Proactivator function of human plasmin as shown by lysine esterase assay.

Tillett and Garner (1) discovered that an extracellular protein elaborated by certain strains of streptococci produces the lysis of clots which contain human fibrinogen or thrombin. M&tone demonstrated that the bacterial product was not in itself fibrinolytic but interacted with a lytic material in human plasma (2). The situation was greatly clarified and a rational nomenclature introduced by Christensen (3), who showed that the bacterial protein, streptokinase, activated the lytic precursor, plasminogen, to a proteolytic enzyme, plasmin. The concept that streptokinase acts directly on plasminogen to catalyze its conversion to plasmin has given way to the theory that this conversion is a two-step reaction. Streptokinase, which exhibits little activity in any species other than the human, first reacts stoichiometrically with a precursor in human plasma to form a plasminogen activator which then enzymatically causes the transformation of plasminogen to plasmin in virtually all species (4-6). The nature of the precursor has been a matter of considerable controversy. Because of its inseparability from human plasminogen under a variety of conditions, the precursor has been claimed to be plasminogen itself (7-9). Other investigators, to avoid assigning two distinct functions to the same plasminogen molecule, have proposed that the precursor is a unique protein which they called “proactivator” (5). The evidence in this controversy has been recently reviewed (10). Methods for the measurement of proactivator and activator activities have depended on the estimation of plasmin evolved from bovine plasminogen. The production of plasmin to measure these activities involves serious limitations since preformed plasmin or the presence of plasmin inhibitors interferes. In this communication, assays are proposed for the quantitative estimat’ion of proactivator, activator, plasminogen, and plasmin in the presence of each other, based on the hydrolysis of lysine methyl ester. Both plasmin and the activator hydrolyze lysine methyl ester, but evidence will be presented indicating that only the former is inhibited by soybean trypsin inhibitor. The residual activity, therefore, is a measure of the activator. Conditions for the lysine methyl ester assay of plasminogen and of the precursor of the activator have also been defined. Since the various activities can be estimated in the presence of each other, it was possible to study the interconversion of components of the streptokinase-induced fibrinolytic system. The findings strongly point to plasmin as a precursor which re-