Thyroglobulin, iodotyrosines

The coupling of two DIT molecules to form T4—or of an MIT and DIT to form T3—occurs within the thyroglobulin molecule. A separate coupfing enzyme has not been found, and since this is an oxidative process it is assumed that the same thyroperoxidase catalyzes this reaction by stimulating free radical formation of iodotyrosine. This hypothesis is supported by the observation that the same drugs which inhibit H oxidation also inhibit coupfing. The formed thyroid hor-... 

Fig. 1 Thyroid hormone synthesis in a thyroid follicular cell. NIS and TPO (organification and coupling reaction) have been marked in red dashed line as the two main targets for direct thyroid gland function disrupters. DEHALl iodotyrosine dehalogenase 1, DIT diiodotyrosine, DUOX2 dual oxidase 2, MIT monoiodotyrosine, Na/K-ATPase sodium-potassium ATPase, NIS sodium-iodide symporter, PSD pendrin, TG thyroglobulin, TPO thyroperoxidase. Reprinted from [7] with permission from Elsevier...

Thyroxine is actually a simple derivative of the aromatic amino acid tyrosine (see Section 13.1), but is believed to be derived by degradation of a larger protein molecule containing tyrosine residues. One hypothesis for their formation invokes suitably placed tyrosine residues in the protein thyroglobulin being iodinated to di-iodotyrosine. These residues then react together by phenolic oxidative coupling. 

FICURn 10.21 Iodine uptake and thyroid hormone s>mthesis by the thyroid gland. Thyroglobulin is broken down to yield T4 and T3 as wot] as iodotyrosine byproducts. These byproducts are further broken down in the thyroid to yield iodide. 

Serum total T4 concentrations were initially determined indirectly, using methods that measured the amount of iodine in a protein precipitate of serum (protein-bound iodine, PBI). In addition to hormonal iodine, the PBI tests also measured iodoproteins, iodotyrosines, inorganic iodine, and thyroglobulin. More specific T4 procedures involved the measurement of hormonal iodine in either a butanol extract of a serum protein precipitate (butanol-extractable iodine) or in a purified fraction of serum (I4 by column). These methods were useful because the iodine in T4 normally accounts for 80% to 90% of all iodine in serum. Both... 

It may be significant that tyrosine, di-iodotyrosine and thyroxine all occur as iV-terminal groups in pork thyroglobulin (720a). 

Thionamides are the most important class of antithyroid compounds in clinical practice used in nondestructive therapy of hyperthyroidism. These agents are potent inhibitors of TPO, which is responsible for the iodination of tyrosine residues of thyroglobulin and the coupling of iodotyrosine residues to form iodothyronines. These drugs have no effect on the iodide pump or on thyroid hormone release. The most clinically useful thionamides are thioureylenes, which are five- or six-membered heterocyclic derivatives of thiourea and include the thiouracil 6-n-propyl-2-thiouracil (PTU) and the thioimidazole 1-methyl-2-mercaptoimidazole (methimazole, Tapazole, MMI). The uptake of these drugs into the thyroid gland is stimulated by TSH and inhibited by iodide. 

This peroxidase catalyzes two important reactions in the thyroid hormone synthesis (i) the iodination of tyrosines in thyroglobulin to yield protein-bound mono- and di-iodotyrosines... 

MMI, TZD (6), PYSH (7), PMT (10), MNA (8) and MBA (9) on the other hand that strongly bind to I or are oxidized to disulfides [15] most probably interfere in the formation of mono-iodotyrosine (MIT), di-iodotyrosine (DIT) by the tyrosine residues of thyroglobuline Tyr(TG), competing with active iodine. 

It is not well known at what stage of the thyroglobu-lin molecule s biosynthesis tyrosine is iodinated. Three proposals have been submitted (1) iodination of the amino acid before its incorporation into the polypeptide chain (but puromycin does not impair iodination, and no enzymes capable of activating iodotyrosine have been found) (2) iodination of the finished tetra-meric globulin and (3) iodination of the 12 S subunits with concomitant condensation of the unit to yield new protein. However, there seems to be no doubt that thyroglobulin continues to be iodinated even after its excretion into the colloid, indeed the ratio... 

A number of inborn errors have been described in patients with sporadic cretinism (1) a defect in the iodine-trapping mechanism (2) an inability to convert inorganic iodide to iodine (3) a lack of thyroid peroxidase [178] (4) an inability to couple iodotyrosine to form iodothyronines and thyroglobulins (5) a lack of dehalogenase (6) an interference with thyroglobulin metabolism and (7) a defect in thyrotropin secretion [179-180]. 

The mechanism by which the absence of the dehalogenase leads to low thyroxine levels and cretinism is not clear. Two different theories have been proposed. The first postulates the existence of an additional defect namely, an inability to couple iodotyrosine to form T3 and T4. The second proposes that the absence of dehalogenase leads to a glandular hyperfunction in which hormone precursors are released before they can be used for thyroglobulin biosynthesis. It has now been established that the dehalogenase defect results from the absence of a single autosomal recessive gene. 

Iodine is taken up by the thyroid gland where it is converted to di-iodotyrosine and thyroxine. The thyroid hormone is stored in the form of thyroglobulin. In normal persons, the rate of iodine uptake by the thyroid is proportional to the blood concentration and averages about 2.5% per hour. In hyperthyroidism, the uptake may reach 20 % per hour. The release of thyroid hormone from the gland is under pituitary regulation. 

Synthesis of thyroid hormones has several stages. In the thyroid gland, iodine ions are oxidised to the active form (cation 1+) by the action of a specific thyroid peroxidase (thyroperoxidase), which reacts with tyrosyl residues of thyroglobulin to form 3-iodotyrosine. Subsequent iodisation of 3-iodotyrosine yields 3,5-diiodotyrosine. The condensation reaction of 3,5-diiodotyrosine with 3-iodotyrosine in the colloid of the thyroid follicle yields 3,5,3 -triiodothyronine. Two molecules of 3,5-diiodotyrosine combine to form thyroxine. These hormones, bound to thyroglobulin, are then released into the blood as a result of thyroglobulin proteolysis regulated by thyrotropin. In the blood, normal concentrations of 3,5,3 -triiodothyronine can vary by as much as 1-1.5 p,g/l, and thyroxine concentrations range from 60 to 120 (xg/l. 

MIT, DIT, and Tx in the thyroid extracts of rats injected 48 hours previously with I were first identified as free amino acids along with three unknown substances (16,31). One of the unknowns was later shown to be TRITh (63,67) and traces of MIH were regularly found by the writers. Autography is the best technique for studying the free amino acid of thyroid because the amino acids contain only a very small part of total radioactivity (a few per cent). Since the proteolysis of thyroglobulin seems to be complete and does not cause liberation of iodothyronines faster than iodotyrosines, the composition of the mixture of free iodinated amino acids is similar to that of samples obtained by in vitro hydrolysis of the protein... 

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