Two types of iodothyronine 5'-deiodination have been characterized previously in rat tissues. They can be distinguished by inhibition of type I but not type II by 6-npropylthiouracil, by the relative suitability of T4 and rT3 as substrates, rT3 much better than T4 for type I and T4 as good as, or better than, rT3 for type II, and by the concentration of T4 required to inhibit deiodination of rT3, 1-10 µm for type I and 1-10 nm for type II. Type I activity (6-n-propylthiouracil sensitive) is most abundant in liver and kidney. Type II activity has, to date, been identified only in the pituitary, central nervous system, and brown adipose tissue. Iodothyronine tyrosyl deiodination has also been identified in homogenates of rat brain, liver, and placenta. It is not clear how many different enzymes carry out this latter reaction. In the present studies, we have extended previous work by determining maturational patterns of the deiodinating pathways in several thyroid hormone-responsive rat tissues, possible modulation of those patterns by glucocorticoids, and the age of onset of responsivity of the deiodinases to hypothyroidism. Iodothyronine 5'-deiodinating activity was found in rat lung and eye, and the reaction was all or nearly all type I in both. Activity in the eye was virtually absent from the lens and vitreous humor. In immature rat cerebrum, pituitary, lung, and eye, between gestational day 17 and postnatal day 21, there was a uniform pattern of an increase in type 15'-deiodination activity over time, until adult levels were attained. The ages at which adult activity levels were reached varied from tissue to tissue, however. Type II activity was present at the earliest ages tested in the cerebrum (gestational day 17), pituitary, and brown adipose tissue (day of birth). In cerebral cortex, type II activity was highest at day 21 postnatally and equal at birth and in adulthood, and in pituitary and brown adipose tissue it was higher in adulthood than at birth. T3 tyrosyl ring deiodinating activity was several times greater in homogenates of eye and placenta than in cerebral homogenates. In all three tissues, there was similar, dose-dependent inhibition of [125I]T3 tyrosyl deiodination by 5 nm and 20 nm nonradioactive T3. In the eye and brain, T3 tyrosyl deiodination rates decreased progressively with age from gestational day 17 to postnatal day 7. Short term in utero hypothyroidism caused significant increases in type II5'-deiodination in pituitary and brown adipose tissue by the day of birth, similar to previous findings for cerebrum. However, this treatment caused no significant alteration in type I activity in pituitary or lung. Chronic hypothyroidism in adult rats caused a significant decrease in type I activity in lung, but had no effect on type I activity in the eye. Treatment with corticosterone acetate, 10 µg/g BW sc daily for 7 or 21 days beginning the day after birth, had no effect on type I or type II iodothyronine 5'-deiodination in cerebral cortex, pituitary, lung, or eye, or T3 tyrosyl deiodination in cerebral cortex, but did significantly inhibit hepatic rT3 5'-deiodination in 7-day rats. These results further demonstrate the diversity of responses to individual regulatory factors of the iodothyronine deiodinases in different tissues.
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