Inhibition of Iodoamino Acid Synthesis in Isolated Thyroid Epithelial Cells by High Concentrations of Inorganic Iodide

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Abstract
The effect of various concentrations of 127I- on iodoamino acid synthesis by isolated bovine thyroid follicular cells (ITC) and slices from the same thyroid gland were compared during a 3-hr. incubation with 131I. Higher concentrations of medium 127I- were required to inhibit iodoamino acid synthesis in ITC than in slices but, when the conentration of 127I- exerted equivalent inhibition in both ITC and slices. Increasing the concentration of medium 127I- to > 3 [mu]g/ml increased the DIT/MIT [diiodotyrosine/monoidotyrosiner] ratio in ITC but not in slices. High concentrations of 127I- depressed DIT and MIT synthesis in both. No organically bound 131I was detected in the slices when a medium 127I- concentration of > 300 [mu]g/ml was employed or in the ITC with a medium 127I- concentration of 500 [mu]g/ml. It is concluded that intact thyroid follicles containing colloid are not essential for high concentrations of iodide to inhibit organic iodination and that this inhibition is probably exerted directly at the level of the thyroid epithelial cell.