Xenopus laevis and human type 3 iodothyronine deiodinase enzyme cross-species sensitivity to inhibition by ToxCast chemicals.

Deiodinase enzymes are critical for tissue-specific and temporal control of activation or inactivation of thyroid hormones during vertebrate development, including amphibian metamorphosis. We previously screened ToxCast chemicals for inhibitory activity toward human recombinant Type 3 iodothyronine deiodinase enzyme (hDIO3) and subsequently produced Xenopus laevis recombinant dio3 enzyme (Xldio3) with the goals to identify specific chemical inhibitors of Xldio3, to evaluate cross-species sensitivity and explore whether the human assay results are predictive of the amphibian. We identified a subset of 356 chemicals screened against hDIO3 to test against Xldio3, initially at a single concentration (200 μM), and further tested 79 in concentration-response mode. Most chemicals had IC values lower for hDIO3 than for Xldio3 and many had steep Hill slopes (a potential indication of non-specific inhibition). However, eight of the most potent chemicals are likely specific inhibitors, with IC values of 14 μM or less, Hill slopes near -1 and curves not significantly different between species likely due to conservation of catalytically active amino acids. Controlling for assay conditions, human in vitro screening results can be predictive of activity in the amphibian assay. This study lays the groundwork for future studies using recombinant non-mammalian proteins to test cross-species sensitivity to chemicals. DISCLAIMER: The views expressed in this paper are those of the authors and do not necessarily reflect the views or policies of the U.S. Environmental Protection Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.