The Inhibition of Urokinase by Alpha₂-Macroglobulin

1. Purified preparations of α₂-macroglobulin inhibited the ability of urokinase to induce lysis of a fibrin plate, to activate plasminogen in a purified system, and to hydrolyse N-α-acetyl glycyl-l-lysine methyl ester.

2. Inhibition of urokinase by α₂-macroglobulin was immediate: no progressive inhibition could be detected using fibrinolytic and esterolytic assays. Inhibition was competitive.

3. The fibrinolytic activity of euglobulin precipitate prepared from exercise plasma was also inhibited by α₂-macroglobulin.