FACTOR-V

Factor V is a single chain glycoprotein that plays an essential role i n the regulation of blood coagulation. After initiation of coagulation , factor V is converted into factor Va through limited proteolysis. Fa ctor Va acts as protein cofactor in the prothrombin-activating complex , which is comprised of the serine protease factor Xa, Ca2+ ions and a procoagulant membrane surface, Factor Va accelerates factor Xa-cataly sed conversion of prothrombin into thrombin more than 10(4)-fold. The cofactor activity of factor Va in prothrombin activation is down-regul ated by activated protein C (APC). The physiological importance of thi s regulatory pathway is demonstrated by the occurrence of hereditary t hrombophilia in individuals with a genetic defect that makes factor Va less sensitive to proteolytic inactivation by APC (APC resistance). C opyright (C) 1997 Elsevier Science Ltd.