Evidence, by in vitro enzymatic iodination of thyroglobulin, that the efficiency of coupling is determined by the initial iodide concentration.

The rate of iodination and the extent and the efficiency of coupling, assessed by the T4 residues formed per atom of iodine bound, have been studied by in vitro enzymatic iodination of iodine-poor (0.01% I) human goiter thyroglobulin (Tgb; 1 microM) with varying concentrations of iodide (0.75 X 10(-5)-10(-4) M I-), lactoperoxidase (0.1-5 micrograms/ml), and hydrogen peroxide (H2O2) generated from glucose (1 mg/ml) and glucose oxidase (0.025-2.5 mU/ml). Under the experimental conditions used, variations in the H2O2 concentration affect the rate of iodination, but not the extent of coupling. Variations in the peroxidase concentration do not affect the rate of iodination or the coupling efficiency. Variations in the iodide concentration do not affect the rate of iodination, but do affect the coupling efficiency, which is higher the lower the initial iodide concentration. Both iodination and coupling take place almost simultaneously. They are catalyzed by peroxidase, and iodination is completed earlier the lower the iodide concentration. Therefore, once iodination is complete, coupling alone, with high efficiency, takes place. This suggests that the active peroxidase compounds formed will be mainly directed toward the catalysis of iodination or coupling, depending on the actual iodide concentration present in the incubation medium. Provided that initially nonlimiting noninhibiting H2O2 and iodide concentrations are used and that iodination and coupling are allowed to proceed to completion, the extent of coupling is a function of the degree of iodination of Tgb achieved, and its efficiency is determined not just by the native structure of Tgb, but also by the initial iodide concentration used.