[37] Radioiodination of human alpha interferons by the chloramine T method

This chapter discusses the radioiodination of human alpha interferons by the chloramines T method. The chloramine T method has long been used for incorporating iodine into the tyrosine residues of proteins. The reactants generated are strong oxidants, and for sensitive proteins gentler methods may be needed. The chloramine T reaction involves a slow hydrolysis to generate hypochlorite followed by fast reactions with iodide to form oxidized species that substitute rapidly at positions ortho to the tyrosine hydroxyl, the first substitution facilitating the second. The iodine atom has dimensions similar to those of the phenolate ion, so substitution would likely lead to local distortions in protein structure. The method was originally devised to keep hypochlorite and molecular iodine concentrations as low as possible during the reaction, thus reducing damage and the escape of volatile iodine. In a properly screened and ventilated hood, the reaction poses no great hazard, but particular care is needed to avoid contamination because, in the method described, iodide is allowed to oxidize before it is allowed to react.