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A wide range of chemicals with diverse structures derived from plant and environmental origins are reported to have hormonal activity. The potential for appreciable exposure of humans to such substances prompts the need to develop sensitive screening methods to quantitate and evaluate the risk to the public. Yeast cells transformed with plasmids encoding the human estrogen receptor and an estrogen responsive promoter linked to a reporter gene were evaluated for screening compounds for estrogenic activity. Relative sensitivity to estrogens was evaluated by reference to 17 beta-estradiol (E2) calibration curves derived using the recombinant yeast cells, MCF-7 human breast cancer cells, and a prepubertal mouse uterotrophic bioassay. The recombinant yeast cell bioassay (RCBA) was approximately two and five orders of magnitude more sensitive to E2 than MCF-7 cells and the uterotrophic assay, respectively. The estrogenic potency of 53 chemicals, including steroid hormones, synthetic estrogens, environmental pollutants, and phytoestrogens, was measured using the RCBA. Potency values produced with the RCBA relative to E2 (100) included estrone (9.6), diethylstilbestrol (74.3), tamoxifen (0.0047), alpha-zearalanol (1.3), equol (0.085), 4-nonylphenol (0.005), and butylbenzyl phathalate (0.0004), which were similar to literature values but generally higher than those produced by the uterotrophic assay. Exquisite sensitivity, absence of test compound biotransformation, ease of use, and the possibility of measuring antiestrogenic activity are important attributes that argue for the suitability of the RCBA in screening for potential xenoestrogens to evaluate risk to humans, wildlife, and the environment.ImagesFigure 1.Figure 2.Figure 3.Figure 4.

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Imizol injection (imidocarb) is used for the prevention and treatment of babesiosis in cattle. Studies in sheep indicate that imidocarb is retained in edible tissues (Aliu et al.). In the present study we have set up and validated a high-performance liquid chromatography based method to investigate the retention of imidocarb in cattle liver. Imidocarb was still detectable 224 d after a single therapeutic dose with a half-life of 42.7 d. The mechanism of imidocarb retention by bovine liver was modelled using isolated bovine hepatocytes. Incubations with isolated hepatocytes indicated that [14C]imidocarb binding was dependent on hepatocyte number and showed signs of saturation. Bound [14C]imidocarb could be eluted from hepatocytes with buffer and extracted with solvents. Equilibrium dialysis under denaturing conditions (Sun and Dent) indicated that 3% of the [14C]imidocarb was covalently bound to macromolecules. Although the hepatocyte preparations demonstrated the capacity for phase I and II 7-ethoxycoumarin metabolism no metabolites of [14C]imidocarb were found. Further in vitro binding studies involving sub-cellular fractionation indicated that [14C]imidocarb is partitioned largely in the nuclear fraction of bovine liver homogenates and that it binds to deoxyribonucleic acid.