Extraction, partial purification and characterization of Xanthosoma caracu polyphenol oxidase

Polyphenol oxidase, a group of copper-containing enzymes responsible for the oxidation of monophenols to diphenols and diphenols to quinones are very important as their action sometimes produces undesirable results. In this study, polyphenol oxidase was extracted from Xanthosoma caracu tuber and its characteristics elucidated. The activity of X. caracu polyphenol oxidase was assayed by monitoring the increase in absorbance at 420nm at 30sec interval for 5mins using a spectrophotometer. The partially purified preparation of polyphenol oxidase extracted from X. caracu was purified 2.094 fold, recovered 37.33% andhad a specific activity of 1.60u min-1mg-1. The km and Vmax values, were found to be 27.03mM±4.31 and 0.17umin-1for catechol, 51.64mM±10.12 and 011umin-1 for p-cresol, and 31.25mM±3.58 and 0.14umin-1forpyrogallol.Maximum activity of X. caracu polyphenol oxidase was observed at temperature of 40°C and pH 7.0 with catechol and pyrogallol substrates, while with p-cresol substrate was 45°C and pH 6.5.The results on inhibition studies revealed mixed type inhibition by citric acid, while ascorbic acid was noncompetitive. Inhibitor constant with ascorbic and citric acids was 0.65mM and 1.7mM respectively. The presence of the browning enzyme, polyphenol oxidase was established in X. caracu with kinetic characteristics similar to those in other plant sources. Catechol was the best substrate, while ascorbic acid was the most potent inhibitor. At appropriate concentrations citric and ascorbic acids may be used to prevent enzymatic browning in X. caracu due to the action of polyphenol oxidases.