Streptokinase-dependent delayed activation of horse plasminogen

Complete activation of purified horse plasminogen to plasmin was obtained with a 1:10 molar ratio of streptokinase to plasminogen after 5 min of incubation at 37 degrees C. At a 1:1 molar ratio, maximal activity did not appear until 15-30 min, while at a ratio of 6:1 complete activation was delayed for 120-180 min. Gel filtration studies of isotopically labeled streptokinase and horse plasminogen suggest that the delay was due to impaired formation of a streptokinase-plasminogen complex. The predominant streptokinase moiety within the streptokinase-plasmin complex which forms from the streptokinase-plasminogen complex had a molecular weight of about 25000. The streptokinase-horse plasmin complex activated bovine plasminogen and was relatively stable. Native streptokinase was rapidly modified by horse plasmin predominantly to a fragment with a molecular weight comparable to that of the streptokinase moiety within the horse streptokinase-plasmin complex, about 25000 daltons. Partial characterization of horse plasminogen revealed no striking differences from human plasminogen in terms of molecular weight, N-terminal analysis and amino acid composition. However, horse plasminogen did not react with antibodies to human plasminogen, and its isoenzymes were more acidic than those of the human. Further characterization of horse plasminogen will be required to ascertain whether activation by streptokinase can serve as a model for the altered kinetics which have recently been described for the activation of aberrant types of human plasminogen.