Circumstantial evidence suggested that normal human plasma contained a substance regulating the neutralization of F.Xa by F.Xa inhibitor(XaI), (Yin et.al.,Adv.Exper. Med. & Biol., 52 : 239, 1975, Plenum Press, N.Y.).This plasma component has now been isolated and partially purified in our laboratory, and tentatively designated as “Anti-XaI”.In experiments employing purified components, when Anti-XaI was incubated at 37°C with F.Xa, Xal and heparin for two minutes at pH7.5, the amount of F.Xa inhibited was inversely proportional to the Anti-XaI concentration. But, when the F.Xa was replaced by thrombin in the incubation mixture, the neutralization of thrombin clotting activity was undisturbed.Anti-XaI was found to be neither PF3 nor PF4.These and other data strongly suggest that the “Antithrombin III pathway” is more complex than currently believed to be. In circulating blood an equilibrium state must exist between Anti-XaI and XaI.Under certain conditions when the Anti-XaI activity is predominant the rate of F.Xa neutralization bv XaI then becomes slower than the activation of prothrombin to thrombin by F.Xa.
