Decoy Plasminogen Receptor Containing a Selective Kunitz-Inhibitory Domain

[Image: see text] Kunitz domain 1 (KD1) of tissue factor pathway inhibitor-2 in which P2′ residue Leu17 (bovine pancreatic trypsin inhibitor numbering) is mutated to Arg selectively inhibits the active site of plasmin with ∼5-fold improved affinity. Thrombin cleavage (24 h extended incubation at a 1:50 enzyme-to-substrate ratio) of the KD1 mutant (Leu17Arg) yielded a smaller molecule containing the intact Kunitz domain with no detectable change in the active-site inhibitory function. The N-terminal sequencing and MALDI-TOF/ESI data revealed that the starting molecule has a C-terminal valine (KD1(L17R)-V(T)), whereas the smaller molecule has a C-terminal lysine (KD1(L17R)-K(T)). Because KD1(L17R)-K(T) has C-terminal lysine, we examined whether it could serve as a decoy receptor for plasminogen/plasmin. Such a molecule might inhibit plasminogen activation as well as the active site of generated plasmin. In surface plasmon resonance experiments, tissue plasminogen activator (tPA) and Glu-plasminogen bound to KD1(L17R)-K(T) (K(d) ∼ 0.2 to 0.3 μM) but not to KD1(L17R)-V(T). Furthermore, KD1(L17R)-K(T) inhibited tPA-induced plasma clot fibrinolysis more efficiently than KD1(L17R)-V(T). Additionally, compared to ε-aminocaproic acid KD1(L17R)-K(T) was more effective in reducing blood loss in a mouse liver-laceration injury model, where the fibrinolytic system is activated. In further experiments, the micro(μ)-plasmin–KD1(L17R)-K(T) complex inhibited urokinase-induced plasminogen activation on phorbol-12-myristate-13-acetate-stimulated U937 monocyte-like cells, whereas the μ-plasmin–KD1(L17R)-V(T) complex failed to inhibit this process. In conclusion, KD1(L17R)-K(T) inhibits the active site of plasmin as well as acts as a decoy receptor for the kringle domain(s) of plasminogen/plasmin; hence, it limits both plasmin generation and activity. With its dual function, KD1(L17R)-K(T) could serve as a preferred agent for controlling plasminogen activation in pathological processes.