Microheterogeneity in Purified Broad Bean Polyphenol Oxidase

Polyphenoloxidase was purified from chloroplasts of broad bean leaves (Vicia faba L.) to apparent homogeneity. The enzyme was composed of two proteins with an apparent mass of 65 and 68 kilodaltons after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isolated enzyme contained covalently attached carbohydrates and bound concanavalin A, Phaseolus vulgaris erythroagglutinin, and Ricinus communis agglutinin lectins. Under native isoelectric focusing, several charged isoforms were present in the pH range of 4 to 6. Many, if not all, of the isoforms separated by isoelectric focusing were glycosylated and bound concanavalin A. All these isoforms shared a 65 kilodalton protein in common, and some of the isoforms were associated with both a 65 and 68 kilodalton protein. Isoforms separated by isoelectric focusing in the presence of 9 molar urea followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a similar pattern of proteins within a slightly higher pH range from 5 to 6.5.