triiodothyronine-glucuronide and thyroxine-glucuronide

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

An enzymatic method for synthesis of labelled thyroxine glucuronide (T4G) and triiodothyronine glucuronide (T3G) from labelled thyroxine (T4) and triiodothyronine (T3) is presented. The synthetic glucuronides are completely digested by beta-glucuronidase, with recovery of the parent T4 or T3. They have distinctive elution patterns on HPLC and on Sephadex G25 chromatography, and can be clearly separated from T4 and T3 as well as from synthetic T4 sulfate (T4S) and T3 sulfate (T3S). On LH 20 chromatography, elution of T4G and T3G is intermediate between that of T4 and T3 and that of T4S and T3S. T3G can be well separated from other thyronines by HPLC alone, but T4G coelutes with rT3 on HPLC; these are then separated by adding a Sephadex G25 chromatography step. Biosynthetic 131I-T3G and 125I-T4G from the bile of a cat given 131I-T3 and 125I-T4 had similar HPLC chromatographic patterns to those of synthetic T3G and T4G. That the identified peaks from analysis of the bile were indeed T3G and T4G was confirmed by recovery of the parent T3 and T4 after beta-glucuronidase digestion.

Topics: Animals; Cats; Chromatography; Chromatography, High Pressure Liquid; Dextrans; Glucuronates; Glucuronidase; In Vitro Techniques; Iodine Radioisotopes; Sulfates; Thyroxine; Triiodothyronine