Endotoxin (Lipopolysaccharide)

Lipopolysaccharide has three parts: a lipid A anchor embedded in the outer membrane, a core oligosaccharide, and a variable O-antigen chain. Lipid A is the biologically active portion, delivered by CD14 to the TLR4 and MD-2 complex on myeloid cells, triggering NF-kB and a rapid burst of tumour necrosis factor, interleukin-1 and interleukin-6. Its practical menace is chemical robustness: endotoxin survives autoclaving and passes through a 0.22 micron sterilising filter, so a solution can be perfectly sterile and still pyrogenic.

Endotoxin is quantified in units of roughly 0.1 to 0.2 nanograms of reference lipopolysaccharide. The compendial limit for most parenteral products is 5 endotoxin units per kilogram of body weight per hour, tightened to 0.2 units per kilogram for intrathecal routes. Detection uses the Limulus amebocyte lysate assay or a recombinant factor C equivalent.

This is the immunology term that matters most when reading a peptide certificate of analysis: sterile and endotoxin-free are unrelated claims, and research-grade material is typically tested for neither. An endotoxin specification is also meaningless in isolation, since the limit derives from the intended dose per kilogram per hour: one figure in units per milligram is acceptable for a small exposure and far out of specification for a larger one.

The specific error is treating a chromatographic purity figure as a cleanliness figure. HPLC purity measures peptide-related substances and is blind to lipopolysaccharide, so endotoxin from a recombinant host, contaminated water or unclean glassware never appears on the chromatogram a listing advertises. The inference fails in reverse too: an unexplained fever or inflamed injection site after unregulated material is commonly attributed to the compound's pharmacology when a pyrogen is the simpler explanation.