I-125 (iodine-125) is a gamma-emitting radionuclide with a half-life of about 59.4 days, incorporated into peptides by electrophilic substitution on the phenol ring of tyrosine — or, when no tyrosine is present, on histidine or via a conjugated Bolton-Hunter reagent. Chloramine-T and Iodogen are the standard oxidising methods.
Why it is still used
Specific activity is the reason. A mono-iodinated peptide can approach 2,200 Ci/mmol, which lets receptor binding and RIA work proceed at true tracer concentrations that do not perturb the equilibrium being measured. Saturation, competition and kinetic binding studies were built on I-125 tracers, and much of the receptor-affinity literature for peptide hormones rests on them.
Practical constraints
Everything about the format is governed by decay. Activity is quoted with a reference date and must be decay-corrected; a tracer is typically usable for one to two half-lives before specific activity and radiolytic damage make it unreliable. Use requires a radioactive materials licence, a gamma counter, shielded handling and a waste route. Radiolysis also degrades the peptide itself, so tracers are stored dilute, often with a radical scavenger, and repurified before demanding work. Peptides lacking tyrosine may need a sequence modification, which must be validated against the parent.
Related terms and material
biotin label · FAM/FITC label · half-life. Reference material: labeled peptides, assay kits. Further reading: labeled peptides guide.