Journal of Nutrition & Food Sciences

Journal of Nutrition & Food Sciences
Open Access

ISSN: 2155-9600



Kinetic Studies of Polyphenol Oxidase from White Yam (Dioscorea rotundata Poir) Tuber

Paul Chidoka Chikezie

Enzymatic browning is engendered by conversion of monophenols (cresolase activity) and ortho-diphenols (catecholase activity) to reactive ortho-quinones in the presence of molecular oxygen by polyphenol oxidases (PPOs). The present study ascertained the inhibition kinetics of PPO extracted from Dioscorea rotundata tuber in the presence of non-toxic sulfhydryl amino acid (L-cysteine) and appraised other connecting kinetic and thermodynamic properties of D. rotundata PPO (DR-PPO). DR-PPO was extracted and purified by (NH4)2SO4 precipitation, ultrafiltration and dialysis. Thermodynamic parameters and DR-PPO activity were measured by standard methods. Kinetic analyses showed that 4.0 mM L-cysteine displayed non-competitive inhibition prototype with DR-PPO experimental substrate. The affinity of L-cysteine for DR-PPO inhibitor binding site was ≈ 25 folds greater than that of catechol for the enzyme active site. Arrhenius plot gave an approximate first-order reaction with activation energy (Ea)=60.07 kJ mole-1. DR-PPO activity upon incubation at T=50°C for 5 min resulted to 28.11 ± 0.05% decay in relative enzyme activity. Likewise, within experimental pH values (pH 7.5-10.5 units), decay in relative DR-PPO activity ranged between 19.39 ± 0.01% - 65.11 ± 0.05%. Furthermore, pH/DR-PPO activity profile displayed three peak values, which suggests the presence of DR-PPO isoforms. The present study propose the use of L-cysteine as an alternative inhibitor for DR-PPO activity alongside with thermal inactivation (T>70°C for 15 min) and pH adjustment (pH<7.5 units for 15 min) to alleviative and serve as control measures against enzymatic browning reactions in D. rotundata tuber.