The Plasmin Inhibitor of Human Plasma

Purified al-proteolytic inhibitor from human plasma was shown to inhibit trypsin, chymotrypsin, plasmin, and thrombin. This inhibitor is probably identical with the inhibitor previously known as serum trypsin inhibitor, al-antitrypsin, or antiplasmin. The enzymes were affected by the inhibitor via two different mechanisms. Trypsin and chymotrypsin reacted instantaneously in stoichiometric manner, and the enzyme substrate had no effect on the reaction. The inhibition of plasmin and thrombin was time-dependent and of nonstoichiometric nature, and it was retarded by the presence of enzyme substrate. Even so, kinetic studies failed to show a competition reaction between the substrate and the inhibitor on a single site of the enzyme molecule. It is therefore suggested that the inhibition mechanism might be an enzymatic inactivation of the proteolytic enzymes concerned. This inactivation was irreversible in the sense that no enzymatic activity could be recovered after acid dissociation of the enzyme-inhibitor complex. It was shown by differential titration that the inhibitory activity of the inhibitor towards all four enzymes is due to a single molecular entity; this conclusion is also suggested by the high purity of the inhibitor preparation. On the basis of its stoichiometric combination with trypsin, the cq-proteolytic inhibitor was estimated to have a molecular weight of 47,000 and to comprise about 2% of plasma proteins.