Deiodination of Iodinated Amino Acids by Pig Thyroid Microsomes

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Abstract
Studies were carried out on the metabolism of iodinated amino acids in the pig thyroid with the following results: 1. It was confirmed that NADPH-dependent, iodotyrosine-deiodi-nating activity is localized mainly in the microsomal fractions. 2. L-Monoiodotyrosine was revealed to be present as an intermediate during the deiodination of L-diiodotyrosine. This fact, together with oxygen independency and requirement of reduced nicotinamide nucleo-tide for this reaction, indicates that the deiodination of iodotyrosines is of a reductive nature. 3. Iodotyrosine deiodinase is highly specific for L-iodotyrosines. At the substrate concentration of 1× 10minus6M, the rate of deiodination of L-diiodotyrosine was about one fifth the rate of deiodination of L-mono-iodotyrosine. At the optimal pH of 8.0, the Km for L-diiodotyrosine was greater than that for L-monoiodotyrosine, the maximal velocity being the same for each iodotyrosine. 4. Some other enzymatic properties of the enzyme such as cofactor requirement, and sensitivity toward inhibitors were investigated. 5. L-Tyrosine, and keto acid analogues of monoiodotyrosine and diiodotyrosine were demonstrated to act as competitive inhibitors of the deiodination reaction. 6. Peroxidase [EC 1.11.1.7], NADPH-cytochrome c reductase and DT-diaphorase [EC 1.6.99.2], each derived from pig thyroid, do not appear to be concerned in the deiodination of iodotyrosines.

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