Wholesale Oxytocin — Bulk Research Supply

Oxytocin is a nine-residue peptide closed into a ring by a disulfide bridge between two cysteines, with a short tail extending from the ring. That cyclic topology is not incidental — it is what holds the molecule in the conformation OXTR recognises, and reducing the bridge does not weaken the compound so much as abolish the shape the research depends on.

In practical terms that rules out buffers containing reducing agents such as DTT or β-mercaptoethanol during preparation.

Signalling proceeds through OXTR, a G-protein-coupled receptor working via phospholipase C, which is why the same compound appears across smooth-muscle, central-nervous-system and autonomic research rather than in one narrow field. Its close structural relationship to vasopressin — the two differ at only two of nine positions — is a standard subject of receptor-selectivity work, and is worth knowing because cross-reactivity between the two systems is a real consideration in interpreting results.

Supplied lyophilised; stored at -20°C protected from light, reconstituted solution refrigerated at 2-8°C. General reagent handling only, not human or veterinary use instruction.

The most detailed public data on how oxytocin degrades comes from Hawe, Poole, Romeijn, Kasper, van der Heijden and Jiskoot at Leiden (Pharmaceutical Research, 2009). They held oxytocin at pH 2.0, 4.5, 7.0 and 9.0 and at 40, 55, 70 and 80°C, followed loss of the intact peptide by RP-HPLC, and identified the products by ESI-MS/MS.

Loss followed pseudo-first-order kinetics with Arrhenius temperature dependence. Degradation was fastest at pH 9.0, then 7.0, then 2.0, and slowest at pH 4.5, where the activation energy was highest at 116.3 kJ/mol. The pathways differed with pH. At pH 2.0 the main route was deamidation of oxytocin's three amide groups (Gln4, Asn5 and the C-terminal glycinamide). At pH 4.5 and above the products were dominated by the disulfide: tri- and tetrasulfide species, disulfide-linked and dityrosine-linked dimers, β-elimination products and larger aggregates. Two details follow from the sequence. Oxytocin has a single tyrosine, Tyr2, so every dityrosine dimer is cross-linked through that residue rather than through sulfur. And β-elimination at the Cys1–Cys6 cystine is the route that opens the ring itself.

For anyone characterizing reconstituted oxytocin, that list is effectively the impurity map: species at +32 Da (an extra sulfur), dimers near twice the parent mass, and deamidation (+1 Da) under acidic conditions. The finished pharmaceutical shows the same priorities. The US Pitocin label describes a solution buffered with acetic acid to a target pH of 3.5 that may contain up to 16% total impurities.

Oxytocin's heat sensitivity has shaped a whole line of analog chemistry. Carbetocin, (2-O-methyltyrosine)-deamino-1-carba-oxytocin (C₄₅H₆₉N₁₁O₁₂S), removes the N-terminal amino group and replaces one sulfur of the disulfide with a CH₂ group, turning the bridge into a thioether. Those two edits take away the sites attacked by aminopeptidases and disulfidases, and they remove the S–S bond that Hawe et al. found at the center of oxytocin's degradation at neutral pH.

The practical stakes were tested in the WHO CHAMPION trial (Widmer et al., New England Journal of Medicine, 2018). Its background states that oxytocin requires cold storage that is not available in many countries. The trial randomized 29,645 participants at 23 sites in 10 countries to a heat-stable carbetocin formulation or oxytocin, with both kept at 2 to 8°C to preserve blinding. For research use the lesson is narrower: native oxytocin, unlike its carba analog, is limited by its disulfide, and storage conditions should be chosen with that in mind.

The enzyme that carbetocin's deamination guards against is named after oxytocin. Leucyl-cystinyl aminopeptidase (LNPEP; UniProt Q9UIQ6) is also called oxytocinase, placental leucine aminopeptidase and insulin-regulated aminopeptidase (IRAP). It removes N-terminal residues, cleaving before cysteine among others, and degrades oxytocin and vasopressin. The antagonist side of the family went the same way: atosiban, authorized in the EU as Tractocile, is a desamino-oxytocin analog that acts as a competitive vasopressin/oxytocin receptor antagonist.