Precursor / peptide
hCAP18 vs LL-37
They are not the same molecule. hCAP18 is the complete 18 kDa precursor protein encoded by the CAMP gene — an N-terminal cathelin domain joined to a C-terminal antimicrobial domain — and as a whole protein it is antimicrobially inert. LL-37 is the 37-residue peptide released when a protease cuts that C-terminal domain free.
The cut happens outside the cell, it is performed by different enzymes in different tissues, and in seminal plasma it produces a different peptide entirely. Treating the two names as interchangeable discards all of that.
The distinction
What each name actually denotes
| Property | hCAP18 | LL-37 |
|---|---|---|
| What it is | Precursor protein, ~18 kDa | Mature peptide, 37 residues |
| Structure | Cathelin domain + C-terminal antimicrobial domain | The C-terminal domain alone |
| Antimicrobial activity | None — inert as the whole protein | Yes |
| Net charge | Not the operative property | Approximately +6 |
| Where it is stored | Neutrophil granules; seminal plasma; epithelia | Not stored — generated on release |
| Made by | Transcription of CAMP, a direct vitamin D receptor target | Proteolytic cleavage of hCAP18 |
| Alternative product | Can also yield ALL-38 instead | — |
Primary sourceProcessing enzymes, compartments and peptide lengths from the studies cited on the main resource.
Why it is not pedantry
Three places the conflation causes a real error
Reading an assay
An immunoassay may detect the precursor, the mature peptide, or both, depending on the epitope. “Circulating cathelicidin” in one paper and in another need not be the same measurement. Where a study correlates a cathelicidin level with an outcome, the species detected is part of the result, not a footnote to it.
Reasoning from expression to activity
MechanismVitamin D signalling raises CAMP transcription, which raises precursor availability. That is upstream of activation and does not by itself produce mature peptide — the protease still has to be there, in the right compartment. Two dials; moving one does not move the other.
Carrying animal findings across
Bovine and porcine cathelicidins are cleaved by elastase; the human precursor is cleaved by proteinase 3. The paper that established this said so in as many words — the generation of active peptide from a common proprotein occurs differently in related species. A rodent or porcine result about activation is not a human result.
The settling evidence
A human condition that removes the cut
LiteratureIf the distinction were merely formal, taking the cut away would change little. It can be taken away, and it changes a great deal.
Papillon-Lefèvre syndrome arises from loss-of-function mutations in the cathepsin C gene. Cathepsin C activates the neutrophil serine proteases — proteinase 3 among them. Patients transcribe CAMP normally and carry hCAP18; what they cannot do is process it. The published finding is exactly that: absent cathelicidin processing, and with it an essential role for LL-37 in periodontal homeostasis. A separate study of a Papillon-Lefèvre patient examined the same defect against species-dependent requirements for neutrophil defence.
MechanismWhat the natural experiment isolates
Gene: intact. Transcription: normal. Vitamin D pathway: untouched. Precursor: present. One activating enzyme: missing. The result is severe, early periodontal destruction — which makes this about as clean a demonstration as human biology offers that possessing the precursor and possessing the peptide are different states.
The complication
And a third name: ALL-38
The precursor does not have one destination. In seminal plasma, hCAP18 is abundant and unprocessed — carried inert. The prostate-derived protease gastricsin cleaves it, but only at the pH of the vagina, and it cuts at a different point. The product is ALL-38, 38 residues, with reported antimicrobial activity against the organisms tested equal to that of LL-37.
So the honest formulation is not “hCAP18 becomes LL-37”. It is: hCAP18 becomes whichever mature peptide the local protease and local conditions produce. LL-37 is the most studied of those, not the only one.
That is the whole case for naming a resource after the precursor rather than the peptide. The precursor is where the decision is still open.
Questions
Frequently asked
Are hCAP18 and LL-37 the same thing?
No. hCAP18 is the whole precursor and is antimicrobially inert; LL-37 is the 37-residue peptide cut from its C-terminus. The same precursor also yields ALL-38 by a different enzyme in a different compartment.
Does measuring hCAP18 tell you how much LL-37 there is?
Not by itself. Expression and activation are separate controls, so a precursor level constrains the ceiling without establishing the amount of mature peptide. Any study reporting “cathelicidin” should state which species its assay detects.
Which is the active one?
The mature peptide. The known biological functions reside in the C-terminus, which has to be cleaved from the holoprotein before anything happens — that is the structural logic of the whole cathelicidin family.
Why is the cleavage outside the cell?
Because that is where it was observed. After phagocytosis, hCAP18 inside the cell showed no detectable cleavage; it was cleaved in exocytosed material. Whatever the evolutionary reason, the practical consequence is that activation is governed by the extracellular environment rather than by the cell that carried it.
How do I tell which one a paper means?
Look for the molecular weight, the assay epitope, and the compartment. A paper reporting an 18 kDa band, seminal plasma, or a granule fraction is usually discussing the precursor; one reporting a 37-residue peptide, a synthetic sequence, or an activity assay is usually discussing the mature form. Where the paper does not say, that ambiguity is a limitation of the paper.
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