Monoiodotyrosines

Figure 42-11. Model of iodide metabolism in the thyroid follicle. A follicular cell is shown facing the follicular lumen (top) and the extracellular space (at bottom). Iodide enters the thyroid primarily through a transporter (bottom left). Thyroid hormone synthesis occurs in the follicular space through a series of reactions, many of which are peroxidase-mediated. Thyroid hormones, stored in the colloid in the follicular space, are released from thyroglobulin by hydrolysis inside the thyroid cell. (Tgb, thyroglobulin MIT, monoiodotyrosine DIT, diiodotyro-sine Tj, triiodothyronine T4, tetraiodothyronine.) Asterisks indicate steps or processes that are inherited enzyme deficiencies which cause congenital goiter and often result in hypothyroidism.

Several compounds can be oxidized by peroxidases by a free radical mechanism. Among various substrates of peroxidases, L-tyrosine attracts a great interest as an important phenolic compound containing at 100 200 pmol 1 1 in plasma and cells, which can be involved in lipid and protein oxidation. In 1980, Ralston and Dunford [187] have shown that HRP Compound II oxidizes L-tyrosine and 3,5-diiodo-L-tyrosine with pH-dependent reaction rates. Ohtaki et al. [188] measured the rate constants for the reactions of hog thyroid peroxidase Compounds I and II with L-tyrosine (Table 22.1) and showed that Compound I was reduced directly to ferric enzyme. Thus, in this case the reaction of Compound I with L-tyrosine proceeds by two-electron mechanism. In subsequent work these authors have shown [189] that at physiological pH TPO catalyzed the two-electron oxidation not only L-tyrosine but also D-tyrosine, A -acetyltyrosinamide, and monoiodotyrosine, whereas diiodotyrosine was oxidized by a one-electron mechanism. 

The iodinated tyrosine residues monoiodotyrosine (MIT) and diiodoty-rosine (DIT) combine (couple) to form iodothyronines in reactions catalyzed by thyroid peroxidase. Thus, two molecules of DIT combine to form T4, and MIT and DIT join to form T3. 

Iodination (sometimes referred to as organification of Tgb) reactions form, as shown in Figure 4.5, monoiodotyrosine (MIT) and diiodotyrosine (DIT). 

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...

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. 

Experimental design Groups of 8-11 male rats were treated with 0, 1, 3, or 6 mg/kg/day doses of an unspecified mixture of PBBs in lecithin liposomes by gavage for 10 days. Plasma was assayed on treatment days 10 and 20. Other end points were evaluated on treatment day 20 these included plasma TSH levels, 5-hour thyroid uptake of I, incorporation of into monoiodotyrosine, diiodotyrosine, I, or T4, amount of intrathyroidal iodide, thyroid and liver weights, and body weights. Differences between mean values for the measured parameters in the control and PBB-treated groups were analyzed with the Student s Mest, with aP value of 0.05 considered as statistically significant. 

Takatera and Watanabe [41] used this technique for the speciation of iodide ion, I-, and five iodo amino acids (monoiodotyrosine (MIT), diiodotyrosine (DIT), 3,3,5-triiodothyromine (T3), 3,3,5 -triiodothyromine (rT3), and thyroxine (T4)) which are all found in thyroid hormones. The speciation of these compounds in clinical samples such as blood plasma and urine may assist in the identification of thyroid diseases. The RPLC-ICP-MS system was able to detect all of the I-containing compounds with no interferences. Detection limits were in the range 35-130 pg for the six compounds using a 50% methanol eluent. Detection limits were better for species eluted at a shorter retention time since the peak shapes were sharper. The detection limits calculated were an order of magnitude lower than for methods where UV absorbance detection was used. 

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... 

FIGURE 31-1 Structure of the thyroid hormones triiodothyronine (T3] and thyroxine (T4X Addition of one iodine atom [I] to tyrosine produces monoiodotyrosine addition of a second iodine atom produces diiodotyrosine. A monoiodotyrosine and diiodotyrosine combine to form triiodothyronine (T3X Coupling of two diiodotyrosines forms thyroxine (T4X... 

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. 

Abbreviations 3,5,3, 5 -tetraiodothyronine or thyroxine, T4 3,5,3-triiodothyronine, T, 3,3, 5 -triio-dothyronine, reverse T, or r-T3 3,5,3, 5 -tetraiodothyroacetic acid, TETRAC 3,5,3 -triiodothyroacetic acid, TRIAC 3,5-diiodotyrosine, DIT 3-monoiodotyrosine, MIT thyrotropic hormone, TSH thyreo-liberin, TRH growth hormone, GH microtubule associated proteins, MAPs TAU protein, one of the brain MAPs. 

Abbreviations BAT, brown adipose tissue BrAc, /V-bromoacetyl- BSA, bovine serum albumin CNS, central nervous system DEP, diethylpyrocarbonatc DIT, diiodotyrosine DTT, dithiothreitol G, glu-curonide Grx, glutaredoxin GSH, reduced glutathione GSSG, oxidized glutathione IRD. inner ring deiodination MIT, monoiodotyrosine ORD, outer ring deiodination PTU, propylthiouracil S, sulfate rT3, 3,3, 5 -triiodothyronine (reverse T,) Trx, thioredoxin T2, diiodothyronine T3, 3,3, 5-triiodothyronine T4, 3,3, 5,5 -tetraiodothyronine (thyroxine). 

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. 

The thyroid protein, thyroglobulin, contains 3-monoiodotyrosine, 3,5-diiodotyrosine and several iodinated thyronine derivatives (Salvatore and Edelhoch 1973). lodohistidines may also be present in this protein. 3-Bromotyrosine has been reported in serum proteins (Firnau and Fritze 1972). Methods for separating and analyzing these and other halogenated tyrosines are presented in 2.2.3. 

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). 

The thyroid gland secretes two hormones, thyroxine (3,5,3, 5 -L tetraiodothyronine) and triiodothyronine (3,5>3 -L-triiodothyronine), which are commonly known as T4 and T3, respectively (Table 52-1). In addition, the thyroid gland secretes small amounts of biologically inactive 3,3, 5 -L-triiodothyronine (reverse T3 [rTa]) and minute quantities of monoiodotyrosine (MIT) and diiodotyrosine (DIT), which are precursors of T3 and T4. The structures of these compounds are shown in Figure 52-1. 

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