Diiodotyrosine formation

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... 

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-... 

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. 

Thyroid hormone synthesis requires oxidation of dietary iodine, followed by iodination of tyrosine to mono- and diiodotyrosine coupling of iodotyrosines leads to formation of the active molecules, tetraiodo-tyrosine, (T or L-th3rroxine) and triiodotyrosine (Tj or L-thyronine). 

There are three plausible pathways for the biosynthesis of triiodothyronine. It could be formed from one molecule each of mono- and diiodotyrosine in the same way as thyroxine can be formed from two molecules of diiodotyrosine or it could be formed by incomplete iodination of preformed thryonine or diiodothyronine or it could be formed by deiodina-tion of thyroxine. Formation by deiodination is favored by English work-... 

At low concentrations of 1 and the absence of other substrates, the enzyme is slowly iodinated. In thyroid peroxidase a ternary complex that contains substrates such as tyrosine adjacent to the active site may occur. This provides an attractive hypothesis for the synthesis of intermediates leading to diiodotyrosine and thyroxine formation. 

We have seen that the coupling of diiodotyrosine to give thyroxine can occur in vitro to a small extent. However, the protection of both the amino and carboxyl groups of diiodotyrosine leads to the formation... 

With regard to the fate of the iodinated amino acids in the thyroid, the following concluraons have been reached. Monoiodotyrosine and diiodotyrosine do not leave the thyroid gland after proteolytic hydrolysis of thyroglobulin, but are enzymically deiodinated with the formation of iodide this iodide is available for re-utilization in hormone synthesis. Thyroxine and triiodothyronine are released into the circulation. 

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 formation of thyroxine from 3,5-diiodotyrosine by oxidation with hypoiodite (HIO) was first demonstrated by von Mutzenbecher, although the yield was extremely minute. This was subsequently verified, and Harington was able to obtain a greatly increased yield by carrying out the oxidation with H2O2 and removing the thyroxine formed from the sphere of reaction by virtue of its solubility in butyl alcohol. 

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... 

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. 

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.

Saturated a-acylamino acids are prepared by acylation of o-amino acids or, less frequently, by reduction of the corresponding unsaturated compounds. It sometimes is possible to effect both the preparation of the o cylamino acid and the formation of the azlactone by heating the amino add with a large excess of acetic anhydride. Leucine and phenylalanine give excellent yields of azlactones under these conditions. However, this method is not satisfactory with alanine, diiodotyrosine, isovaline," or a-amiiioisobutyric acid," which yield products of high molecular weight. 

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. 

M. E. Morton, I. L. Chaikoff and S. Rosenfeld. Inhibition effect of inorganic iodide on the formation in vitro of thyroxine and diiodotyrosine by surviving thyroid tissue. J. Biol. Chem. 154 381 (1944)... 

---