Every other entry in this library is about something somebody is selling. This one is about a piece of biological offcut that has no market, no vendor and no dose — and that is measured in laboratories more often than almost any other peptide in medicine.
Where it comes from
Insulin is not made as insulin. The pancreas first builds a single continuous chain of 110 amino acids called preproinsulin, and then cuts it up. Taking the annotated human sequence and reading off the boundaries gives the whole story in four pieces:
| Piece | Residues | Length | Fate |
|---|---|---|---|
| Signal peptide | 1–24 | 24 aa | Removed during synthesis |
| Insulin B chain | 25–54 | 30 aa | Kept — half of insulin |
| C-peptide | 57–87 | 31 aa | Cut out, released into the blood |
| Insulin A chain | 90–110 | 21 aa | Kept — the other half of insulin |
Between the kept pieces sit two short linkers, Arg-Arg and Lys-Arg, which mark where the enzymes cut.
The arithmetic is worth pausing on. Insulin, the hormone, is the A and B chains held together: 21 + 30 = 51 residues. The connecting peptide that gets discarded is 31. The piece thrown away is roughly 38% of the finished product by residue count — this is not a trimmed end, it is a substantial part of the molecule.
Its sequence is EAEDLQVGQVELGGGPGAGSLQPLALEGSLQ, and computed from that sequence it weighs about 3,020 daltons.
What it is for, structurally
The name is literal. While proinsulin is folding, the A and B chains have to end up in precisely the right orientation relative to each other so that the disulphide bonds form in the right places. The connecting peptide is what holds them there — a molecular jig. Once the folding is done and the bonds are made, the jig is no longer needed, so it is cut out and released.
That is the classical account, and it explains the shape of the molecule: a flexible, unremarkable stretch rich in glycine and alanine, with no obvious business end. It was designed by evolution to be a spacer, and for a long time it was assumed to do nothing else. Whether that assumption is right is still an open research question, and this entry does not attempt to settle it.
Why laboratories care about an offcut
Because of an accident of accounting that turns out to be extremely useful.
The pancreas releases one C-peptide for every one insulin. They leave together, in equal numbers. But they do not stay in equal concentrations, because insulin is cleared from the blood considerably faster — which is why published insulin-to-C-peptide molar ratios sit well below 1, commonly cited around 0.05 to 0.2.
Then comes the part that makes it a test: manufactured insulin contains no C-peptide at all. A syringe delivers the hormone without its connecting piece. So:
- Insulin and C-peptide, roughly in proportion → the insulin came from the pancreas
- Insulin present, C-peptide low or absent → the insulin came from somewhere else
The published literature treats a molar ratio above 1.0 as evidence of exogenous or factitious insulin. It also documents exceptions — recent work on pre-term neonates reports inverted ratios in infants who received no insulin at all, which is the kind of finding that keeps a rule of thumb from hardening into a law.
This entry reports that convention because it explains why the test exists. It does not interpret anyone's result, and it is not capable of doing so: reference ranges differ between laboratories and assays, and results are read alongside glucose and clinical history rather than alone. That reading belongs to the clinician who ordered the test.
The largest literature in this library, by far
16,300 PubMed records name C-peptide in the title or abstract. For comparison, the entries in this library that report a large literature report a few hundred. Several report single digits, and a number report zero registered trials of any kind.
So the ranking is almost exactly inverted from the market's. The molecule with no vendors, no marketing and nothing to buy has been studied more than the compounds with the busiest sales pages put together — because it is a routine clinical measurement made millions of times a year, and every study that measures it adds to the pile.
Why this entry exists at all
Because two completely different readers type the same word. Someone whose doctor has ordered a C-peptide test, and someone reading about injectable peptides, can both end up searching "c peptide" — and they have almost nothing in common.
They are not looking at the same subject. C-peptide shares no target, no mechanism, no route and no market with the compounds in the rest of this library. It shares a word. What a peptide actually is covers why that word describes chemistry and nothing else — and C-peptide is the sharpest demonstration of it, because "peptide" here means a diagnostic number on a laboratory report.
Related entries
For the chemistry that makes a 31-residue chain weigh what it does, see the amino acid chart. For the hormones that the metabolic entries in this library actually concern, see semaglutide and tirzepatide — and for a name in that family with no molecule behind it, GLP-3.