A CHIMERIC STREPTOKINASE WITH UNEXPECTED FIBRINOLYTIC SELECTIVITY

Chimeric 59D8-SK was designed to confer fibrin-selectivity to streptok inase by fusion of the Fab fragment of anti-fibrin antibody 59D8 to th e N-terminus of streptokinase (SK: Ile(1)-Lys(414)). It was expressed in a mouse hybridoma cell line and purified by affinity chromatography on a 59D8-antigen column. Chimeric 59D8-SK is a disulfide-linked hete rodimer composed of an antibody light chain (Mr 27,000) and a N-glycos ylated chimeric heavy chain (Mr 90,000). The fibrin targeting by 59D8 increased plasma clot lysis by 2-fold, but connecting 59D8 to SK has p rovided 59D8-SK several unique properties: (i) 59D8-SK activated human Glu-plasminogen with a significant lag period that coincided with lim ited proteolysis of 59D8-SK similar to that observed for wild-type SK. In a kinetic study, both gave very similar kinetic parameters for the activation of Glu-plasminogen even though 59D8-SK was N-glycosylated in its SK portion. (ii) 59D8-SK was relatively inactive in human plasm a, compared to SK, but it became activated in the presence of clots; ( iii) 59D8-SK lysed clots slowly but completely whereas SK lysed clots rapidly but incompletely. Even though the mechanism behind these new p roperties is not fully understood, they are characteristics of a secon d-generation plasminogen activator.