ANTIOXIDANT ACTIVITIES OF ISOFLAVONES AND THEIR BIOLOGICAL METABOLITES IN A LIPOSOMAL SYSTEM

Genistein and daidzein, the two major soy isoflavones, principally occ ur in nature as their glycosylated or methoxylated derivatives, which are cleaved in the large intestine to yield the free aglycones and fur ther metabolites. The objective of this study was to compare the antio xidant activities of genistein and daidzein with their glycosylated an d methoxylated derivatives and also those of their human metabolites. The abilities of these compounds to inhibit lipid peroxidation in a li posomal system were evaluated using fluorescence spectroscopy, and str uctural criteria that enhance antioxidant activity were established, T he peroxidation initiators employed in the study were Fe(II) and Fe(II I) metal ions and aqueous-phase, ate-derived peroxyl radicals. Both th e parent isoflavonoids and their metabolites were more effective at su ppressing metal-ion-induced peroxidations than the peroxyl-radical-ind uced peroxidation, Antioxidant activities for the isoflavone metabolit es were comparable to or superior to those for the parent compounds. E quol and its 4-hydroxy and 5-hydroxy derivatives were the most potent antioxidants in the study, suggesting that absence of the 2,3-double b ond and the 4-oxo group on the isoflavone nucleus enhances antioxidant activity. Additionally, the number and position of hydroxyl groups we re determining factors for isoflavonoid antioxidant activity, with hyd roxyl substitution being of utmost importance at the C-4' position, of moderate importance at the C-5 position, and of little significance a t the C-7 position. (C) 1998 Academic Press.