Purification and properties of an antiactivator fraction from bovine serum.

: A method is described for the purification of antiactivator from bovine euglobulin-free serum by means of gelfiltration and ion exchange chromatography. The purified antiactivator has no antifibrinolytic activity. It has a molecular weight of about 115,000 and it appears to be a gamma globulin. The dissociation constant of its complex with urokinase is 1.6 x 10(-9) M and the maximum urokinase binding is close to 2000 CTA units per mg. Its concentration in bovine serum is 0.37%. Flufenamate displaces urokinase from the antiactivator at very low concentrations, about 10(-4) M. Cysteine restores its activity if lost by standing. Also an antifibrinolysin fraction is obtained free of antiactivator activity.