Other · Disulphide-cyclised octapeptide; μ-opioid receptor antagonist
CTOP
Also known as D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH₂
A disulphide-cyclised, ornithine-containing octapeptide used to probe μ-opioid receptor-dependent signalling.
- Length
- 8 amino acids
- Half-life
- Not well characterised
- Mechanism
- CTOP acts as a selective, competitive μ-opioid receptor antagonist in established experimental systems. It occupies the receptor and opposes agonist-dependent signalling, allowing the μ-receptor contribution to a measured response to be tested.
- Studied in
- Receptor-binding studies, isolated tissues, neuronal preparations and rodent models use CTOP to investigate μ-opioid receptor involvement in experimental responses.
About CTOP
CTOP is a synthetic cyclic octapeptide with the sequence D-Phe–Cys–Tyr–D-Trp–Orn–Thr–Pen–Thr–NH₂. A disulphide bond joins cysteine at position two to penicillamine at position seven, constraining the central portion of the molecule. Penicillamine is a thiol-containing, non-proteinogenic amino acid with two methyl substituents on its β carbon. The peptide emerged from investigations of conformationally restricted, somatostatin-related scaffolds that could recognise opioid receptors without reproducing the agonist activity of conventional opioid peptides.
Its main laboratory role is pharmacological blockade of μ-opioid receptors. Researchers compare agonist responses with and without CTOP, ideally alongside receptor-defined controls, to assess subtype involvement. CTOP differs from CTAP at position five, where CTOP contains ornithine rather than arginine. This seemingly small change affects the side-chain chemistry within the constrained ring. Selectivity remains concentration- and system-dependent, so a blocked response alone is not conclusive proof of exclusive μ-receptor mediation.
Laboratory notes
- Avoid reducing reagents unless disulphide cleavage is an intended experimental variable.
- Use chromatographic and mass-spectrometric checks to assess purity and the expected oxidised molecular species.
- Include antagonist-only controls and an appropriate equilibration period when characterising agonist response shifts.
Storage and reconstitution basics: storage guide.
Questions about CTOP
What makes CTOP cyclic?
An intramolecular disulphide bond connects Cys² and Pen⁷; it is not a head-to-tail cyclised peptide.
How is CTOP different from CTAP?
CTOP contains ornithine at position five, whereas CTAP contains arginine at that position.
Does CTOP blockade establish absolute receptor specificity?
No. Interpretation should account for concentration, receptor expression and potential off-target effects, preferably using orthogonal controls.
Is CTOP for human use?
No. It is supplied strictly for in-vitro laboratory research and is not for human or animal consumption.
Related research peptides
All products are sold for laboratory research use only. Not for human consumption, not a medicine, not for diagnosing, treating or preventing any condition.