Potent inhibitor of pancreatic carboxypeptidase A with a Ki of 27 nM, and of the serine proteases trypsin, chymotrypsin and pancreatic elastase, with Ki values of 6.9 nM, 1.83 nM and 4 nM, respectively. Does not inhibit cysteine and aspartic proteases. Expressed in the tentacle crown. Not detected in annelid body. Contains 9 disulfide bonds.