TISSUE-TYPE PLASMINOGEN-ACTIVATOR (TPA) INTERACTS WITH UROKINASE-TYPEPLASMINOGEN-ACTIVATOR (UPA) VIA TPAS LYSINE BINDING-SITE - AN EXPLANATION OF THE POOR FIBRIN AFFINITY OF RECOMBINANT TPA UPA CHIMERIC MOLECULES/

Differential scanning calorimetry was used to study the domain structu re and intramolecular interactions of tPA/uPA chimeras. A high tempera ture transition centered near 90 degrees C was observed upon melting o f the tPA/uPA chimera (amino acids 1-274 of tPA and 138-411 of uPA) an d its variant lacking the finger and epidermal growth factor-like modu les (residues 1-3 and 87-274 of tPA and 138-411 of uPA). Since neither of the two parent plasminogen activators display such a stable struct ure, one may suggest that a new stabilizing intramolecular interaction occurs in the chimeras, We found that occupation of the lysine bindin g site of tPA by a lysine or arginine side chain from the urokinase mo iety is responsible for the high temperature transition as well as for the failure of the chimeras to exhibit the expected fibrin binding pr operties, All uPA species, single- and two-chain high molecular weight uPA (pro-Uk and HMW-Uk) and two-chain low molecular weight uPA (LMW-U k), interact intermolecularly with tPA and its kringle-containing deri vatives. This intermolecular interaction was strongly inhibited by eps ilon-aminocaproic acid indicating that the lysine binding site of tPA is involved. The binding of uPA with the fluorescein-labeled A-chain o f tPA, registered by changes in fluorescence anisotropy, was estimated to have a K-d range of 1-7 mu M. The interaction of tPA with uPA dete rmined by solid-phase assays appeared to be tighter, with a K-d range of 50-300 nM. Two synthetic peptides, with and without carboxyl-termin al lysine, corresponding to urokinase residues 144-158 and 144-157, we re similar to 100-fold more potent than epsilon-aminocaproic acid with respect to inhibition of the tPA-uPA interaction, indicating that the tPA binding site on urokinase is located within this sequence, close to the activation site Lys(158)-Ile(159). The discovered intermolecula r interaction may be related to the reported synergistic effect of sim ultaneous administration of these two plasminogen activators.