This research proposal is an attempt to define in biochemical terms the mechanism and structural changes which take place as prothrombin converts to thrombin during the clotting of human blood. Studies will continue on the sequence of changes which take place as the prothrombin molecule is proteolyzed to thrombin; the activation fragments which are split from the zymogen will be isolated and characterized as to NH2 and COOH terminal amino acids, molecular weight and carbohydrate composition. Using this system as a guide, other mammalian prothrombin- thrombin systems will be studied by similar techniques in the hope of obtaining animal models useful for in vivo experiments. In addition the amino acid sequence of the "A" peptide chain of a number of different mammalian thrombin preparations will be analyzed in order to determine if there is an area on homology on the "A" chain of different species which might serve as a "recognition site" which governs its narrow substrate specificity on the enzyme. The binding of prothrombin activation fragments to other plasma proteins will be studied with particular attention given to the role of alpha-2 macroglobulin and antithrombin III (antifactor Xa) and, finally, in conjunction with other studies we hope to determine the mechanism and role which other blood clotting factors may have on the structural changes which take place during prothrombin activation and transformation. This proposed work should ultimately lead to a better understanding of the conversion of prothrombin to thrombin during human blood coagulation, the role of this system in hemostasis and the pathogenesis and pharmacologic control of hemorrhagic and thrombotic disease.