3′5′-Diiodothyronine to 3′-Monoiodothyronine Conversion in the Fed and Fasted Rat: Enzyme Characteristics and Evidence for Two Distinct 5′-Deiodinases

The present study investigated the 5′-monodeiodination of 3′5′-diiodothyronine (3′, 5′T₂) to 3′-monoiodothyronine (3′T₁), a reaction involving deiodination at the same position as is needed for T₄ to T₃ and for rT₃ to 3, 3′-diiodothyronine (3, 3′T₂) conversion. 3′5′T₂ to 3′T₁ conversion appears to be enzymatic in nature, being dependent on temperature and pH. Consistent with previous observations for T₄ and rT₃, 3′5′T₂ 5′-deiodination occurred primarily in the liver and kidney, but, unlike these other iodothyronines, 3′5′T₂ to 3′T₁ 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 × 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 × 10^-7 M. The activity of both subcellular enzymes was inhibited by iopanoic acid and propylthiouracil. Dithiothreitol, while stimulating a 5-to 7-fold increase in microsomal 3′5′T₂ 5′-deiodination, produced less than a 2-fold increase in cytosol activity. Fasting, a condition known to inhibit both T₄ and rT₃ 5′-deiodination, produced a 68% decrease (1.16 ± 0.29 vs. 3.61 ± 0.97 ng/mg protein; mean ± SE) in hepatic T₄ to T₃ conversion, yet had no effect on 3′, 5′T₂ to 3′T₁ conversion in rat liver or kidney homogenates. Cellular fractionation studies indicated that liver microsomal 3′, 5′T₂ 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 T4 (0.8 μg/100 g BW.day) failed to correct the reduced 3′, 5′T₂ 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′T₂ 7- and 12-fold in the fed and fasted animals, respectively. These results indicate that: 1) 3′5′T₂ to 3′T₁ conversion proceeds enzymatically in liver and kidney, 2) enzyme activities having different kinetics exist in both microsomes and cytosol, 3) fasting inhibits 3′5′T₂ deiodination in microsomes but not in homogenates or cytosol, and 4) microsomal activity is restored by a sulfhydryl group-reducing agent, but not by physiological doses of T₄. The data strongly suggest the existence of more than one 3′5′T₂ 5′-deiodinase in rat liver and kidney.