Substrate specificity, heat inactivation and inhibition of polyphenol oxidase from different aubergine cultivars

Abstract

The effects of substrate specificity, heat inactivation, temperature, pH and inhibitors on polyphenol oxidase (PPO) activity obtained from three different aubergine cultivars were investigated to identify the most appropriate aubergine cultivar for dried preparations. PPO obtained from different aubergine cultivars (cultivars I, II and III) was partially purified by (NH4)2SO4 precipitation followed by dialysis. PPO showed activity with catechol and 4-methylcatechol but not with L-tyrosine. The best substrate for cultivar I (Vmax: 3333.3 EU min-1 mL-1, Km: 8.7 mM and Vmax/Km: 384.9 min-1) and cultivar III (Vmax: 1000 EU min-1 mL-1, Km: 9.3 mM and Vmax/Km: 107.5 min-1) was catechol, but 4-methylcatechol was the best substrate for cultivar II (Kmax: 5000 EU min-1 mL-1, Km: 35.5 mM and Vmax/Km: 140.8 min-1). The optimum pH for aubergine PPO was 7.0 with catechol as a substrate and 6.0 with 4-methylcatechol. Heat inactivation studies showed a decrease in enzyme activity at temperatures above 40 °C. For catechol and 4-methylcatechol substrates, the optimum temperature for maximum PPO activity was 30 °C for all aubergine cultivars except cultivar I using catechol which was 20 °C. The effects of compounds such as tropolone, D,L-dithiothreitrol and glutathione as inhibitors of the reactions catalysed by PPO were tested. Generally, tropolone was the most effective inhibitor.