The Lactoperoxidase Method for Radiolabeling Protein
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This method, introduced by Marchalonis (1), employs lactoperoxidase in the presence of a trace of hydrogen peroxide to oxidize the radioactive iodide 125r to produce the reactive species 125I2 or 125I+. These reactive species substitute mainly into tyrosine residues of the protein, although substitution into other amino acid residues can occur under certain conditions. The oxidation can be stopped by simple dilution. Although the technique should result in less chance of denaturation of susceptible proteins than the chloramine T method, it is more technically demanding and is subject to a more marked variation in optimum reaction conditions.