Iodine metabolism

Yıl 2012, Cilt: 29 Sayı: 4S, 277 - 279, 07.05.2013

Ali Okuyucu Hasan Alaçam

https://doi.org/10.5835/jecm.omu.29.s4.014

Öz

The major source of iodine contained in our body comes from our diet. If the iodine taken via diet is in the molecular form (I2), it is absorbed by facilitated diffusion, if in the iodide form, then the (I-) absorption takes place with the help of sodium-iodide symporter carrier protein present in the gastric mucosa. Since, in humans, no information exists on tubular secretion or active transport, renal clearance first of all depends on the glomerular filtration. Inorganic iodine is ingested into the cell through iodide symporter which is located in the basolateral membrane, close to the capillary, of follicle cells and enables cell ingestion of two Na+ iodine and one I- iodine against electrochemical gradient. The activity of this pump is enhanced by TSH and this pump is the rate limiting step in the synthesis of thyroid hormones. The second transport system required for transporting the iodide into the follicle lumen is the passive transport which enables it to enter into colloid after passing through thyroid cell apical membrane. The first step of the thyroid hormone synthesis is the addition of iodide to the tyrosyl residues in the thyroglobuline. This addition takes place at the boundary of apical plasma membrane and follicle lumen and in existence of H2 O2 , thyroperoxidase (TPO), iodide and thyroglobuline. First iodide becomes oxidised and then combines with tyrosyl residues of thyroglobuline. The next step in the hormone synthesis is the amalgamation of two neighbouring monoiododetyrosyl (MIT) residues to generate diiodothyronine (DIT). Two DITs amalgamate to generate T4, and amalgamation of one DIT and one MIT produces T3. TPO catalyzes this reaction.

Anahtar Kelimeler

Iodine, T3, T4, Thyroperoxidase, TSH

İyot metabolizması

Öz

Vücudumuzda bulunan iyotun major kaynağı diyettir. Diyetle alınan iyot moleküler (I2) formda ise kolaylaştırılmış difüzyonla emilirken, iyodür formunda ise (I-) emilimi gastrik mukozada bulunan sodyum-iyodür simporter taşıyıcı proteini yardımı ile gerçekleşir. İnsanlarda, tubuler sekresyonu veya aktif transportuna dair bilgi olmadığından, renal klirensi öncelikle glomerüler filtrasyona bağlıdır. İnorganik iyot, folikül hücrelerinin kapillere yakın bazolateral membranında yerleşmiş olan ve elektrokimyasal gradyente karşı iki Na+ iyonu ve bir I- iyonunun hücre içine geçişini sağlayan sodyum-iyodür simporter’i ile hücre içine alınır. Bu pompanın aktivitesi tiroit stimülan hormon (TSH) ile arttırılır ve bu pompa tiroit hormonlarının sentezinde hız sınırlayıcı basamaktır. İyodürün folikül lümenine geçmesi için gerekli ikinci transport sistemi; tiroit hücresi apikal membranını geçerek kolloide girmesini sağlayan pasif transporttur. Tiroit hormon sentezinin ilk adımı tiroglobulindeki tirozil kalıntılarına iyodürün eklenmesidir. Bu eklenme apikal plazma membranı-folikül lümeni sınırında, H2 O2, thyroperoxidase (TPO), iyodür ve tiroglobulin varlığında oluşur. İlk olarak iyodür okside olur ve daha sonra tiroglobulinin tirozil kalıntısı ile birleşir. Hormon sentezinde sonraki adım komşu iki monoiyodotirozil (MIT) kalıntısının diiyodotironin (DIT) oluşturmak üzere birleşmesidir. İki DIT birleşerek tiroksini (T4), bir DIT ve bir MIT birleşerek triiyodotironini (T3) oluşturur. Bu reaksiyonu TPO katalizler.

Anahtar Kelimeler

İyot, T3, T4, Tiroit peroksidaz, TSH

Kaynakça

• Barrett, K.E., Barman, S.M., Boitano, S., Brooks, H.L., 2010. Ganong’s Review of Medical Physiology. The McGraw-Hill Education (Asia), Singapore. pp. 301-314.
• Berndorfer, U., Wilms, H., Herzog, V., 1996. Multimerization of thyroglobulin (TG) during extracellular storage: Isolation of highly cross-linked TG from human thyroids. J. Clin. Endocr. Metab. 81, 1918-1926.
• Chambard, M., Verrier, B., Gabrion, J., Mauchamp, J., 1983. Polarization of thyroid cells in culture: Evidence for the basolateral localization of the iodide "pump" and of the thyroid-stimulating hormone receptor-adenyl cyclase complex. J. Cell Biol. 96, 1172-1177.
• Dai, G., Levy, O., Carrasco, N., 1996. Cloning and characterization of the thyroid iodide transporter. Nature. 379, 458-460.
• DeGroot, L.J., 1966. Kinetic analysis of iodine metabolism. J. Clin. Endocr. Metab. 26, 149.
• Grotzke, M., 2010. Chapter 22: The Thyroid Gland. Clinical Chemistry. Bishop, M.L., Fody, E.P., Schoeff, L.E. Sixth eds. Lippincott Williams&Wilkins, pp: 490-495.
• Ingbar, S.H., 1955. Simultaneous measurement of the iodide-concentrating and protein-binding capacities of the normal and hyperfunctioning human thyroid gland. J. Clin. Endocr. Metab. 15, 238.
• Ishii, H., Inada, M., Tanaka, K., Mashio, Y., Naito, K., Nishikawa, M., Matsuzuka, F., Kuma, K., Imura, H., 1983. Induction of outer and inner ring monodeiodinases in human thyroid gland by thyrotropin. J. Clin. Endocr. Metab. 57, 500-505.
• Johanson, V., Ofverholm, T., Ericson, L.E., 1988. Forskolin-induced elevation of cyclic AMP does not cause exocytosis and endocytosis in rat thyroid follicle cells in vivo. Mol. Cell Endocrinol. 59, 27-34.
• LoPresti, J.S., Fried, J.C., Spencer, C.A., Nicoloff, J.T., 1989. Unique alterations of thyroid hormone indices in the acquired immunodeficiency syndrome (AIDS). Ann. Intern. Med. 110, 970-975.
• Newsome, S., Hickmen, P.E., 2010. Chapter 49: Thyroid. Clinical Chemistry. Kaplan, A.L., Pesce, J.A. Fifth eds. Elsevier, pp: 948-960.
• Pennington, J.A.T., Young, B.E., 1991. Total diet study nutritional elements, 1982-1989. J. Am. Diet. Assoc. 91, 179-83.
• Perry, W.F., Hughes, J.F.S., 1952. The urinary excretion and thyroid uptake of iodine in renal disease. J. Clin. Invest. 31, 457.
• Spitzweg, C., Harrington, K.J., Pinke, L.A., Vile, R.G., Morris, J.C., 2001. Clinical review 132: The sodium iodide symporter and its potential role in cancer therapy. J. Clin. Endocr. Metab. 86, 3327-3335.
• Swain, P.A., 2005. Bernard Courtois (1777-1838) famed for discovering iodine (1811), and his life in Paris from 1798. Bull. Hist. Chem. 30, 103-111.
• Weaver, J.C., Kamm, M.L., Dobson, R.L., 1960. Excretion of radioiodine in human milk. JAMA. 173, 872.
• Whitley, R.J., 2001. Chapter 40: Thyroid function. Tietz Fundamentals of Clinical Chemistry fifth edition. Carl A. Burtis, Edward R. Ashwood.
• W.B. Saunders Company, Philadelphia. 839-856.
• Wolff, J., 1964. Transport of iodide and other anions in the thyroid gland. Physiol. Rev. 44, 45-90.
• Wynn, J.O., 1961. Components of the serum protein-bound iodine following administration of I131-labeled hog thyroglobulin. J. Clin. Endocr. Metab. 21, 1572-1961.