Primary structure of a 21-kD protein from potato tubers

A 21-kD protein isolated earlier from potato tubers (Solanum tuberosum L.) has two isoforms, with pI 6.3 and 5.2, which were separated by fast protein ion-exchange chromatography on a Mono Q column. The primary structures of the two forms consisted of 187 and 186 amino acid residues. Both isoforms are composed of two polypeptide chains, designated A and B, linked by a single disulfide bond between Cys-146 of the A chain and Cys-7 of the B chain. The amino acid sequences of the A chains of the two forms, consisting of 150 residues each, differ in a single amino acid residue at position 52 (Val --> Ile), while the B chains, containing 37 and 36 residues, respectively, have substitutions at nine positions (Leu-8 --> Ser-8, Lys-25--Asp-26 --> Asn-25--Glu-26, Ile-31--Ser-32 --> Val-31--Leu-32, Lys-34--Gln-35--Val-36--Gln-37 --> Gln-34--Glu-35--Val-36). Both isoforms form stable inhibiting complexes with human leukocyte elastase and are less effective against chymotrypsin and trypsin.