Daidzein and genistein glucuronides in vitro are weakly estrogenic and activate human natural killer cells at nutritionally relevant concentrations

Daidzein and genistein glucuronides (DG and GG), major isoflavone metabolit es, may be partly responsible for biological effects of isoflavones, such a s estrogen receptor binding and natural killer cell (NK) activation or inhi bition. DG and GG were synthesized using 3-methylcholanthrene-induced rat l iver microsomes. The K-m and V-max for daidzein and genistein were 9.0 and 7.7 mu mol/L, and 0.7 and 1.6 mu mol/(mg protein min), respectively. The ab sence of ultraviolet absorbance maxima shifts in the presence of sodium ace tate confirmed that the synthesized products were 7-O-glucuronides. DG and GG were further purified by a Sephadex LH-20 column. DG and GG competed wit h the binding of 17 beta-(H-3) estradiol to estrogen receptors of B6D2F1 mo use uterine cytosol. The concentrations required for 50% displacement of 17 beta-(H-3) estradiol (CB50) were: 17 beta-estradiol, 1.34 nmol/L; diethyls tilbestrol, 1.46 nmol/L; daidzein, 1.6 mu mol/L; DG, 14.7 mu mol/L; geniste in, 0.154 mu mol/L; GG, 7.27 mu mol/L. In human peripheral blood NK cells, genistein at <0.5 mu mol/L and DG and GG at 0.1-10 mu mol/L enhanced NK cel l-mediated K562 cancer cell killing significantly (P < 0.05). At > 0.5 mu m ol/L, genistein inhibited NK cytotoxicity significantly (P < 0.05). The glu curonides only inhibited NK cytotoxicity at 50 mu mol/L. Isoflavones, and e specially the isoflavone glucuronides, enhanced activation of NK cells by i nterleukin-2 (IL-2), additively. At physiological concentrations, DG and GG were weakly estrogenic, and they activated human NK cells in nutritionally relevant concentrations in vitro, probably at a site different from IL-2 a ction.