Thyroxine 5′-monodeiodinase activity in hepatocytes of rainbow trout, Salmo gairdneri: Distribution, effects of starvation, and exogenous inhibitors

l-Thyroxine (T 4) 5′-monodeiodinase activity (MDA) of hepatocyte cell fractions of rainbow trout was evaluated by 125I − generation following incubation with [ 125I-3′ or 125I-5′]T 4 at 12°. 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 T 4 deiodination in trout to removal of a single outer-ring iodine atom. T 3 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 > N-ethyl maleimide > propylthiouracil > sodium salicylate > 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 V max 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 T 3 generation, it may exert a key role in regulating peripheral thyroidal status under both natural and experimental conditions.