A systematic approach to the preparation of 125I-labeled gastrointestinal regulatory peptides with high specific radioactivities

A systematic approach is outlined for the preparation of a whole series of immunoreactive 125I-labeled gastrointestinal peptides with high specific radioactivities. In our hands, the theoretically superior Iodo-gen method has no more to offer than the harsher chloramine-T method in the iodination of secretin, vasoactive intestinal polypeptide, gastric inhibitory polypeptide and motilin; whereas the gentler Iodo-gen method has to be used to obtain fully immunoreactive cholecystokinin39 (CCK39) and Tyr1-somatostatin tracers. By applying the iodination mixtures on a Sephadex G-15 or a Sephadex G-10 column followed by a SP Sephadex C-25 column - being eluted under so-called ''finite adsorption equilibrium'' between the peptides to be purified and the adsorbent - highly purified tracers are obtained with unusually high specific radioactivities. Stored at -20 deg C in diluted aliquots of 200 to 500 μl, these tracers can be used for radioimmunoassay purposes without rechromatography for at least 60 days