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Kinetics of Thyroglobulin Iodination and Thyroid Hormone Synthesis Catalyzed by Peroxidases: The Role of H 2 O 2

1981; Wiley; Volume: 117; Issue: 1 Linguagem: Inglês

10.1111/j.1432-1033.1981.tb06308.x

1432-1033

Alain Virion, Jacques Pommier, D. Dème, Jacques Nunez,

Thyroid Disorders and Treatments

When thyroglobulin iodination by thyroid peroxidase or lactoperoxidase is studied with optimal concentrations of H 2 O 2 ‐generating system, thyroid hormone synthesis (coupling reaction) begins after a constant lag period which is independent of the concentration of iodide, thyroglobulin or enzyme. Increasing the concentration of the H 2 O 2 ‐generating system inhibited the iodination reaction whereas the length of the lag period was decreased. When thyroglobulin containing iodinated tyrosine residues but no hormone was incubated with increasing concentrations of iodide and limiting amounts of the H 2 O 2 ‐generating system the iodination reaction was stimulated and the coupling reaction was inhibited. To analyze whether the iodination and coupling reactions require different enzyme‐H 2 O 2 species the iodination and coupling reactions and the absorbance at 430 nm (compound II) were measured simultaneously with lactoperoxidase. Under these conditions: (a) the coupling reaction occurred only in the presence of sufficient amounts of H 2 O 2 whereas the level of iodination was the same; (b) an increase in absorbance at 430 nm was observed only when the iodination reaction leveled off, i.e. just before the beginning of the coupling reaction; (c) during the lag period, i.e. the iodination step, the only enzyme species seen in the Soret region was the native enzyme; and (d) adding iodinated thyroglobulin containing no hormone to preformed compound II or III resulted in a prompt coupling reaction. These data suggest that the iodination and coupling reactions are catalyzed by different enzyme‐H 2 O 2 species. When thyroid peroxidase or lactoperoxidase were preincubated for varying periods of time in the presence of the H 2 O 2 ‐generating system their activity towards guaiacol decreased progressively: with thyroid peroxidase the K m for guaiacol was increased 10‐fold after 5 min of preincubation and 40‐fold with lactoperoxidase. The activity of the preincubated enzymes was restored by iodide. Preincubated lactoperoxidase had the Soret spectrum of compound III. Similarly, the rate of iodination of thyroglobulin was inhibited with the preincubated enzyme, but increasing the iodide concentration restored the iodinating activity. The proposal is made that different enzyme‐H 2 O 2 species are the active agents in the catalysis of the iodination and coupling reaction respectively.