Serine protease inhibitor that is strongly active against trypsin (1900 IU/mg) and moderately active against plasmin. Also shows weak inhibition against chymotrypsin (70%), elastase (38%) and the metalloprotease thermolysin (14%). Expressed by acrorhagi. Does not inhibit potassium channels (Kv), as well as cysteine proteases (papain and bromelain) (PubMed:18450492). Is not active against kallikrein (PubMed:9440231). Belongs to the venom Kunitz-type family. Sea anemone type 2 potassium channel toxin subfamily.