Chloramine-T in radiolabeling techniques - III. Radioiodination of biomolecules containing thioether groups

A previously reported method for iodination of the tyrosine moiety of oxida tion-sensitive biomolecules was found to cause unacceptable damage to biomo lecules containing thiols and thioether groups. This was due to the oxidati on of the sulfur-containing residues by molecular iodine (I-2), To selectiv ely iodinate the tyrosine moiety with minimum oxidation to the sulfur funct ionality, studies of the kinetics of the reactions between I-3(-) and vario us amino acids and small peptides at various pH values in phosphate buffer were undertaken. Within the pH range studied (5.5-8.2), the results showed that the iodination reaction is strongly catalyzed by hydroxide ions, where as the oxidation of the sulfur group was insensitive to pH, The results als o showed that both reactions are strongly catalyzed by HPO4- ion. In a comp lex molecule, such as methionine- enkephalin, oxidation of the methionine r esidue (undesirable reaction) proceeds in parallel with iodination of the t yrosine residue (desirable reaction). If such a molecule was iodinated in 0 .01 M phosphate buffer at pH values above 7.5, the iodination reaction woul d proceed much more rapidly than the oxidation reaction, resulting in a hig h yield of iodinated substrate with little oxidative damage. (C) 2000 Acade mic Press.