Thyroid Hormone Sulfation in Chicken and Axolotl

Abstract

Next to deiodination, glucuronidation and sulfation of the phenolic hydroxyl group are important pathways of iodothyronine metabolism. Although in general conjugation serves to facilitate the urinary and biliary excretion of lipophilic substances, sulfation of thyroid hormones has another important function. Sulfation protects the tissues against an excess of active thyroid hormone and it can also be used as a reservoir from which active hormone is released by sulfatases when and where it is required. Interaction between sulfation and deiodination has already been described in several mammalian tissues. After an initial characterization of the sulfotransferases we determined the changes in sulfation during chicken embryonic development and induced metamorphosis of the axolotl, a neotenous amphibian. Sulfation assays demonstrate the presence of sulfotransferases in liver, kidney and brain cytosol in one-day-old chicken. Similar to the situation in mammals, the enzyme(s) show a substrate preference for 3,3'-diiodothyronine (3,3'-T2) over 3,5,3'triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) over thyroxine (T4). Measurements of sulfation using 3,3'-T2 as substrate in tissues from 16-day-old embryonic chickens (E16) to one-day-old posthatch chicks (C1) demonstrate that sulfation activity varies independently in liver, kidney and brain, suggesting that the activity is regulated in a tissue specific manner. We also detected sulfation activity in liver, kidney and brain of the axolotl. The highest activity was found in the liver, hence we used this tissue to study the profile of activity during induced metamorphosis. We found that the activity in the liver remains stable during the full length of metamorphosis.