Inhibition of glutathione reductase by flavonoids. A structure-activity study.

A structure-activity study of fourteen chemically related flavonoids was conducted to evaluate their abilities to inhibit glutathione reductase (GR). By comparing the I50 values of flavonoids from different classes possessing an identical hydroxyl configuration, we determined the following order of potency for inhibition of GR: anthocyanidin > dihydroflavonol = chalcone > flavonol > catechin. Enzyme inhibition by delphinidin chloride and myricetin was partially prevented in a N2 atmosphere which implicates a role for oxygen in the mechanism of inhibition. To determine the role of oxygen species in enzyme inhibition, GR was preincubated with either mannitol, diethylenetriaminepenta-acetic acid (DETAPAC), superoxide dismutase (SOD), catalase (CAT), or SOD and CAT prior to assays for enzyme inhibition by flavonoids. Enzyme inhibition by delphinidin chloride and myricetin was suppressed by the addition of SOD, suggesting that superoxide (O2-.) is involved. However, inhibition by quercetin and morin was not sensitive to antioxidants. To further investigate the role of O2-. in GR inhibition, a superoxide generating system was utilized in the presence and absence of flavonoid. The O2-. generating system failed to inhibit GR in the absence of flavonoid but enhanced the inhibition by myricetin, indicating that the O2-. did not directly inhibit GR but reacted directly with certain flavonoids to form a reactive intermediate which, in turn, inhibited GR. These findings suggest that the mechanism of inhibition of GR by flavonoids is complex and may have oxygen-dependent and oxygen-independent components.