Fibrin-dependent activation of plasminogen by a proteolytic digest of streptokinase

Among four enzymatic digests of streptokinase (SK), the smallest peptide with plasminogenolytic activity was in a tryptic digest; it had a molecular weight of 29,000. A complex of this peptide, SK29, and human plasminogen hydrolysed human fibrin, but a complex of native streptokinase and human plasminogen hydrolysed both human and bovine fibrin. The complex with SK29 caused amidolysis of the synthetic substrate S-2251 in the presence of human fibrin, but was inactive in the presence of human fibrinogen, bovine fibrinogen or bovine fibrin. Analysis of the amino terminal sequence of SK29 indicated that cleavage by trypsin was on the carboxyl side of lysine, the 59th amino acid of streptokinase. These results suggest that the conformational changes caused by human fibrin formation resulted in the generation of an active site of human plasminogen by SK29.