Summary
The novel mutant streptokinase, SK-K59E, can activate human plasminogen as efficiently
as the purified commercially available streptokinase. Several peptide bonds including
Lys59-Ser60 in native streptokinase were hydrolyzed in reaction with plasmin and peptides of
small molecular masses were generated. The plasminogen activator activity of native
streptokinase in reaction with human plasmin declined to 25% of the original activity
in a 120-min incubation. On the other hand, the NH2-terminal peptide of SK-K59E remained intact in reaction with plasmin and the activator
activity of streptokinase decreased to 75% of the original activity in 120 min. The
major degraded peptide fragments of native streptokinase in reaction with plasmin
had molecular masses of 36 and 30 kDa. However, two major peptide fragments of 42
and 34 kDa were observed in the reaction of SK-K59E with human plasmin. The 42 kDa
peptide fragment, which contained NH2-terminal of streptokinase, could activate human plasminogen as efficiently as the
native streptokinase. SK-K59E can induce greater degree of caseinolysis and fibrinolysis
than the native streptokinase. In conclusion, the results demonstrate that the prevention
of cleavage at Lys59 of streptokinase prolongs the half-life of streptokinase in complex with plasmin
and that the NH2-terminal of streptokinase (Ile1-Lys59) plays an important role in maintaining its stability.
