Interactions and inhibition of blood coagulation factor Va involving residues 311–325 of activated protein C

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Abstract
Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. The synthetic peptide‐(311–325) (KRNRTFVLNFIKIPV), derived from the heavy chain sequence of APC, potently inhibited APC anticoagulant activity in activated partial thromboplastin time (APTT) and Xa‐1‐stage coagulation assays in normal and in protein S‐depleted plasma with 50% inhibition at 13 μM peptide. In a system using purified clotting factors, peptide‐(311–325) inhibited APC‐catalyzed inactivation of factor Va in the presence or absence of phospholipids with 50% inhibition at 6 μM peptide. However, peptide‐(311–325) had no effect on APC amidolytic activity or on the reaction of APC with the serpin, recombinant [Arg358]α1‐antitrypsin. Peptide‐(311–325) surprisingly inhibited factor Xa clotting activity in normal plasma, and in a purified system it inhibited prothrombinase activity in the presence but not in the absence of factor Va with 50% inhibition at 8 μM peptide. The peptide had no significant effect on factor Xa or thrombin amidolytic activity and no effect on the clotting of purified fibrinogen by thrombin, suggesting it does not directly inhibit these enzymes. Factor Va bound in a dose‐dependent manner to immobilized peptide‐(311–325). Peptide‐(311–315) inhibited the binding of factor Va to immobilized APC or factor Xa. These data are consistent with the hypothesis that residues 311–325 in APC bind to factor Va at a site that can bind either APC or factor Xa, and that peptide‐(311–325) interferes with both APC inactivation of factor Va and expression of factor Xa activity in the prothrombinase complex by binding to this site.