Histidine iodination reaction

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... 

The reaction of IODO-GEN with iodide ion in solution results in oxidation with subsequent formation of a reactive, mixed halogen species, IC1 (Fig. 266). Either 125I or 13 1 can be used in this reaction. The IC1 then rapidly reacts with any sites within target molecules that can undergo electrophilic substitution reactions. Within proteins, any tyrosine and histidine side-chain groups can be modified with iodine within... 

Radiolabelling is often performed by oxidation of iodide by different agents for iodination of tyrosine and histidine residues in peptides. However, unfavourable oxidation reactions can occur simultaneously, e.g., cysteine and methionine can be oxidized to their corresponding sulphone or sulphonic acid, so that neutral amino acids are converted to highly acidic groups, thus changing the electrostatic distribution within the structme of the... 

Tryptophan can also be iodinated directly. Both mono and diiodinated derivatives are known. Reaction is slower than with tyrosine and histidine, and hence occurs rarely. 

Advantages The reaction mixture remains free of enzymes. The iodined glucoseoxidase/ lactoperoxidase molecules (self-iodination) remain in the reaction vessel. Important Azid inhibits the lactoperoxidase. If the peptide/protein to be marked has no tyrosine residue, the experimenter tries to iodize a histidine resid ue. Above pH 8 to 8.5, iodine preferentially substitutes the imidazole ring of histidine. 

If the experimenter wishes for monoiodized proteins, however, just a single iodine can keep the protein from binding. In this case, the experimenter changes the reaction conditions to try to iodize another tyrosine residue or a histidine residue, in the hope that iodine in the new position does not interfere with the binding. If this hope turns out to be unfulfilled, or you do not want to get involved in endless screening of reaction conditions, Bolton-Hunter reagent or marking remain an option. 

The experimenter separates heterooligomers before cleaving them into subunits. Then she denatures the purified subunit and reduces possible disulfide bridges. A carboxymethylation with iodoacetic acid protects against the reoxidation of the cysteines (Lind and Baker 1982). This reaction requires a sure instinct, in that other amino acids such as methionine, lysine, and histidine also react at high concentrations of iodine acetate or at the wrong pH. If you just need a few partial sequences, you can skip the carboxymethylation. 

Before leaving the subject of the iodination of amino acids in the thyroid gland, it may be of interest to mention the presence of mono-iodohistidine, discovered by Roche, Lissitzky, and Michel (1961) in labeled rat thyroglobulin hydrolyzates. This amino acid is present only in small amounts and might be considered as a chemical accident, merely resulting from the ease in which histidine can be iodinated. This reaction has been studied in vitro by Roche, Lissitzky, Michel, and Michel (1951) the iodination of histidine is considerably slower than that of tyrosine. 

In addition to all the other groups noted by them, Blum and Strauss (282) also reported a disappearance of unoxidized sulfur, and stated that the iodide ion formed during iodination was over four times that expected from a straight substitution reaction. They suggested that cystine sulfur had been oxidized. They also found that the biuret value was lower in the iodinated protein. Sometimes considerably more iodine is bound to proteins than can be accounted for on the baas of thmr tyrosine and histidine content (286, 287). 

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