In vitro study of hepatic iodothyronine deiodination in rainbow trout, Salmo gairdneri

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Abstract
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.degree. 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.degree.-20.degree. C incubation temperature modified both Km and Vmax values but did not alter the pH optimum. Deiodination ws 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.degree. C and weak deiodination at 20.degree. C (pH optimum 6.8-7.4). In conclusion, trout liver possesses a T4 5''-monodeiodinase with several properties similar to those of its mammalian counterpart but with little tendency to deiodinate T3.