Iodothyronine sulfotransferase activity in rat uterus during gestation

In developing mammals, we and others demonstrated that sulfation is an impo rtant pathway in the metabolism of thyroid hormone, and there is significan t fetal-maternal transfer of sulfated iodothyronine. In the present study, we characterized a novel iodothyronine sulfotransferase (IST) in pregnant r at uterus. I-125-labeled 3,3'-diiodothyronine (T-2), T-3, rT(3), and T-4 we re used as substrates with unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor. Sulfated iodothyronine products were separated by Sephadex LH-20 column and further identified on reverse phase HPLC. We measured IST activity in pregnant rat uterus by incubating 1 muM substrate, 50 muM PAPS, and 50 mug cytosol protein, pH 7.2, 30 min at 37 degreesC. Th e results show that the substrate preference of the uterine IST activity is : T-2 > rT(3) > T-3 > T-4; the pH optimum is 6.0 for T-2. The K-m and V-max (for gestational day 21 uterus) for T-2 are 0.62 muM and 3466 pmol/mg prot ein/h, respectively; for PAPS the values are 2.6 muM and 1523 pmol/mg pmtei n/h, respectively. During pregnancy, the total activities exhibit a U-shape d curve with minimum activity at day 13 of gestation; while a thermostable activity increases significantly near term. In summary, there is significan t uterine IST that varies during pregnancy. The role of this uterine sulfot ransferase activities in regulating the bioavailability of thyroid hormone in the developing fetus remains to be elucidated.