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

Abstract

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 ( μM), 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, 36°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⋙T 3>T 4.