The Role of Proaccelerin in Human Blood Coagulation . Evi - dence that Proaccelerin

In his original description of a patient with parahemophilia and his studies on the nature of the defect, Owren (1) proposed that the missing factor, proaccelerin (Factor V), was converted, in the presence of calcium, into a prothrombinconverting substance ("prothrombinase," Factor VI) by the action of tissue thromboplastin. Other investigators, notably Ware and Seegers (2) in a series of studies that they summarized in 1948, concluded that proaccelerin was convrerted to an active substance, sometimes called "accelerin," by thrombin and that the "accelerin" so formed was an accessory factor during prothrombin conversion and was not essential to this process. The exact role of proaccelerin in human blood coagulation has not been settled with certainty. Several investigators (3-6) supported Ware and Seegers' hypothesis, but others (7-9) propose that prothrombin cannot be converted to thrombin in the absence of proaccelerin and that proaccelerin manifests its action after activation by some clotting factor other than thrombin (10). Previous studies from this laboratory on the site of action of a circulating anticoagulant found