Serine protease inhibitor that inhibits trypsin (Ki=73.8 nM) and chymotrypsin (Ki=993 nM). Contains three disulfide bonds. Does not inhibit potassium channels (Kv1.1/KCNA1, Kv1.2/KCNA2, Kv1.3/KCNA3, Kv1.4/KCNA4, Kv1.5/KCNA5, Kv1.6/KCNA6, Shaker, Kv2.1/KCNB1, Kv3.1/KCNC1, Kv4.2/KCND2, Kv4.3/KCND3, hERG/KCNH2) and TRPV1, the capsaicin receptors. Does not inhibit the serine proteases plasmin, thrombin, kallikrein, the cysteine proteinase papain, and the aspartic protease pepsin. Belongs to the venom Kunitz-type family. Sea anemone type 2 potassium channel toxin subfamily.