RAPID SULFATION OF 3,3',5'-TRIIODOTHYRONINE IN NATIVE XENOPUS-LAEVIS OOCYTES

Sulfation is an important metabolic pathway facilitating the degradati on of thyroid hormone by the type I iodothyronine deiodinase. Differen t human and rat tissues contain cytoplasmic sulfotransferases that sho w a substrate preference for 3,3'-diiodothyronine (3,3'-T-2) > T-3 > r T(3) > T-4. During investigation of the expression of plasma membrane transporters for thyroid hormone by injection of rat liver RNA in Xeno pus laevis oocytes, we found uptake and metabolism of iodothyronines b y native oocytes. Groups of 10 oocytes were incubated for 20 h at 18 C in 0.1 mi medium containing 500,000 cpm (1-5 nM) [I-125]T-4, [I-125]T -3, [I-125]rT(3), or [I-125]3,3'-T-2. In addition, cytosol prepared fr om oocytes was tested for iodothyronine sulfotransferase activity by i ncubation of 1 mg cytosolic protein/ml for 30 min at 21 C with 1 mu M [I-125]T-4, [I-125]T-3, [I-125]rT(3), or [I-125]3,3'-T-2 and 50 mu M 3 '-phosphoadenosine-5'-phosphosulfate. Incubation media, oocyte extract s, and assay mixtures were analyzed by Sephadex LH-20 chromatography f or production of conjugates and iodide. After 20-h incubation, the per centage of added radioactivity present as conjugates in the media and oocytes amounted to 0.9 +/- 0.2 and 1.0 +/- 0.1 for T-4, less than 0.1 and less than 0.1 for T-3, 32.5 +/- 0.4 and 29.3 +/- 0.2 for rT(3), a nd 3.8 +/- 0.3 and 2.3 +/- 0.2 for 3,3'-T-2, respectively (mean +/- SE M; n = 3). The conjugate produced from rT(3) was identified as rT, sul fate, as it was hydrolyzed by acid treatment. After injection of oocyt es with copy RNA coding for rat type I iodothyronine deiodinase, we fo und an increase in iodide production from rT(3) from 2.3% (water-injec ted oocytes) to 46.2% accompanied by a reciprocal decrease in rT(3) su lfate accumulation from 53.7% to 7.1%. After 30-min incubation with cy tosol and 3'-phosphoadenosine-5'-phosphosulfate, sulfate formation amo unted to 1.8% for T-4, less than 0.1% for T-3, 77.9% for rT(3), and 2. 9% for 3,3'-T-2. These results show that rT(3) is rapidly metabolized in native oocytes by sulfation. The substrate preference of the sulfot ransferase activity in oocytes is rT(3) much greater than 3,3'-T-2 > T -4 > T-3. The physiological significance of the high activity for rT(3 ) sulfation in X. laevis oocytes remains to be established.