The mechanism of inactivation of human factor V and human factor Va by activated protein C.

The cleavage of human factor V and human factor Va by human activated protein C (APC) was analyzed in the absence and presence of phospholipid vesicles containing 75% phosphatidylcholine (PC) and 25% phosphatidylserine (PS). Membrane-bound human factor V (250 nM) is cleaved by APC (2.5 nM) to give M(r) = 200,000, 70,000, 45,000, and 30,000 fragments and an M(r) = 22/20,000 doublet. These fragments are released after four sequential cleavages of the membrane-bound procofactor at Arg306, Arg506, Arg679, and Lys994. No cofactor activity is observed following thrombin treatment of the membrane-bound APC-cleaved procofactor. In the absence of a membrane surface, no cleavage of factor V by APC is observed, and following thrombin activation factor Va retains full cofactor activity. Membrane-bound human factor Va (600 nM) loses more than 90% of its initial cofactor activity after 10 min of incubation with APC (10.9 nM), and virtually no cofactor activity is observed after 1 h of incubation. Under similar conditions but in the absence of PCPS vesicles, factor Va is cleaved but retains approximately 80% of its initial cofactor activity after 2 h of incubation with APC. In the presence of PCPS vesicles, the APC related loss of activity is correlated with cleavage of the heavy chain and appearance of fragments of M(r) = 45,000, 30,000, and of 28/26,000, and 22/20,000 doublets. These products correspond to three cleavages of the heavy chain (at Arg306, Arg506, and Arg679). Cleavage at Arg506 of factor Va precedes and appears to be required for cleavage at Arg306 and Arg679. In the absence of membrane, proteolysis at Arg506 produces an M(r) = 75,000 fragment which corresponds to the NH2-terminal portion of the human factor Va heavy chain (residues 1-506), and a carboxyl-terminal doublet of M(r) = 28/26,000 (residues 507-709) which is cleaved by APC at Arg679 to generate an M(r) = 22/20,000 doublet and an M(r) = 6,000 peptide. No cleavage of the light chain of the human cofactor is observed in the presence or absence of PCPS vesicles following 2 h of incubation with APC. Our data demonstrate that inactivation of human factor V and human factor Va only occurs in the presence of a membrane surface after cleavage at Arg306. However, while this cleavage site is exposed on membrane-bound human factor V, cleavage at Arg506 on the heavy chain of factor Va appears necessary for complete exposure of the cleavage site at Arg306.