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  • Title: Deiodination and deconjugation of thyroid hormone conjugates and type I deiodination in liver of rainbow trout, Oncorhynchus mykiss.
    Author: Finnson KW, McLeese JM, Eales JG.
    Journal: Gen Comp Endocrinol; 1999 Sep; 115(3):387-97. PubMed ID: 10480990.
    Abstract:
    We studied the hepatic in vitro deconjugation and deiodination of glucuronide (G) and sulfate (S) conjugates of the thyroid hormones (TH) thyroxine (T(4)), 3,5,3'-triiodothyronine (T(3)), and 3,3', 5'-triiodothyronine (rT(3)) in trout. These conversions have not been studied in nonmammals. Deconjugation of T(4)G, T(3)G, rT(3)G, or rT(3)S was negligible in all subcellular fractions. Some T(4)S desulfation occurred but T(3)S was desulfated to the greatest extent by freshly isolated hepatocytes and by the mitochondrial/lysosomal and microsomal fractions. Deiodination of T(4)G, T(3)G, rT(3)G, T(4)S, T(3)S, and rT(3)S (1 or 1000 nM) was negligible in control trout and in trout treated with T(3) to induce inner-ring deiodination (IRD) but simultaneously tested rat microsomes rapidly deiodinated T(4)S, T(3)S, and rT(3)S. Furthermore, T(4)S, T(3)S, and rT(3)S (1-100 nM) were less effective than their unsulfated forms in competitively inhibiting trout hepatic outer-ring deiodination (ORD) of T(4) (0.8 nM), and rT(3)ORD (100 nM) was not competitively inhibited by T(4)S, T(3)S, or rT(3)S (100 nM) or by T(4) or T(3) (1 microM). Thus, there is no evidence in trout liver for THS deiodination, which is a key property of rat type I deiodination. We therefore studied other properties of trout hepatic high-K(m) deiodination, which has been considered homologous to rat type I deiodination. We found that it resembled rat type I deiodination in its rT(3)ORD ability, its optimum pH (7.0), and its requirement for dithiothreitol (DTT). However, it differed from rat type I deiodination not only in its negligible deiodination of T(4) and THS but also in its low DTT optimum (2.5 mM), its low apparent K(m) for rT(3) (200 nM), its lack of IRD ability, its extremely weak propylthiouracil inhibition (IC(50), 1 mM), its weaker inhibition by iodoacetate (IC(50), 10 microM) and aurothioglucose (IC(50), <3 microM), its activation by fasting, and its unresponsiveness to T(3) hyperthyroidism. We conclude that most conjugated TH are neither deconjugated nor deiodinated by trout liver and are therefore eliminated in bile. However, T(3)S can be desulfated. Substrate preference and other properties suggest that trout hepatic high-K(m) ORD shares some properties with rat type I deiodination but differs functionally in several other respects and may contribute negligibly to hepatic T(3) production in trout.
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