Isolation and Characterization of Polyphenol Oxidase Isozymes of Clingstone Peach

Abstract

The polyphenol oxidase system in clingstone peach (Prunus persica) was investigated. Polyacrylamide disc-gel electrophoresis indicated four bands with polyphenol oxidase activity in extracts from acetone powder of clingstone peach. These four isozymes were then isolated from a buffer extract of peach acetone powder by cold acetone precipitation, followed by diethylaminoethyl cellulose column chromatography. All isozymes had different heat stabilities. At 55 C, polyphenol oxidases A, B, and D had half-lives of 5.4, 14.6, and 14.1 minutes, respectively. Polyphenol oxidase C was stable over a period of 50 minutes of incubation at 55 C, but had a half-life of 2.2 minutes at 76 C. None of the isozymes had monophenolase activity, and they varied in their specificity for several diphenols. The following values were found for polyphenol oxidases A, B, C, and D, respectively, with catechol as substrate: optimal pH: 6.8, 6.5, 7.2, and 7.0; Michaelis constant: 6.6, 4.2, 7.0, and 36 mm; V_max/(E₀): 4.95, 39.4, 2.16, and 80.0 (ΔA min⁻¹ mg⁻¹). Each isozyme showed a different amount of inhibition by NaHSO₃, NaCl, NaCN, l-ascorbic acid, glutathione, ethylenediaminetetraacetate, and sodium diethyldithiocarbamate.