Tpo

Capillary

FIGURE 6-7 Process of thyroid hormone biosynthesis and secretion by a thyroid follicular cell. Abbreviations: IT, iodide transporter; ER, endoplasmic reticulum; GA, Golgi apparatus; SG, secretory granules; EV, exocytotic vesicles; NO, NADPH oxidase; TPO, thyroid peroxidase; MV, microvilli (which are present all over the apical surface but have been omitted for clarity); MPV, micropinocytotic vesicles (involved in Tg reabsorption); PP, pseudopods; CD, colloid droplets; EE, early endosomes; LE, late endosomes. The deiodination of iodotyro-sine, iodide recycling, Tg conversion to T3, and T4 + T3 secretion are also indicated. [Modified with permission from Gentile, F., DiLauro, R., and Salvatore, G. (1995). Biosynthesis and secretion of thyroid hormones. In "Endocrinology" (L. J. DeGroot et al., eds.), Vol. 1, Chapt. 34, pp. 517-542. W. B. Saunders Philadelphia, PA.]

Capillary

FIGURE 6-7 Process of thyroid hormone biosynthesis and secretion by a thyroid follicular cell. Abbreviations: IT, iodide transporter; ER, endoplasmic reticulum; GA, Golgi apparatus; SG, secretory granules; EV, exocytotic vesicles; NO, NADPH oxidase; TPO, thyroid peroxidase; MV, microvilli (which are present all over the apical surface but have been omitted for clarity); MPV, micropinocytotic vesicles (involved in Tg reabsorption); PP, pseudopods; CD, colloid droplets; EE, early endosomes; LE, late endosomes. The deiodination of iodotyro-sine, iodide recycling, Tg conversion to T3, and T4 + T3 secretion are also indicated. [Modified with permission from Gentile, F., DiLauro, R., and Salvatore, G. (1995). Biosynthesis and secretion of thyroid hormones. In "Endocrinology" (L. J. DeGroot et al., eds.), Vol. 1, Chapt. 34, pp. 517-542. W. B. Saunders Philadelphia, PA.]

Although free iodine is capable of chemically iodinat-ing tyrosine residues, reaction b is not believed to be a significant pathway since the Km for iodide conversion to I2 is ~100X greater than the Km of iodide for ioaination of a tyrosine residue (reaction a).

In addition, TPO catalyzes a third reaction, the coupling of Tg-bound iodotyrosines into Tg-bound thyronines.

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