The underlying goal of the project is to elucidate the reactions of antithrombin III with thrombin and other plasma procoagulants, and to define the mechanisms by which heparin inhibits blood coagulation. Since antithrombin and heparin affect a number of reactions, results of these studies should contribute greatly to our understanding of the process that regulate and control blood clotting. One of the subjects to be investigated during the next year will be the mechanism of formation and degradation of the primary complex formed by the interaction of thrombin with antithrombin in the presence and absence of heparin. Specifically, it will be our aim: (a) to determine whether formation of the complex involves the release of an oligopeptide from antithrombin and (b) to determine the sites in the complex that are cleaved when it undergoes degradation. Another aim of the project will be to define the mechanism of action of the high-activity heparin isolated previously in this laboratory. Studies will also be made on the effect of the preparation in accelerating the inhibition of thrombin and factor Xa, by antithrombin. Additionally, functional group modification studies will be performed to define the structural unit in heparin that is critical in its action for promoting the inhibition of thrombin and other procoagulants. A third area of study will be to determine the kinetic aspects of the effect of heparin in inhibiting the neutralization of thrombin by alpha1-proteinase inhibitor. Quantitative studies will also be made on the action of heparin in decreasing the rate of proteolysis of the thrombin-antithrombin complex by thrombin.