Oxytocin is a cyclic nonapeptide hormone with a single disulfide bridge, produced in the hypothalamus and released from the posterior pituitary. As a synthetic peptide it is the reference agonist for the oxytocin receptor (OXTR), a Gq-coupled G protein-coupled receptor, and it is used in receptor binding, calcium signalling and smooth muscle cell assays. NuVion supplies Oxytocin as a laboratory chemical for in vitro research use only.
Key facts
| Type/class | Synthetic cyclic peptide, neurohypophysial hormone, OXTR agonist |
| Amino acid count | 9 |
| Sequence | Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, disulfide Cys1-Cys6 (CYIQNCPLG-NH2) |
| Molecular formula | C43H66N12O12S2 |
| Molecular weight | 1007.2 g/mol |
| CAS number | 50-56-6 |
| Synonyms | OT, OXT, ocytocin, oxytocin acetate (salt form) |
| Supplied form | Lyophilised powder in a sealed vial |
Structure and chemistry
Oxytocin is encoded by the OXT gene as part of a preprohormone that also contains its carrier protein, neurophysin I. In magnocellular neurons of the paraventricular and supraoptic nuclei the precursor is cleaved by prohormone convertases, trimmed by carboxypeptidase E, and finished by peptidylglycine alpha-amidating monooxygenase, which converts the C-terminal glycine-extended intermediate into the Gly-NH2 amide. The peptide was the first peptide hormone to have its sequence determined and to be made by total chemical synthesis, work published by du Vigneaud in 1953.
The molecule has two parts. Residues 1 to 6 form a 20-membered ring closed by the disulfide between Cys1 and Cys6, and residues 7 to 9 (Pro-Leu-Gly-NH2) form a flexible tail. Arginine vasopressin has the same architecture and differs at only two positions, Phe3 in place of Ile3 and Arg8 in place of Leu8, which is why the two hormones cross-react at each other’s receptors and why selectivity has to be checked in any assay. Tyr2 and the ring conformation are the main determinants of receptor affinity; the tail contributes to activation.
Chemically the peptide is most stable in mildly acidic solution, around pH 3 to 5. At neutral and alkaline pH the disulfide can undergo exchange to give dimers and scrambled species, Gln4 and Asn5 deamidate, and the amide at the C-terminus can hydrolyse slowly. Reducing agents such as dithiothreitol or TCEP open the ring and abolish receptor activity, so they are kept out of any buffer that will contain the peptide. The tyrosine makes the compound light-sensitive. In serum-containing media it is also cleaved between Cys1 and Tyr2 by leucyl-cystinyl aminopeptidase (oxytocinase), which shortens its half-life in culture.
Mechanism of action
OXTR is a class A G protein-coupled receptor that binds oxytocin with low-nanomolar affinity and couples primarily to Gq/11. Agonist binding activates phospholipase C beta, which hydrolyses phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3) and diacylglycerol. IP3 releases calcium from the endoplasmic reticulum, diacylglycerol activates protein kinase C, and the calcium signal is sustained by store-operated and voltage-gated calcium entry. In smooth muscle cells, including myometrial cell lines, the rise in calcium activates myosin light chain kinase, and phosphorylation of the regulatory light chain drives contraction; a parallel RhoA-Rho kinase pathway inhibits myosin phosphatase and raises calcium sensitivity. OXTR activation also stimulates phospholipase A2 and induces cyclooxygenase-2 expression, linking the receptor to prostaglandin synthesis in cultured cells.
Several features of the receptor shape experimental design. OXTR can also couple to Gi, and the Gq and Gi arms both feed into ERK1/2 phosphorylation. Sustained agonist exposure leads to GRK-mediated phosphorylation, beta-arrestin recruitment and internalisation, so cells that have been exposed to oxytocin for hours show a smaller response on re-stimulation. High-affinity binding depends on membrane cholesterol, which acts as an allosteric modulator, and on divalent cations, with magnesium in particular increasing agonist affinity. Finally, oxytocin binds the vasopressin V1a receptor with roughly ten to one hundred-fold lower affinity than OXTR, so responses in cells that express both receptors are assigned using selective antagonists such as atosiban or L-368,899 for OXTR and SR49059 for V1aR. A cryo-EM structure of oxytocin bound to OXTR in complex with Gq shows the ring seated in the orthosteric pocket between the transmembrane helices, with the tail reaching toward the extracellular loops.
Research applications
Oxytocin sits in NuVion’s Reproductive Signalling category. Typical in vitro contexts include:
- Competitive radioligand binding at recombinant OXTR and V1a, V1b and V2 receptors using tritiated oxytocin or iodinated ornithine vasotocin analogue, to derive Ki values and selectivity ratios for new ligands
- IP1 accumulation (HTRF) and calcium mobilisation assays (Fluo-4, Fura-2) in HEK293-OXTR or CHO-OXTR cells, with oxytocin as the reference full agonist for EC50 determination
- Beta-arrestin recruitment, receptor internalisation and desensitisation assays used to profile biased agonists and antagonists at OXTR
- Smooth muscle cell models: collagen gel contraction and myosin light chain phosphorylation in myometrial cell lines, and COX-2 induction measured by qPCR or western blot
- Membrane biophysics: cholesterol and magnesium dependence of binding in reconstituted membranes, and NMR or molecular dynamics of the disulfide ring conformation
- Analytical reference standard for RP-HPLC and LC-MS methods on disulfide-containing peptides, including forced degradation studies of deamidation and dimer formation
Handling in the laboratory
Reconstitute the lyophilised powder with bacteriostatic water, adding the water gently and swirling until the solid dissolves; the peptide is freely water-soluble. The reconstitution calculator converts the vial content and the volume added into a stock concentration. Keep the stock away from light because of the tyrosine, keep it free of reducing agents because of the disulfide, and avoid alkaline buffers, which accelerate disulfide exchange and deamidation. Keep the sealed vial dry, away from light and refrigerated as described in the product documentation, and keep reconstituted solution refrigerated and use it within the period the documentation specifies. Each lot is characterised by RP-HPLC for purity and by mass spectrometry for identity; a disulfide-scrambled dimer appears at twice the monomer mass and as a later-eluting peak, and deamidated species at +1 Da, so both are easy to spot on a certificate.
Testing and supply from NuVion
NuVion’s Oxytocin comes from GMP-audited manufacture and is supplied lyophilised in sealed vials. Most batches are independently tested by Janoshik Analytical for purity by RP-HPLC and identity by mass spectrometry, and the Certificate of Analysis for a tested batch is published on the product page and in the COA library. Orders are dispatched from within Australia.
Related compounds
Kisspeptin is the KISS1R (GPR54) agonist that shares the Gq-phospholipase C-calcium signalling route, and PT-141 is the cyclic melanocortin analogue that covers the MC4R and MC3R side of the same category.
Frequently asked questions
What is Oxytocin used for in research?
It is the endogenous reference agonist for OXTR. Laboratories use it to define binding affinity and functional potency in receptor-expressing cell lines, to trigger calcium and contractile responses in smooth muscle cell models, and as a selectivity comparator for vasopressin receptor ligands. It is also a common standard for disulfide peptide analysis.
How is Oxytocin different from vasopressin?
Both are nine-residue peptides with a Cys1-Cys6 disulfide ring and an amidated three-residue tail. Oxytocin has Ile3 and Leu8 where vasopressin has Phe3 and Arg8. Those two changes shift affinity from the V1a, V1b and V2 vasopressin receptors toward OXTR, although each peptide still binds the other’s receptors at higher concentrations.
Is Oxytocin a therapeutic good in Australia?
No. Oxytocin from NuVion is a laboratory chemical for in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the TGA. It is not for human or veterinary use.
How should Oxytocin be stored?
Keep the lyophilised powder sealed, dry, away from light and refrigerated according to the product documentation. After reconstitution with bacteriostatic water, refrigerate the solution, protect it from light, keep it away from reducing agents and alkaline buffers, and use it within the period stated in the documentation.
Research use only. This product is a laboratory chemical supplied for in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. It is not for human or veterinary use, and nothing on this page is a representation about therapeutic use.
