Effect of Specific Cleavage of Immunoglobulin G by Plasmin on the Binding and Activation of Plasminogen

A method of ELISA for measuring the binding of different samples of immunoglobulin (IgG) and its fragments to human plasminogen (Pg) has been developed. Instead of plasminogen, the heavy chain of plasminogen (Pg-H) containing five ligand-binding kringle domains, immobilized on the surface of the plate, was used in this method as a detector. It was found that IgG treated with plasmin (IgG_Pm-t) binds to the immobilized Pg-H 2.84 times more strongly than intact IgG. Both IgG samples showed a weak nonspecific binding to the immobilized light chain of plasminogen (Pg-L). It was shown that 0.2 M L-lysine inhibits the binding of IgG_Pm-t and does not affect the nonspecific binding of intact IgG to the immobilized Pg-H, indicating the involvement of lysine-binding regions of Pg-H in binding to IgG_Pm-t. A preliminary treatment of IgG samples with carboxypeptidase В (CPB) inhibited the binding of IgG_Pm-t and did not affect the nonspecific binding of intact IgG to the immobilized Pg-H, which indicates a key role of the С-terminal lysine of IgG_Pm-t in the specific binding to the lysine-binding sites of Pg. The study of the effects of intact IgG and IgG_Pm-t on the rate of activation of Glu- and Lys-forms of Pg (Glu-Pg and Lys-Pg) by a tissue activator of Pg (tPA) and urokinase (uPA) in buffer showed that intact IgG completely inhibited the activation of Glu-Pg and Lys-Pg with both tPA and uPA. Presumably, the inhibitory effect of intact IgG is due to steric hindrances that it creates for protein–protein interactions of the activators with the zymogen. IgG_Pm-t accelerated the generation of plasmin from Pg. In this case, the stimulatory effect of IgG_Pm-t on the activation of Glu-Pg under the action of tPA was ∼25% higher than on the activation of Lys-Pg, which is explained by more significant conformational changes in the Glu-Pg molecule compared with the Lys-Pg molecule after their binding to IgG_Pm-t. The results suggest that the specific cleavage of IgG by plasmin may be one of the ways by which the plasminogen/plasmin system is involved in various physiological and pathological processes.