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 Km of 8.6 × 10-9 M. Cytosolic activity occurred with a narrower pH peak (pH 5–6.5), and had a different apparent Km 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 mi...