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Evidence is presented that free iodide in the reaction medium during iodination of thyroglobulin by lactoperoxidase or thyroidperoxidase in vitro and a limiting Hz02 supply actively regulates the efficiency of the simultaneously occurring synthesis of iodothyronines. Our arguments are as follows.1. For any level of iodination achieved in vitro, the amount of iodine incorporated in newly formed iodothyronine residues (but not that of the overall iodination) was linearly and inversely correlated to the logarithm of free iodide remaining in the incubation medium. This finding holds true for iodide concentrations in the pM range, regardless of the native iodine content of a 19-S thyroglobulin sample, of the concentration of the enzymes and of the incubation time.2. When thyrogiobulins, iodinated either in the absence or in the presence of excess iodide, are compared the latter contained (for similar degrees of iodination) more newly synthesized diiodotyrosine residues but fewer iodothyronine residues, although over 50 "/, of these diiodotyrosine residues were hormonogenic.3. An additional important increase in hormone synthesis was obtained by extending the reaction time beyond the point were all iodide was organified. The latter phenomenon is but another demonstration that coupling always lags behind iodination.

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