Endotoxin is the lipopolysaccharide (LPS) component of the outer membrane of Gram-negative bacteria, composed of a lipid A anchor, a core oligosaccharide and a variable O-antigen polysaccharide. It is released on bacterial lysis and is a potent activator of the innate immune system, acting principally via the TLR4-MD2 receptor complex.
Even trace endotoxin contamination can confound in-vitro experiments and produces a pyrogenic response in vivo. Endotoxin content in a peptide preparation is quantified by the LAL (Limulus amebocyte lysate) assay and reported in Endotoxin Units per milligram (EU / mg) on the Certificate of Analysis.
Endotoxin levels are one of the key quality parameters distinguishing research-grade peptides from low-quality material and are essential to interpretable cell-culture data.