Characterization of the estrogenic properties of a nonsteroidal estrogen, equol, extracted from urine of pregnant macaques.

Recent reports of substantial urinary levels of equol in pregnant macaques and humans pose a concern, because equol poisoning in the ovine is characterized by an often permanent failure of reproductive processes. Equol (Fig. 1), a metabolite of phytoestrogens, is thought to act through estrogen receptors. The present study made a direct comparison of the estrogenic activity of equol from macaque urine, (+/-) equol and 17 beta-estradiol (E2) in vitro and in vivo. Relative binding affinity of equol for rat uterine receptor was 1% that of E2, and the dissociation rate of equol from the receptor was very high. Consistent with equol's binding properties in vitro, it was ineffective in stimulating rat uterine weight gain and possessed limited ability to increase progesterone receptor. Uterine nuclear receptors after doses of equol sufficient to produce depletion and replenishment of cytosol estrogen receptor were not measurable by exchange assay. No antiestrogenic activity of equol could be demonstrated. Equol's weak potency and lack of antiestrogenic activity are difficult to reconcile with its ability to induce ovine infertility. We conclude species differences at some level other than classical estrogen receptor as defined in the rat model are responsible for variability in equol's impact.