Deiodination and Deconjugation of Thyroid Hormone Conjugates and Type I Deiodination in Liver of Rainbow Trout, Oncorhynchus mykiss

Abstract

We studied the hepatic in vitro deconjugation and deiodination of glucuronide (G) and sulfate (S) conjugates of the thyroid hormones (TH) thyroxine (T4), 3,5,3′-triiodothyronine (T3), and 3,3′,5′-triiodothyronine (rT3) in trout. These conversions have not been studied in nonmammals. Deconjugation of T4G, T3G, rT3G, or rT3S was negligible in all subcellular fractions. Some T4S desulfation occurred but T3S was desulfated to the greatest extent by freshly isolated hepatocytes and by the mitochondrial/lysosomal and microsomal fractions. Deiodination of T4G, T3G, rT3G, T4S, T3S, and rT3S (1 or 1000 nM) was negligible in control trout and in trout treated with T3 to induce inner-ring deiodination (IRD) but simultaneously tested rat microsomes rapidly deiodinated T4S, T3S, and rT3S. Furthermore, T4S, T3S, and rT3S (1–100 nM) were less effective than their unsulfated forms in competitively inhibiting trout hepatic outer-ring deiodination (ORD) of T4 (0.8 nM), and rT3ORD (100 nM) was not competitively inhibited by T4S, T3S, or rT3S (100 nM) or by T4 or T3 (1 μM). 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-Km deiodination, which has been considered homologous to rat type I deiodination. We found that it resembled rat type I deiodination in its rT3ORD 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 T4 and THS but also in its low DTT optimum (2.5 mM), its low apparent Km for rT3 (200 nM), its lack of IRD ability, its extremely weak propylthiouracil inhibition (IC50, 1 mM), its weaker inhibition by iodoacetate (IC50, 10 μM) and aurothioglucose (IC50, <3 μM), its activation by fasting, and its unresponsiveness to T3 hyperthyroidism. We conclude that most conjugated TH are neither deconjugated nor deiodinated by trout liver and are therefore eliminated in bile. However, T3S can be desulfated. Substrate preference and other properties suggest that trout hepatic high-Km ORD shares some properties with rat type I deiodination but differs functionally in several other respects and may contribute negligibly to hepatic T3 production in trout.