Inhibitors of plasminogen activation in human blood.

Plasma contained two inhibitors of plasminogen activation by urokinase when fractioned by gel filtration on Sephadex G-200. The inhibitor in the lower molecular weight fractions was separate from the principal protease inhibitors of plasma: alpha1-antitrypsin, alpha2-macroglobulin, C1 inactivator and antithrombin III, and from factor XIII. This activation inhibitor was present in both plasma and serum and its recovery was not reduced by preincubating the serum for 6 h at 37 degrees C. Its inhibitory activity was stable for several days at 4 degrees C, and was enhanced in the presence of an increased saline concentration. Preparations of the inhibitor, active in a clot lysis system, failed to inhibit the esterase activity of urokinase on N-alpha-acetyl-glycyl-L-lysine methyl ester.