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Protein Protocols Protein Protocols

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972 Conlon<br />

Fig. 1. The structure of IODO-GEN (1,3,4,6-tetrachloro-3α,6α-diphenylglycoluril).<br />

peptide from the unreacted starting material in order to obtain a tracer of sufficiently<br />

high specific activity to be of use in radioimmunoassay, radioreceptor assay, or autoradiography.<br />

Reversed-phase high-performance liquid chromatography (RP-HPLC) combines<br />

rapidity and ease of operation with optimum separation of labeled and unlabeled<br />

peptide. The availability of wide-pore C 3 and C 4 columns generally permits good<br />

recoveries of labeled proteins with molecular mass (M r) > 10,000. Techniques such as<br />

selective adsorption to diatomaceous materials (e.g., talc or microfine silica) and gel<br />

permeation chromatography give tracers of low specific activity, and ion-exchange chromatography<br />

is time consuming and results in a sample dilution that may be unacceptable.<br />

2. Materials<br />

2.1. Apparatus<br />

No specialized equipment is required to carry out the iodination, but the reaction<br />

should be carried out in an efficient fume hood. Reaction takes place in a 1.5-mL naturalcolored<br />

polypropylene microcentrifuge (Eppendorf tube) (see Note 1), immersed in an<br />

ice bath. A nitrogen or argon cylinder is required for removal of solvent. Liquids are<br />

dispensed with 10- and 100-µL pipets (e.g., Gilson Pipetman) that, because of inevitable<br />

contamination by radioactivity, should be dedicated to the reaction and stored in<br />

a designated area.<br />

For HPLC, a system capable of generating reproducible linear two solvent gradients<br />

is required. Again, because of contamination by radioactivity, an injector, for example,<br />

Rheodyne Model 7125 with 1-mL sample loop; a 1-mL leak-free injection syringe, for<br />

example, Hamilton Gastight (cat. no. 1001); a 25 × 0.46 cm analytical reversed-phase<br />

column (see Note 2 on column selection); and a fraction collector capable of collecting<br />

a minimum of 70 samples, for example, Frac 100 (Pharmacia, Uppsala, Sweden) should<br />

be dedicated to the radioiodination procedure. A UV detection system and chart<br />

recorder/integrator are not necessary unless it is important to demonstrate that the<br />

radiolabeled component has been completely separated from the starting material.

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