Streptokinase(SK) is an extracellular protein produced by many strains of hemolytic streptococci, which lyses blood clots by activating plasminogen to plasmin. The complex of SK and human plasminogen is the functional activator that converts plasminogen to plasmin. It was reported that peptide bond at $Lys^{59}-Ser^{60}$, Lys^{387}-Asp^{388}$ and $Lys^{333}-Ala^{334}$ of streptokinase were cleaved when streptokinase forms complex with plasminogen. The 36-KDa peptide ($Ser^{60}-Lys^{387}$) shows 1/6 plasminogen activation activity of native streptokinase. However, when this 36-KDa peptide was preincubated with N-terminal peptide ($Ile^1-Lys^{59}$), this mixture shows plasminogen activation activity near the native streptokinase.In this study. the 59th residue Lys of streptokinase was mutated to Ala,Gln and Glu for the elucidation whether the cleavage of the peptide bond at $Lys^{59}-Ser^{60}$ was essential for the activation activity. Inferring from the unreduced SDS-PAGE, K59E and K59Q mutant showed similar cleavage pattern to wild type streptokinase. However,K59A mutant wasn't cleaved. From the specificity of plasmin, the site of cleavage of mutant proteins might be not the 59th residue. The specific activities of mutant proteins were all lower than wild type streptokinase. However, the faster the cleavage at the peptide bond near $Lys^{59}$, the higher the plasminogen activation activity of mutated streptokinase. From these results, it is possible that the cleavage of the peptide bond at $Lys^{59}-Ser^{60}$ of streptokinase is required for the full plasminogen activation activity to some extent.
