Biochemical Characterization of Two Polyphenol Oxidase Purified from Edible Yam (Dioscorea cayenensis-rotundata cv. Kponan) Cultivated in Côte d’Ivoire

Abstract

Yam (Dioscorea cayenensis-rotundata cv. Kponan) is a staple food in many tropical regions. A biochemical characterization study of crude polyphenol oxidase from yam (PPO) was carried out to provide useful information for food processing operations. Two polyphenol oxidases of edible yam (Dioscorea cayenensis-rotundata cv. Kponan) cultivated in Côte d’Ivoire were purified to homogeneity. The purification procedure consisted of ammonium sulphate fractionation, ion-exchange, size exclusion and hydrophobic interaction chromatography. The enzymes designated PPO1 and PPO2 had native molecular weights of approximately 113.7±0.34 and 115.65 ±1.78 kDa, respectively, and functioned as dimeric (PPO2) and monomeric (PPO1) structures. The two isoforms isolated had different optimum pHs and temperatures. The maximal activity of PPO1 occurred at 35ºC and pH 6.0. On the other hand, PPO2 had a maximum activity at 25ºC and pH 6.6. The enzymes were stable at their optimal temperatures (25ºC and 35ºC) and their pH stability was in the range of 5.6–7.0. Polyphenol oxidases (PPO1 and PPO2) remained their full activity in the presence of ion Mn2+, Fe2+, Na+ and Cu2+ but were inhibited strongly by the reducing agents as beta-mercaptoethanol, L-ascorbic acid, sodium bisulphite and L-cysteine. The values Vmax/Km showed that the enzymes have the greatest reactivity towards dopamine among the substrates used. The present work is therefore the first report on the purification to homogeneity of a yam tuber PPOs