Characterization of hepatic low-K-m outer-ring deiodination in red drum (Sciaenops ocellatus)

The more biologically active thyroid hormone 3,5,3'-triiodothyronine (T-3), is primarily derived from peripheral deiodination of thyroxine (T-4). We c haracterized hepatic deiodination for a commercially important, warmwater t eleost fish, the red drum (Sciaenops ocellatus). Low K-m outer-ring deiodin ation (ORD) activity was determined by production of free iodide (I-125) up on incubation of hepatic microsomes with radiolabeled T-4. HPLC analysis de monstrated that I-125, and T-3 were produced in equal amounts, thereby vali dating I-125 as a measure of T-3 production. A small amount of 3,3',5'-trii odothyronine (reverse T-3) was also produced by inner-ring deiodination. Pr oduction of I-125 was linear over a range of 0-100 mug protein/ml and for i ncubations of 30 min-4 h. Maximal ORD activity was measured at pH 6.6, 50 m M dithiothreitol (DTT) and an incubation temperature of 20 degreesC. Double reciprocal plots demonstrated that the average apparent K-m was 5.1 nM and the average V-max was 3.7 pmol T-4 converted/h per mg protein. ORD was not inhibited by propylthiouracil but was 50% inhibited by 90 muM Of iodoaceti c acid and 7 muM of gold thioglucose. The substrate analog preference was T -4 = tetraiodoacetic acid = reverse T-3 > triiodoacetic acid much greater t han T-3. In relation to other tissues, ORD for liver > gill > intestine > k idney. Similar hepatic deiodination activity was present in adult wild, aqu acultured and laboratory-reared red drum, but in adult wild red drum the op timum temperature was higher. Red drum hepatic low-K-m deiodination activit y appears to most closely resemble rainbow trout hepatic and mammalian Type II deiodination. Evidence of inner-ring T-4, deiodination suggests a more active hepatic iodothyronine catabolic pathway than in other teleost specie s. (C) 2001 Elsevier Science Inc. All rights reserved.