Iodogen-catalyzed iodination of transferrin.

Transferrin (human, rabbit) labels at low efficiency (1%-10%) with 125I when reaction of 0.5-0.7 ng of I- (8-10 microCi) with 20 micrograms of the protein is catalyzed by iodogen in a constant volume of 0.1 ml. Microiodination by this technique was therefore analyzed with regard to the relative proportions of the reactants, oxidant requirement, and timing. In vials giving a reaction volume-to-active surface ratio of 0.88, efficiency was independent of the amount of iodogen in the range from 1 microgram to 15 micrograms, and prolongation of the reaction beyond 1 min failed to improve yields. In contrast, the amount of I- present was decisive. Butanol/NH4OH chromatograms of iodination reactions carried out with 0.6 ng or 20 ng of I- showed 3-4 radioactivity peaks, the relative proportions of which markedly depended on the amount of I- present originally. A link was established between labeling efficiency and chromatographic profile of the I- derivatives formed during oxidation. Dual-label experiments in rats showed that transferrin (20 micrograms) can be labeled using iodogen (1-5 micrograms, 1 min) to behave indistinguishably from its IC1-labeled counterpart. However, prolonged exposure to more oxidant progressively damaged the protein. The damage was independent of substituting I and it manifested itself in increased protein binding to the anion exchange resin, Dowex 1-X8. Over 99.5% of the labeled residues in iodotransferrin were mono- and diiodotyrosines (MIT, DIT). DIT content of the protein increased linearly with the number of I atoms substituted. At comparable levels of substitution, more label was present as MIT after using iodogen than after using IC1. Electrophoretic data are presented regarding homogeneity of the label as obtained after iodinating transferrin by different methods and to varying extents.