Is Iodide Required for Peroxidase-Catalyzed Coupling in Thyroglobulin?
There is controversy regarding the possible role of iodide in the coupling reaction. We first reported (1,2) that chemically iodinated thyroglobulin (Tgb), containing almost no iodothyronines and dialyzed to remove the remaining excess of iodide, would give rise to a significant amount of thyroid hormones when incubated in the presence of a peroxidase system (thyroid or lactoperoxidase and glucose, glucose oxidase as H2O2 generating system) in the absence of added iodide, presumably by coupling of the preformed iodotyrosines. It was subsequently reported (3) that chemically iodinated Tgb releases iodide when incubated with peroxidase, suggesting that iodide is needed for coupling to occur and that maximal coupling rates are obtained with an iodide concentration as high as 100 μ M. More recently (4), it has been reported that free iodide is an inhibitor of iodothyronine synthesis in vitro and that, in the presence of limiting H2O2, it actively regulates the efficiency of coupling.
KeywordsCoupling Reaction Glucose Oxidase Paper Chromatography Multinodular Goiter Iodide Concentration
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