Expression of Type I Iodothyronine 5′-Deiodinase in Xenopus laevis Oocytes

The enzymes mediating the conversion of thyroxine to 3,5,3'-triiodothyronine have proved difficult to purify and characterize biochemically. In this report we describe the successful expression in Xenopus laevis oocytes of type I iodothyronine 5'-deiodinase. Oocytes injected with rat liver mRNA and then incubated for 5 days demonstrated a progressive increase in 5'-deiodinase activity. This activity: (a) manifested a Km for 3,3',5'-triiodothyronine of 0.24 microM, (b) was sensitive to inhibition by 6-n-propyl-2-thiouracil, and (c) was associated with the membrane fraction of oocyte homogenates. Size fractionation of the mRNA by agarose gel electrophoresis under non-denaturing conditions resulted in the identification of a 1.5-2.0-kilobase fraction capable of inducing type I 5'-deiodinase activity. This oocyte assay system should provide a mechanism for identifying cDNA(s) encoding the enzymes involved in the peripheral metabolism of thyroid hormones.