జర్నల్ ఆఫ్ న్యూట్రిషన్ & ఫుడ్ సైన్సెస్
అందరికి ప్రవేశం

నైరూప్య

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.