Role of the membrane in the inactivation of factor Va by activated protein C.

Bovine factor Va inactivation by activated protein C (APC) was evaluated in the presence and absence of phospholipid vesicles and protein S. Following a 30-min incubation with APC (10 nM), membrane-bound factor Va (200 nM) is completely inactivated, whereas in the absence of phospholipid vesicles, after a 2-h incubation, the cofactor retains 60% of its initial cofactor activity. The complete loss of activity of membrane-bound factor Va is associated with the appearance of M(r) 40,000, 28,000, and 20,000 fragments derived from the heavy chain of the cofactor which correspond to cleavage at Arg306, Arg505, and Arg662. In the absence of a lipid bilayer, cleavage at Arg505 and Arg662 results in a cofactor with reduced activity. No difference is observed in the cleavage of the light chain of the cofactor by APC in the presence or absence of phospholipid vesicles. The rate of the cleavage of factor Va heavy chain at Arg306, Arg505, and Arg662 as well as the rate of the membrane-bound cofactor inactivation by APC were enhanced in the presence of protein S. Our data demonstrate that the anionic lipid-dependent cleavage of factor Va by APC at Arg306 is required for the complete inactivation of the cofactor.