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 a nimals, Like other plant phenolics, PEs may have antioxidant properties thr ough hydrogen/electron donation via hydroxyl groups. They might therefore a ct as free radical scavengers and inhibit development of coronary heart dis ease and cancers. The hydrogen-donating ability of a range of phytoestrogen s was assessed using electron spin resonance spectroscopy, the ferric-reduc ing ability of plasma assay, and the Trolox equivalent antioxidant capacity . in addition, the ability of compounds to inhibit lipid peroxidation was e xamined in vitamin E-deficient Fiver microsomes. Genistein had the highest activity of the isoflavones; however, the isoflavones were relatively poor hydrogen donors compared with the other estrogenic compounds examined. Coum estrol and equol were more effective antioxidants than genistein but had re latively limited activity in comparison with Trolox. The only estrogenic co mpound 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 significa nce as antioxidants in vivo. (C) 1998 Academic Press.