Inhibition of glutathione reductase by plant polyphenols

The effects of forty-one plant polyphenols on the activity of glutathione reductase (GSH-RD) were studied. These polyphenols showed varying degrees of concentration-dependent inhibition on the enzyme, with IC50 values that varied from approximately 40 mu M to 1 mM. 4'-Hydroxychalcone and tannic acid were among the more potent inhibitors, with IC50 values of 47.3 and 50.4 mu M, respectively, Different classes of polyphenols varied in potency in the following order: chalcones > tannic acid > flavonoids > coumarins > catechins. Analysis of structure-activity relationships showed certain chemical structures to be important for the inhibition of GSH RD: (a) C-5 and C-7 hydroxylations in the A-ring, a carbonyl group at C-4, and the B-ring attached to C-2 in flavonoids; (b) C-2' and C-4' hydroxylations in chalcones; and (c) C-6 and C-7 hydroxylations in coumarins. The inhibition of GSH-RD by tannic acid and quercetin was time dependent and irreversible, whereas that by 4'-hydroxychalcone and esculin was reversible but not time dependent. Enhanced inhibition of GSH RD by the four polyphenols 4'-hydroxychalcone, quercetin, butein, and acacetin was observed in the presence of NADPH. Kinetic studies showed that both tannic acid and 4'-hydroxychalcone exhibited non-competitive inhibition on GSH-RD towards glutathione disulfide. (C) 1997 Elsevier Science Inc.