Glossary
Endotoxin
Endotoxin is bacterial lipopolysaccharide that acts at picogram levels and never shows on HPLC. Why a low-endotoxin grade is its own separate specification.
Endotoxin is the lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria. It strongly activates Toll-like receptor 4 and sets off inflammatory signalling at concentrations measured in picograms per millilitre. It is quantified in endotoxin units (EU); about 10 EU equal 1 ng of reference LPS.
Why it needs its own specification
Routine purity analysis is blind to it, so it has to be specified separately. A peptide may reach 99% purity on HPLC and yet contain enough endotoxin to swamp an immunological endpoint, because LPS is not a peptide-related impurity and never appears on the chromatogram. In cell culture, macrophage and immune assays, the contamination triggers cytokine responses that are easily credited to the peptide under test — a well-known cause of results that cannot be reproduced.
Most contamination arrives by one of two paths: recombinant production in bacterial hosts, where LPS is carried through purification from the host organism, and water or glassware in workflows that are otherwise synthetic. Endotoxin withstands heat, including autoclaving, so it has to be kept out rather than destroyed afterwards. It is tested with the LAL assay and appears on the certificate of analysis only if specifically requested. See reading a COA and third-party testing.
Related terms
purity