Serine protease inhibitor. Inhibits activated protein C (APC) with an IC(50) of 3.5 nM in the presence of heparin (to which it strongly binds) and an IC(50) of 88.9 nM in its absence. Other targets are also significantly inhibited in presence of heparin: trypsin (PRSS1) (45%), coagulation FXIa (F11) (40%), and plasmin (PLG) (70%). In vivo, synergizes with RVV-X in promoting coagulation in mice and by extension in Russell's viper bite patients. Expressed by the venom gland. Belongs to the venom Kunitz-type family.