Evaluation of the Estrogenic Activity of Pueraria (Kudzu) Flower Extract and Its Major Isoflavones Using ER-Binding and Uterotrophic Bioassays

Abstract

Pueraria flower extract (PFE) is a hot water extract of the Kudzu flower ( Pueraria thomsonii ). Tea made from dried Kudzu flower is widely used inChina, and PFE is utilized as a nutritional supplement inJapan. PFE contains unique isoflavones such as 6-hydroxygenistein 6,7-di- O -glucoside (6HGDG), tectorigenin 7- O -xylosylglucoside (TGXG), and tectoridin. 6HGDG is known to be metabolized into 6-hydroxygenistein, and TGXG and tectoridin are known to be metabolized into tectorigenin in the digestive tract. Isoflavones typically mimic the effects β -estradiol has on estrogen receptors (ERs) and may influence the female genital system in the case of excessive intake. As a result, the upper limit of safe daily consumption of soy isoflavones has been enforced inJapan. In the present study, ER-binding assays were performed using the EnBio estrogen receptor/cofactor assay system to compare the estrogenic activity of 6-hydroxy genistein and tectorigenin to that of the soy isoflavone genistein. In addition, uterotrophic bioassays were performed to investigate the estrogenic effects of PFE in vivo . The ER-binding assays revealed that the ER-binding affinities of 6-hydroxygenistein and tectorigenin were approximately 0.01 - 0.04 that of genistein. Soy isoflavone products also induced an increase in uterine wet and blotted weight at doses of 500 mg/kg and 1000 mg/kg, whereas PFE did not cause adverse estrogenic effects, even at a dose of 1000 mg/kg. Based on these results, PFE does not appear to contain com pounds with strong estrogenic activity or cause adverse estrogenic effects in vivo . Importantly, the results of this study confirm the safety of PFE as a food supplement.