The Active Site of Bovine Plasminogen Activator

Bovine plasminogen activator preparations were isolated at 0° and 25° from a mixture of streptokinase and human plasminogen. The degree of incorporation and inhibition with radioactive 32P-labeled diisopropyl phosphofluoridate (DFP) demonstrated a definite difference between these two preparations. Incorporation of approximately 1 mole of DFP-32P per mole of bovine plasminogen activator prepared at 25° and per mole of human plasmin indicated that bovine plasminogen activator and human plasmin each contains a single active site which may be identical. Incorporation of 0.50 mole of DFP-32P per mole of bovine plasminogen activator prepared at 0° and its retention of partial activity indicated that this activator preparation was partly a human plasminogen-streptokinase complex. Acid treatment of this preparation and inhibition studies with e-aminocaproic acid also indicated that this preparation contains a plasminogen-streptokinase complex. Complex formation also occurred when streptokinase was allowed to react with DFP-32P-treated human plasminogen and plasmin. Starch gel electrophoretic analyses indicated that all the complexes formed under these conditions possessed the same mobility. Dissociation of the 25° activator complex inhibited with DFP-32P after its isolation revealed that the radioactivity was located in the plasmin moiety of the activator complex.