Tyrosine iodination reaction

Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone. 

Isoflavones have been implicated in goiter induction. Soybean extracts inhibit reactions catalyzed by thyroid peroxidase (TPO), essential to the synthesis of thyroid hormones (Divi et al., 1997). Genistein and daidzein (at about 1-10 p,M of IC50) may act as alternative substrates for tyrosine iodination (Divi et al., 1997). Furthermore, genistein and daidzein have also been shown to cause the irreversible inactivation of TPO in the presence of hydrogen peroxide. Genistein also inhibits thyroxine synthesis in the presence of iodinated... 

Directing the iodination reaction toward histidine residues in proteins, as opposed to principally tyrosine modification, is possible simply by increasing the pH of the lodobeads reaction from the manufacturer s recommended pH 7.0-8.2 (Tsomides et ai, 1991). No reducing agent is required to stop the iodination reaction as is the case with chloramine-T and other methods. 

Directing the iodination reaction toward histidine residues in proteins, as opposed to principally tyrosine modification, is possible simply by increasing the pH of the... 

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,... 

For PA, the conditions of the iodination reaction can be crucial. I(or I)-labeled PA generally has been prepared by labeling tyrosine residues by use of the chloramine-T method.Lactoperoxidase also has been used. However, since all four tyrosine groups of PA are necessary for maximum functional activity, loss of Fc binding ability results from incorporation of more than 1 iodine atom per molecule.Loss of activity also results from chemical damage unless limited amounts of chloramine-T are used and the reaction time is kept short. When this method is used, experimental details must be followed exactly. 

Antithyroid drug prototype inhibits tyrosine iodination and coupling reactions orally active. Tox rash, agranulocytosis (rare). Methimazole is similar but more lipid-soluble. 

Iodination reactions on peptides lead to monoiodinated (if the peptide has several tyrosine residues, reference is to positional isomers) and polyiodinated products. The latter results from ... 

The mechanism of thyroglobufin (Tg) iodination has been intensely debated and remains an open question the specific controversy focuses on whether or not iodination of Tg tyrosyl groups occurs via an enzyme-bound intermediate, or by freely diffusing iodination equivalents released from TPO. It is clear, however, that TPO actuates nonspecific iodination reactions in the thyroid. The two chemical species of iodine that can iodinate biomolecules under physiologic conditions are HOI and I2. Dunford and Ralston (1983) demonstrated that HOI, not I2, is the primary species that iodinates tyrosine in aqueous environments that contain both I2 and HOI. I2 is likely to be the species that is responsible for the iodination of lipids. The distinct pharmacological and toxicological properties of iodide and oxidized iodide (I2) have been a topic of some interest in the literature, as some activity not associated with THs has been ascribed to I2. 

An iodination reaction often creates a mixture of different molecules. For example, the iodination of proteins that contain several tyrosine residues yields monoiodized, di-iodized. 

D-Tyrosine solution (10 mg/ml) Prepare in water, store at -l-4°C, and use to quench the iodination reaction. 

The length of the iodination reaction can be modified to increase or decrease the specific activity of the labeled protein however, there is a trade-off between achieving a protein of higher specific activity and the possibility of either protein denatur-ation or modification of key tyrosine residues that may affect the protein s activity. We have used reaction times of 5-10 min with successful results. In addition, we have performed assays under various conditions of specific activity (from 2.0 X 10 to 1.3 X 10 cpm//xg), with no apparent effect on the specificity of the interaction (see Crawford et al., 1992). 

Suppose that sequence analysis of a protein has provided the data that the given protein contains 7 residues of t5Toslne. React the protein with radioactive iodine - 131 in a sultabk reaction mixture so that tyrosine residues pick up the iodine. After this precipitate the protein with the help of trichloroacetic acid. Count the precipitate. Suppose that the count given Is 4000 cpm. Now denature amother aliquot of the protein and subject this too to iodination reaction in the presence of radioactive iodine 131. After iodination is over, precipitate the protein and count for radioactivity. Suppose now that the count is 9000 cpm. 

Additional kinetic evidence supporting molecular iodine as an iodinating species is sparse. Li325 found that the iodination of tyrosine in acetate buffers at 25 °C showed the mixed inverse dependence on iodide ion concentration noted above, so that part of the reaction appeared to involve the molecular species. Subsequently, Doak and Corwin326 found that the kinetics of the iodination of (N-Me)-4-carboethoxy-2,5-dimethyl- and (N-Me)-5-carboethoxy-2,4-dimethyl-pyrroles in phosphate buffers in aqueous dioxane at 26.5 °C obeyed equation (162), viz. 

Paul and Treybal studied iodination of L-tyrosine. The overall scheme of iodination is shown in Fig. 5.3-3. The reactions can be summarized as ... 

The reactions are parallel with respect to iodine (fi) and consecutive with respect to iodine-substituted derivatives of L-tyrosine (R). 

Tyrosine may be targeted specifically for modification through its phenolate anion by acylation, through electrophilic reactions such as the addition of iodine or diazonium ions, and by Mannich condensation reactions. The electrophilic substitution reactions on tyrosine s ring all occur at the ortho position to the —OH group (Figure 1.11). Most of these reactions proceed effectively only when tyrosine s ring is ionized to the phenolate anion form. 

Each bead can iodinate up to 500 pg of tyrosine-containing protein or peptide. This translates into an oxidative capacity of about 0.55 pmol per bead. The rate of reaction can be controlled by changing the number of beads that are used and altering the sodium iodide concentration added to the reaction. Reaction volumes of 100-1,000 pi are possible per bead. The following protocol is suggested for iodinating proteins. Optimization should be done to determine the best incorporation level to obtain good radiolabel incorporation with retention of protein activity. 

Add from 5 pg to 500 pg of a tyrosine-containing peptide or protein dissolved in iodination buffer to the reaction mixture. 

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