2.5- diiodotyrosine derivative

Horseradish peroxidase (HRP) has also been used in an enzymatic process to create aryl-aryl bonds, especially for the synthesis of diiodotyrosine derivatives in a possibly biomimetic transformation [30, 100]. Here again, the regioselectivity of bond formation is an important issue and, depending on both the conditions and the substrate, C-C or C-O-coupling can occur. The effect of substituents ortho to the alcohol group has been studied by Sih and co-... 

Electrochemical oxidation of 3,5-dihalogenated tyrosine derivatives provided the C—O or C—C conpled dimers depending on the halogen snbstitnents. As shown in Scheme 11, electrochemical oxidation of both dichloro- and dibromotyrosines provided the corresponding diaryl ethers snch as 61, while the diaryl (63) was selectively produced from the 3,5-diiodotyrosine derivative. Quite interestingly, almost the same results have been obtained by enzymatic oxidation. ... 

It should be noted that the reaction discussed is purely a chemical one, and there is no evidence that the reaction occurs in the same way enzymatically. In fact little is known about the enzymes concerned with thyroxine formation, and it is not certain that any enzymes are required at all. As already mentioned and further shown, diiodotyrosine derivatives... 

An additional reason for the higher reactivity of the phenolic group compared to the sterically hindered threonine hydroxy group may be its free accessibility. In contrast, the reactivity of ring-substituted tyrosines, such as 3-iodo- or 3,5-diiodotyrosine, is relatively low, thus allowing the use of these derivatives without side-chain protection, even in solid-phase synthesis. ... 

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. 

It is frequently important to separate the different derivatives of the iodinated protein obtained upon iodination. In almost all cases the iodinated reaction mixture contains a mixture of uniodinated protein and different iodinated derivatives (mono-, diiodinated, etc.). It is usually not difficult to separate the purified protein on the basis of the number of iodine atoms incorporated. The pH of the phenol group changes by approximately 2 pH units when the tyrosine phenol is converted to iodotyro-sine and another 2 units when it is converted to the diiodotyrosine. Thus,... 

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. 

I. They conclude that the strong T- "S transition in diiodotyrosine and thyroxine derivatives makes these compounds very efficient in the transfer... 

Primary or secondary aliphatic amines in aqueous or methanolic solution may replace the ammonia, for these do not form explosive iodo derivatives.468,469 This method can be recommended for preparative purposes and has proved very valuable in the preparation of thyroxine from 3,5-diiodo-thyronine in 20% aqueous ethylamine (KI3), of thyroxine methyl ester from 3,5-diiodothyronine methyl ester in 1 2 butylamine-methanol (I2)4 9 and of 3,5-diiodotyrosine in 20% aqueous ethylamine (Nal3),470 as well as in iodination... 

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. 

L-3-Iodotyrosine, L-3,5-diiodotyrosine, L-triiodothyronine and L-thyroxine are the most important iodinated phenylpropanoids found in nature. L-3-Iodo-tyrosine and L-3,5-diiodotyrosine are derivatives of L-tyrosine. L-Triiodothyro-nine and L-thyroxine are derived from the chemically related amino acid L-thyronine. 

Theoretically, the condensation of mono- and diiodotyrosine could yield four different compounds 3,3 -diiodothyronine, 3,3, 5 -triiodothyronine, 3,5,3 -triio-dothyronine, and thyroxine (see Fig. 8-4). The exact mechanism of the formation of these various iodinated derivatives is not clear, but it has been suggested that they are formed by the condensation of two molecules of iodotyrosines with loss of a side chain. In this reaction, the iodinated tyrosine is converted by the loss of two atoms of hydrogen to the quinoid form, which acts as a free radical and condenses with other molecules of iodinated tyrosine to yield a diphenyl ring. During such a reaction, an oxygen bridge is formed, and one of the side chains of tyrosine is split to yield an a-amino acrylic acid. The a-amino acrylic acid is further metabolized to yield ammonia and pyruvic acid. 

The cells of the thyroid gland fix I ions and oxidize them enzymically to iodine. They contain the specific enzyme systems for the formation of iodotyrosine and iodohistidine, and for the condensation of the iodotyro-sines into iodothyronines, especially the two thyroid hormones, l-3 5 3 -triiodothyronine and L-3 3 -diiodothyronine. Another thyroid hormone and the most important quantitatively is L-thyroxine, derived from the coupling together of two molecules of L-3 5-diiodotyrosine. 

It must be evident to the reader that more work is needed to determine the amino acids other than tyrosine in proteins which react with iodine. Since iodine has a powerful effect on some absorbing groups, spectro-photometric analyses coupled with variations of pH and other factors might prove very useful. In the case of tyrosine, the ionization constants, or pKs of phenolic groups of the iodo derivatives have been determined spectrophotoroetrically (75, 307). For tyrosine (75, 308), mono iodo-tyrosine (308), and diiodotyrosine (75, 308) the pKs are 10.1, 8.2 and 6.4 respectively. 

Fig. 10. Formation of 3,5,3 -triiodothyronine and of thyroxine by the action of I + on 3,5-diiodotyrosine, followed by radioactivity measurements on the products of the reaction separated by paper chromatography (44). Solvent, iso-pentanol saturated with 6 N NH OH. Abscissa, number of labeled I atoms reacting with one molecule of 3,5-diiodothyronine ordinate, number of labeled I atoms fixed by 3,5-diiodothyronine as tri- or tetrasubstituted derivatives.

Study of the formation of iodinated derivatives by thyroid tissue has revealed that monoiodotyrosine is formed first followed hy diiodotyrosine (266). Later thyroxine and triiodothyronine are found (266) (see Fig. 17). Iodine concentration and organic iodine binding by the thyroid gland are separate processes. They can be distinguished by the fact that concentration of iodine is inhibited by thiocyanate, while thiourea permits iodine concentration, but inhibits the organic binding of the iodine. 

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