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  • Title: Sulfation of thyroid hormones by liver of rainbow trout, Oncorhynchus mykiss.
    Author: Finnson KW, Eales JG.
    Journal: Comp Biochem Physiol C Pharmacol Toxicol Endocrinol; 1998 Oct; 120(3):415-20. PubMed ID: 9827058.
    Abstract:
    We studied hepatic sulfation of thyroid hormones (TH) in rainbow trout, Oncorhynchus mykiss. Sulfation of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) 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 T4 and T3 by the cytosolic fraction depended on protein concentration and time. The pH profiles for T4- and T3-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) (microM), Vmax (pmol/mg cytosolic protein per hour) and catalytic efficiency (Vmax/K(m)) values were 3,5',3'-triiodothyronine (reverse T3, rT3) = 0.7, 583 and 832; T4 = 1.7, 46 and 27; T3 = 11.5, 840 and 73. Inhibitor profiles for both T4- and T3-sulfation were not significantly different with a common inhibitor preference of rT3 > pentachlorophenol > triiodothyroacetic acid > tetraiodothyroacetic acid T4 = T3 = 3,5-diiodothyronine. T4-, T3- and rT3-sulfation activity decreased with increasing pre-incubation temperature (12, 24, 36 degrees C); however, there were no significant differences in T4-, T3- and rT3-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 T4, T3 and rT3 may be catalyzed by a single form of ST preferring rT3 as substrate and with a catalytic efficiency of rT3 >>> T3 > T4.
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