Sulfation of thyroid hormones by liver of rainbow trout, Oncorhynchus mykiss

We studied hepatic sulfation of thyroid hormones (TH) in rainbow trout, Oncorhynchus mykiss. Sulfation of thyroxine (T-4) and 3,5,3'-triiodothyronine (T-3) was detected in the cytosolic (63-67%), microsomal (12-16%), nuclear (12-14%) and mitochondrial/ lysosomal (7-8%) fractions. Using 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as a sulfate donor, sulfation of T-4 and T-3 by the cytosolic fraction depended on protein concentration and time. The pH profiles for T-4- and T-3-sulfation were broad and overlapping with optimal pH values of about 6.5 and 7.0 U respectively. At pH 7.0, apparent K-m (mu M), V-max (pmol/mg cytosolic protein per hour) and catalytic efficiency (V-max/K-m values were 3,5',3'-triiodothyronine (reverseT(3), rT(3)) = 0.7, 583 and 832; T-4 = 1.7, 46 and 27; T-3 = 11.5, 840 and 73. Inhibitor profiles for both T-4- and T-3-sulfation were not significantly different with a common inhibitor preference of rT(3) > pentachlorophenol > triiodothyroacetic acid > tetraiodothyroacetic acid T-4 = T-3 = 3,5-diiodothyronine. T-4-, T-3,- and rT(3)-sulfation activity decreased with increasing pre-incubation temperature (12, 24, 3 degrees C); however, there were no significant differences in T-4-, T-3- and rT(3)-sulfation activity at each pre-incubation temperature. We conclude that: (i) in trout, hepatic sulfation of TH is enzymatic and obeys Michaelis-Menten kinetics; (ii) like mammalian hepatic sulfotransferases (STs), trout hepatic STs are heat-sensitive cytosolic proteins using PAPS as a sulfate donor; (iii) unlike mammalian sulfation of TH, trout hepatic sulfation of T-4, T-3 and rT(3) may be catalyzed by a single form of ST preferring rT(3) as substrate and with a catalytic efficiency of rT(3) much greater than T-3 > T-4. (C) 1998 Elsevier Science Inc. All rights reserved.