title: Thyroxine 5'-monodeiodinase activity in hepatocytes of rainbow trout, Salmo gairdneri: distribution, effects of starvation, and exogenous inhibitors.
abstract: L-Thyroxine (T4) 5'-monodeiodinase activity (MDA) of hepatocyte cell fractions of rainbow trout was evaluated by 125I- generation following incubation with [125I-3' or 125I-5']T4 at 12 degrees. Produced in approximately equal proportions, 3,5,[125I-3'] triiodo-L-thyronine and 125I- were the sole labeled products detected by gel permeation on G-25 Sephadex columns, confirming restriction of T4 deiodination in trout to removal of a single outer-ring iodine atom. T3 underwent no significant outer-ring deiodination. MDA activity, located mainly in the microsome fraction, was optimal at a pH of approximately 7.0 and was enhanced by dithiothreitol but not by reduced glutathione. Azide, thiocyanate, thiourea, and KCl exerted no significant influence on MDA, but MDA was inhibited by: 8-anilino-1-naphthalene sulfonic acid greater than N-ethyl maleimide greater than propylthiouracil greater than sodium salicylate greater than KI. Starvation for 2 weeks depressed MDA to 46% of the level of trout fed 1% of body wt once per day. This was due to a decreased Vmax of MDA. In conclusion, trout hepatic microsomal MDA is acutely and chronically susceptible to both exogenous and endogenous factors; as an enzyme responsible for extrathyroidal T3 generation, it may exert a key role in regulating peripheral thyroidal status under both natural and experimental conditions.
mesh_terms: D000818:Animals; D004229:Dithiothreitol / Q000494:pharmacology; D005978:Glutathione / Q000494:pharmacology; D007453:Iodide Peroxidase / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008247:Lysosomes / Q000201:enzymology; D008862:Microsomes, Liver / Q000201:enzymology; D008930:Mitochondria, Liver / Q000201:enzymology; D012487:Salmonidae / Q000378:metabolism*; D013217:Starvation; D014337:Trout / Q000378:metabolism*
chemical_list: D007453:Iodide Peroxidase; D005978:Glutathione; D004229:Dithiothreitol