Key Event Title
|Level of Biological Organization|
Key Event Components
|catalytic activity||type I iodothyronine deiodinase||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|DIO1i posterior swim bladder||MolecularInitiatingEvent|
|DIO1i anterior swim bladder||MolecularInitiatingEvent|
|DIO1 inhib alters metamorphosis||MolecularInitiatingEvent|
|fathead minnow||Pimephales promelas||NCBI|
Key Event Description
Disruption of the thyroid hormone system is increasingly being recognized as an important toxicity pathway, as it can cause many adverse outcomes. Thyroid hormones do not only play an important role in the adult individual, but they are also critical during embryonic development. Thyroid hormones (THs) play an important role in a wide range of biological processes in vertebrates including growth, development, reproduction, cardiac function, thermoregulation, response to injury, tissue repair and homeostasis. Numerous chemicals are known to disturb thyroid function, for example by inhibiting thyroperoxidase (TPO) or deiodinase (DIO), upregulating excretion pathways or modifying gene expression. The two major thyroid hormones are triiodothyronine (T3) and thyroxine (T4), both iodinated derivatives of tyrosine. The synthesis of the thyroid hormones is a process that involves several steps. Thyroglobulin, the thyroid hormone precursor, is produced by the thyroid epithelial cells and transported to the lumen via exocytosis. Then thyroperoxidase (TPO) plays an essential role in the production of mainly T4. The prohormone T4 is then released in the circulation under the influence of thyroid stimulating hormone (TSH), in order to be transported to the various tissues, including the liver, the kidneys and the heart. Most TH actions depend on the binding of T3 to its nuclear receptors. Active and inactive THs are tightly regulated by enzymes called iodothyronine deiodinases (DIO). The activation occurs via outer ring deiodination (ORD), i.e. removing iodine from the phenolic ring of T4 to form T3, while inactivation occurs via inner ring deiodination (IRD), i.e. removing iodine from the inner tyrosol ring of T4 or T3.
Three types of iodothyronine deiodinases (DIO1-3) have been described in vertebrates that activate or inactivate THs and are therefore important mediators of TH action. All deiodinases are integral membrane proteins of the thioredoxin superfamily that contain selenocysteine in their catalytic centre. Type I deiodinase is capable to convert T4 into T3, as well as to convert rT3 to the inactive thyroid hormone 3,3’ T2, through outer ring deiodination. rT3 is the preferred substrate for DIO1 (Hennemann G, Visser TJ 1997). Type II deiodinase (DIO2) is only capable of ORD activity with T4 as a preferred substrate. DIO3 can inner ring deiodinate T4 and T3 to the inactive forms of THs, reverse T3, (rT3) and 3,3’-T2 respectively
How It Is Measured or Detected
At this time, there are no approved OECD or EPA guideline protocols for measurement of DIO inhibition. Deiodination is the major pathway regulating T3 bioavailability in mammalian tissues. In vitro assays can be used to examine inhibition of deiodinase 1 (DIO1) activity upon exposure to thyroid disrupting compounds.
Several methods for deiodinase activity measurements are available. A first in vitro assay measures deiodinase activities by quantifying the radioactive iodine release from iodine-labelled substrates, depending on the preferred substrates of the isoforms of deiodinases. Another assay uses a chromatography-based method coupled to mass spectroscopy to measure products of thyroxin resulting from deiodinase type-1 activity (Butt et al., 2010). A colorimetric method (Renko et al., 2012), the Sandell-Kolthoff method, that measures the release of iodine from T4 is also available. Each of these assays requires a source of deiodinase which can be obtained for example using unexposed pig liver tissue (available from slaughterhouses) or rat liver tissue. Hornung et al. (2018) on the other hand used an adenovirus expression system to produce the DIO1 enzyme and developed an assay for nonradioactive measurement of iodide released using the Sandell-Kolthoff method in a 96-well plate format. This assay was then used to screen the ToxCast Phase 1 chemical library. The specific synthesis of DIO1 through the adenovirus expression system provides an important advantage over other methods where activity of the different deiodinase isoforms needs to be distinguished in other ways, such as based on differences in enzyme kinetics.
Domain of Applicability
Deiodination by DIO enzymes is known to exist in a wide range of vertebrates and invertebrates.
Evidence for Perturbation by Stressor
Overview for Molecular Initiating Event
Butt, C.M., Wang, D., Stapleton, H.M. 2011. Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases in human liver. Toxicological sciences 124: 339-347.
Hornung, M.W., Korte, J.J., Olker, J.H., Denny, J.S., Knutsen, C., Hartig, P.C., Cardon, M.C., Degitz, S.J., 2018. Screening the ToxCast Phase 1 Chemical Library for Inhibition of Deiodinase Type 1 Activity. Toxicological Sciences 162, 570-581.
Renko, K., Hoefie, C.S., Hiller, F., Schomburg, L., Köhrle, J. 2012. Identification of Iopanoic acid as substrate of type 1 deiodinase by a novel nonradioactive iodide-release assay. Endocrinology, 153: 2506-2513.
Visser, T.J., Van Overmeeren, E., Fekkes, D., Docter, R., Hennemann, G. 1979. Inhibition of iodothyronine 5'-deiodinase by thioureylenes: structure-activity relationship. FEBS Letters, 103, 2.