propylthiouracil and thyroxine-sulfate

Recently, we identified significant amounts of thyroxine sulfate (T4S) in fetal sheep serum, meconium, bile, and amniotic and allantoic fluids. Little is known, however, about sulfate conjugation of thyroxine in humans. In this study, we employed a novel, sensitive T4S RIA to address this question. The rabbit antiserum was quite specific; T4, T3, rT3, and 3,3'-T2 showed less than 0.002% cross-reactivity. Other analogs cross-reacted less than 0.0001%. Only rT3S and T3S cross-reacted significantly (9.9% and 2.0%, respectively). The mean serum T4S concentration (ng/dL) was 8.6 in euthyroid subjects, 14.4 in hyperthyroid subjects, 5.0 in hypothyroid subjects, 5.9 in pregnancy, and 4.5 in patients with nonthyroid illnesses. T4S concentration in amniotic fluid from women at 18-19 weeks of gestation (25.5 ng/dL) was higher than that at 14-15 weeks of gestation (14.3 ng/dL). A significant rise in serum T4S was detected in hyperthyroid patients 1 day after ingestion of 1 g of ipodate. These data suggest that T4S is a normal component of human serum and amniotic fluid, and it is mostly derived from T4 peripherally and accumulates when type I 5'-monodeiodinating activity is low in fetuses or inhibited by drugs, such as ipodate.

Topics: Amniotic Fluid; Analysis of Variance; Female; Graves Disease; Humans; Ipodate; Pregnancy; Propylthiouracil; Radioimmunoassay; Sensitivity and Specificity; Thyroxine

The liver metabolizes T4 by deiodination and conjugation to T4 glucuronide (T4G), but little information exists about the formation of T4 sulfate (T4S) in vivo. We have examined the excretion of T4G, T4S, T3 and rT3 glucuronide (T3G and rT3G) in bile, collected under pentobarbital anesthesia 0-8 h or 17-18 h after iv [125I]T4 injection to control and 6-propyl-2-thiouracil (PTU)-treated rats. Radioactivity in bile, plasma, feces, and urine was analyzed by Sephadex LH-20 chromatography and HPLC. PTU induced a 2-fold increase in the biliary excretion of total radioactivity (26.6% vs. 15.0% dose between 0-8 h; 2.0% vs. 1.0% dose between 17-18 h). Biliary metabolites, 17-18 h after T4 injection, in control vs. PTU rats amounted to (percent dose): T4G, 0.44 vs. 0.75; T3G, 0.19 vs. 0.07; rT3G, 0.02 vs. 0.15; and T4S, 0.06 vs. 0.32. Similar results were obtained for control rats when bile was collected between 7-8 h after iv T4. The excretion rate of T3G was lower and that of rT3G higher when bile was continuously collected for 8 h immediately after T4 administration, probably due to prolonged experimental stress. However, regardless of the period of bile collection, PTU induced a more than 24-fold decrease in the T3G/rT3G ratio and a 5-fold increase in T4S excretion. In the animals killed 18 h after T4 injection, PTU treatment increased plasma T4 retention by 50%, reduced urinary I- excretion by 74%, and increased fecal radioactivity by 47%. No conjugates were detected in feces, and the distribution of fecal T4:T3:rT3 was 70:18:2 in control and 68:7:6 in PTU-treated rats. The results indicate that 1) the glucuronidative clearance of T4 is not affected by PTU; 2) the T3G/rT3G ratio in bile is a sensitive indicator of type I deiodinase inhibition; 3) T4 undergoes significant sulfation in rats in vivo, and 4) biliary excretion of T4S is enhanced if its type I deiodination is inhibited.

Topics: Animals; Bile; Chromatography, High Pressure Liquid; Glucuronates; Propylthiouracil; Rats; Thyroxine; Triiodothyronine

Previous studies have shown that the inner ring deiodination (IRD) of T3 and the outer ring deiodination (ORD) of 3,3'-diiodothyronine are greatly enhanced by sulfate conjugation. This study was undertaken to evaluate the effect of sulfation on T4 and rT3 deiodination. Iodothyronine sulfate conjugates were chemically synthetized. Deiodination was studied by reaction of rat liver microsomes with unlabeled or outer ring 125I-labeled sulfate conjugate at 37 C and pH 7.2 in the presence of 5 mM dithiothreitol. Products were analyzed by HPLC or after hydrolysis by specific RIAs. T4 sulfate (T4S) was rapidly degraded by IRD to rT3S, with an apparent Km of 0.3 microM and a maximum velocity (Vmax) of 530 pmol/min X mg protein. The Vmax to Km ratio of T4S IRD was increased 200-fold compared with that of T4 IRD. However, formation of T3S by ORD of T4S could not be observed. The rT3S formed was rapidly converted by ORD to 3,3'-T2 sulfate, with an apparent Km of 0.06 microM and a Vmax of 516 pmol/min X mg protein. The enzymic mechanism of the IRD of T4S was the same as that of the deiodination of nonsulfated iodothyronines, as shown by the kinetics of stimulation by dithiothreitol or inhibition by propylthiouracil. The IRD of T4S and the ORD of rT3 were equally affected by a number of competitive inhibitors, suggesting a single enzyme for the deiodination of native and sulfated iodothyronines. In conjunction with previous findings on the deiodination of T3S, these results suggest that sulfation leads to a rapid and irreversible inactivation of thyroid hormone.

Topics: Animals; Diiodothyronines; Dithiothreitol; Iodide Peroxidase; Iodine Radioisotopes; Kinetics; Microsomes, Liver; Peroxidases; Propylthiouracil; Rats; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse