INCREASE IN THE RESISTANCE OF PLASMIN TO THE INACTIVATION BY ALPHA(2)-PLASMIN INHIBITOR ON INCUBATION

Plasmin was found to acquire resistance to the inhibition by alpha2-pl asmin inhibitor (alpha2PI) on incubation at 37-degrees-C in 50% glycer in whereas its amidase activity was preserved. This resistance is ascr ibable to the reduction of the affinity of plasmin to alpha2PI, since the ability of plasmin to form the complex with alpha2PI was proved to decrease on incubation. The reduced affinity of pre-incubated plasmin was recovered by incubation with alpha2PI. The pre-incubation presuma bly causes a reversible change in the conformation of the alpha2PI-bin ding site of plasmin. The use of freshly prepared plasmin solution is recommended for the assay of plasma alpha2PI if such an acquired resis tance of plasmin is undesirable.