Surface plasmon resonance-based immunoassay for 17β-estradiol and its application to the measurement of estrogen receptor-binding activity

A rapid and simple surface plasmon resonance (SPR)-based immunoassay for detection of 17 beta-estradiol was developed. The assay was designed as an inhibitive format, in which 17 beta-estradiol-BSA conjugates are immobilized on an SPR sensor chip and the binding of antibody to the chip is measured. The binding was inhibited by 17 beta-estradiol in the concentration range 0.468 to 21.4 nmol L-1 with an IC50 value of 2.29 +/- 0.10 nmol L-1. Although not as sensitive as traditional radioimmunoassay (RIA) and enzyme-linked immunoassay (ELISA), this method requires no separation and washing after addition of the antibody, steps which are relatively time-consuming. Estrogen receptor (ER)-binding was then investigated using this SPR immunoassay for the determination of the amount of unbound 17 beta-estradiol after competition with test compounds for the ER-binding. Inhibition of the binding of 17 beta-estradiol to ER by diethylstilbestrol (DES) was successfully measured by injecting the reaction mixture into the SPR sensor after addition of the antibody. This binding assay requires no separation of unbound 17 beta-estradiol from the mixture and no radioisotope- or fluorescence-labeling of 17 beta-estradiol. These results show the potential usefulness of the SPR sensor both detecting 17 beta-estradiol and evaluating the ER-binding activity of xenoestrogens such as DES in a single assay system.