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 properties and possible regulation of the
sulfotransferase(s) involved in the sulfation of
thyroid hormone. We have developed a convenient method for the analysis of
iodothyronine sulfotransferase activity in tissue cytosolic fractions, using radioiodinated
3,3'-diiodothyronine (3,3'-T2) as the preferred substrate, unlabeled 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the
sulfate donor, and
Sephadex LH-20 minicolomns for separation of the products. We found that
iodothyronine sulfotransferase activity in rat liver cytosol is 1) higher in male than in female rats; 2) optimal at pH 8.0; 3) characterized (at 50 microM PAPS and pH 7.2) by apparent Michaelis-Menton (Km) values for 3,3'-T2 of 1.77 and 4.19 microM, and Vmax values of 1.94 and 1.45 nmol/min per mg
protein in male and female rats, respectively; 4) characterized (at 1 microM 3,3'-T2 and pH 7.2) by apparent Km values for PAPS of 4.92 and 3.80 microM and Vmax values of 0.72 and 0.31 nmol/min per mg
protein, in males and females, respectively; 5) little affected by
hyperthyroidism in both male and female rats, but significantly decreased by
hypothyroidism in males but not in females; and 6) not affected by short-term (3 days) fasting in both male and female rats, but significantly decreased 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 hepatic
iodothyronine sulfotransferase activity in male vs. female rats, as well as the decreases induced in males by
hypothyroidism and long-term food restriction, represents differences in the expression of the male-dominant
isoenzyme rSULT1C1.