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Chemical Inhibition of the Iodide Recycling Enzyme Iodotyrosine Deiodinase, with Human and Xenopus Compariso
Citation:
Olker, J., J. Korte, J. Haselman, M. Hornung, AND S. Degitz. Chemical Inhibition of the Iodide Recycling Enzyme Iodotyrosine Deiodinase, with Human and Xenopus Compariso. SETAC North America, Fort Worth, TX, November 15 - 19, 2020. https://doi.org/10.23645/epacomptox.13114172
Impact/Purpose:
Iodotyrosine deiodinase (IYD) has an important role in vertebrate thyroid hormone homeostasis through catalyzing iodide recycling and promoting retention of iodide in thyroid follicular cells. A screening assay was recently developed to evaluate chemical inhibition of IYD using recombinant human enzyme in a 96-well plate format. This mammalian-based assay was used to screen over 1,800 chemicals from the ToxCast phase 1_v2, phase 2, and e1k chemical libraries. This presentation describes the comparison of the results for ~150 chemicals from the human IYD inhibition assay to those from a complementary assay testing inhibition of amphibian IYD. The strong cross-species agreement in inhibition of IYD enzyme activity suggest that results from the mammalian-based assay would be protective of chemical effects on the amphibian IYD enzyme. These assays can be used for future screening of large chemical libraries for inhibition of IYD activity. This work supports the U.S. EPA Endocrine Disruptor Screening Program’s need for higher-throughput screening assays to address additional molecular initiating events, beyond those currently in the ToxCast suite of assays, with the potential to disrupt normal thyroid function.
Description:
The enzyme iodotyrosine deiodinase (dehalogenase, IYD) is an important component for maintaining thyroid function through its role in conserving iodide, a scarce micronutrient critical to thyroid hormone’s molecular structure. Although historically there has been limited research on IYD, there are clear negative consequences with genetic mutations that cause failure of IYD, as described in human clinical studies dating back to the 1950s. Chemical inhibition of IYD in rats and amphibians in vivo has resulted in thyroid disruption, including reduced circulating thyroid hormones and altered development. Until recently, the susceptibility of IYD to chemical disruption by environmentally relevant chemicals remained largely unknown. The aim of this study was to expand evaluation of chemical inhibition of IYD and compare cross-species sensitivity to IYD inhibition. With recombinant human IYD (hIYD) and Xenopus laevis liver microsomal fractions (xIYD), 96-well plate in vitro assays were developed to screen chemicals for inhibition of mammalian and amphibian IYD enzyme activity. Screening of over 1,800 U.S. EPA ToxCast chemicals with the hIYD assay resulted in less than 200 (~11%) identified as potential IYD inhibitors (inhibited enzyme activity by 20% or greater when tested at 200 μM). 154 chemicals were further tested in concentration-response with both the hIYD and xIYD assays to determine IC50s, rank-order potency, and cross-species concordance of IYD inhibition. The vast majority of chemicals exhibited similar inhibition in both assays, with less than 25% variation in median inhibition for 120 of 154 chemicals and 85% concordance in categorization of ‘inactive’ versus potential IYD inhibitor. Rank-order potency was similar for many of the top inhibitors; however, IC50s varied between species across the full set of chemicals, with a few extreme examples. Most differences resulted from greater maximum inhibition or higher chemical potency in hIYD, suggesting that results from this mammalian-based assay would be protective of chemical effects on amphibian IYD. These two assays expand the coverage of molecular targets for which chemicals can be screened for potential thyroid disruption, and greatly increase the number of compounds that have been evaluated for inhibition of IYD. The strong cross-species agreement in IYD inhibition suggests that, in general, response of IYD activity to potential chemical inhibitors is conserved across these vertebrate species.
URLs/Downloads:
DOI: Chemical Inhibition of the Iodide Recycling Enzyme Iodotyrosine Deiodinase, with Human and Xenopus Compariso