Primary structure of potato Kunitz-type serine proteinase inhibitor

The serine proteinase inhibitor (PSPI-51) isolated from potato tubers (Sola num tuberosum L,) comprises two protein species with pi 5.2 and 6.3, denote d as PSPI-21-5.2 and PSPI-21-6.3, respectively. They were separated by anio n exchange chromatography on a Mono Q FPLC column. Both species tightly inh ibit human leukocyte elastase, whereas their interaction with trypsin and c hymotrypsin is substantially weaker. The sequences of both PSPI-21-5.2 and PSPI-21-6.3 were determined by analysis of overlapping peptides obtained fr om the oxidized or reduced and S-pyridylethylated proteins after digestion with trypsin or pepsin, Both species of PSPI-21 are composed of two chains, named chains A and B, which are linked by a disulfide bridge between Cys(1 46) and Cys(157). The other disulfide bridge is located within the A chains between Cys(48) and Cys(97). The amino acid sequences of the large A chain s of the two forms, consisting of 150 amino acids residues each, differ in a single residue at position 52. The small chains B, containing 37 and 36 r esidues in PSPI-21-6.3 and PSPI-21-5.2, respectively, have nine different r esidues. The entire amino acid sequences of the two inhibitors show a high degree of homology to the other Kunitz-type proteinase inhibitors from plan ts.