Regulation of the initiation of coagulation by factor VII.

Factor VII, a single-chain zymogen, has sufficient proteolytic activity to initiate blood coagulation. The reason that coagulation does not occur continuously is that the zymogen like its 2-chain derivative enzyme, factor VIIa, absolutely requires tissue factor. The latter, a lipid-dependent glycoprotein, is not normally present in the blood. Upon tissue injury, however, coagulation is initiated by the activation of factors IX and X. The relationship of these events to the possibility that hemophilia A and B are tissue factor-dependent diseases is discussed.