CHARACTERIZATION OF IODOTHYRONINE SULFOTRANSFERASE ACTIVITY IN RAT-LIVER

Sulfation is an important pathway in the metabolism of thyroid hormone because it strongly facilitates the degradation of the hormone by the type I iodothyronine deiodinase. However, little is known about the p roperties and possible regulation of the sulfotransferase(s) involved in the sulfation of thyroid hormone. We have developed a convenient me thod for the analysis of iodothyronine sulfotransferase activity in ti ssue cytosolic fractions, using radioiodinated 3,3'-diiodothyronine (3 ,3'-T-2) as the preferred substrate, unlabeled 3'-phosphoadenosine-5'- phosphosulfate (PAPS) as the sulfate donor, and Sephadex LH-20 minicol omns for separation of the products. We found that iodothyronine sulfo transferase activity in rat Liver cytosol is 1) higher in male than in female rats; 2) optimal at pH 8.0; 3) characterized (at 50 mu M PAPS and pH 7.2) by apparent Michaelis-Menton (K-m) values for 3,3'-T-2 of 1.77 and 4.19 mu M, and V-max values of 1.94 and 1.45 nmol/min per mg protein in male and female rats, respectively; 4) characterized (at 1 mu M 3,3'-T-2 and pH 7.2) by apparent K-m values for PAPS of 4.92 and 3.80 mu M and V-max values of 0.72 and 0.31 nmol/min per mg protein, i n males and females, respectively; 5) little affected by hyperthyroidi sm in both male and female rats, but significantly decreased by hypoth yroidism in males but not in females; and 6) not affected by short-ter m (3 days) fasting in both male and female rats, but significantly dec reased by long-term (3 weeks) food restriction to one-third of normal intake in males but not in females. It is suggested that the higher he patic iodothyronine sulfotransferase activity in male us. female rats, as well as the decreases induced in males by hypothyroidism and long- term food restiction, represents differences in the expression of the male-dominant isoenzyme rSULT1C1.