Structure and properties of the potato chymotrypsin inhibitor.

The potato tubers contain proteins that inhibit serine proteinases and belong to subfamily of potato Kunitz-type proteinase inhibitors (PKPI). New highly purified protein had been isolated from mature potato tubers (Solanum tuberosum L., cv. Zukov's Jubilee). The protein is а single polypeptide chain of molecular, weighing 23 kDa. The 20 N-terminal amino acid residues of the protein were determined by automatic Edman procedure. On the basis of N-terminal sequence structure and the molecular mass it is indicated that the inhibitor is a potato Kunitz-type serine proteinase inhibitor that belongs to group B. It was denoted as PKCI (potato Kunitz-type chymotrypsin inhibitor). The PKCI was able to inhibit chymotrypsin and trypsin with the same degree of effectiveness. It formed equimolar complexes with both enzymes. The protein PKCI is a double-headed proteinase inhibitor and is able to bind simultaneously two molecules of different enzymes. The probable cDNA of the inhibitor, denoted as PKPIJ-B consists of a 579-bp open reading frame, encodes protein out of 193-amino acids with a signal peptide (10 residues), indicating that this protein is synthesized as a preprotein. Deduced amino acid sequence of the cDNA is aligned with the respective sequences of already known PKPI, belonging to group B.