The Inhibition of Human Plasmin by Human Antithrombin-Heparin Cofactor

The interaction of purified human antithrombin-heparin cofactor and human plasmin was studied in the presence and absence of heparin. Antithrombin is a progressive, timedependent inhibitor of the proteolytic and esterolytic activities of plasmin. Incubation of plasmin with antithrombin for 15 to 30 min results in 90 to 100% inhibition of both activities of the enzyme. The presence of heparin dramatically accelerates the rate of interaction of antithrombin and plasmin, with nearly complete inhibition within 30 s of incubation. Sodium dodecyl sulfate gel electrophoresis of reduced and nonreduced proteins indicates that antithrombin functions as a potent antiplasmin by forming an undissociable complex which is stable in the presence of denaturing or reducing agents (or both). This complex represents a 1:1 stoichiometric combination of enzyme and inhibitor. Heparin increases the rate of formation of this complex without affecting its dissociability or stoichiometry.