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 microM [125I]T4, [125I]T3, [125I]rT3, or
[125I]3,3'-T2 and 50 microM 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