Alteration of enzyme activities and kinetic properties of GST and NQO1 with naturally occurring phenolic compounds

Objective: Glutathione S-transferase (GST) and NAD(P)H:quinine oxidoreductase 1 (NQO1) are the enzymes important in cytoprotection and bioactivation of chemicals. This study has addressed effects of polyphenolic compounds; ellagic acid, quercetin, naringenin, resveratrol, rutin and hesperidin on rabbit liver GST and NQO1 enzyme activities. Methods: Cytosolic fractions were obtained from homogenized liver tissues of rabbit by differential centrifugation. After calculating IC₅₀ values of individual enzymes for phenolic compounds, both Lineweaver-Burk and Dixon plots were drawn to determine the effect of phenolics on enzyme activity. Michaelis-Menten constant (K_m), maximum velocity (V_max), and inhibition constant (K_i) were calculated from Lineweaver-Burk and Dixon plots, respectively. Results: Resveratrol was found to be the most potent inhibitor for rabbit hepatic cytosolic GST activity with 75.9±2.06 μM IC₅₀ value while naringenin was the least potent one with IC₅₀ value of 260±1.92 μM. Hesperidin and quercetin were found to be the most and least potent inhibitors for NQO1 enzyme activity with IC₅₀ values of 2.7±0.85 μM and 13.8±0.91 μM, respectively. Resveratrol and naringenin inhibited GST activity noncompetitively and mixed type with K_i of 6.2 μM and 245 μM, respectively; while both hesperidin with 0.64 μM K_i value and quercetin with 3.5 μM K_i value inhibited NQO1 activity in a competitive manner. Conclusion: These results indicate that phenolic compounds may modulate Phase II enzyme, GST and NQO1. Moreover, they can influence the metabolic activation of xenobiotic and toxic compounds metabolized by this enzyme.