Chloramine-T in Radiolabeling Techniques: III. Radioiodination of Biomolecules Containing Thioether Groups

A previously reported method for iodination of the tyrosine moiety of oxidation-sensitive biomolecules was found to cause unacceptable damage to biomolecules containing thiols and thioether groups. This was due to the oxidation of the sulfur-containing residues by molecular iodine (I2). To selectively iodinate the tyrosine moiety with minimum oxidation to the sulfur functionality, studies of the kinetics of the reactions between I−3 and various 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, whereas the oxidation of the sulfur group was insensitive to pH. The results also showed that both reactions are strongly catalyzed by HPO−4 ion. In a complex molecule, such as methionine–enkephalin, oxidation of the methionine residue (undesirable reaction) proceeds in parallel with iodination of the tyrosine residue (desirable reaction). If such a molecule was iodinated in 0.01 M phosphate buffer at pH values above 7.5, the iodination reaction would proceed much more rapidly than the oxidation reaction, resulting in a high yield of iodinated substrate with little oxidative damage.