The Activation of Factor X by Extracts of Rabbit Brain.

By YALE NEMERSON AND THEODORE H. SPAET W HEN BLOOD is allowed to clot in vitro without the addition of other reagents, a complex sequence of events occurs which involves the participation of numerous coagulation factors, all of which are derived from the blood itself. This process is now generally known as “intrinsic coagulation.” Extracts of tissues markedly accelerate coagulation by bypassing several steps of the “intrinsic” system. Such extracts have been collectively called “tissue thromboplastin”; they are particularly active when derived from lung, placenta, or brain. The clotting that occurs in the presence of tissue thromboplastin is called “extrinsic coagulation,” and it is only in these reactions that factor VII participates. It has been suggested that the final cornmon pathway of both the intrinsic and extrinsic systems is activation of factor X ( Stuart-Prower factor ) . Activated factor X subsequently reacts with phosphatides and factor V to produce an agent capable of converting prothrornhin to thrombin. Fractionation of brain thromboplastin was an early step in coagulation research. Howell in 19122 prepared a chloroform-soluble reagent ( cephalin) which accelerated clotting, and is now known to serve as an alternative to