A Comparison of Glu- and Lys-Plasminogens: Binding to Fibrin and Effect on Lysis Rates

Partially activated (lys-) plasminogen (plgn) exhibits a greater affinity than native (glu-) plgn for fibrin. To complement previous data obtained using radio-labelled plgn we have studied a range of plgn concentrations, used functional assays (S-2251, caseino-lysis and binding to lysine sepharose), ensured that the fibrin is totally cross-linked, correlated affinities of plgn for fibrin with lysis rates when plgn was activated by SK, and compared buffer with plasma and serum systems containing native inhibitors. When fibrinogen or plasma was clotted in the presence of plgn then lys-plgn gave greater incorporation and faster lysis rates than glu-plgn. When washed fibrin was immersed in plgn lys-plgn again showed greater affinity than glu-plgn for fibrin, but on transfer to SK lysis rates were identical. In the most potent lysis system, immersion of fibrin in SK then transfer to plgn, lysis was slightly fasterin glu- than lys-plgn. Incorporation of purified glu-plgn into plasma fibrin exceeded that of native plasma olqn and the former promoted lysis more effectively. Binding of lys-plgn to fibrin was the more susceptible to inhibition by plasma or serum. The pertinence of these data to in vivo requirements for thrombolytic therapy is speculative, but clearly it cannot be assumed that lys-plgn must be more effective than glu-plgn, in promoting fibrinolysis, because of superior affinity for purified fibrin.