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In this study, the endocrine‐disrupting (ED) potency of metabolites from brominated flame retardants (BFRs) was determined. Metabolites were obtained by incubating single‐parent compound BFRs with phenobarbital‐induced rat liver microsomes. Incubation extracts were tested in seven in vitro bioassays for their potency to compete with thyroxine for binding to transthyretin (TTR), to inhibit estradiol‐sulfotransferase (E2SULT), to interact with thyroid hormone‐mediated cell proliferation, and to (in‐)activate the androgen, progesterone, estrogen, or aryl hydrocarbon receptor. For most BFRs, TTR‐binding potencies, and to a lesser extent E2SULT‐inhibiting potencies, significantly increased after biotransformation. Microsomal incubation had less pronounced effects on other ED modes of action, due to low biotransformation efficiency and background activities determined in control incubations without BFRs …