Equol: a contributor to enigmatic immunoassay measurements of estrogen

The efficacy of radioimmunoassay (RIA) for the measurement of estradiol-17 beta (E2) in murine plasma was investigated. When Sephadex LH-20 or celite column chromatography was used to separate E2 from estrone (E1) and other cross-reacting compounds, the results were erratic if small volumes of mouse plasma were resolved. Assay of a diethyl ether extract of plasma (500 microL) was the most practical method for estimating the concentration of estradiol-17 beta in mice. This method was used to determine the pattern of estrogen secretion during the estrous cycle, on the day of implantation and during pregnancy. No convincing change in estrogen secretion was observed in the diestrous/proestrous mouse. By comparison, estrogen levels were elevated during pregnancy. Taken together, these results implied that cross-reactive components in plasma masked low levels of endogenous estrogen. Further evaluation of mouse plasma and urine using a co-chromatography technique to examine estrogen elution from a reverse-phase HPLC system followed by GC/MS analysis indicated the presence of equol [7-hydroxy-3-(4-hydroxyphenyl)chroman], a phytoestrogen metabolite with a ring structure similar to estradiol-17 beta. Equol and possibly other cross-reactive components of plasma may account for the apparent lack of increased estrogen secretion during the mouse estrous cycle and on the day of implantation as determined by the radioimmunoassay of ether extracts of plasma.