Polypeptide composition and amino-terminal sequence of broad bean polyphenoloxidase

Polyphenoloxidase was purified from broad bean (Vicia faba) leaves. The purified enzyme contained two immunocross-reactive proteins of approximately 60 to 65 and 43 to 45 kilodaltons. Further electrophoretic separation resolved these proteins into doublets with molecular mass of 61.5, 60, 44.5, and 43 kilodaltons, respectively. Each of the four polypeptides was transferred to Immobilon and subjected to microprotein sequencing. All the polypeptides showed the same amino acid sequence up to residue 9 but some variations occurred thereafter. The amino-terminal sequence contained a large number of proline and serine residues. These results suggest that the four polypeptides were derived from a common parent form and that a posttranslational modification(s) must have occurred to account for the difference in their apparent size.