The steady-state kinetic characteristics of the amidolytic and plasminogen activator activities of equimolar streptokinase (SK)-human plasminogen (HPg) and SK-human plasmin (HPm) complexes have been determined, exploiting the generation and use of cleavage site resistant mutants of HPg to stabilize plasminogen within the complex. Whereas amidolytic kinetic constants for equimolar complexes of SK with the following proteins, viz., plasma HPm, insect (i) cell-expressed wild-type (wt) recombinant (r) HPm, R561E-irHPg, and Chinese hamster ovary cell (c)-expressed R561S-crHPg, are similar, it has been found that the various SK-HPg complexes are far better enzymes than SK-HPm complexes for activation of bovine plasminogen, a species of plasminogen that is resistant to activation by SK, alone. In addition, it is emphasized that as a result of mutating the cleavage site in plasminogen, it is possible to express this protein in mammalian cells, and thus provide it for use in complex with SK as a more efficient plasminogen activator than plasma plasminogen, which is rapidly converted to HPm within the SK complex. This finding has important implications in the assessment of thrombolytic therapeutic reagent employing SK-plasminogen and SK-plasmin complexes.
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