Activator activities of the transient forms of the human plasminogen-streptokinase complex during its proteolytic...

When human plasminogen and the bacterial protein streptokinase are mixed, a tight equimolar complex is formed in which an active center of well defined hydrolytic activity developes; this event precedes the cleavage of the plasminogen chain, i.e. the conversion to plasmin. Immediately after the formation of the complex, a series of proteolytic transformations occurs which, within a few minutes, results in at least two cleavages in the plasminogen, and at least five cleavages in the streptokinase peptide chains. None of the fragments so created seem to dissociate from the main body of the complex, but the activator activity, when measured by a rapid bovine clot-lysis system, undergoes a characteristic pattern of fluctuation coincident with the fragmentation of the two components. When the latter process is followed by sodium dodecyl sulfate gel electrophoresis, the state of fragmentation of the activator can be correlated with the measured activator activities. By manipulating the temperature, and by the introduction of inhibitors, it was possible to slow down, or temporarily arrest, the fragmentation at certain stages, allowing the identification in a number of cases of the predominant activator species, and the determination of a characteristic relative activator activity for it. By the use of such relative activities, it was possible to carry out a calculation, based on electrophoretic analysis alone, which predicted reasonably successfully the kinetics of activator fluctuation.