Mammalian plasma contains an enzyme precursor termed plasminogen (profibrinolysin) ( 1 ) wh.ich can be activated by various agents in vitro to form plasmin (fibrinolysin) ( 1 ) , an enzyme capable of dissolving fibrin clots. Streptokinase,* a commonly employed activator of human plasminogen, is incapable of activating some animal plasminogens (2). Geiger (3) first reported that more than one factor was involved in the activation of certain animal plasminogens by SK. Later, Miillertz and Lassen (4) showed that although bovine plasminogen was refractory to activation by SK, the addition of human globulin to the system resulted in the activation of the bovine plasminogen. As a result of their investigations, Miillertz and Lassen postulated that SK reacts with a proactivator (human factor) to form a plasminogen activator which converts plasminogen (bovine) to bovine plasmin. The studies of Troll et aZ. (5) and Troll and * Streptokinase-abbreviated SK.--1 1 " Sherry (6) indicated that this 2-stage activation process also applied to the SK activation of human plasminogen, the first step being a stoichiometric combination of SK with proactivator to form activator; the second stage, the enzymatic conversion of plasminogen to plasmin by this activator. Employing synthetic substrates, they concluded that both the plasminogen activator and plasmin hydrolyze lysine esters, whereas plasmin hydrolyzes arginine esters as well.It is the purpose of this paper to present data which indicate that the proactivator described by Mullertz and Lassen may be human plasminogen. This conclusion is based on evidence obtained from a comparison of the solubility, stability and purification characteristics of proactivator and human plasminogen. Further evidence supporting this conclusion is provided through studies of the autocatalytic conversion of human plasminogen to human plasmin.Prebarations. Human idasmino Pen.
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