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Role of Sulfated Tyrosines of Thyroglobulin in Thyroid Hormonosynthesis

Institut National de la Santé et de la Recherche Médicale Unité 555 (N.V., O.C.), Faculté de Médecine, Université de la Méditerranée, 13385 Marseille, France; Northwestern University (M.-C.N.), Pulmonary Division & Critical Care Medicine, Chicago, Illinois 60611; and The Scripps Research Institute (D.M.C.), Department of Molecular and Experimental Medicine, La Jolla, California 92037

Address all correspondence and requests for reprints to: Odile Chabaud, Institut National de la Santé et de la Recherche Médicale Unité 555, Faculté de Médecine, Université de la Méditerranée, 27 Boulevard J. Moulin, 13385 Marseille, Cedex 05, France. E-mail: Odile.Chabaud{at}medecine.univ-mrs.fr.

Our previous studies showed that sulfated tyrosines (Tyr-S) are involved in thyroid hormone synthesis and that Tyr₅, the main hormonogenic site of thyroglobulin (Tg), is sulfated. In the present paper, we studied the role of Tyr-S in the formation and activity of the peroxidase-Tg complex. Results show that noniodinated ³⁵SO₃-Tg specifically binds (Kd = 1.758 µM) to immobilized lactoperoxidase (LPO) via Tyr-S linkage by using saturation binding and competition experiments. We found that NIFEY-S, a 15-amino acid peptide corresponding to the NH₂-end sequence of Tg and containing the hormonogenic acceptor Tyr₅-S, was a better competitor than cholecystokinin and Tyr-S. ³⁵SO₃-Tg, iodinated without peroxidase, bound to LPO with a Kd (1.668 µM) similar to that of noniodinated Tg, suggesting that 1) its binding occurs via Tyr-S linkage and 2) Tyr-S requires peroxidase to be iodinated, whereas nonsulfated Tyr does not. Iodination of NIFEY-S with [¹²⁵I]iodide showed that Tyr₅-S iodination increased with LPO concentration, whereas iodination of a nonsulfated peptide containing the donor Tyr₁₃₀ was barely dependent on LPO concentration. Enzymatic hydrolysis of iodinated Tg or NIFEY-S showed that the amounts of sulfated iodotyrosines also depended on LPO amount. Sulfated iodotyrosines were detectable in the enzyme-substrate complex, suggesting they have a short life before the coupling reaction occurs. Our data suggest that after Tyr-S binding to peroxidase where it is iodinated, the sulfate group is removed, releasing an iodophenoxy anion available for coupling with an iodotyrosine donor.