Radioimmunoassay Analysis of Phytoestrogens of Isoflavonoid Series

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R. Hampl*, O. Lapcik*, K. Wahala**, L. Starka*, and H. Adlercreutz***

* Institute of Endocrinology, Prague, ** Department of Chemistry, University of Helsinki, *** Department of Clinical Chemistry, University of Helsinky, Helsinki, Finland


Radioimmunoassay systems for the determination of 3 phytoestrogens of isoflavonoid series, daidzein, genistein, and equol, have been developed and evaluated. The determination of the first two isoflavonoids uses rabbit antisera to daudzein-4'-O-(carboxymethyl)ether, daidzein -7-O-(carboxymethyl)ether, genistein-7-O-(carboxymethyl)ether, and bridge and position homologous radioiodinated derivatives with tyrosine methyl ester as radioligands. The reagents for RIA of equol were prepared via 7-O-position. The former methods were suitable for assessment of free (unconjugated) daidzein and genistein in human serum and also for following the isoflavonoid content in sera of subjects after consumption of a meal from soy beans. The antisera prepared via the 4'-position recognized also 4'-methoxy derivatives of daidzein and genistein, formononetin, and biochanin A. In the case of genistein, very close results were obtained with the reagents prepared both via 4'-O- and 7-O-position. The bridge effect in equol determination, occurring when the bridge homologous reagents were used, could be overcome using position homologous but bridge heterologous reagents, differing in the length of the spacer used in the preparation of immunogen or tracer. The RIA system for equol was not sensitive enough for the detection of this compound in human serum.