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Rapid Sulfation of 3,3',5'-Triiodothyronine in Native Xenopus laevis Oocytes

Departments of Internal Medicine III (E.C.H.F., R.D., E.P.K., M.E.E., G.H., T.J.V.) and Nuclear Medicine (E.P.K.), Erasmus University Medical School, Rotterdam, The Netherlands

Address all correspondence and requests for reprints to: Dr. Theo J. Visser, Department of Internal Medicine III, Room Bd234, Erasmus University Medical School, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. E-mail: visser{at}inw3.azr.nl

Sulfation is an important metabolic pathway facilitating the degradation of thyroid hormone by the type I iodothyronine deiodinase. Different human and rat tissues contain cytoplasmic sulfotransferases that show a substrate preference for 3,3'-diiodothyronine (3,3'-T₂) > T₃ > rT₃ > T₄. During investigation of the expression of plasma membrane transporters for thyroid hormone by injection of rat liver RNA in Xenopus laevis oocytes, we found uptake and metabolism of iodothyronines by native oocytes. Groups of 10 oocytes were incubated for 20 h at 18 C in 0.1 ml medium containing 500,000 cpm (1–5 nM) [¹²⁵I]T₄, [¹²⁵I]T₃, [¹²⁵I]rT₃, or [¹²⁵I]3,3'-T₂. In addition, cytosol prepared from oocytes was tested for iodothyronine sulfotransferase activity by incubation of 1 mg cytosolic protein/ml for 30 min at 21 C with 1 µM [¹²⁵I]T₄, [¹²⁵I]T₃, [¹²⁵I]rT₃, or [¹²⁵I]3,3'-T₂ and 50 µM 3'-phosphoadenosine-5'-phosphosulfate. Incubation media, oocyte extracts, and assay mixtures were analyzed by Sephadex LH-20 chromatography for production of conjugates and iodide. After 20-h incubation, the percentage of added radioactivity present as conjugates in the media and oocytes amounted to 0.9 ± 0.2 and 1.0 ± 0.1 for T₄, less than 0.1 and less than 0.1 for T₃, 32.5 ± 0.4 and 29.3 ± 0.2 for rT₃, and 3.8 ± 0.3 and 2.3 ± 0.2 for 3,3'-T₂, respectively (mean ± SEM; n = 3). The conjugate produced from rT₃ was identified as rT₃ sulfate, as it was hydrolyzed by acid treatment. After injection of oocytes with copy RNA coding for rat type I iodothyronine deiodinase, we found an increase in iodide production from rT₃ from 2.3% (water-injected oocytes) to 46.2% accompanied by a reciprocal decrease in rT₃ sulfate accumulation from 53.7% to 7.1%. After 30-min incubation with cytosol and 3'-phosphoadenosine-5'-phosphosulfate, sulfate formation amounted to 1.8% for T₄, less than 0.1% for T₃, 77.9% for rT₃, and 2.9% for 3,3'-T₂. These results show that rT₃ is rapidly metabolized in native oocytes by sulfation. The substrate preference of the sulfotransferase activity in oocytes is rT₃ >> 3,3'-T₂ > T₄ > T₃. The physiological significance of the high activity for rT₃ sulfation in X. laevis oocytes remains to be established.

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