Histidine, iodination

Cephalins Ethyl oleate Ferrous citrate L-Glutamic acid L-Histidine Iodine L-lsoleucine... 

Copper hydroxide (ic) Copper iodide (ous) Cupric sulfate anhydrous Ferric chloride Ferric chloride hexahydrate Ferrous sulfate anhydrous L-Glutamine Griseofulvin L-Histidine Iodine Iron ammonium citrate Manganese acetate (ous) Manganese chloride (ous), tetrahydrate Manganese citrate (ous) Phenothiazine... 

Figure 12.2 The iodination of tyrosine or histidine residues in proteins by H2OI+.

The addition of a radioactive iodine atom to a protein molecule typically has little effect on the resultant protein activity, unless the active center is modified in the process. The size of an iodine atom is relatively small and does not result in many steric problems with large molecules. The sites of potential protein modification are tyrosine and histidine side chains. Tyrosine may be modified with a total of two iodine atoms per phenolate group, whereas histidine can incorporate one iodine. Sulfhydryl modification at cysteine residues is typically unstable. 

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. 

Figure 12.5 IODO-GEN is a water-insoluble oxidizing agent that can react with 1251 - to form a highly reactive mixed halogen species, 125IC1. This intermediate can add radioactive iodine atoms to tyrosine or histidine side chain rings.

Figure 12.6 The immobilized glucose oxidase/lactoperoxidase system radioiodinates proteins through the intermediate formation of hydrogen peroxide from the oxidation of glucose. H2O2 then reacts with iodide anions to form reactive iodine (I2). This efficiently drives the formation of the highly reactive H2OI+ species that is capable of iodinating tyrosine or histidine residues (see Figure 12.2).

Fig. 18. Iodination of tyrosine (top) and histidine (bottom) amino acid residues in proteins by electrophilic substitution.

The histidine residue in position 12 is crucial for activity. Its destruction by photooxidation or modification by iodination (No. 13, Table XIB) or carboxymethylation in the 3 position on the ring (No. 24, Table XIA) all destroy potential activity. The first two also substantially lower the association constant while the latter has no effect or may even increase it slightly. The CM group in the 3 position is, of course, easily accommodated and the interaction of the free carboxyl group with other positive charges nearby, e.g., His 119, may explain the increase in association constant. Conversion to a pyrazolyl residue destroys activity but not binding (No. 19). 

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

Figure 265 IODO-BEADS contains immobilized Chloramine-T functional groups that can react with radioactive iodide in aqueous solution to form a highly reactive intermediate. The active species may be an iodosulfonamide derivative, which then can iodinate tyrosine or histidine residues in proteins.

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... 

The main kinetic features of the iodination of imidazole—which show a suggestive resemblance to those of the iodination of phenols102, 103—have been substantially confirmed by a study of Havinga and co-workers99,100 on the kinetics of iodination of histidine and some other imidazole derivatives. 

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