These four appear adjacent in catalogs because they relate to the same area of physiology. Chemically they could hardly be more different: a decapeptide, a decapeptide fragment, a cyclic nonapeptide and a large heterodimeric glycoprotein. The last of these is not made by peptide synthesis at all.
A hierarchy, not a group
Three of the four sit at different levels of one signalling cascade, which is the clearest way to hold them apart.
- Kisspeptin acts at KISS1R, also called GPR54, on neurons upstream of GnRH neurons. It sits at the top of the cascade as a regulator of what follows.
- Gonadorelin is GnRH itself, a decapeptide acting at the GnRH receptor on pituitary gonadotrophs.
- HCG acts at the LH receptor, downstream, where it is studied because it engages the same receptor as luteinising hormone.
Oxytocin is not part of that cascade at all. It is a posterior pituitary hormone acting at its own receptor, filed here by association rather than by pathway.
Kisspeptin-10 is a fragment
The catalog item is kisspeptin-10, which is the C-terminal decapeptide of the larger parent peptide kisspeptin-54. The fragment retains receptor activity, which is why it is the one synthesised.
This is the fragment notation described elsewhere: the number is the residue count of the fragment, and a buyer should confirm which species a lot is, since the parent and the fragment have very different masses.
Oxytocin and the two-residue difference
Oxytocin is a nonapeptide closed into a ring by a disulfide bridge between the cysteines at positions 1 and 6, leaving a short tail. That cyclic structure is part of its identity, and as with any cyclic compound, incomplete cyclisation is a real impurity with a different mass.
The detail worth knowing is that vasopressin differs from oxytocin at only two of the nine positions. Two closely related nonapeptides with distinct receptor profiles is a textbook illustration of how little sequence change is needed to redirect activity, and it is also a practical caution: these two are close enough that identity confirmation is not a formality.
A further consequence of the disulfide bridge is that reducing conditions will open the ring. Reducing agents in a buffer will convert the cyclic compound into its linear form, which is chemically a different molecule.
HCG is not a synthetic peptide
Human chorionic gonadotropin is a heterodimeric glycoprotein: two different subunits, an alpha shared with other glycoprotein hormones and a beta that confers specificity, associated non-covalently and carrying extensive glycosylation.
Several things follow that set it apart from everything else on this page.
- It is not made by solid-phase synthesis. Production is by recombinant expression or by extraction, because a glycosylated two-subunit protein cannot be assembled residue by residue.
- Glycosylation is heterogeneous, so the material is a population of related glycoforms rather than a single molecular species. There is no single exact mass to confirm.
- Activity depends on the subunits remaining associated and correctly folded, so a chemically intact preparation can still be inactive.
- It is therefore quantified in international units by bioassay rather than in milligrams, which is why catalog entries are listed in IU.
That last point is the practical one. An IU is a unit of activity, not of mass, and the two cannot be converted without a stated specific activity. Comparing an IU figure to a milligram figure is a category error.
What to check, by compound
- Gonadorelin and kisspeptin-10: observed mass against theoretical, and the fragment designation stated.
- Oxytocin: mass confirming the cyclic rather than linear form, given the disulfide bridge.
- HCG: potency in IU with the assay named, plus the production route, since recombinant and extracted material are different products.
All material is supplied strictly for laboratory research.
Four molecules, four different characterisation problems
Grouping compounds by the system they are studied in hides how different they are as materials. Chain length varies widely across this group, one member is a glycoprotein rather than a short peptide, and the characterisation that counts as complete differs accordingly.
For purchasing that means a certificate format adequate for one of these is not automatically adequate for another, and the identity method matters more as the molecule gets larger and more modified.
What to confirm per compound
- Whether the material is a synthetic peptide or a biologically produced product, since the analytical panel differs.
- The identity method, and whether it reads a sequence or only a mass.
- Which salt form is supplied, which changes the mass of powder without changing the peptide.
- That the certificate names the lot on the vial you will receive.
Peptide or protein covers why the first question changes what the rest of the certificate can establish.
Why one of these is not like the others
Material produced biologically rather than by synthesis is characterised differently, because it carries heterogeneity that a synthetic peptide does not. Glycosylation varies between batches by nature rather than by error, and a purity figure means something different against that background.
A certificate adequate for a short synthetic peptide is therefore not automatically adequate here, and the methods named on it matter more than usual. Peptide or protein covers why larger and more modified molecules need more than a mass.
Storage differences within the group
Chain length, modification and format vary widely across these four, and so does how they behave in solution. Reading storage requirements from the category rather than from the compound is the error to avoid, since one heading covers materials with quite different stabilities.
For where this sits among the other molecules a catalogue carries, not everything in a peptide catalog is a peptide covers how the classes differ.