The role of iodide and of free diiodotyrosine in enzymatic and non-enzymatic thyroid hormone synthesis.

Abstract

The respective role of iodide and of free diiodotyrosine on thyroid hormone synthesis (coupling reaction) has been analyzed under a variety of enzymatic and non‐enzymatic conditions. The coupling reaction was studied either during the course of the iodination of poorly‐iodinated thyroglobulin or with thyroglobulin containing uncoupled iodotyrosine residues. Free diiodotyrosine stimulated the coupling reaction not only with thyroid peroxidase but also with lactoperoxidase and horseradish peroxidase. Similar effects of free diiodotyrosine were obtained under non‐enzymatic conditions with iodine + iodide at pH 7.4. In contrast the iodination of the tyrosine residues of poorlyiodinated thyroglobulin was not modified by this orthohalophenol either under enzymatic or non‐enzymatic conditions. Preformed iodotyrosine residues were efficiently coupled with thyroid peroxidase only in the presence of either iodide, or free diiodotyrosine, alone or combined. In the presence of thiocyanate, an anion with the same size and charge as iodide, the coupling of preformed iodotyrosine residues was also observed. However the effect of thiocyanate was not additive with that of free diiodotyrosine. Hormonogenic but uncoupled iodotyrosine residues present in iodinated thyroglobulin may also couple non‐enzymatically, but only in the presence of iodine + iodide, efficient coupling being seen only with iodine concentrations greater than 10 μH. In contrast, in the presence of low concentrations of free diiodotyrosine (1 μM), significant coupling was obtained with much lower iodine concentrations, but no effect of free diiodotyrosine was seen in the absence of iodine. These results suggest that the hormonogenic iodotyrosine residues can couple to thyroid hormone only after they are oxidized either by iodine (or another oxidized species of iodine, probably I+) or, more efficiently, by an oxidized derivative of free diiodotyrosine (itself produced only in the presence of iodine). The iodinating system chloramine‐T + iodide was also able to couple preformed iodotyrosine residues of thyroglobulin. Chloramine‐T (0.1 mM) alone was inefficient, whereas when used in combination with diiodotyrosine very efficient coupling was observed. In contrast with high concentrations of both chloramine‐T (0.1 mM) and iodide (0.1 mM), free diiodotyrosine remained without effect. The proposal is made therefore that an oxidized species either of iodine (I+) or of diiodotyrosine is required to oxidize the hormonogenic residues of thyroolobulin. Maximal stimulation of the coupling reaction catalyzed by thyroid peroxidase was obtained with about 0.5 μM free diiodotyrosine. The same level of stimulation was obtained under non‐enzymatic conditions with the iodine + iodide system only with 2.5 μM free diiodotyrosine. These data suggest that free diiodotyrosine or iodide catalyze the coupling reaction after being oxidized either enzymatically or non‐enzymatically. The proposal is made that, with thyroid and lactoperoxidase, both iodide and diiodotyrosine would facilitate the transfer of electrons from the substrate to the heme. Under nonenzymatic conditions of coupling the oxidized species of iodine or of diiodotyrosine would also catalyze the oxidation of the two hormonogenic iodotyrosine residues present at the surface of the molecule of thyroglobulin.