Iodothyronine 5'-deiodinase is present in mouse sublingual gland

Homogenates from male ddy mouse sublingual glands were incubated with 100 nM [125I]rT3 in the presence of 2 mM dithiothreitol (DTT). Metabolites were analyzed by HPLC or Dowex-50 minicolumn assay. 3,3'-Diiodothyronine and I- were the only appreciable products. Iodide release from rT3 was compatible with the enzymatic nature. The Km and maximum velocity from three separate determinations were 326, 356, and 629 nM and 29.8, 42.1, and 31.3 pmol I- release/mg protein.min, respectively. The deiodinase activity was DTT dependent and had higher affinity for rT3 than for T4 or T3. 6-Propyl-2-thiouracil inhibited the deiodination, which was competitively overcome by DTT. Sublingual 5'-deiodinase activity was approximately 80% of that in the liver, while the submandibular gland showed no deiodination. Our results show the presence of 5'-deiodinase (type I) in mouse sublingual gland for the first time. Selective localization and abundance of the enzyme suggest a previously unrecognized role of the sublingual gland in thyroid hormone physiology.