A NOVEL RADIOIMMUNOASSAY FOR DAIDZEIN

A radioimmunoassay for daidzein was established, based on polyclonal a ntibodies against daidzein-4'-O-(carboxymethyl)ether-BSA. The sensitiv ity of the assay was 0.4 pg/tube; the intra- and interassay coefficien ts of variation varied from 4.1 to 11.5% and from 5.6 to 21.7%, respec tively, depending upon the method (direct or extraction) and concentra tion of daidzein in the sample. The cross reactivities with other chem ically related compounds, with the exception of 4'-derivatives of daid zein, were 2.4% for dihydrodaidzein, 1.3% for genistein, 1.5% for bioc hanin A, and 1.6% for equol, respectively. The method was used for mea surement of daidzein levels in 105 normal human subjects and in three volunteers after consumption of a meal prepared from 125 g of cooked w hole soybeans. The daidzein values obtained following diethyl ether ex traction of human sera was only 8% of that obtained by direct radioimm unoassay. We suggest that this difference is caused by cross-reacting daidzein 4'-glucuronides and -sulfates present in serum. Using ether e xtraction, the basal serum levels of free daidzein were 0.11 ng/mL (0. 43 nmol/L), with 14 subjects showing no detectable levels. Levels were detectable in all subjects with the direct assay with a mean value of 7.1 ng/mL (28.0 nmol/L). Peak levels were reached 4 hours after inges tion of the soybeans. The levels were 10.3 +/- 3.6 ng/mL (40.4 +/- 14. 3 nmol/L) for free daidzein and 129.4 +/- 36.1 ng/mL (509 +/- 142 nmol /L) for total immunoreactive compounds. After 24 hours, the levels wer e still clearly distinguishable from the basal levels; the concentrati ons were 0.43 +/- 0.15 ng/mL (1.69 +/- 0.59 nmol/L) and 24.36 +/- 6.07 ng/mL (95.9 +/- 23.9) for free and total immunoreactive material, res pectively. It is concluded that this is the first immunoassay for a ph ytoestrogen in human biological fluids, and for the first time serial assays of unconjugated daidzein in plasma have been possible. (C) 1997 by Elsevier Science Inc.