Partial Purification and Characterisation of Polyphenol Oxidase from Two Commonly Consumed Eggplants ( Solanum mellongena depressum and Solanum gilo ) in Nigeria

Abstract

Polyphenol oxidase (PPO) catalyzes in the presence of oxygen, the oxidation of mono- and di- phenols to o-quinones. The browning in fruits, vegetables and their processed products occurs as a result of this oxidation reaction. PPO in this study was isolated and characterized from two species of garden egg; Solanum mellongena depressum and Solanum gilo. Extracts were purified using a combination of ammonium sulphate precipitation, ion exchange chromatography on DEAE sephadex and gel filtration on Sephadex G-200 column. The effect of pH and temperature were carried out, pH and thermal stability, kinetic and substrate specificity, the effect of inhibitors and activator on the enzyme activity was investigated. About 6% protein yield for both species and a purification fold of 30.0 and 41.0 for S. depressum and S. gilo respectively was achieved. The optima pH of activity were found to be 4.0 – 4.5 and 7.0 for S. depressum and 4.0 and 8.0 for S. gilo while 4.5 and 8.0 were obtain in presence of SDS, the activity of the PPO increased in the presence of small concentration of SDS in all the pH investigated. The temperature optimum for both species was observed at 30°C. The PPO were stable at 30°C retaining about 88% and 87% of initial activity after 60minutes for S. depressum and S. gilo respectively while PPO from S. depressum was inactivated after 40min and 80°C and 70°C. A minimal remaining activity of 5% was observed at 80°C after 60min incubation time. PPO was fairly stable at pH 6.0 – 8.0 retaining a percentage remaining activity of 40.3% to 50% for S depressum and 45.5% to 52.1% for S. gilo. The PPO exhibited a marked activity towards o-diphenol and lower activities for monophenols. Ascorbic acid, EDTA and SDS inhibited enzymatic activity while the Km values using catechol, DOPA and catechin as substrate were 0.3mM, 0.095mM and 1.09mM for S. depressum and 1.9mM, 0.414mM and 0.56mM for S. gilo. PPO from S. depressum exhibited a higher enzymatic activity compared to S. gilo while S. gilo retained a fairly more stable activity.