Endotoxin Testing for Research Peptides: How to Read LAL Results
In brief. Endotoxin is a fragment of bacterial cell wall that triggers strong immune responses in cells and animals at tiny concentrations. It is not removed by filtration or destroyed by ordinary autoclaving, so purity and sterility do not rule it out. Endotoxin testing, usually by the LAL method, is a separate measurement reported in endotoxin units (EU).
Purity, identity and sterility get most of the attention on a certificate of analysis. Endotoxin is the quieter fourth question, and for anyone running immune, inflammation or cell-signalling work, it can be the one that decides whether the results mean anything.
What endotoxin is
Endotoxin is lipopolysaccharide (LPS) from the outer membrane of Gram-negative bacteria. It is released when bacteria die or divide, and it is extremely stable. Immune cells recognise it through the TLR4 receptor and respond by releasing cytokines. Macrophages and monocytes in culture can react to very small amounts.
Why it can be present in a pure peptide
- Sources. Water, buffers, glassware and handling can introduce bacteria at any stage before final processing.
- Sterile does not mean endotoxin-free. A 0.22 µm filter removes bacteria but lets endotoxin fragments pass through.
- Heat resistance. Normal autoclaving does not destroy it. Depyrogenation of glassware generally requires dry heat at around 250 °C.
- Invisible to HPLC. Endotoxin does not show up as a peak in a purity analysis.
How the tests work
The classic method uses Limulus amebocyte lysate (LAL), an extract from horseshoe crab blood cells that clots in the presence of endotoxin. It comes in three formats:
| Method | What it reads | Output |
|---|---|---|
| Gel clot | Whether a firm clot forms | Pass or fail at a set limit |
| Kinetic turbidimetric | Rate of cloudiness | Quantitative EU value |
| Kinetic chromogenic | Colour from a synthetic substrate | Quantitative EU value |
Recombinant factor C (rFC) assays reproduce the first step of the LAL cascade without animal-derived lysate and are increasingly accepted as an alternative.
Reading the number
Results are given in endotoxin units, usually per milligram of material (EU/mg) or per millilitre of solution (EU/mL). A lower figure is better. A result reported as “less than” a value means endotoxin was below the test’s detection limit at that dilution.
What counts as acceptable depends on the experiment. Sensitive immune-cell assays need far lower levels than, for example, a binding assay with purified protein. Work out the endotoxin your assay will actually receive by multiplying EU/mg by the amount of peptide used.
A note on peptide interference
Some peptides interfere with the LAL reaction, either suppressing or enhancing it. Properly run tests include a positive product control, also called spike recovery: a known amount of endotoxin is added to the sample to confirm the test can still detect it. Recovery is generally expected to fall between 50% and 200%. If a report includes this figure, it tells you the result is trustworthy for that material.
What to check
- Which method was used, and its detection limit.
- Units, and whether the value is per mg or per mL.
- Spike recovery, if reported.
- That the lot number matches your vial.
Lot reports are posted on our COA / Lab Reports page. For related reading, see net peptide content, and our guide to reading Resolve documentation.
Keeping it in scope
This article covers laboratory quality testing. Resolve materials are for research use only; see the Research Use Only policy.