Iodine-125 has a specific activity of 2170 Ci per matom (80 TBq per matom) and therefore the incorporation of one Iodine atom per peptide or protein molecule will yield material with a specific activity of 2170 Ci per mmole.
Labelling with Iodine-125 normally involves incorporation of Iodine-125 into tyrosine or histidine residues within the peptide or protein. This is done by treatment of the sample with Na125I in the presence of an oxidising reagent to generate a source of electrophilic Iodine. Usually, one or two Iodine atoms are added to each tyrosine residue depending on conditions used. The standard oxidant is chloramine-T but milder, more selective reagents are now available such as iodogen or the enzyme lactoperoxidase. For peptides and proteins lacking accessible tyrosine residues, free amines can be derivatived with Bolton-Hunter reagent prior to iodination.
Both of these methods result in quite drastic modification of the physical properties of the peptide concerned and although the specific activities of such compounds may be high they do not necessarily possess the biological characteristics of the parent compounds. There may also be a considerable degree of heterogeneity caused by the conditions of labelling. As valid receptor binding studies require labelled ligands with physical properties and biological activities identical to those of the unlabelled ligand, labelling with tritium is often preferred. This leads to a molecule possessing virtually identical properties to those of the parent compound, as there has been no gross modification of its primary structure, whilst affording a material of appreciable specific radioactivity.