COMBINED TOXIC EFFECTS OF FIVE ESTROGENS ASSESSED BY YEAST ESTROGEN SCREEN ASSAY

Due to the wide presence of estrogens in aquatic environment and their negative ecological effects on wildlife and humans, they have become a major concern worldwide. However, the joint effect and reaction mechanism are not very clear, especially for various estrogens coexisting as a mixture at environmental levels. This study investigated the single and combined effects of five estrogens at different toxic units and molar ratios by employing the estrogen receptor binding as the endpoint. The results showed that all chemicals studied at single exposure could bind with estrogen receptor with a relative estrogenic effect of E-2>EE2>DES>E-1>E-3>OP>NP>BPA. The combined effects of binary, ternary, quaternary and quinary mixture were assessed by toxic unit, additive index and mixture toxicity index method. The result indicated that the types and strengths of combined effects were related to kinds and contents of chemicals in the mixture. Most estrogens mixture in this study exhibited a synergic effect. However, for EE2-E-1 mixture, it exhibited partial addition effect at EE2/E-1 toxic unit ratio of 1:1 and 1:4, while antagonistic effect at ratio of 4:1. We also found that the strength of synergic effect did not always increase with estrogen numbers in the mixture. The present study will provide useful information and reference for the ecological effect research and risk assessment of estrogens in actual aquatic environment.