Formation and properties of the activator of plasminogen and of human and bovine plasmin

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Abstract
An activator of plasminogen is formed by the interaction of strepto-kinase and a component, a proactivator, in human blood. At suboptimum concentrations of streptokinase, only a partial conversion of proactivator into activator was obtained, followed by a slow deterioration of the activator formed. The amount of activator formed in solutions containing varying proportions of streptokinase and of proactivator indicated a stoicheiometric reaction proceeding to an equilibrium. Experiments on the effect of dilution suggested that streptokinase and proactivator interact to form two components, an activator and an inactive component. The activator was very labile at acid and alkaline reactions, with an optimum of stability between pH 6.5 and 8.0, differing in this respect from other known components of the proteolytic-enzyme system in blood. A complete conversion of bovine plasminogen into plasmin was accomplished by addition of the activator. The preparations of human and bovine plasmin were purified by a treatment at acid reaction, which produced a destruction of streptokinase, activator and inhibitory substances. Human plasmin did not catalyse the conversion of human and bovine plasminogen into plasmin. Human and bovine plasmin showed nearly identical relationships between pH and activity on casein, with an optimum between pH 7.5 and 8.3. They showed a remarkably high stability at acid reaction. At neutral reaction human plasmin was fairly stable, while bovine plasmin was rather labile. Plasmin was stabilized by casein.