Antioxidant efficacy of phytoestrogens in chemical and biological model systems.

Phytoestrogens (PEs) are diphenolic compounds from plants which can bind to estrogen receptors and have estrogen and antiestrogen effects in man and animals. Like other plant phenolics, PEs may have antioxidant properties through hydrogen/electron donation via hydroxyl groups. They might therefore act as free radical scavengers and inhibit development of coronary heart disease and cancers. The hydrogen-donating ability of a range of phytoestrogens was assessed using electron spin resonance spectroscopy, the ferric-reducing ability of plasma assay, and the Trolox equivalent antioxidant capacity. In addition, the ability of compounds to inhibit lipid peroxidation was examined in vitamin E-deficient liver microsomes. Genistein had the highest activity of the isoflavones; however, the isoflavones were relatively poor hydrogen donors compared with the other estrogenic compounds examined. Coumestrol and equol were more effective antioxidants than genistein but had relatively limited activity in comparison with Trolox. The only estrogenic compound with significant antioxidant activity was kaempferol which is better known as a dietary antioxidant than a phytoestrogen. As the concentrations of PEs used in this study exceed the estimated serum concentrations, their relatively poor antioxidant ability in vitro may indicate little significance as antioxidants in vivo.