Partial Purification and Characterisation of Polyphenol oxidase from Two Species of Bitter Yam ( Discorea dumentorum )

Abstract

Polyphenol oxidases (PPO) are group of copper proteins, widely distributed phylogenetically from bacteria to mammals that catalyze the oxidation of phenolics to quinones which produces brown pigments in wounded tissues. PPO was isolated and partially characterized in white and yellow bitter yam (Discorea dumentorum). Extracts were partially purified using ammonium sulphate. The effects of optimum pH and temperature were investigated, while thermal and pH stability was also determined. The substrate specificity, kinetic and inhibition studies were also carried out. The optimal pH in the absence and presence of SDS were found to be 5.0 and 6.0, and 6.0 and 8.0 for white and yellow bitter yam respectively, while the optimum temperature for both white and yellow bitter yam was at 30°C. The enzyme was stable with over 90% activity after 3 hours incubation time at pH of 4.0 and 5.0, and 4.0 for yellow and white bitter yam respectively. The enzyme revealed over 70% remaining activities at all temperature investigated for yellow and at 30, 40 and 50°C for white bitter yam after 1 hour incubation. The Km values of 6.8 x 10-2mM and 8.0 x 10-2mM, and 3.3 x 10-1mM and 4.0 x 10-1mM with L-DOPA and tyrosine as substrates for PPO from white and yellow bitter yam respectively. Higher activity was observed with L-DOPA than with tyrosine.