The binding of human plasminogen to fibrin and fibrinogen.

The binding of human plasminogen to fibrin was studied using 125Lplasminogen and uniform suspensions of noncross-linked or cross-linked human fibrin. Of the two types of plasminogen, the proteolytically modified form (NJ&-terminal lysine; Lys-plasminogen) showed a high binding affinity for both forms of fibrin, having a dissociation constant of 0.32 p~ and two binding sites per fibrin monomeric unit. Binding could be inhibited by w-aminocarboxylic acids such as lysine or e-aminocaproic acid (EACA). Nonlinear least squares computer analysis of the competition curve with EACA indicated that dissociation of Lys-plasminogen from fibrin occurred through the binding of one EACA molecule per plasminogen with a K d Of 18 pM. Native plasminogen (NHz-terminal glutamic acid; Glu-plasminogen) and its isolated elastase digestion fragments, K123, K4, and Vah-plasminogen, also bound to cross-linked fibrin with about 2 binding sites per monomeric unit but with much lower affinities. Dissociation constants were 38 p~ for Glu-plasminogen, 61 p~ for K123, 160 pM for K4, and 170 pM for v a h plasminogen. Computer analysis of the inhibition of this binding by EACA indicated that only one lysinebinding site was involved in the interaction between any of the plasminogen fragments and fibrin but that two sites were involved in the Glu-plasminogen-fibrin interaction, with K d values of 10 pM and 440 p. With only the high affinity lysine-binding site on Glu-plasminogen blocked by EACA, fibrinogen still enhanced the rate of Glu-plasminogen activation over &fold. Hence, while two lysine-binding sites may be involved in binding to fibrin, it is the lower affinity binding site that interacts with fibrin(ogen) to accelerate activation. By sucrose density ultracentrifugation Glu-plasminogen did not bind to fibrinogen, while Lys-plasminogen and fibrinogen did form a complex which had a dissociation constant of about 1 m ~ . Of the fibrinogen plasmic degradation fragments, only Fragment E bound Lys-plasminogen, having a dissociation constant of about 8.3 q. The elastase digestion fragment of plasminogen, K123, also formed a complex with fibrinogen as well as with Fragment E. Unlike Lys-plasminogen, a difference was noted in the binding of the two carbohydrate forms of K123 to fibrinogen. Only the K123 which lacked the asparagine-linked carbohydrate chain bound to fibrinogen, but both K123 forms bound F’ragment E. Just as in the plasminogen-fibrin binding studies, all of these complexes could be inhibited by aaminocarboxylic acids.