Type 1 iodothyronine deiodinase is a selenium-containing enzyme

Expression cloning in the Xenopus oocyte has led to the isolation of a complimentary DNA for Type I iodothyronine deiodinase. Site-directed mutagenesis confirmed the presence of a selenocysteine amino acid in the deiodinase protein. These findings provide a molecular explanation of the effect of Se status on thyroid hormone activity.