title: Iodination of monoclonal IgG antibodies at a sub-stoichiometric level: immunoreactivity changes related to the site of iodine incorporation.
abstract: Thirteen monoclonal antibodies (MAbs) were labeled with 125I to a different degree such as to cover the range from 0.5-20 microCi/micrograms. By SDS polyacrylamide gel electrophoresis, the amount of iodine incorporated into heavy (h) and light (l) chains was determined. Comparing different MAbs, h:1 ratios varied from 0.6-34.6, but virtually no variation was observed with individual MAbs labeled at different levels. Immunoreactivity of labeled MAbs was analyzed with antigen-positive tumor cells according to the Lineweaver Burk method. Immunoreactive fractions were found to decrease with increasing iodine incorporation in 9/12 MAbs, while binding affinities decreased in 5/12 MAbs; only 1 MAb was stable in both respects. Immunoreactivity changes were not linked to preferential h or 1 chain labeling, nor to the isotype. This result indicated incorporation of the first iodine atom to take place at individually distinct residues, with a minimum estimate of two or four sites, depending on whether preferential chain labeling or random incorporation took place. In cases where increasing labeling led to a gradual decrease of binding affinity, a shift in the spectrum of acceptor residues has to be assumed.
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000302:isolation & purification; D006801:Humans; D007074:Immunoglobulin G / Q000302:isolation & purification; D007143:Immunoglobulin Heavy Chains; D007147:Immunoglobulin Light Chains; D007457:Iodine Radioisotopes; D007700:Kinetics; D008545:Melanoma / Q000000981:diagnostic imaging; D051379:Mice; D011865:Radioisotope Dilution Technique; D011877:Radionuclide Imaging; D051381:Rats
chemical_list: D000911:Antibodies, Monoclonal; D007074:Immunoglobulin G; D007143:Immunoglobulin Heavy Chains; D007147:Immunoglobulin Light Chains; D007457:Iodine Radioisotopes