Kinetics Properties of Polyphenol Oxidase in Hawthorn (<i>Crataegus spp</i>)

Abstract

Polyphenol oxidase (PPO) from hawthorn was extracted and partially purified through (NH4)2SO4 precipitation, dialysis and ion exchange chromatography. The activity of polyphenol oxidase was investigated in Crataegus spp. Spectrophotometric method was used to assay the enzyme activity and the kinetic constants - maximum enzyme velocity (Vmax) and Michealis - Menten constant (Km). Of the substrates tested, catechol was the best substrate for PPO with a Km value of 2.2 mM. The optimum pH for PPO activity was found to be 7. The enzyme showed high activity over a broad pH range of 4 - 8. The optimal pH and temperature for enzyme activity were found to be 7 and 40-45 °C, respectively. km value for hawthorn PPO is calculated 22 mM for catechol and 6.7 mM for pyrogallol and 9.7 mM for L-dopa. As can be seen, affinity of PPOs for various substrates varies widely. The enzyme showed a broad activity over a broad pH and temperature range. The thermal inactivation studies showed that the enzyme is heat resistant. The enzyme showed the highest activity toward pyrogallol and no activity toward tyrosine. Of the inhibitors tested, the most potent inhibitors were kojic acid, cysteine and glycine , respectively