I-125 Radiolabel

I-125 (iodine-125) is a radionuclide that emits gamma radiation and has a half-life of roughly 59.4 days. It is introduced into peptides by electrophilic substitution on the phenol ring of tyrosine — or, if the sequence has no tyrosine, on histidine or through a coupled Bolton-Hunter reagent. The usual oxidising methods are Chloramine-T and Iodogen.

Why labs still rely on it

The deciding advantage is specific activity. A peptide carrying a single iodine can reach close to 2,200 Ci/mmol, allowing receptor-binding and RIA experiments to run at genuine tracer levels that leave the measured equilibrium undisturbed. Saturation, competition and kinetic binding assays were developed around I-125 tracers, and a large part of the receptor-affinity data on peptide hormones is based on them.

Working limitations

Decay dictates every aspect of the format. Activity is stated for a reference date and needs decay correction; a tracer generally remains usable for one to two half-lives before falling specific activity and radiolytic damage undermine it. Working with it demands a licence for radioactive materials, a gamma counter, shielding and a waste disposal route. Radiolysis damages the peptide as well, so tracers are kept dilute, frequently with a radical scavenger, and purified again ahead of critical experiments. A peptide without tyrosine may need its sequence altered, and that change has to be validated against the parent molecule.

Related terms and material

biotin label · FAM/FITC label · half-life. Reference material: labeled peptides, assay kits. Further reading: labeled peptides guide.