Oxytocin

Overview

Oxytocin is a nonapeptide hormone produced in the hypothalamus and released from the posterior pituitary. It is among the most thoroughly studied molecules in endocrinology, in routine clinical use worldwide, and historically the most important compound documented in this library — though not for anything it does in the body.

It is not part of our catalogue. It is documented because peptide chemistry began with it.

Chemical identity

• Sequence: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂
• CAS number: 50-56-6
• Molecular formula: C₄₃H₆₆N₁₂O₁₂S₂
• Molecular weight: 1007.2 g/mol
• Classification: Posterior pituitary nonapeptide hormone

The two sulfurs are cysteines at positions 1 and 6, joined by a disulphide bridge that closes the first six residues into a ring. A three-residue tail extends from it, ending in an amide. That ring-plus-tail architecture is characteristic of the neurohypophysial hormones.

The first one

In 1953 Vincent du Vigneaud and colleagues determined oxytocin’s sequence and then synthesised it chemically, demonstrating that the synthetic material had the biological activity of the natural hormone. He received the Nobel Prize in Chemistry in 1955 for the work.

It is difficult to overstate what that established. Before it, peptide hormones were substances isolated from tissue in tiny quantities and characterised indirectly. Afterwards, a peptide hormone was a defined sequence that could be built from amino acids and verified by whether it worked — a molecule rather than an extract.

Every compound in this library depends on that shift. Epithalon, BPC-157, the GLP-1 analogues and the rest exist because sequences can be written down, synthesised and modified deliberately. Oxytocin is where that became possible.

The synthetic methods have changed completely — solid-phase synthesis, which made everything else practical, came a decade later — but the underlying proposition was settled here.

Two residues from a different hormone

Oxytocin’s closest relative is vasopressin, the antidiuretic hormone, and comparing them is one of the clearest lessons in structure and function available.

• Oxytocin: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂
• Vasopressin: Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH₂

Seven of nine residues are identical, including the entire disulphide-bonded ring architecture. The differences are at positions 3 and 8.

Those two substitutions produce hormones with almost nothing in common functionally. Oxytocin acts on uterine smooth muscle and mammary myoepithelium; vasopressin governs renal water retention and vascular tone. Two molecules, 78% identical, doing unrelated jobs.

The practical lesson recurs throughout this library: peptide function is exquisitely sensitive to sequence, and a compound that is “almost” the right molecule is not a slightly weaker version of it. It is something else. That is why the verification points on these pages dwell on substitutions that are easy to make and hard to detect.

Mechanism of action

Oxytocin acts at the oxytocin receptor, a G-protein coupled receptor signalling through phospholipase C and intracellular calcium release. Its two classical peripheral roles follow directly: calcium-driven contraction of uterine smooth muscle during labour, and contraction of the myoepithelial cells surrounding mammary alveoli, which ejects milk.

Oxytocin receptors are also expressed in the central nervous system, and oxytocin released centrally acts as a neuromodulator rather than a circulating hormone. That central role is the basis for the behavioural research discussed below — and also for much of the difficulty with it.

Clinical use, and a caution about the behavioural literature

Oxytocin is in routine obstetric use worldwide for labour induction and augmentation and for postpartum haemorrhage. Its peripheral pharmacology is not in question: it is among the best-characterised hormone–receptor systems in medicine.

The research on social behaviour is a different matter, and warrants an explicit caution because it is widely cited and widely misrepresented.

From the mid-2000s a large literature reported effects of intranasal oxytocin on trust, empathy, generosity and social cognition. Much of it has since proved difficult to replicate. Later work has raised methodological problems including small sample sizes, publication bias toward positive findings, and — most fundamentally — genuine uncertainty about how much intranasally administered oxytocin reaches the brain at all.

None of this means central oxytocin does nothing; the receptors are there and the animal literature is substantial. It means the popular account, in which oxytocin is a “trust hormone” with reliable behavioural effects, outran the evidence considerably. Anyone citing that literature should check whether the specific finding has replicated.

Handling and storage

• Avoid reducing agents. The disulphide bridge holds the ring closed; DTT, beta-mercaptoethanol or TCEP will open it and destroy the structure.
• The single tyrosine gives mild photosensitivity; keep vials dark.
• No methionine, so oxidation is a lesser concern than for many peptides here.
• Lyophilised storage: sealed, refrigerated, protected from light; freeze for long-term storage.
• Reconstitution: add diluent slowly down the vial wall and allow to dissolve undisturbed.

Purity and analytical verification

Oxytocin is a well-established pharmaceutical substance with pharmacopoeial monographs, reference standards and validated assay methods — a level of analytical infrastructure that almost nothing else in this library has.

The compound-specific check is the disulphide. An intact bridge and a reduced open-chain form differ by two hydrogen atoms — two mass units out of 1,007. That is detectable on a good instrument but easy to overlook, and the reduced form has an entirely different conformation and no meaningful receptor activity, since the ring is what the receptor recognises.

The same consideration applies to hGH Fragment 176-191, which also carries a structural disulphide. For any cyclic peptide, confirming that the ring is closed is a distinct question from confirming that the sequence is correct, and only the second is what a purity figure describes.

References

• du Vigneaud V, Ressler C, Swan JM, et al. The synthesis of an octapeptide amide with the hormonal activity of oxytocin. Journal of the American Chemical Society, 1953. doi:10.1021/ja01115a553
• Gimpl G, Fahrenholz F. The oxytocin receptor system: structure, function, and regulation. Physiological Reviews, 2001. PMID 11274341
• Meyer-Lindenberg A, Domes G, Kirsch P, Heinrichs M. Oxytocin and vasopressin in the human brain: social neuropeptides for translational medicine. Nature Reviews Neuroscience, 2011. PMID 21852800

Summary

Oxytocin is a cyclic nonapeptide hormone, Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂, CAS 50-56-6, molecular formula C₄₃H₆₆N₁₂O₁₂S₂, molecular weight 1,007.2 g/mol, with a disulphide bridge closing a six-residue ring. It was the first peptide hormone to be sequenced and chemically synthesised, by du Vigneaud in 1953, work recognised with a Nobel Prize and foundational to every compound in this library. It differs from vasopressin at only two of nine positions despite having unrelated functions — a durable illustration of how little sequence change is needed to make a different molecule. Its peripheral pharmacology is exceptionally well established; its social-behaviour literature has substantial replication problems. We do not stock it.

This page is provided for research reference. Compounds described in this library are for laboratory research use only, are not drugs, foods or cosmetics, and may not be used for any purpose other than in vitro or non-human laboratory research.