Preparation of125I-Labeled Gastric Inhibitory Polypeptide, Using Water-Insoluble 1, 3, 4, 6-Tetrachloro-3α, 6α-Diphenylglycoluril (Iodo-gen) as the Oxidizing Agent

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Abstract
The preparation and evaluation of a 125I-labeled gastric inhibitory polypeptide (GIP) by means of a modified Iodo-gen method are described. The portal vein fasting plasma GIP concentration was determined in 6 groups of rats with 10 rats in each group. In three of the groups, three separately prepared Iodo-gen tracers were used in the radioimmunoassay, and in the three other groups three separately prepared chloramine-T tracers were used. The mean fasting portal vein plasma GIP concentrations in the three groups were 45.0 ± 5.0, 41.9 ± 3.9, and 39.6 ± 2.7 pmol/l, respectively, with the Iodo-gen tracers and 80.3 ± 4.6, 78.4 ± 5.4, and 85.7 ± 3.7 pmol/l with the chloramine-T tracers. The values obtained in each of the groups with the Iodo-gen tracers were significantly lower than those obtained in each of the groups with the chloramine-T tracers. This may be due to a better preserved immunoreactivity of the Iodo-gen tracer than of the chloramine-T tracer. The explanation of this may be the water insolubility of Iodo-gen, which may prevent oxidation of easily oxidized residues of the peptide during the radioiodination procedure.