Inhibitor of papain (cysteine protease). Does not inhibit trypsin, chymotrypsin nor elastase (serine proteases). May protect the plant by inhibiting proteases of invading organisms. Tubers. Does not accumulate in tubers infected with Phytophthora infestans. Has a single chain structure. Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.