Inhibitory Spectrum of α2-plasmin Inhibitor

α2-Plasmin inhibitor (α2PI) has been recently characterized as a fast-reacting inhibitor of plasmin in human plasma and appears to play an important role in the regulation of fibrinolysis in vivo. We have studied the effect of purified α2PI upon various proteases participating in human blood coagulation and kinin generation. At physiological concentration (50 μ g/ml), α2PI inhibited the clot-promoting and prekallikrein-activating activity of Hageman factor fragments, the amidolytic, kininogenase, and clot-promoting activities of plasma kallikrein, and the clot-promoting properties of activated plasma thromboplastin antecedent (PTA, Factor XIa) and thrombin. α2PI had minimal inhibitory effect on surface-bound activated PTA and activated Stuart factor (Factor Xa). α2PI did not inhibit the activity of activated Christmas factor (Factor IXa) or urinary kallikrein. Heparin (1.5-2.0 units/ml) did not enhnace the inhibitory function of α2PI. These results suggest that, like other plasma protease inhibitors, α2PI possesses a broad in vitro spectrum of inhibitory properties.