Deiodination and deconjugation of thyroid hormone conjugates and type I deiodination in liver of rainbow trout, Oncorhynchus mykiss

We studied the hepatic in vitro deconjugation and deiodination of glucuroni de (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. Deconjugatio n of T(4)G, T(3)G, rT(3)G, or rT(3)S was negligible in all subcellular frac tions. 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 T(4)G, T(3)G, rT(3)G, T4S, T3S, a nd rT(3)S (1 or 1000 nM) was negligible in control trout and in trout treat ed with T-3 to induce inner-ring deiodination (IRD) but simultaneously test ed rat microsomes rapidly deiodinated T4S, T3S, and rT(3)S. Furthermore, T4 S, T3S, and rT(3)S (1-100 nM) were less effective than their unsulfated for ms 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 4S, T3S, or rT(3)S (100 nM) or by T-4 or T-3 (1 mu M). Thus, there is no ev idence 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 n egligible 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 e xtremely weak propylthiouracil inhibition (IC50, 1 mM), its weaker inhibiti on by iodoacetate (IC50, 10 mu M) and aurothioglucose (IC50 <3 mu M), its a ctivation by fasting, and its unresponsiveness to T-3 hyperthyroidism. We c onclude that most conjugated TH are neither deconjugated nor deiodinated by trout liver and are therefore eliminated in bile. However, T3S can be desu lfated. Substrate preference and other properties suggest that trout hepati c high-K-m ORD shares some properties with rat type I deiodination but diff ers functionally in several other respects and may contribute negligibly to hepatic T-3 production in trout. (C) 1999 Academic Press.