Key result
Trout liver T4 5'-monodeiodinase mimics mammalian enzymes but exhibits minimal T3 deiodination.
Trout liver possesses a T4 5′-monodeiodinase similar to mammals but with little tendency to deiodinate T3.
Trout T4 deiodinase resembles mammalian enzyme yet spares T3; extends comparative endocrinology while leaving clinical translation open.
A rapid method involving radioiodide separation by miniature G-25 Sephadex columns was used for in vitro assay of outer-ring deiodination of labeled thyroxine ([125I]T4) and 3,5,3′-triiodo-L-thyronine ([125I]T3) by liver homogenates of rainbow trout acclimated at 12 °C. T4 deiodination depended on time, enzyme (protein) level, substrate (T4) level, and pH (optimum, 6.8–7.4). Boiling eliminated deiodination. Over the range 12–20 °C incubation temperature modified both Km and Vmax values but did not alter the pH optimum. Deiodination was accompanied by [125I]T3 production, but 125I-labeled reverse T3 was not detected. T4 deiodination was unaltered by addition of carrier T3 or T3 removal by immunosequestration. T3 itself underwent no outer-ring deiodination at 12 °C and weak deiodination at 20 °C (pH optimum 6.8–7.4). In conclusion, trout liver posseses a T4 5′-monodeiodinase with several properties similar to those of its mammalian counterpart but with little tendency to deiodinate T3.
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Pimlott et al. (1983) studied this question. In vitro assay of outer-ring deiodination was evaluated on Outer-ring deiodination of labeled thyroxine and T3. Trout liver possesses a T4 5'-monodeiodinase with properties similar to its mammalian counterpart but with little tendency to deiodinate T3.
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