The phytoestrogen equol increases nitric oxide availability by inhibiting superoxide production: an antioxidant mechanism for cell-mediated LDL modification

Estrogen replacement therapy (ERT) is reported to lower the incidence of cardiovascular disease in postmenopausal women. ERT also lowers the levels of oxidatively modified low-density lipoprotein (LDL). Because modified LDL can mediate the development of atherosclerosis by inflammatory processes, ERT may exert its LDL protective effect through enhanced antioxidant activity in vascular tissues. Plant sources of estrogenic compounds have been used as alternatives for ERT because they avoid a number of negative health effects produced by estrogen. In this study, the antioxidant properties of the soy isoflavone metabolite, equol (an estrogenic metabolite of daidzein) were studied. Equol has a greater antioxidant activity than the parent isoflavone compounds genistein and daidzein, found in high concentration in soy. Equol inhibits LDL oxidation in vitro and LDL oxidative modification by J774 monocyte/macrophages to LDL−, an electronegative modified LDL found in human plasma. An antioxidant effect of equol was found to be mediated by inhibition of superoxide radical (O2−•) production and manifested through enhanced levels of free nitric oxide (NO) that prevents LDL modification. Thus, when NO levels were increased by donor agents, generators, or compounds that facilitate nitric oxide synthase activity, LDL− formation by J774 cells was strongly inhibited. Conversely, inhibition of NO production enhanced LDL− formation, and the combination of reduced NO and increased O2−• production yielded maximum LDL− formation. Pretreatment of cells with equol inhibited production of O2−• by J774 cells apparently via the inactivation of the reduced nicotinamide adenine dinucleotide phosphate oxidase complex. Decreased O2−• production resulted in increased free NO levels (but not total NO production) indicating that decreased reactions between O2−• and NO are an outcome of equol’s antioxidant activity in cell culture.