Summary
Human, chimpanzee and baboon plasminogens, in that order, were found to be decreasingly susceptible to streptokinase (SK) activation. The relative activation of the three primate plasminogens remained the same when species specific fibrin or casein was used as substrate and when the following conditions were varied: 1. SK concentration; 2. plasminogen concentration; and 3. length of incubation of the SK-plasminogen-substrate reaction mixture. Under the same conditions, the three primate plasminogens were readily activated by human urokinase (UK) and no consistent order of activation by UK was observed. Our findings suggest that the site on plasminogen which interacts with SK has undergone changes during evolution whereas the site at which UK activates each of the three plasminogens has changed little. It remains obscure whether the susceptibility of primate plasminogen to SK activation is a specifically selected genetic trait or is simply a laboratory indication of structural changes in the plasminogen molecule that have occurred for other reasons.