The present study investigated the 5'-monodeiodination of 3',5'-diiodothyronine (3',5'T 2) to 3'-monoiodothyronine (3'T 1), a reaction involving deiodination at the same position as is needed for T 4 to T 3 and for rT 3 to 3,3'-diiodothyronine (3,3'T 2) conversion. 3',5'T 2 to 3'T 1 conversion appears to be enzymatic in nature, being dependent on temperature and pH. Consistent with previous observations for T 4 and rT 3, 3',5'T 2 5'-deiodination occurred primarily in the liver and kidney, but, unlike these other iodothyronines, 3',5'T 2 to 3'T 1 conversion was active in cytosol as well as in microsomes. Characteristics of the microsomal enzymatic activity included heat inactivation (56°C), a broad pH plateau (pH 4.9-7.6), and an apparent K(m) of 8.6 x 10 -9 M. Cytosolic activity occurred with a narrower pH peak (pH 5-6.5), and had a different apparent K(m) of 2.4 x 10 -7 M. The activity of both subcellular enzymes was inhibited by iopanoic acid and prophylthiouracil. Dithiothreitol, while stimulating a 5- to 7-fold increase in microsomal 3',5'T 2 5'-deiodination, produced less than a 2-fold increase in cytosol activity. Fasting, a condition known to inhibit both T 4 and rT 3 5'-deiodination, produced a 68% decrease (1.16 ± 0.29 vs. 3.61 ± 0.97 ng/mg protein; mean ± SE) in hepatic T 4 to T 3 conversion, yet had no effect on 3',5'T 2 to 3'T 1 conversion in rat liver or kidney homogenates. Cellular fractionation studies indicated that liver microsomal 3',5'T 2 deiodination was diminished by 46% (71.6 ± 19.8 vs. 131.6 ± 14.4 pg/mg protein; P < 0.025) after a 3-day fast, whereas cytosol activity was unchanged (23.7 ± 10.2 vs. 22.8 ± 4.9 pg/mg protein). Treatment with T 4 (0.8 μg/100 g BW.day) failed to correct the reduced 3',5'T 2 5'-deiodinase activity in microsomes from fasted rats [79.7 ± 17.4 (fasted) vs. 311.9 ± 33.8 (control) pg/mg protein; P < 0.001]. However, the addition of 5 mM dithiothreitol to the incubation medium increased hepatic microsomal 5'-deiodinase activity for 3',5'T 2 7- and 12-fold in the fed and fasted animals, respectively. These results indicate that: 3',5'T 2 to 3'T 3 conversion proceeds enzymatically in liver and kidney, enzyme activities having different kinetics exist in both micromes and cytosol, fasting inhibits 3',5'T 2 deiodination in microsomes but not in homogenates or cytosol, and microsomal activity is restored by a sulfhydryl group-reducing agent, but not by physiological doses of T 4. The data strongly suggest the existence of more than one 3',5'T 2 5'-deiodinase in rat liver and kidney.
|Original language||English (US)|
|Number of pages||10|
|State||Published - 1981|
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism