Iodine/iodide transport

The method of choice for the preparation of Pa metal is a somewhat modified van Arkel-De Boer process, which uses protactinium carbide (Section II,C) as the starting material. The carbide and iodine are heated to form protactinium iodide, which is thermally dissociated on a hot filament 12-15). An elegant variation is to replace the filament with an inductively heated W or Pa sphere 109). A photograph of a 1.4-g sample of Pa metal deposited on a radiofrequency-heated W sphere is shown in Fig. 6. From the analytical data presented in Table V, the impurities present before and after application of this modified iodide transport process (Sections II,D and III,C) can be compared. 

The biosynthesis of thyroid hormones involves the trapping of circulating iodide (iodide transport) by the thyroid gland, the incorporation of iodine into tyrosine, and the coupling of iodinated tyrosyl residues to form the thyronines (T4 and... 

The expression of the sodium iodide symporter is perhaps nowhere more important than in the thyroid gland. A complete review of the physiological importance of the thyroid is beyond the scope of this chapter. It is sufficient to say that the symporter provides the iodine needed for normal thyroid function. Once the symporter has been trafficked to the basolateral surface of the thyrocyte, it can transport iodine from the blood into the cell. Once inside the cells, iodine is transported to the apical membrane where it is organified through attachment to a tyrosine residue and incorporated into the thyroid hormone thyroglobulin. The thyroglobu-lin is then stored inside thyroid follicles as colloid, to be released into the bloodstream as thyroid hormones (thyroxine and triiodothyronine) via TSH stimulation. 

Large amounts of thioq anate are generated in people with a high intake of cyanide from tobacco smoking, from cyanide in food, or from industrial pollution of the environment with cyanide. Thiocyanate may also be directly consumed with certain foods. Thiocyanate is a competitive inhibitor of the sodium iodide symporter (NIS) at thiocyanate levels normally found in blood. Thereby, it worsens iodine deficiency by inhibition of thyroidal iodide accumulation and by inhibition of iodide transport into breast milk for infant nutrition. Cessation of smoking, reduction of industrial pollution and improved diet will reduce the role of thiocyanate in thyroid disease. In individuals exposed to high levels of thiocyanate, adverse effects may be prevented by an increase in iodine intake. 

In sows given rapeseed, it has been shown that the low iodine transfer via sows milk to piglets caused by rapeseed inhibition of iodide transport into the milk leads to hypothyroidism and developmental defects in the piglets, and that this is preventable by iodine administration (Schone et al.. 

No studies of humans have demonstrated inhibition of iodide transport into breast milk of mothers from thiocyanate in diet. In many countries the most important determinant of thiocyanate levels in blood is tobacco smoking, as discussed above, and smoking is associated with similar increases in thiocyanate in blood as may be found from diets with high cyanide content. We therefore investigated the effect of maternal smoking on iodine transfer to the breast-fed neonate. 

Metabolism and Transport of Iodine Iodide, supplied by food, is bound to serum proteins, especially to albumin. 

Figure 101.2 Thyroid iodide transport and organification. A schematic of a thyroid follicular cell showing the key aspects of thyroid iodine transport and thyroid hormone synthesis. TSHR, TSH receptor NIS, sodium iodide symporter TPO, thyroid peroxidase Tg, thyroglobulin.

Iodide in the apical cells is released into the follicular lumen via an apical iodide transporter (Rodriguez et ai, 2002) and pendrin, the product of Pendreds syndrome gene (Royaux et aL, 2000 Yoshida et ai, 2004). Iodine is then transferred onto thyroglobulin (Tg), which is located... 

In the thyroid follicular cell, intracellular iodide taken up from blood is bound in organic form in a few minutes, so less than 1 % of the total iodine of the gland is found as iodide. Therefore, inhibition of the iodide transport system requires blockade of organic binding. This can be achieved by the use of antithyroid drugs, of which n-propyl-6-thiouracil and 1-methyl-2-mercaptoimidazole (methimazole) are the most potent. 

Price, D.J. and Sherwin, J.R. (1986) Autoregulation of iodide transport in the rabbit absence of autoregulation in fetal tissue and comparison of maternal and fetal thyroid iodination products. Endocrinology 119 2547. 

In discussing iodide metabolism, we need only be concerned with the fate of inorganic iodides because organic iodides (tyrosine derivatives) are absorbed and enter the normal plasma pool of organic iodides produced by the thyroid. The site and the mechanism of absorption of organic iodides in the intestine are not exactly known, but somehow the dietary iodine is transported from the intestinal lumen into the blood. 

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