Radioimmunoassay (RIA) labeling: Which radionuclide is most commonly used to label ligands or antibodies in classic RIA protocols?

Introduction / Context:
RIA detects antigens or antibodies at very low concentrations by using a radiolabeled component. Selecting an appropriate isotope balances half-life, detection efficiency, labeling chemistry, and safety.

Given Data / Assumptions:

• Typical RIA labels peptide/protein hormones or antibodies.
• Isotopes considered: tritium, carbon-14, iodine-125.
• Solid-phase separation or double-antibody methods are used.

Concept / Approach:
Iodine-125 is preferred for labeling proteins because iodine can be efficiently introduced into tyrosine residues, it emits gamma rays detectable by standard gamma counters, and it has a practical half-life (~60 days). Tritium and carbon-14 are common in metabolic labeling and autoradiography but are less ideal for routine RIA of proteins.

Step-by-Step Solution:
Consider labeling chemistry → iodination of tyrosines is straightforward for proteins.Consider detection → I-125 gamma emissions are easily counted with high sensitivity.Match the isotope most commonly used in RIA → Iodine-125.

Verification / Alternative check:
Classic hormone RIAs (e.g., TSH, insulin) use I-125-labeled ligands or antibodies.

Why Other Options Are Wrong:

• Tritium and carbon-14 emit low-energy beta particles and are used differently; they are not the standard choice for protein RIA labeling.
• “All of these” overstates usage patterns.

Common Pitfalls:
Confusing general radiolabeling in biochemistry with the specific needs of RIA (protein-friendly chemistry and gamma detection).

Final Answer:
Iodine-125