Iodination of Peptides and Proteins

and Lazdunski, M. (1992). Purification, Affinity Labeling, and Reconstitution of Voltage-sensitive 

During the iodination with the oxidizer chloramine T, all reactants are present in solution (one-phase system). Pierce offers oxidizers that were applied to a solid phase (two-phase system iodobeads, iodogen). lodobeads are N-chlorobenzene sulfonamides attached to polystyrene beads. Iodogen is a hydrophobic chloramine T derivative applied to the wall of the reaction vessel. After the reaction with iodobeads and iodogen, the solid phase with the oxidizer can easily be separated from the reaction mixture. Hence, the addition of reducing agent (bisulfite) is unnecessary, which spares the sensitive disulfide bridges of some proteins. In addition, N-chlorobenzene sulfonamide is a milder oxidizer than chloramine T. 

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. 

After iodination, the free iodine needs to be separated from iodined (and uniodined) protein or peptide. Traditionally, small ion exchangers or gel filtration columns (for proteins) are used for is purpose, but HPLC (for smaller stable proteins and peptides) is gaining popularity. HPLC is worth using if you iodine often, because after a nm with 1 mCi iodine the 

If you have to iodize a peptide that can withstand pH 3 and 100° C, you can iodize it first with iodine. After separating out the free iodine, you substitute iodine for the iodine you incorporated into the peptide (Breslav et al. 1996). The advantage The products of the iodination (see Section 2.1.4) can comfortably be separated on the (cold) reversed-phase HPLC and then examined for their biological activity. For the active iodine derivative, you then switch the iodine with Todine. You can also skip the iodination reaction and instead synthesize the peptide with suitable iodine amino acid at the desired site (peptide synthesizer). 

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