Sulfation appears to be an important pathway for the reversible inactivation of thyroid hormone during fetal development. The rat is an often used animal model to study the regulation of fetal thyroid hormone status. The present study was done to determine which sulfotransferases (SULTs) are important for iodothyronine sulfation in the rat, using radioactive T 4, T3, rT3, and 3,3Ј-T2 as substrates, 3Ј-phosphoadenosine-5Ј-phosphosulfate (PAPS) as cofactor, and rat liver, kidney and brain cytosol, and recombinant rat SULT1A1, -1B1, -1C1, -1E1, -2A1, -2A2, and -2A3 as enzymes. Recombinant rat SULT1A1, -1E1, -2A1, -2A2, and -2A3 failed to catalyze iodothyronine sulfation. For all tissue SULTs and for rSULT1B1 and rSULT1C1, 3,3Ј-T 2 was by far the preferred substrate. Apparent K m values for 3,3Ј-T2 amounted to 1.9 M in male liver, 4.4 M in female liver, 0.76 M in male kidney, 0.23 M in male brain, 7.7 M for SULT1B1, and 0.62 M for SULT1C1, whereas apparent K m values for PAPS showed less variation (2.0-6.9 M). Sulfation of 3,3Ј-T 2 was inhibited dose dependently by other iodothyronines, with similar structure-activity relationships for most enzymes except for the SULT activity in rat brain. The apparent K m values of 3,3Ј-T2 in liver cytosol were between those determined for SULT1B1 and -1C1, supporting the importance of these enzymes for the sulfation of iodothyronines in rat liver, with a greater contribution of SULT1C1 in male than in female rat liver. The results further suggest that rSULT1C1 also contributes to iodothyronine sulfation in rat kidney, whereas other, yet-unidentified forms appear more important for the sulfation of thyroid hormone in rat brain. thyroid hormone; sulfation; rat sulfotransferase 1B1; rat sulfotransferase 1C1 SULFATION IS A METABOLIC REACTION that facilitates the excretion of endogenous and exogenous hydrophobic compounds in bile and urine by increasing their water solubility (5,9,16,35). Biliary excretion of iodothyronines is also increased by sulfation. More importantly, however, sulfation appears to be a key step in the inactivation of thyroid hormone. The prohormone thyroxine (T 4 ) is converted by outer-ring deiodination (ORD) to the biologically active 3,3Ј,5-triiodothyronine (T 3 ) or by inner-ring deiodination (IRD) to the inactive 3,3Ј,5Ј-triiodothyronine (rT 3 ) (42). By sulfation, T 3 loses its affinity for the thyroid hormone receptors (41). Additionally, T 3 sulfate (T 3 S) is subject to accelerated degradation, as sulfation facilitates the IRD of T 3 by the type I deiodinase (D1) (42). Sulfation also facilitates the inactivating IRD of T 4 by D1, whereas the activating ORD of T 4 by D1 is completely blocked by sulfation (42). Therefore, an important function of sulfation is to facilitate the irreversible degradation of thyroid hormone. Furthermore, under conditions in which the deiodinative clearance of sulfates is impaired, sulfation may be reversed by sulfatases. Because T 3 S and T 4 S levels in the human fetal circulation are high (4, 38, 40), it has been speculated ...

Sulfation is an important pathway of thyroid hormone metabolism that facilitates the degradation of the hormone by the type I iodothyronine deiodinase, but little is known about which human sulfotransferase isoenzymes are involved. We have investigated the sulfation of the prohormone T 4 , the active hormone T 3 , and the metabolites rT 3 and 3,3Ј-diiodothyronine (3,3Ј-T 2 ) by human liver and kidney cytosol as well as by recombinant human SULT1A1 and SULT1A3, previously known as phenol-preferring and monoaminepreferring phenol sulfotransferase, respectively. In all cases, the substrate preference was 3,3Ј-T 2 ϾϾ rT 3 Ͼ T 3 Ͼ T 4 . The apparent K m values of 3,3Ј-T 2 and T 3 [at 50 mol/L 3Ј-phosphoadenosine-5Ј-phosphosulfate (PAPS)] were 1.02 and 54.9 mol/L for liver cytosol, 0.64 and 27.8 mol/L for kidney cytosol, 0.14 and 29.1 mol/L for SULT1A1, and 33 and 112 mol/L for SULT1A3, respectively. The apparent K m of PAPS (at 0.1 mol/L 3,3Ј-T 2 ) was 6.0 mol/L for liver cytosol, 9.0 mol/L for kidney cytosol, 0.65 mol/L for SULT1A1, and 2.7 mol/L for SULT1A3. The sulfation of 3,3Ј-T 2 was inhibited by the other iodothyronines in a concentration-dependent manner. The inhibition profiles of the 3,3Ј-T 2 sulfotransferase activities of liver and kidney cytosol obtained by addition of 10 mol/L of the various analogs were better correlated with the inhibition profile of SULT1A1 than with that of SULT1A3. These results indicate similar substrate specificities for iodothyronine sulfation by native human liver and kidney sulfotransferases and recombinant SULT1A1 and SULT1A3. Of the latter, SULT1A1 clearly shows the highest affinity for both iodothyronines and PAPS, but it remains to be established whether it is the prominent isoenzyme for sulfation of thyroid hormone in human liver and kidney. (J Clin Endocrinol Metab 84: 1357-1364, 1999 S ULFATION is a detoxication reaction that increases the water solubility of a variety of endogenous and exogenous lipophilic compounds, thus facilitating their excretion in bile and/or urine (1-3). Sulfation is also an important pathway for the metabolism of thyroid hormone, increasing the hydrophilicity and the biliary excretion of the hormone. However, the major purpose of sulfation of thyroid hormone is to facilitate its degradation by the type I iodothyronine deiodinase (D1) (4,5). This selenoenzyme catalyzes the outer ring deiodination (ORD) as well as the inner ring deiodination (IRD) of different iodothyronines, including the ORD of the prohormone T 4 to the active hormone T 3 and the IRD of T 4 and T 3 to the inactive metabolites rT 3 and 3,3Ј-diiodothyronine (3,3Ј-T 2 ), respectively (6, 7). The preferred substrate for D1 is rT 3 , which is converted by ORD to 3,3Ј-T 2 (6, 7).An intriguing characteristic of D1 is that its deiodination of a number of iodothyronines is accelerated by sulfation of their phenolic hydroxyl group (4, 5). Thus, IRD of both T 4 sulfate (T 4 S) and T 3 sulfate (T 3 S) by rat D1 is 40 -200 times faster than deiodination of the nonsulfated substrates...

In conditions associated with high serum iodothyronine sulfate concentrations, e.g. during fetal development, desulfation of these conjugates may be important in the regulation of thyroid hormone homeostasis. However, little is known about which sulfatases are involved in this process. Therefore, we investigated the hydrolysis of iodothyronine sulfates by homogenates of V79 cells expressing the human arylsulfatases A (ARSA), B (ARSB), or C (ARSC; steroid sulfatase), as well as tissue fractions of human and rat liver and placenta. We found that only the microsomal fraction from liver and placenta hydrolyzed iodothyronine sulfates. Among the recombinant enzymes only the endoplasmic reticulum-associated ARSC showed activity toward iodothyronine sulfates; the soluble lysosomal ARSA and ARSB were inactive. Recombinant ARSC as well as human placenta microsomes hydrolyzed iodothyronine sulfates with a substrate preference for 3,3'-diiodothyronine sulfate (3,3'-T(2)S) approximately T(3) sulfate (T(3)S) >> rT(3)S approximately T(4)S, whereas human and rat liver microsomes showed a preference for 3,3'-T(2)S > T(3)S >> rT(3)S approximately T(4)S. ARSC and the tissue microsomal sulfatases were all characterized by high apparent K(m) values (>50 microM) for 3,3'-T(2)S and T(3)S. Iodothyronine sulfatase activity determined using 3,3'-T(2)S as a substrate was much higher in human liver microsomes than in human placenta microsomes, although ARSC is expressed at higher levels in human placenta than in human liver. The ratio of estrone sulfate to T(2)S hydrolysis in human liver microsomes (0.2) differed largely from that in ARSC homogenate (80) and human placenta microsomes (150). These results suggest that ARSC accounts for the relatively low iodothyronine sulfatase activity of human placenta, and that additional arylsulfatase(s) contributes to the high iodothyronine sulfatase activity in human liver. Further research is needed to identify these iodothyronine sulfatases, and to study the physiological importance of the reversible sulfation of iodothyronines in thyroid hormone metabolism.