Endocrinology, Vol 131, 479-484, Copyright © 1992 by Endocrine Society

ARTICLES

Selenium deficiency and type II 5'-deiodinase regulation in the euthyroid and hypothyroid rat: evidence of a direct effect of thyroxine

JP Chanoine, M Safran, AP Farwell, P Tranter, DM Ekenbarger, S Dubord, S Alex, JR Arthur, GJ Beckett and LE Braverman
Department of Nuclear Medicine, University of Massachusetts Medical Center, Worcester 01655.

Selenium deficiency in rats is characterized by elevated serum T4 and decreased serum T3 concentrations, and low liver type I (5'D-I) and brain type II (5'D-II) iodothyronine 5'-deiodinase activities. These findings are partially explained by the demonstration that type I 5'D is a selenoprotein; however, 5'D-II does not contain selenium. Since 5'D-II varies inversely with serum T4 concentrations, and serum T4 is elevated in selenium deficiency, the decreased cerebrocortical 5'D-II activity may be secondary to the increased serum T4 levels. To determine the mechanism(s) by which selenium influences 5'D-II activity, we examined the effects of altered selenium intake on brain 5'D-II levels and enzyme turnover in euthyroid and thyroidectomized rats. Rats were fed a selenium-supplemented or selenium-deficient diet for 5 weeks from weaning; half of the animals were also thyroidectomized 3 weeks before death. Selenium deficiency was confirmed by decreased liver and brain glutathione peroxidase activities. In euthyroid rats, selenium deficiency caused a 38% increase in serum T4, and 91% and 39% decreases in 5'D-I and 5'D-II, respectively, compared to those in selenium-supplemented rats. In the thyroidectomized hypothyroid rats, selenium deficiency caused a 60% decrease in 5'D-I, but had no effect on 5'D-II activity, fractional turnover of the enzyme, or the calculated enzyme synthesis rate. The lack of effect of selenium deficiency on 5'D-II levels in hypothyroid rats is consistent with the finding that 5'D-II is not a seleno-enzyme. Thus, the decrease in brain and pituitary 5'D-II activity in selenium- deficient euthyroid rats is due to the T4-dependent increase in the turnover of the enzyme polypeptide.

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