Interaction of Streptokinase with Human Plasminogen

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The development of caseinolytic, fibrinolytic, and esterolytic activities from human plasminogen was examined using various amounts of crude and purified streptokinase. Streptokinase was purified by ethanol precipitation at pH 5.6.
High esterase activity (with tosyl-L-arginine methyl ester as substrate) was obtained with a catalytic amount of crude or purified streptokinase. Further addition of streptokinase resulted in only a slight increase in this elevated activity.
Fibrinolytic activity was obtained with a catalytic amount of purified streptokinase and the activity was uneffected by further addition of the enzyme.
On the other hand, the maximum caseinolytic activity was obtained with a catalytic amount of crude streptokinase and the activity decreased on further addition of streptokinase.
However, when purified streptokinase was added, no decrease of caseinolytic activity was observed. Therefore, it seemed that the observed decrease of caseinolytic activity on addition of a large amount of streptokinase was due to an inhibitor present in the crude streptokinase. This was confirmed by experiments with purified streptokinase (ethanol precipitate), combined with the ethanol supernatant. Moreover, the decrease of caseinolytic activity by a high concentration of crude streptokinase was temperature-sensitive; the lower the temperature, the higher the inhibition.