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 regulatory 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 an 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 degrees C in diluted aliquots of from 200 to 500 microliter, these tracers can be used for radioimmunoassay purposes without rechromatography for at least 60 days.