The inhibition of human glutathione S-transferases activity by plant polyphenolic compounds ellagic acid and curcumin

Glutathione S-transferases (GSTs) are multifunctional detoxification proteins that protect the cell from electrophilic compounds. Overexpression of GSTs in cancer results in resistance to chemotherapeutic agents and inhibition of the over expressed GST has been suggested as an approach to combat GST-induced resistance. The inhibition of human recombinant GSTs by natural plant products was investigated in this study. Using 1-chloro-2,4 dinitrobenzene (CDNB) as a substrate, ellagic acid and curcumin were shown to inhibit GSTs A1-1, A2-2, M1-1, M2-2 and P1-1with IC50 values ranging from 0.04 to 5 μM whilst genistein, kaempferol and quercetin inhibited GSTs M1-1 and M2-2 only. The predominant mode of inhibition with respect to the G and H-sites were mixed inhibition and uncompetitive to a lesser extent. The Ki (Ki′) values for ellagic acid and curcumin with respect to GSH and CDNB were in the range 0.04–6 μM showing the inhibitory potency of these polyphenolic compounds. Ellagic acid and curcumin also showed time- and concentration-dependent inactivation of GSTs M1-1, M2-2 and P1-1 with curcumin being a more potent inactivator than ellagic acid. These results facilitate the understanding of the interaction of human GSTs with plant polyphenolic compounds with regards to their role as chemomodulators in cases of GST-overexpression in malignancies.