alpha2-Plasmin inhibitor and alpha2-macroglobulin were allowed to compete for the protease plasmin. The binding of the enzyme to these inhibitors was assessed by two different but comparable methods. The interactions were completed in 10 s of incubation, and transfer of plasmin from one inhibitor to the other did not occur. Almost as much plasmin was bound to alpha2-plasmin inhibitor in mixtures containing a large molar excess of alpha2-macroglobulin relative to plasmin or alpha2-plasmin inhibitor, as was bound in mixtures not containing alpha2-macroblobulin. These studies demonstrate directly the effectiveness of alpha2-plasmin inhibitor in binding and inhibiting plasmin in the presence of alpha2-macroglobulin, and suggest that the alpha2-plasmin inhibitor may be the major circulating plasmin inhibitor.

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