Relative Affinity of Plasmin for the α1-Macroglobulin, Cl Inactivator and α1-Antitrypsin Inhibitors

The quantitative contribution of three major plasma protease inhibitors in binding plasmin has been studied. Mixtures of plasmin and each of the purified inhibitors were analyzed by SDS-acrylamide gel electrophoresis. Plasmin remained bound to its inhibitors in the presence of SDS and urea. A 1 : 1 molar ratio for complex formation was established, and treatment of the complexes with a disulfide bond reducing agent showed that the light chain of plasmin contained the binding sites for both CĪ inactivator and α-antitrysin. Limited degradation of all three inhibitors by plasmin was observed, and the altered inhibitor remained complexed to the enzyme. The competitive binding of 125I plasmin to mixtures of these inhibitors was followed by sucrose density ultracentrifugation and by SDS-gel electrophoresis. In mixtures containing physiologic molar ratios of enzyme and inhibitors, over 80% of the bound plasmin was complexed to the α2-macroglobufui (α2M). No evidence for an exchange of plasmin between the inhibitors was obtained.