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:''Not to be confused with Thyroxine-binding globulin, a carrier protein responsible for carrying the thyroid hormones in the blood.'' Thyroglobulin (Tg) is a 660 kDa, dimeric protein produced by the follicular cells of the thyroid and used entirely within the thyroid gland. Thyroglobulin protein accounts for approximately half of the protein content of the thyroid gland. Each thyroglobulin molecule contains approximately 100-120 tyrosyl (Tyrosine) residues. However, because only a small number (>20) of these tyrosine residues are subject to iodination by thyroperoxidase in the follicular colloid, each Tg molecule is only able to form very small amounts of thyroid hormone (5-6 molecules of either T4 and T3).〔 == Function == Tg is used by the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). The active form of triiodothyronine, 3, 5, 3' triiodothyronine, is produced both within the thyroid gland and in the periphery by 5'-deiodinase (which has been referred to as tetraiodothyronine 5' deiodinase). It is presumed that Tg and thyroid are also an important storage of iodine for all body needs, in particular, for many iodine-concentrating organs such as breast, stomach, salivary glands, thymus, choroid plexus and cerebrospinal fluid, etc. (see iodine in biology). Tg is produced by the thyroid epithelial cells, called thyrocytes, which form spherical follicles. Tg is secreted and stored in the follicular lumen. Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). * Thyroxine is produced by combining two moieties of DIT. * Triiodothyronine is produced by combining one molecule of MIT and one molecule of DIT. Small globules of the follicular colloid (Tg) are endocytosed (hormone (TSH)-mediated) and proteases in lysosomes digest iodinated thyroglobulin, releasing T3 and T4 within the thyrocyte cytoplasm. The T3 and T4 are then transported across (TSH-mediated) the basolateral thyrocyte membrane, into the bloodstream, by an unknown mechanism while the lysosome is recycled back to the follicular lumen. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thyroglobulin」の詳細全文を読む スポンサード リンク
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