The streptokinase-human plasminogen activator complex. Composition and identity of a subcomponent with activator activity.

Reactions between purified plasminogen and streptokinase, labelled with (131)I and (125)I respectively, were investigated by polyacrylamide-gel discontinuous electrophoresis. A molecular complex consisting of both (131)I-labelled plasminogen and (125)I-labelled streptokinase migrated between plasminogen and streptokinase. This complex contained bovine plasminogen activator activity. The relative quantities of (131)I-labelled plasminogen and (125)I-labelled streptokinase in this complex were markedly affected by reaction conditions. A fragment that retained 50% or more of the parent activator activity was released from the complex after exposure to mercaptoethanol. This subcomponent had an estimated molecular weight of 70000, and contained both (131)I-labelled plasminogen and (125)I-labelled streptokinase.