Endocrine Abstracts

Enzymatic 5′- and 5-deiodination represent key pathways for local and systemic and local activation and inactivation of iodothyronines and iodothyronamines. Expression of the three respective deiodinases (DIO) is regulated by thyroid parameters, local and systemic inflammatory status as well as by selenium supply. In addition, the DIO isoenzymes are intended but also unanticipated target proteins of pharmacological, nutritive and environmental substances that might interfere with their proper function (endocrine disruptors). Therefore, determination of tissue-specific DIO activity is pertinent both for in vitro screening tests and for mammalian animal models which intend to systematically address safety issues with respect to the thyroid hormone axis. The classical radioactive DIO activity assay depends on the availability of ¹²⁵I-labeled substrates (e.g. ¹²⁵-rT₃) and monitors the release of 125-iodide. Therefore, an easily accessible, photometric method for determination of iodide release based on the Sandell–Kolthoff-reaction was combined with the classical deiodination assay. The protocol was optimized to minimize handling efforts, reaction volume and time. Since this method is not depending on radioactively labelled components, it expands the spectrum of the classical assay regarding the substrates to additional iodinated compounds. Comparison of DIO1 activities in tissue samples from hypo- and hyperthyroid mice and concentration–response experiments with PTU, an established Dio1 inhibitor, yielded the expected results. Incubation of iopanoic acid, a well-known, non-selective pan-inhibitor of all three DIO, with active DIO1 preparations revealed a significant release of iodide that was suppressible by PTU and exhibited features of an enzymatic reaction. We conclude that iopanoic acid itself might serve as substrate for DIO1, thereby contributing to the mobilization of iodide from the X-ray contrast agent and its re-utilization by the thyroid gland. This reaction might enhance the persistent iodide overload observed after administration of iodinated X-ray contrast agents.

This work is supported by the Bundesministerium für Bildung und Forschung.