Inhibits trypsin and chymotrypsin (serine proteases). Does not inhibit elastase, subtilisin, cathepsin L nor papain (serine and cysteine proteases). Protects the plant by inhibiting proteases of invading organisms, decreasing both hyphal growth and zoospores germination of Phytophthora infestans. Tubers. By infection with Phytophthora infestans. Has a single chain structure. Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.