Tyrosine phosphorylation blocks tyrosine free radical formation and, hence, the hormonogenic iodination reaction

It is known that a tyrosine free radical is produced during the hormonogenic iodination reaction of tyrosine residues on thyroglobulin. In the hormonogenic region of thyroglobulin, phosphorylated tyrosine residues have been detected. Using an in vitro tyrosine iodinating system we report that the hormonogenic reaction cannot go off if tyrosine becomes phosphorylated. Enzymatic dephosphorylation of the modified amino acid restored the ability of the molecule to become iodinated. Considering the mechanism of the tyrosine free radical formation, these observations are due to the inability of the phosphorylated amino acid to form a free radical. Our data may suggest a putative regulatory mechanism in thyroid hormone synthesis by phosphorylation of hormonogenic tyrosine residues on thyroglobulin.