Diiodotyrosine residues

FIGURE 3. Schematic diagram of coupling of 3,5-diiodotyrosine residues of thyroglobulin to form thyroxine. Subsequent to coupling, proteolytic and hydrolytic reactions produce free thyroxine and pyruvic acid. Reproduced from Reference 29 by permission of John Wiley Sons, Inc. 

L-Triiodothyronine and L-thyroxine are synthesized in higher animals and humans in the cells of the thyroid gland. These cells accumulate iodide ions from the blood with great efficiency. A halogenoperoxidase (C 2.4.2) reacts with L-tyrosine residues present in thyroglobulin, one of the thyroid proteins, to form L-monoiodo- and L-diiodotyrosine residues (Fig. 293). These residues, w hile still incorporated into thyroglobulin, condense in a radicaHc reaction with each other to L-triiodothyronine and L-thyroxine residues, which are released by hydrolysis of thyroglobulin and secreted in the blood-stream. 

The thyroid follicular cells transport I across the cell and secrete the precursor protein, Tg, into the follicular lumen. In addition, these cells contain an apical membrane-bound enzyme, thyroperoxidase (TPO), and the enzymatic machinery to produce hydrogen peroxide (H2O2). In the presence of H2O2, TPO catalyzes the incorporation of L into tyrosyl residues of Tg to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) and the coupling of these iodotyrosyl residues to form T4 and Tj. 

The coupling reaction by which the aromatic group from one residue of mono- or diiodotyrosine is joined in ether linkage with a second residue is also catalyzed readily by peroxidases. One dehydroalanine residue is formed for each molecule of hormone released.108 A possible mechanism involves formation of an electron-deficient radical, which can undergo (3 elimination to produce a dehydroalanine residue and an aromatic radical. The latter could couple with a second radical to form triiodothyronine or thyroxine. However, as depicted in Eq. 25-6, the radical coupling may occur prior to chain cleavage. While P elimination (pathway... 

The chemical structures of thyroxine and triiodothyronine are shown in Figure 31—1. As shown in the figure, thyroid hormones are synthesized first by adding iodine to residues of the amino acid tyrosine. Addition of one iodine atom creates monoiodotyrosine, and the addition of a second iodine creates diiodotyrosine. Two of these iodinated tyrosines are then combined to complete the thyroid hormone. The combination of a monoiodotyrosine and a diiodotyrosine yields triiodothyronine, and the combination of two diiodoty-rosines yields thyroxine.55... 

Antithyroid drugs such as propylthiouracil (PTU) and methimazole inhibit the iodination of tyrosyl residues as well as the coupling of monoiodotyrosine and diiodotyrosine into T3 and T4. In addition, PTU inhibits the peripheral conversion of T4 to T3. Unfortunately, there is a very high relapse rate with these drugs and a cure is very infrequent, if at all. The final alternative is thyroidectomy. In this case it is very difficult to determine exactly how much is to be excised. Not surprisingly, a consequence of surgery is the production of a hypothyroid individual who will require daily levothyroxine for the remainder of their life. 

Only two amino acids, tyrosine and histidine, form stable derivatives as the result of peroxidase-catalyzed iodination. All the tyrosine and histidine residues in a protein are not identical with respect to their reactivity or their geographic position. The residue which will be iodinated by lactoperoxidase must have the proper geometric position, while other methods of halogenation are influenced only by reactivity. The reactivity depends upon the microenvironment of the residue. There is an inverse relationship between the extent of tyrosine iodination and the dielectric constant of the environment of the tyrosine. Tyrosine iodination increases with decreasing dielectric constant. Steric factors also influence iodination since the relatively large iodine atom may be blocked in either the production of monoiodotyrosine or the formation of diiodotyrosine. 

Iodine, concentrated in the follicular cells by a pump in the cell membrane, is oxidized by a peroxidase. Iodination of tyrosine residues in thyroglobulin produces monoiodotyrosine (MIT) and diiodotyrosine (DIT), which undergo coupling reactions to produce 3,5,3-triiodothy-ronine (T3) and 3,5,3, 5 -tetraiodothyronine (T4). 

IR spectra of 382, 383 rearrangement of 818, 822 1,2-Tungstophosphoric acid 619 Tyrosinase 955, 969, 978, 979, 981 biosensors based on 974-977 recombinant 977 Tyrosine protein residues 105 Tyrosines 15—see also Diiodotyrosine,... 

The iodinated tyrosine residues monoiodotyrosine (MIT) and diiodotyrosine (DIT) combine to form iodothyronines (Fig. 73-3). Thus, two molecules of DIT combine to form T4, whereas MIT and DIT constitute T3. In addition to its role in iodine organification. 

Fig. 43.10. Synthesis of the thyroid hormones (T3 and T4). The protein thyroglobulin (Tgb) is synthesized in thyroid follicular cells and secreted into the colloid, lodination and coupling of tyrosine residues in Tgb produce T3 and T4 residues, which are released from Tgb by pinocytosis (endocytosis) and lysosomal action. The coupling of a monoiodotyrosine with a diiodotyrosine (DIT) to form triiodothyronine (Tj) is not depicted here.

The oxidation of intracellular iodide is catalyzed by thyroid peroxidase (located at the apical border of the thyroid acinar cell) in what may be a two-electron oxidation step forming 1 (iodinium ion), lodinium ion may react with a tyrosine residue in the protein thyroglobulin to form a tyrosine quinoid and then a 3 -monoiodotyrosine (MIT) residue. It has been suggested that a second iodide is added to the ring by similar mechanisms to form a 3,5-diiodotyrosine (DIT) residue. Because iodide is added to these organic compounds, iodination is also referred to as the organification of iodide. ... 

T3 and synthesis The thyroid follicular cell traps inorganic iodide and oxidizes it to iodine. Iodine binds to tyrosine residues of thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT). Then, either two DIT molecules couple to form T or MIT couples with DIT, forming T3. T production exceeds T3 production in the thyroid gland and T is converted to T3 in the periphery. 

The coupling of two molecules of diiodotyrosine to give one molecule of thyroxine leads to the over-all loss of one alanine side chain, but the form in which this 3-carbon residue is removed is still not yet determined. Up till now, no work has been done on this reaction in the thyroid itself or in isolated thyroid tissues. All the evidence comes from in vitro incubations of diiodotyrosine or its derivatives modelled on von Mutzenbecher s experiments. Johnson and Tewkesbury (1942) found that after such incubations pyruvic acid and ammonia could be detected in the reaction mixture Ohno (see Roche and Michel, 1951) with the aid of chromatography was able to detect serine. Both of these compounds are likely results of the reaction, but at present neither of them has been confirmed. Experiments are still in progress (Pitt-Rivers, unpubhshed) to determine the nature of the alanine residue obtained during aerobic incuba tion of diiodotyrosine or its derivatives. It must, however, be emphasized that even if the nature of this residue were established in in vitro experiments, it would still be necessary to demonstrate that the same reaction takes place in the thyroid gland itself. 

It is believed that thyroxine is formed in nature from the amino acid tyrosine (180) through the stage of diiodotyrosine, 181. When iodine is taken into the body, it is covalently bound to tyrosine residues in thyroglobulin molecules, and the enzyme thyroperoxidase converts the bound tyrosine to monoiodotyrosine (MIT, 181) and diiodotyrosine, 18. This transformation is clearly an enzyme-mediated electrophilic aromatic substitution reaction. Other enzymes convert 182 to 179. 

Extract with w-butanol, in slightly acidic solution, and wash the extract with a solution containing 16 per cent sodium hydroxide and 5 per cent sodium bicarbonate. The excess of 3,5-diiodotyrosine is thus removed by these operations.Distil off the butanol and dissolve the residue in a solution containing 0-5 N sodium carbonate, 5 per cent tetra-methylammoniumbromide and 20 per cent isopropanol. Determine the content of thyroxine from the most positive wave at —1 2 V. The two other waves at —1-4 V and —1-7 V coincide with the reduction waves of 3,5-diiodotyrosine. 

Although earlier workers iodinated proteins, Ostwald in 1910 provided the first unequivocal proof that tyrosine in proteins was iodinated. He isolated diiodotyrosine from iodinated albumin (273), gliadin (274), and casein (275)-. Since then papers on serum albumin, egg albunun, and serum globulin (276), pepsin (6, 7), insulin (125), and the lactogenic hormone (277) all indicate that the action of iodine on these proteins in neutral or slightly alkaline solution is one of substitution on the tyrosine residues. The reaction is as follows ... 

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