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Outer and Inner Ring Monodeiodination of Thyroxine by Dog Thyroid and Liver: A Comparative Study Using a Particulate Cell Fraction^*

PETER LAURBERG and NIELS BOYE

Second University Clinic of Internal Medicine, Kommunehospitalet, DK-8000 Aarhus C, Denmark

Abstract

In previous studies using perfused dog thyroids we have found evidence for a modulation of thyroid secretion by intrathyroidal thyroxine deiodinases. The purpose of the present study was to examine in detail the in vitro characteristics of thyroidal 5- and 5'-deiodinases of T₄and to make a comparison to T₄ deiodinases from liver tissue. The sources of deiodinases were crude microsomal fractions prepared from thyroid and liver tissues obtained from the same dogs.

Deiodinases from the two tissues behaved very similarly with respect to the time course of T₃ and rT₃ generation, dependency on the amount of added microsomes and substrate, and influence of temperature, pH, dithiothreitol, propylthiouracil, methimazol, ipodate, and added rT₃. At pH 7.4, both T₃ and rT₃ were stable when added to the microsomal preparations. The apparent kinetic constants estimated from Lineweaver-Burk plots of T₄ deiodination to T₃ and rT₃ by liver and thyroid microsomes, were similar for all reactions, (kinetic constant, 6.1-25.1 µM). T₃ generation was maximal at pH 6.2, and rT₃ generation was maximal at pH 8.0. Even at pH 8.0, the generation of T₃ was severalfold higher than that of rT₃. In contrast to previous studies employing rat tissue, it was found that the addition of rT₃ did not modify the deiodination of T₄ to T₃ nless the rT₃ concentration exceeded that of T₄.

Thus, the canine thyroid contained T4 deiodinases very similar to those of a peripheral thyroid hormone target organ, the liver. These thyroidal deiodinases are probably responsible for the relatively high secretion of T₃ and rT₃ compared to that of T₄ found in our previous studies. Further, the discrepancies between these and previous data obtained using rat tissues demonstrate that in such in vitro studies, great species variations are present in the stability of generated rT₃ and the effect of rT₃ on outer ring deiodination of T₄ to T₃.

Footnotes

^* This work was supported by the Danish Medical Research Council, the P. Carl Pedersens Foundation, and the Institute of Experimental Clinical Investigations, University of Aarhus.

To whom requests for reprints should be addressed

Received May 28, 1981.

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