A Thyroid Hormone-Regulated Gene in Xenopus laevis Encodes a Type III Iodothyronine 5-Deiodinase

• Published in: Proceedings of the National Academy of Sciences - PNAS 1994-08, Vol.91 (16), p.7767-7771
• Main Authors: St Germain, D L, Schwartzman, R A, Croteau, W, Kanamori, A, Wang, Z, Brown, D D, Galton, V A
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Base Sequence; Biology; Cell lines; Cells, Cultured; Codons; Complementary DNA; Cultured cells; DNA Mutational Analysis; Enzymes; Gene Expression Regulation - drug effects; Genes; Iodide Peroxidase - genetics; Messenger RNA; Metalloproteins - genetics; Metalloproteins - metabolism; Metamorphosis, Biological - genetics; Molecular Sequence Data; Oocytes; RNA; Selenium; Sequence Homology, Amino Acid; Tadpoles; Tail - metabolism; Thyroid gland; Thyroid hormones; Thyroid Hormones - pharmacology; Triiodothyronine, Reverse - metabolism; Xenopus laevis - embryology; Xenopus laevis - genetics
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.91.16.7767

The type III iodothyronine 5-deiodinase metabolizes thyroxine and 3,5,3'-triiodothyronine to inactive metabolites by catalyzing the removal of iodine from the inner ring. The enzyme is expressed in a tissue-specific pattern during particular stages of development in amphibia, birds, and mammals. Recently, a PCR-based subtractive hybridization technique has been used to isolate cDNAs prepared from Xenopus laevis tadpole tail mRNA that represent genes upregulated by thyroid hormone during metamorphosis. Sequence analysis of one of these cDNAs (XL-15) revealed regions of homology to the mRNA encoding the rat type I (outer ring) 5'-deiodinase, including a conserved UGA codon that encodes selenocysteine in the mammalian enzyme. We report here that the protein encoded by the XL-15 cDNA efficiently catalyzes the (inner ring) 5-deiodination of 3,5,3'-triiodothyronine with a Kmvalue of 2 nM and is resistant to inhibition by propylthiouracil and aurothioglucose. Our analysis confirms that the UGA codon encodes a selenocysteine that is critical for the catalytic activity of the enzyme. In addition, the direct induction of XL-15 mRNA levels by thyroid hormone in X. laevis tadpole tail tissue and cultured cell lines correlates closely with increases in 5- (but not 5'-) deiodinase activity. These findings indicate that the XL-15 cDNA encodes a type III 5-deiodinase and underscores the importance of the trace element selenium in thyroid hormone metabolism.