Native forms with the disulfide bond and C-terminal amide, calculated with the residue masses and pKa set used on this site.
Two hormones, one design
Oxytocin and vasopressin are nine-residue peptides that differ at only two positions. Both begin with a ring of six residues closed by a disulfide bond between Cys1 and Cys6, followed by a three-residue tail ending in a C-terminal glycine amide. Oxytocin has isoleucine at position 3 and leucine at position 8; vasopressin has phenylalanine at 3 and arginine at 8.
The ring, including the disulfide, contains 20 atoms. Position 8 is decisive for function: the basic arginine of vasopressin is required for strong binding to vasopressin receptors, and the neutral leucine of oxytocin for oxytocin receptors. The two hormones nonetheless cross-react to some extent at each other's receptors, which is one reason for the many synthetic analogues designed to separate their actions.
The single substitution at position 8 has a large effect on the calculated properties. In the native forms, both hormones have lost their C-terminal carboxyl to amidation and their two cysteine thiols to the disulfide. Oxytocin retains only the N-terminal amine and Tyr2 as ionisable groups, giving a calculated pI near 9. Vasopressin adds the strongly basic arginine and has a calculated pI above 11. The isoelectric point guide uses oxytocin as its worked example of how modifications move the pI.
Pigs and some related species use lysine instead of arginine at position 8. This form, lysine vasopressin, is fully functional, and the natural variation illustrates that a positive charge at position 8, rather than arginine specifically, is what the receptor needs.
The first hormones to be synthesised
In 1953 Vincent du Vigneaud and his colleagues at Cornell determined the sequence of oxytocin and, in the same year, synthesised it. The synthetic peptide was indistinguishable from the natural hormone in its biological activity. It was the first polypeptide hormone to be made in the laboratory, and it demonstrated that a biological activity could be fully explained by a defined chemical structure. Vasopressin followed shortly after, and du Vigneaud received the Nobel Prize in Chemistry in 1955.
The synthesis was done in solution, one residue at a time, a decade before Merrifield introduced solid-phase synthesis. Today, both peptides can be made routinely on Rink amide resin, followed by oxidation to form the disulfide.
Biosynthesis and release
Both hormones are made mainly by large neurons in two regions of the hypothalamus, the supraoptic and paraventricular nuclei. Each is encoded as part of a larger precursor protein that contains, in order, a signal peptide, the hormone itself, a carrier protein called a neurophysin, and, for vasopressin, a glycopeptide called copeptin.
The hormone sequence in the precursor is followed by Gly-Lys-Arg. Processing enzymes cut after the basic pair, carboxypeptidase E trims the basic residues, and peptidylglycine α-amidating monooxygenase converts the exposed glycine into the C-terminal amide. This is the standard route to amidated hormones described in the guide to post-translational modifications. The processed hormones, bound to their neurophysins, travel down the axons to the posterior pituitary gland, where they are stored and released into the blood in response to nerve signals.
Oxytocin is also released within the brain itself, from dendrites and axon collaterals of the same and other neurons, which is the basis of its actions on behaviour.
What they do
Oxytocin
The best-established peripheral functions of oxytocin are in reproduction. It stimulates contraction of the uterine muscle during labour, and it triggers the milk ejection reflex during breastfeeding by contracting the cells surrounding the milk-producing alveoli. The uterus becomes much more sensitive to oxytocin in late pregnancy, largely because the number of oxytocin receptors increases.
In the brain, oxytocin influences social behaviour. The most detailed evidence comes from animal studies: in prairie voles, which form lasting pair bonds, oxytocin and vasopressin signalling in specific brain regions is required for partner preference, and closely related vole species that do not pair-bond differ in the distribution of these receptors. Oxytocin has since become known in popular accounts as a "love" or "trust" hormone. The human evidence is considerably less clear. Many early studies of intranasal oxytocin had small samples, several prominent findings have not replicated consistently, and how much of an intranasal dose reaches the brain remains debated.
Vasopressin
Vasopressin's older name, antidiuretic hormone, describes its principal role. Released when the blood becomes too concentrated or blood volume falls, it acts on V2 receptors in the collecting ducts of the kidney. These trigger the insertion of aquaporin-2 water channels into the cell membrane, allowing water to be reabsorbed and producing concentrated urine. Without vasopressin, or without functioning V2 receptors, the kidney cannot conserve water.
At higher concentrations vasopressin also constricts blood vessels through V1a receptors, the property that gave it its name. V1b receptors in the pituitary mediate its role in the stress response, where it acts together with corticotropin-releasing hormone.
| Receptor | Main ligand | Main locations | Main effect |
|---|---|---|---|
| OTR | Oxytocin | Uterus, mammary gland, brain | Contraction; milk ejection; social behaviour |
| V1a | Vasopressin | Blood vessels, liver, brain | Vasoconstriction |
| V1b (V3) | Vasopressin | Anterior pituitary | ACTH release in the stress response |
| V2 | Vasopressin | Kidney collecting duct | Water reabsorption |
All four are G protein-coupled receptors. Their ligands are so similar that selectivity is only relative, which is why the analogues below were developed.
An ancient peptide family
Peptides of the oxytocin–vasopressin family are found in almost all vertebrates and in many invertebrates, and they appear to have evolved from a single ancestral gene that duplicated early in vertebrate evolution. Non-mammalian vertebrates use related forms: vasotocin, which combines the oxytocin ring with the vasopressin tail (CYIQNCPRG-NH₂), is the vasopressin-like hormone of birds, reptiles, amphibians and fish, and forms such as mesotocin and isotocin play the oxytocin-like role. The conserved nine-residue, disulfide-closed, amidated design has persisted for hundreds of millions of years.
Synthetic analogues
The small size and well-understood structure–activity relationships of these hormones made them early targets for rational peptide design. Several analogues illustrate strategies covered elsewhere on this site:
| Analogue | Change | Effect |
|---|---|---|
| Desmopressin | N-terminal amine removed (1-deamino); Arg8 → D-Arg | Selective for V2 over V1a; much longer half-life |
| Carbetocin | 1-deamino; disulfide replaced by a thioether; O-methyl-Tyr2 | Oxytocin agonist with longer duration |
| Terlipressin | Three glycines added to the N-terminus of lysine vasopressin | Prodrug, slowly converted to the active hormone |
| Atosiban | Several substitutions including a D-residue | Oxytocin receptor antagonist |
Desmopressin combines two strategies. Removing the N-terminal amine protects against aminopeptidases, and the D-arginine at position 8 blocks cleavage and shifts receptor selectivity, the principle described in the guide to D-amino acids. Carbetocin replaces the disulfide with a more stable thioether, one of the disulfide mimics mentioned in the guide to disulfide bonds.
Stability and half-life
Both native hormones are cleared quickly: the plasma half-life of oxytocin is a few minutes and that of vasopressin somewhat longer. Enzymes that open the ring or remove residues from either end account for most of this. Oxytocin is also degraded by a specific enzyme, oxytocinase (placental leucine aminopeptidase), whose level rises in pregnancy.
In solution, oxytocin is sensitive to heat. Its main degradation routes are deamidation of Gln4, Asn5 and the C-terminal glycine amide, and reactions of the disulfide that form dimers and other products. Pharmaceutical oxytocin is therefore typically stored refrigerated, and heat-stable formulations have been a focus of research for use in regions without reliable refrigeration. The chemistry is described in the guide to peptide stability.
Calculating their properties
Entering CYIQNCPLG in the MW calculator gives 1010.19 Da, the mass of the reduced free acid. The native hormone is 3 Da lighter: subtract 2.016 Da for the disulfide and 0.984 Da for the amide to reach 1007.19 Da. The molecular weight guide works through this calculation step by step. The same corrections applied to CYFQNCPRG give 1084.24 Da for vasopressin.
Frequently asked questions
Is oxytocin really the "love hormone"?
The label oversimplifies. Oxytocin has well-established roles in childbirth and lactation and clear effects on social bonding in some animals, but the human evidence on trust, empathy and bonding is mixed, and many popular claims rest on small or unreplicated studies.
Why do two nearly identical hormones do such different things?
Because they act on different receptors in different tissues. The two-residue difference is enough to give each hormone a strong preference for its own receptors, and the tissue distribution of those receptors determines the effect.
What is copeptin?
The C-terminal part of the vasopressin precursor, released in equal amounts with vasopressin. It is more stable in blood samples than vasopressin itself and is measured as a surrogate marker of vasopressin release.
Why does the calculator give oxytocin a pI of 5.4 when its real pI is near 9?
Because the calculator assumes a linear peptide with a free C-terminal acid and free cysteines, all of which are acidic groups. The native hormone has none of these, so its pI is set by the N-terminal amine and tyrosine.
References
- du Vigneaud V, Ressler C, Swan JM, Roberts CW, Katsoyannis PG, Gordon S (1953) The synthesis of an octapeptide amide with the hormonal activity of oxytocin. Journal of the American Chemical Society 75:4879–4880.
- Gimpl G, Fahrenholz F (2001) The oxytocin receptor system: structure, function, and regulation. Physiological Reviews 81:629–683.
- Young LJ, Wang Z (2004) The neurobiology of pair bonding. Nature Neuroscience 7:1048–1054.
- Walum H, Waldman ID, Young LJ (2016) Statistical and methodological considerations for the interpretation of intranasal oxytocin studies. Biological Psychiatry 79:251–257.