Plasmin activity obtained by incubating streptokinase derivatives with plasminogen also was studied as a function of time with each of the PEG-streptokinase derivatives. By this assay, incubations containing PEG-5-streptokinase and unmodified streptokinase demonstrated comparable activity while reaction mixtures containing PEG-2-streptokinase and PEG4-streptokinase were slightly more active. Streptokinpse incubated with plasminogen at a 1:1 molar ratio was extensively degraded after 30 min whereas PEG-2-streptokinase was resistant to plasmin cleavage. The derivatized proteins were radioiodinated and incubated in plastic microtiter plates that were coated with an immunoglobulin fraction containing antibodies to streptokinase. Binding of the PEG-streptokinase adducts was decreased by >95% compared with unmodified streptokinase. Plasminogen activator complexes were formed by reacting the streptokinases with human plasminogen in vitro and the clearance studied in mice. Radioiodinated plasmin in complex with the PEG-streptokinase adducts cleared at a slower rate than did plasmin complexed with unmodified streptokinase. Catabolism of the protease still occurred by a mechanism that involved reaction with a2-macroglobulin as has been described for nonderivatized streptokinase-plasminogen complex (Gonias, S. L., M. Einarsson, and S. V. Pizzo, 1982, J. Clin. Invest., 70:412-423). When more extensive derivatization procedures were utilized, PEG-2-streptokinase preparations were obtained that further prolonged the clearance of complexed 1251-plasmin;however, these adducts did not uniformly retain comparable activity. It is suggested that PEG-streptokinase complexes with greatly reduced antigenicity may be useful in the treatment of thrombotic disorders.Address reprint requests to Dr. Pizzo.

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