K Titani

Bovine factor IX (christmas factor). Further evidence of homology with factor X (Stuart factor) and prothrombin

Factor IX is a single-chain glycoprotein involved in the intrinsic pathway of blood coagulation. The molecular characteristics of factor IX and the mechanism of its activation by factor XI, (activated factor XI) have recently been described [ 1,2] . In the presence of factor VIII, calcium, and phospholipid, factor IX, catalyzes the conversion of factor X to X,. Factor X, in the presence of factor V, calcium, and phospholipid in turn converts prothrombin to thrombin. Factor X, and thrombin are both inactivated by diisopropylphosphorofluoridate (DFP), but the same has not been established for factor IX,. Factor X, and thrombin are homologous with other mammalian serine proteases [3]. The amino terminal regions of factor IX, prothrombin, and the light chain of factor X are also homologous, but factor IX is immunologically distinct [4]. Nevertheless, evidence is presented here showing that considerable homology exists between bovine factors IX and X and prothrombin, suggesting that factor IX, has evolved from the same precursor and therefore is probably also a serine protease.