This highly disulfide-bonded protein is a potent inhibitor of factor Xa. May have therapeutic utility as an anticoagulant. Also exhibits a strong metastatic activity. Binds to heparin-agarose, binds to sulfated glycoconjugates. At least four isoforms of antistasin have been identified in leech salivary gland extracts, which differ by 1 or 2 amino-acid residues. Belongs to the protease inhibitor I15 (antistasin) family.