Dissociation of thyroglobulin iodination and hormone synthesis catalyzed by peroxidases.

Abstract

The properties of thyroid peroxidase in the catalysis of thyroglobulin iodination and thyroid hormone synthesis (coupling reaction) were compared to those of horseradish peroxidase. The following results were obtained. Whereas the thyroid enzyme catalyzes both reactions at the same pH (∼ 7.4) horseradish peroxidase has a pH optimum for iodination of 5.2 and a complex pH dependency for coupling: two peaks of activity were found at pH ∼ 7.5 and ∼ 8.5 with no activity left below pH 5 and above pH 10. These properties of horseradish peroxidase allow a clear dissociation of the iodination and the coupling reactions. Different experiments were performed in which iodination but not coupling could be obtained with either enzyme; a second incubation of these samples at the same or a different pH and with the same or the other enzyme resulted in different levels of coupling. This procedure revealed the following observations. (a) The low efficiency of horseradish peroxidase in performing the coupling reaction at a low pH is not due to the fact that fewer iodotyrosine residues, potentially able to couple, are formed at this pH. Actually 4 hormonogenic iodotyrosine residues were formed at pH 5.2 with horseradish peroxidase but only two of them were able to couple at this pH. Raising the pH to a more alkaline range resulted in the coupling of these preformed hormonogenic residues, (b) Higher levels of coupling, however, were obtained with horseradish peroxidase only at higher levels of iodination than with thyroid peroxidase suggesting a differential sequence in the iodination by the two enzymes of the different tyrosine residues of thyroglobulin. The coupling reaction was catalyzed by horseradish peroxidase in the absence of iodide whereas the iodide was required for the thyroid enzyme.