a kcat/Km of 0.005 microM-1 x s-1, compared to 0.031 microM-1 x s-1 of immobilized human plasmin (iHPlm) and 0.078 microM-1 x s-1 of HPlm. The SK in the complex could be removed by acid buffer solution, and the iHPlg will remain catalytically active. IHPlm.SK complex could activate bovine plasminogen (BPlg), whereas iHPlg.SK complex could not. IHPlg could be activated by interaction with a combination of SK and HPlg or urokinase. The activated iHPlg.SK complex had the ability to activate BPlg as iHPlm.SK. The reasonable explanation is that iHPlg was converted to iHPlm in reaction with the combination of HPlg and SK. However, iHPlg was converted to a virgin enzyme in reaction with SK alone and could not activate BPlg. A new modified mechanism for the interaction of HPlg or HPlm with SK was proposed.