Potent inhibitor of serine proteases (chymotrypsin and trypsin). Inhibits tightly human leukocyte elastase (HLE). Does not inhibit papain, pepsin nor cathepsin D (cysteine and aspartic proteases). Protects the plant by inhibiting proteases of invading organisms, decreasing both hyphal growth and zoospores germination of Phytophthora infestans. Heterodimer of chains A and B; disulfide-linked. Double-headed inhibitor able to form triple complexes with target proteases, by binding simultaneously one molecule of trypsin and one molecule of chymotrypsin. Tubers. By infection with Phytophthora infestans. Probably synthesized as a single-chain precursor which is cleaved to form chain A and chain B. The serine protease inhibitor PSPI-21 comprises two protein species with pI 5.2 and 6.3, denoted as PSPI-21-5.2 and PSPI-21-6.3. They may be encoded by two alleles at the same gene locus. Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.