Oxytocin Acetate 10mg Peptide

Oxytocin is a cyclic nonapeptide, Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, closed by a disulfide bridge between the cysteine residues at positions 1 and 6. That bridge divides the molecule into a six-residue macrocycle and a three-residue tail ending in a C-terminal glycinamide. The free base is C43H66N12O12S2 at 1,007.2 g/mol, CAS 50-56-6, PubChem CID 439302. Acetate is the counterion on the finished powder, not a change to the peptide.

It is the first peptide hormone whose sequence was determined and then confirmed by total synthesis. Vincent du Vigneaud, Charlotte Ressler and Stanley Trippett published the sequence in the Journal of Biological Chemistry in 1953, and the synthesis followed the same year in the Journal of the American Chemical Society under the title 'The synthesis of an octapeptide amide with the hormonal activity of oxytocin'. The word octapeptide in that title is not an error: cystine was counted as a single residue at the time. The 1955 Nobel Prize in Chemistry followed, and every synthetic peptide catalogue since traces back to that result.

Two very different literatures now sit on top of the same molecule. The peripheral one, covering uterine myometrium and mammary myoepithelium, is close to settled and has underpinned an obstetric medicine for decades. The central one, built almost entirely on intranasal administration in humans and concerning trust, social recognition and affiliation, is contested at every level: whether the peptide reaches the brain in useful quantity, whether the early effects replicate, and whether the effects that survive are central or peripheral in origin. This record covers both, and does not pretend the second is as solid as the first.

Oxytocin acts on a single receptor, OXTR, a 389-residue class A G protein-coupled receptor encoded on human chromosome 3 and catalogued as UniProt P30559. It belongs to the vasopressin and oxytocin receptor subfamily, which is the structural reason oxytocin and vasopressin cross-react on each other's receptors rather than behaving as clean selective ligands. The canonical coupling is to Gq/11, which activates phospholipase C-beta, hydrolyses PIP2 into inositol trisphosphate and diacylglycerol, releases calcium from intracellular stores and opens voltage-gated calcium channels. In smooth muscle the rise in intracellular calcium activates myosin light chain kinase, and cross-bridge cycling follows.

OXTR is not a single-pathway receptor. Gimpl and Fahrenholz established in their 2001 Physiological Reviews survey that the high-affinity state requires both magnesium and cholesterol, which act as allosteric modulators, and that the agonist and antagonist binding regions are distinct. At high agonist concentrations the receptor also couples to Gi and inhibits adenylate cyclase, and it recruits beta-arrestins. Busnelli and colleagues confirmed in 2013 that oxytocin activates both Gq and Gi and recruits beta-arrestin at the mouse receptor, and that the selectivity profile of the natural ligands is conserved across human, rat and mouse orthologues.

The peptide is not synthesised as a free nonapeptide. The OXT gene on chromosome 20 encodes a 125-residue prepropeptide in which oxytocin occupies residues 20 to 28 and neurophysin I occupies residues 32 to 125. The C-terminal amide is generated enzymatically from a trailing glycine, which is why the mature peptide ends in glycinamide rather than a free carboxylate. Neurophysin I is a carrier that binds oxytocin, protects it from degradation during axonal transport from the hypothalamic magnocellular neurons to the posterior pituitary, and is co-secreted with it.

Receptor density, not ligand availability, is what governs the uterine response. Fuchs and colleagues reported in Science in 1982 that myometrial oxytocin receptor concentration rises through pregnancy in women and peaks in early labour, with decidual receptor concentration peaking at parturition. In the same tissue work, oxytocin increased prostaglandin production by decidua but not by myometrium, which is where the dual-pathway model of labour initiation comes from: a direct myometrial contraction signal and an indirect decidual prostaglandin signal.

Before 1953 no polypeptide hormone had been sequenced and then rebuilt from amino acids with its biological activity intact. Du Vigneaud, Ressler and Trippett published the sequence and a proposed structure in the Journal of Biological Chemistry in 1953, and du Vigneaud's group published the synthesis in the Journal of the American Chemical Society the same year. The synthetic material reproduced the hormonal activity of the natural extract, which closed the loop between a structural proposal and a functional proof. The 1955 Nobel Prize in Chemistry recognised the work.

The title of the 1953 synthesis paper calls oxytocin an octapeptide amide. This is a naming convention rather than a mistake: the cystine formed by the Cys1 to Cys6 disulfide was counted as one residue. Under modern counting, which treats each cysteine separately, oxytocin is a nonapeptide. Older literature and some reference tables still carry the eight-residue count, which is one reason residue numbering in oxytocin analogues can look inconsistent between sources.

The practical consequence for anyone handling the compound today is that oxytocin has the longest and most thoroughly characterised synthetic history of any peptide in a research catalogue. Its solution behaviour, its degradation routes, its bioassay standards and its analytical fingerprints have been described continuously since the early 1950s, which is why identity confirmation on a modern lot is a matter of matching a well-documented reference rather than establishing a novel one.

Oxytocin and arginine vasopressin are both nine-residue neurohypophysial peptides with the same Cys1 to Cys6 disulfide architecture, the same proline at position 7 and the same C-terminal glycinamide. They differ at exactly two positions. Oxytocin carries isoleucine at position 3 and leucine at position 8; vasopressin carries phenylalanine at position 3 and arginine at position 8. The genes sit near each other on human chromosome 20 and are widely read as the product of an ancient duplication.

That near-identity has a direct experimental consequence: neither peptide is a selective ligand for its own receptor. OXTR sits in the same GPCR subfamily as V1a, V1b and V2, and at the concentrations produced by supraphysiological administration oxytocin engages vasopressin receptors as well. Manning and colleagues reviewed the peptide and non-peptide agonist and antagonist toolkit for this receptor family in 2012 precisely because the natural ligands are too promiscuous to use as selectivity probes, and they published comparative receptor data as a guide to selectivity and species differences.

This is the most commonly omitted caveat in summaries of intranasal oxytocin work. An intranasal administration produces peripheral concentrations far above the physiological range. At those concentrations, attributing an observed behavioural change specifically to OXTR requires either an OXTR-selective antagonist control or a selective agonist comparator such as the synthetic analogue TGOT, which Busnelli and colleagues characterised as strongly selective for the mouse oxytocin receptor. Most human intranasal studies include neither.

The peripheral actions are the part of the oxytocin record that has held up. Uterine sensitivity to oxytocin rises steeply before labour, and Gimpl and Fahrenholz attribute this to a strong upregulation of oxytocin receptors in the myometrium and, to a smaller degree, in the decidua, under steroid control. Fuchs and colleagues measured the same upregulation directly in human tissue in 1982 and found the myometrial peak in early labour and the decidual peak at parturition.

The milk-ejection side is cleanly established by genetics rather than pharmacology. Mice with a null mutation of the oxytocin gene show normal sexual and maternal behaviour but cannot perform the milk let-down reflex, so knockout pups survive only when nursed by a lactating female with an intact oxytocin system. Winslow and Insel summarised the phenotype in 2002: the milk ejection deficit is obligate, the maternal behaviour deficit is not, and the striking behavioural change is the loss of social recognition rather than any reproductive failure.

The peripheral programme also produced the clearest illustration of oxytocin's chemical fragility. Aqueous oxytocin requires cold storage, which limits its use in settings without a reliable cold chain. That constraint drove the development of heat-stable carbetocin, and the CHAMPION trial randomised 29,645 women across 23 sites in 10 countries to compare the two after vaginal birth. Blood loss of at least 500 mL or use of an additional uterotonic occurred in 14.5 percent of the carbetocin arm and 14.4 percent of the oxytocin arm (relative risk 1.01, 95% CI 0.95 to 1.06). The trial is worth knowing here not for its obstetric conclusion but because it exists only because oxytocin in solution is thermally labile.

The modern central literature begins with a 2005 Nature report that intranasal oxytocin substantially increased the transfers made by investors in an economic trust game, and that the effect was specific to social risk rather than to risk in general. The paper was enormously influential and generated more than a decade of intranasal studies on trust, generosity, in-group bias, face processing and emotion recognition.

The correction has come from three directions. Nave, Camerer and McCullough reviewed the field in Perspectives on Psychological Science in 2015 and concluded that the intranasal trust finding had not replicated well, that plasma oxytocin measurements were compromised by assay methodology, and that large-sample searches for OXTR polymorphisms associated with trust had produced no consistent hit. In 2020 a registered replication with 677 participants, powered above 95 percent and implementing the same minimal social contact condition as the original, found no effect on trusting behaviour. In parallel, two adequately powered clinical trials in autism spectrum disorder returned null primary endpoints: 106 adults over 6 weeks in Japan and 290 children and adolescents over 24 weeks in the United States NEJM trial.

The nulls are informative rather than merely disappointing, because they close off the easy explanations. In the Japanese trial, plasma oxytocin rose from baseline to endpoint only in the active arm, confirming that the peptide was delivered and absorbed. In SOARS-B, the placebo arm improved by almost exactly as much as the active arm, which locates the earlier positive signals in expectancy and small-sample noise rather than in a lost pharmacological effect. What survives is narrower and better specified: in the Japanese trial, repetitive behaviour and gaze fixation on socially relevant regions both separated from placebo as secondary endpoints, with effect sizes of 0.44 and 0.55.

This is the unresolved mechanistic question underneath the entire central literature, and it has two named sides. The sceptical case was set out by Leng and Ludwig in Biological Psychiatry in 2016. Their argument is quantitative: very little of the large amount applied intranasally appears in cerebrospinal fluid, while peripheral concentrations are pushed to supraphysiological levels with likely actions on the gastrointestinal tract, heart and reproductive tract. They also argued that many published oxytocin measurements used discredited assay methodology and that peripheral measurements are a poor index of central release. Their prescription was procedural: preregistration, declared primary and secondary outcomes, open data, proper dose-response work, and control conditions that administer the peptide peripherally or block peripheral action with an antagonist.

The quantitative anchor for that scepticism is old rodent work. Mens and colleagues found in 1983 that roughly 0.002 percent of a peripherally administered dose of oxytocin or vasopressin was present in rat cerebrospinal fluid at 10 minutes, and that centrally administered peptide cleared with a terminal half-time of 19 minutes. Notably, those same authors read their result as demonstrating that the peptides do cross in amounts sufficient for central action. The number is agreed on; the interpretation is not.

The human kinetic data sit awkwardly with both readings. Striepens and colleagues sampled blood and cerebrospinal fluid in the same volunteers after 24 international units intranasally and found significant rises in both compartments, but with completely different time courses: plasma peaked at 15 minutes and had declined by 75, while cerebrospinal fluid needed 75 minutes to reach significance, and the correlation between compartments was below 0.10. Quintana and colleagues reviewed the accumulated human and animal evidence in Molecular Psychiatry in 2021 and argued for converging support for a functionally relevant nose-to-brain route. That review is a position in a live argument, not a settlement of it, and anyone designing work on this compound should treat central delivery as a variable to be measured rather than an assumption.

Solid-phase peptide synthesis followed by reverse-phase HPLC purification leaves the peptide as a salt with whatever acid was used in the mobile phase, most often trifluoroacetic acid. Trifluoroacetate is unwelcome in biological work because it is cytotoxic at achievable concentrations and interferes with several assays, so most research-grade peptide is passed through an ion-exchange or lyophilisation step that swaps the counterion for acetate. The peptide itself is unchanged; oxytocin acetate and oxytocin are the same nonapeptide with a different associated acid.

Oxytocin has an unusually simple salt stoichiometry because it contains no lysine and no arginine. The only strongly basic site is the alpha-amino group of Cys1, so it forms a 1:1 monoacetate. That is why the acetate salt is C45H70N12O14S2 at 1,067.2 g/mol, exactly one acetic acid heavier than the 1,007.2 g/mol free base, and why the two are indexed separately: the free base is CAS 50-56-6, PubChem CID 439302, UNII 1JQS135EYN, while the acetate is CAS 6233-83-6, PubChem CID 12004215, UNII 4NR672T8NL. A specification sheet that prints an acetate product name above CAS 50-56-6 and 1,007.2 g/mol is describing the free base, whatever the label says.

The practical consequence is a 5.6 percent mass discrepancy that vendor tables routinely blur by printing an acetate product name above a free-base molecular weight. If a quantity is specified on a gross salt mass basis, 1,007.2 divided by 1,067.2 means about 94.4 percent of that mass is peptide free base; the remainder is acetic acid, plus residual water, which for a lyophilised peptide is commonly a few percent more. This is also why HPLC purity and peptide content are different numbers reporting different things. HPLC purity is an area percent across the chromatogram and says how much of the peptide present is the right peptide; peptide content, determined by amino acid analysis or nitrogen determination, says how much of the vial's mass is peptide at all.

The disulfide bridge is both the pharmacophore and the weak point. Reduction of the Cys1 to Cys6 bond opens the macrocycle and abolishes uterotonic activity in classical bioassays, so any reducing agent in a buffer is a route to silent potency loss that a mass measurement alone will not catch. The degradation chemistry in solution has been mapped in detail by formulation work aimed at producing a heat-stable liquid. Avanti and colleagues stressed oxytocin in citrate buffer at 70 degrees Celsius for 5 days and identified the products by LC-MS/MS: citrate adducts, trisulfides and tetrasulfides, and dimers, essentially all of them arising from reactions at the cysteine residues. Deamidation at the glutamine in position 4 and the asparagine in position 5 is the other established route.

The same group showed in 2011 that the solution environment is decisive. In buffers at pH 4.5, adding sodium or potassium made no difference, and divalent metal ions in acetate buffer made no difference either. Citrate buffer at 5 to 10 mM combined with at least 2 mM calcium, magnesium or zinc chloride strongly improved stability across 4 weeks at both 4 and 55 degrees Celsius, with isothermal calorimetric measurements confirming a direct interaction between oxytocin and the divalent ion. The mechanism proposed in the follow-up paper is that the metal and citrate complex the peptide and suppress cysteine-mediated intermolecular reactions. This is the practical reason a plain water or plain acetate diluent is a worse environment for the peptide than the buffer composition alone would suggest.

The WHO 4th International Standard for oxytocin, NIBSC code 76/575, adds two handling points from a source with no commercial interest. Its instructions state that no attempt should be made to weigh out a portion of the freeze-dried material before reconstitution, and that where extensive dilution is required the diluent should contain a peptidase-free carrier protein specifically to minimise loss by surface adsorption. Adsorption to container walls is a real and frequently overlooked loss route for a peptide at low concentration. The standard also directs that reconstituted material be subdivided into small containers, frozen rapidly and held below minus 40 degrees Celsius in the dark, which is a stricter regime than any research catalogue describes.

Identity confirmation on a lot is straightforward because the reference data are old and abundant, provided the two mass scales are not mixed up. The monoisotopic mass of the free base is 1,006.436 Da, giving a monoisotopic [M+H]+ at m/z 1007.44 and [M+2H]2+ at m/z 504.22, while the average-mass [M+H]+ sits near 1,008.2. A correct oxytocin electrospray spectrum matches one of those, not both. The intact disulfide is confirmed by a 2 Da difference against material deliberately reduced with dithiothreitol or TCEP: reduced oxytocin is 2 Da heavier, and a lot that already shows the reduced mass has lost the macrocycle. Reverse-phase HPLC on a C18 column with a water and acetonitrile gradient in dilute trifluoroacetic acid resolves the intact peptide from deamidated, dimeric and polysulfide species, and the acetate counterion is quantified separately by ion chromatography rather than inferred from the peptide peak.

Two reporting caveats are worth stating plainly. An HPLC purity figure is an area percent at a single detection wavelength: it reports how much of the peptide present is the right peptide, and it says nothing about how much of the vial mass is peptide at all, which is what peptide content by amino acid analysis or nitrogen determination measures. Separately, Leng and Ludwig's criticism of oxytocin immunoassays was not incidental to their argument. Unextracted plasma immunoassays for oxytocin have reported concentrations orders of magnitude above extracted assays and above what mass spectrometry supports, so results from unextracted assays are not comparable with results from extracted ones.

ClinicalTrials.gov lists 922 interventional studies with oxytocin as an intervention, which is a far larger registered footprint than any research peptide of comparable catalogue price. The distribution is heavily skewed toward obstetrics and lactation, where oxytocin has been in continuous use for decades, with a much smaller psychiatric and social-neuroscience arm dominated by intranasal designs in autism spectrum disorder, schizophrenia, social anxiety and post-traumatic stress.

It is worth stating the regulatory position precisely, because it is a common source of confusion. Oxytocin injection is a long-established prescription obstetric medicine administered under clinical supervision, and it appears on the WHO Model List of Essential Medicines in that role. The material in a research catalogue is a synthetic peptide supplied for laboratory use. It is not that pharmaceutical product, it is not manufactured or released to the same specification, and the existence of a licensed obstetric preparation confers nothing on a research-grade powder. The two share a molecule and nothing else.

For the central research questions, the registry also shows where the field has moved. The large intranasal trials that read out between 2018 and 2021 largely closed the question of whether a fixed intranasal regimen produces a broad social-cognition benefit in autism spectrum disorder, and the newer registrations tend toward narrower, mechanistically specified endpoints: attention to socially relevant stimuli, amygdala reactivity on functional imaging, and pharmacokinetic work on delivery devices. That is the direction the evidence pushed rather than the direction the early enthusiasm pointed.