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SummaryThe rôle of human plasminogen activator was studied during the “spontaneous” activation of a human euglobulin preparation at 25° and 37° C. The lytic activity of the solutions undergoing incubation was tested both on heated and on normal bovine fibrin plates. Taking into account the at least two times higher sensitivity of heated fibrin towards plasmin than of normal one as used in this investigation the respective activator activities were calculated from the diverging lytic effects on the two different types of plates. While with incubation at 25° C only low activator concentrations and slow conversion of plasminogen could be observed, at 37° C the increasing activator activity, which reached its maximum after 60 hours, was accompanied by a correspondingly fast formation of plasmin.

SummaryDuring “spontaneous” activation of a human euglobulin preparation in suitably spaced samples — while plasminogen became progressively converted into piasmin — the proactivator content was tested by addition of equal amounts of streptokinase and evaluation of the lytic activities on heated and normal bovine fibrin plates. Indepedently of the decreasing content of plasminogen the proactivator which could be activated by streptokinase showed no significant change of concentration. These observation indicate that plasminogen is not acting as proactivator and that there exists a separate proactivator-activator system of the fibrinolytic mechanism in human serum.

Summary1. Studies of the fibrinolytic effect of spontaneously activated plasmin preparations on normal and heated bovine fibrin plates showed that human plasmin preparations can be obtained, to which heated fibrin possesses a higher sensitivity than native fibrin.2. The results suggest that spontaneously activated human plasmin prepared from an euglobulin fraction is more or less accompanied by an agent which acts as activator for bovine plasminogen contained in native fibrin plates. It seems that this agent is identical to a serum factor which acts physiologically as plasminogen activator and is also responsible for the socalled “spontaneous activation”.3. The findings emphasize the necessity for standardizing testing procedures. Variations in processing the fibrinogen, variations in fibrinogen concentration, and especially in the duration of the heat treatment of the fibrin plates, may cause significant variations in the resulting sensitivity of the method which in turn may be reflected in diverging apparent activities.