A. Yamada et al., Chloramine-T in radiolabeling techniques - III. Radioiodination of biomolecules containing thioether groups, ANALYT BIOC, 277(2), 2000, pp. 232-235
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.