What is Endotoxin?

Endotoxin is lipopolysaccharide (LPS), the molecule that makes up much of the outer membrane of Gram-negative bacteria. It has three parts: lipid A, a core oligosaccharide and the O-antigen. Innate immune cells detect lipid A through the TLR4–MD-2 receptor complex, which signals through NF-κB to switch on pro-inflammatory cytokine production ^[1]. Endotoxin is heat-stable and remains after the bacteria that shed it are dead, so a solution can be sterile and still carry it.

Why it matters in peptide assays

Endotoxin produces a strong biological signal at concentrations that no purity method would notice, which makes it a classic source of false positives in in-vitro work.

• Commercial reagents can carry it. A screen of reagents used in microglia research found significant LPS in a number of commercial-grade proteins, phospholipids and synthetic peptide preparations, but not in pharmaceutical-grade recombinant proteins. The LPS-binding antibiotic polymyxin B suppressed the activating effect of several commercial preparations ^[2].
• Whole findings have been traced to it. Endothelial activation attributed to commercial C-reactive protein disappeared after extensive dialysis and was reproduced by azide or LPS alone at the concentrations present in the preparations ^[3].
• Species changes the picture. Human and mouse TLR4–MD-2 differ in how they recognise lipid A variants ^[1], so the same contamination can behave differently in human and murine cells.

Assays most exposed are those reading cytokines, NF-κB reporters or activation of monocytes, macrophages, microglia or endothelial cells, including "anti-inflammatory" designs in which LPS is deliberately used as the stimulus and contamination in the test peptide quietly adds to it.

Worked example: endotoxin rides along with the dose

Suppose a peptide (molecular weight 1,500 g/mol) carries an illustrative 1 endotoxin unit (EU) per mg. The endotoxin delivered to the well scales with peptide concentration:

Because contamination climbs a hundredfold across the range, an artefact can masquerade as a concentration–response curve that appears only at the top doses. The molarity calculator handles the µM-to-µg/mL step, and the dose–response design guide covers the controls that separate a real response from a contaminant.

How it is measured

Limulus or Tachypleus amebocyte lysate (LAL/TAL) assays, prepared from horseshoe crab blood, have long been the standard. Recombinant factor C (rFC) assays correlate closely with them, and because rFC does not respond through the factor G (glucan) pathway, they detect endotoxin more selectively ^[4]. Results are reported in EU. Each sample type needs a spiked control to show that the peptide itself does not suppress or enhance the reaction.

Common misunderstandings

• "Sterile means endotoxin-free." Sterile filtration removes bacteria, not the LPS they released.
• "99% HPLC purity covers it." Peptide purity by UV does not detect LPS at the levels that activate immune cells. Unless a certificate of analysis reports a measured endotoxin value, treat it as unknown.
• "A heat-inactivation control will expose it." That control works for proteins, whose activity is heat-labile while LPS is not. Many short peptides survive heating too, so polymyxin B or TLR4-blocking controls discriminate better ^[2].

The immunomodulatory peptides guide covers peptide classes where this confound matters most.