Lumera Labs Journal · Method note
Peptide receptor-binding assays: a short primer
Published 2025-04-12 · Lumera Labs Editorial · Kelowna, BC
Short answer. Three assay platforms cover most peptide-receptor research: cAMP accumulation for Gs/Gi-coupled receptors, β-arrestin recruitment (BRET or PathHunter) for biased-agonism profiling, and competition radioligand binding for direct affinity measurement (Ki). Each answers a different question.
cAMP accumulation
Measures downstream signaling for Gs-coupled receptors (cAMP rises) or Gi-coupled receptors (cAMP falls in the presence of forskolin pre-stimulation). The HTRF (homogeneous time-resolved fluorescence) format is the workhorse: fast, plate-friendly, no radioactivity. Output is dose-response curve, EC50 reporting, and Emax for efficacy comparison.
Best for: initial agonist potency screening on Gs/Gi-coupled GPCRs (GLP-1R, GIPR, GHRH-R, melanocortin receptors).
β-arrestin recruitment
Measures the recruitment of β-arrestin-2 to the agonist-activated receptor — independent of G-protein coupling. The Eurofins PathHunter platform uses enzyme-fragment complementation (β-galactosidase reconstitution); BRET-based methods (NanoBiT, BRET2) give kinetic readouts in real time.
Best for: biased-agonism profiling. Two peptides at the same receptor can have similar cAMP EC50 but different β-arrestin EC50 — that's biased agonism, and it's a real research target for next-generation analog design.
Competition radioligand binding
The classical method: an iodinated reference peptide ([¹²⁵I]-labeled) competes with your test peptide for receptor binding, and the IC50 is converted to Ki via the Cheng-Prusoff correction. Output is direct affinity, not signaling potency — answers "how tightly does this peptide bind" rather than "how strongly does it activate downstream signaling."
Best for: direct affinity comparison across structurally related analogs, KD/Ki reporting for publication, and any work where the cAMP or arrestin readouts diverge from expected behavior.
Choosing the platform
| Question | Platform |
|---|---|
| How potent is this agonist? | cAMP HTRF |
| Is this peptide biased toward arrestin signaling? | cAMP + β-arrestin pair |
| What's the affinity (Ki)? | Competition radioligand binding |
| Does this antagonist block receptor activation? | cAMP HTRF (Schild analysis) |
Reference-standard selection
Platform reproducibility depends on the reference peptide's purity and consistency across batches. Lot-specific COAs are essential — running cAMP curves with two different lots of "the same" peptide that came from different syntheses can produce 2-fold EC50 differences. Lumera publishes per-lot Janoshik COAs at /lab-results/ for every release.
Frequently asked questions
Which assay measures direct affinity?
Competition radioligand binding. Ki from this assay is the closest you'll come to a thermodynamic affinity number outside of biophysical methods like SPR.
What is biased agonism?
When two agonists at the same receptor produce different ratios of G-protein vs β-arrestin signaling. Detected by running cAMP and β-arrestin assays in parallel and comparing EC50 ratios.
Is BRET better than HTRF?
Different. BRET gives kinetics; HTRF gives endpoint. BRET requires transfected cells with the BRET partner; HTRF works on lysates. Pick by what your study design needs.
How does lot consistency affect assay reproducibility?
A 1% shift in HPLC purity can produce 5–10% shift in EC50. Lot-specific COAs and reference-grade purity are essential for cross-experiment comparability.
Where can I see Lumera's lot COAs for these reference peptides?
/lab-results/ has per-lot Janoshik COAs for every release.
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