Deconjugation of soy isoflavone glucuronides needed for estrogenic activity

•Genistein and diadzein glucuronides (GG and DG) are not estrogenic.•Deconjugation of GG and DG to their corresponding aglycones renders them estrogenic.•The slight estrogenic activity of GG and DG in vitro is the result of intracellular deconjugation.•At realistic plasma concentrations of the SIF breast cancer cell proliferation was inhibited.•Deconjugation of GG and DG by rat S9 is 30 times faster than by human breast tissue S9. Soy isoflavones (SIF) are present in the systemic circulation as conjugated forms of which the estrogenic potency is not yet clear. The present study provides evidence that the major SIF glucuronide metabolites in blood, genistein-7-O-glucuronide (GG) and daidzein-7-O-glucuronide (DG), only become estrogenic after deconjugation. The estrogenic potencies of genistein (Ge), daidzein (Da), GG and DG were determined using stably transfected U2OS-ERα, U2OS-ERβ reporter gene cells and proliferation was tested in T47D-ERβ cells mimicking the ERα/ERβ ratio of healthy breast cells and inT47D breast cancer cells. In all assays applied, the estrogenic potency of the aglycones was significantly higher than that of their corresponding glucuronides. UPLC analysis revealed that in U2OS and T47D cells, 0.2–1.6% of the glucuronides were deconjugated to their corresponding aglycones. The resulting aglycone concentrations can account for the estrogenicity observed upon glucuronide exposure. Interestingly, under similar experimental conditions, rat breast tissue S9 fraction was about 30 times more potent in deconjugating these glucuronides than human breast tissue S9 fraction. Our study confirms that SIF glucuronides are not estrogenic as such, and that the small % of deconjugation in the cell is enough to explain the slight bioactivity observed for the SIF-glucuronides. Species differences in deconjugation capacity should be taken into account when basing risk–benefit assessment of these SIF for the human population on animal data.