Lipopolysaccharides: the pathway in the body · Biomarker & Stool Analysis

This page shows the biochemical pathway behind the laboratory value Lipopolysaccharides (LPS / endotoxins): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Lipopolysaccharides (LPS, endotoxins) are components of the outer envelope of Gram-negative bacteria. When they reach the blood, immune cells recognise them via the receptor TLR4, and the liver captures and modifies them. Removing fatty chains strips it of its activity.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Gram-negative bacteria → Lipopolysaccharide LPS, also called endotoxin, is the main component of this outer envelope. It consists of the fatty lipid A, a sugar core and a long sugar chain. Lipid A is the part the immune system recognises. Source 1↕ both, depending on amount For the bacterium, LPS is a dense barrier that holds back bile acids and other substances; magnesium links the molecules. For the body, lipid A is the recognition mark that sets off the immune response. established physiology Source 7, 1
   ⚖ When the balance tips

   too much — If a bacterium sheds much LPS, more lipid A is present that can dock onto TLR4; how strongly it acts depends on the number of its fatty chains.

   too little — If a bacterium forms lipid A with fewer fatty chains, it triggers only a weak signal via TLR4; some bacterial species carry such forms naturally.

   established physiology · Source 1, 4

2. Lipopolysaccharide → LPS in the blood · Chylomicrons When bacteria die, LPS fragments are released. Some of them pass, together with dietary fat in chylomicrons, from the gut into lymph and blood. Via the lymph, this part initially bypasses the liver. Source 2↑ supplies Chylomicrons carry LPS along as they transport fat out of the gut. In cell culture and in the animal model, the amount of LPS absorbed rises with the fat absorbed at the same time. observed in studies Source 2
   ⚖ When the balance tips

   too much — If much LPS reaches lymph and blood, it initially stays bound to lipoproteins; only what is released from them reaches the immune cells.

   too little — If little fat is absorbed, fewer chylomicrons form in the animal model, and less LPS reaches the blood by this route.

   observed in studies · Source 2, 5

3. LPS in the blood → LPS + LBP · LBP In the blood, lipopolysaccharide-binding protein (LBP) picks up individual LPS molecules and hands them on to the surface protein CD14. In large amounts, LBP instead steers LPS to lipoproteins. Source 3↕ both, depending on amount In small amounts, LBP amplifies the signal because it passes LPS on to CD14 and thus to TLR4. In large amounts, it instead steers LPS to lipoproteins, where it stays bound; cell and animal experiments show this. observed in studies Source 3
   ⚖ When the balance tips

   too much — If LBP rises sharply, as the liver produces it in the acute-phase reaction, it captures more LPS and passes it to lipoproteins; the signal to the immune cells is then weaker.

   too little — If little LBP is available, LPS reaches CD14 more slowly in the animal model; small amounts of LPS then trigger hardly any signal.

   observed in studies · Source 3

4. TLR4-MD-2 → NF-κB MyD88 signalling Via the MyD88 signalling chain, the switch NF-κB is released inside the cell. It moves into the nucleus and has certain genes read there. Many immune genes thus switch on at the same time. Source 4↑ supplies NF-κB switches on a whole group of immune genes within a short time. A second route via the protein TRIF starts later and leads to the formation of interferons. established physiology Source 4
   ⚖ When the balance tips

   too much — If much NF-κB is released, the cells read more immune genes, and more messengers reach the blood.

   too little — If little NF-κB is released, for instance because hardly any receptor pairs form, the immune genes stay largely silent.

   established physiology · Source 4

5. NF-κB → Cytokines These genes include messengers such as TNF-α and interleukin-6. They summon further immune cells and switch metabolism over. The liver then forms more LBP, among other proteins. Source 4↕ both, depending on amount TNF-α and interleukin-6 summon further immune cells and make the liver produce acute-phase proteins, among them LBP. In larger amounts, the same messengers switch over metabolism throughout the body. established physiology Source 4, 3, 5
   ⚖ When the balance tips

   too much — If many messengers are released, they act not only locally but throughout the body and switch over heat regulation, metabolism and blood vessels.

   too little — If few messengers are released, the response stays local; whether small amounts of LPS in the blood trigger any change at all is viewed differently in the literature.

   contested · Source 5

6. LPS in the liver → Inactivated LPS AOAH The enzyme AOAH cuts two of the six fatty chains out of lipid A. Without them, the molecule no longer fits into the pocket of MD-2. The immune cells then no longer respond to it. Source 5↓ depletes AOAH removes the activity of LPS without breaking it down completely: with four instead of six fatty chains, lipid A no longer triggers a signal at the receptor. In the animal model the response fades much more slowly without AOAH. observed in studies Source 5, 4
   ⚖ When the balance tips

   too much — If AOAH is very active, LPS is quickly inactivated, and the immune cells receive no further signal from that molecule.

   too little — If little AOAH is active, LPS stays active in the tissue for a long time in the animal model, and the response to it fades only slowly.

   observed in studies · Source 5

7. Inactivated LPS → Excretion The liver releases the remnants into the gut with the bile. Another part stays bound to lipoproteins in the blood and is kept harmless there. Both routes keep LPS away from the immune cells. Source 5↓ depletes LPS leaves the body via the bile; bound to lipoproteins, it stays attached without reaching TLR4. Both routes keep it away from the immune cells. established physiology Source 5
   ⚖ When the balance tips

   too much — If much LPS is bound to lipoproteins, it stays longer in the blood, but without access to TLR4; the liver gradually absorbs it together with the lipoproteins.

   too little — If few lipoproteins are in the blood, less binding capacity is available, and free LPS reaches the immune cells more easily.

   observed in studies · Source 5

Further stations

• Gram-negative bacteria — in the gut Some gut bacteria have a second, outer envelope. This group is called Gram-negative because it does not stain blue with the Gram stain. They release LPS as they grow and die. Source 1↑ supplies Gram-negative bacteria are the source of LPS: they continuously build it into their outer envelope and release parts of it as they grow and die. established physiology Source 1
  ⚖ When the balance tips

  too much — If many Gram-negative bacteria grow in the gut, more LPS is present in the gut contents that can pass into the gut wall.

  too little — If few Gram-negative bacteria are present, less LPS is found in the gut; the other gut bacteria carry no such outer envelope.

  established physiology · Source 1, 5

• TLR4-MD-2 — Receptor on immune cells The receptor TLR4 carries the helper protein MD-2. The fatty chains of lipid A fit into its pocket, so that two receptors pair up. Only this pair sets off the signalling chain inside. Source 4↑ supplies The receptor pair is the trigger: only when two TLR4-MD-2 assemble around one lipid A is the signalling chain inside set off. Even a few LPS molecules are enough for this. established physiology Source 4
  ⚖ When the balance tips

  too much — If many LPS molecules bind, many receptor pairs form and the cell sends a strong signal; afterwards it moves TLR4 from the surface into the cell interior for a while.

  too little — If few LPS molecules bind, or the lipid A fits poorly into MD-2, hardly any receptor pairs form, and the signal stays weak.

  established physiology · Source 4

• LPS in the liver — Kupffer cells Blood from the gut flows through the liver first. There, Kupffer cells capture a large part of the LPS. Less of it thus reaches the rest of the body. Source 5↓ depletes The liver acts as a filter between the gut and the general circulation: Kupffer cells and liver cells absorb LPS and remove it from the blood before it reaches the rest of the body. established physiology Source 5
  ⚖ When the balance tips

  too much — If more LPS arrives than the liver can absorb, part of it passes into the general circulation and reaches immune cells with TLR4 there.

  too little — If little LPS arrives, the Kupffer cells absorb most of it; hardly any free LPS is then found in the general circulation.

  established physiology · Source 5

Cofactors in this pathway

• Zinc — Metal centre of the bacterial enzyme LpxC, which carries out an early step of lipid A synthesis; without zinc it stalls Source 6, 1
• Magnesium — Divalent ions such as magnesium bridge neighbouring LPS molecules in the outer envelope and seal it Source 7In the ORY catalogue as a laboratory value: Magnesium
• Pantothenic acid — Building block of the acyl carrier protein that supplies the fatty chains for lipid A synthesis; no ACP, no fatty chains Source 1
• Triglycerides (dietary fat) — Form chylomicrons, with which LPS passes from the gut into lymph and blood; LPS thus partly bypasses the liver Source 2In the ORY catalogue as a laboratory value: Triglyzeride
• Lipoproteins (HDL) — Bind LPS in the blood and keep it bound, away from TLR4 Source 5In the ORY catalogue as a laboratory value: HDL-Cholesterin

Sources

1. Raetz CR, Whitfield C. Lipopolysaccharide endotoxins. Annu Rev Biochem 2002 · PubMed 12045108
2. Ghoshal S, Witta J, Zhong J et al. Chylomicrons promote intestinal absorption of lipopolysaccharides. J Lipid Res 2009 · PubMed 18815435
3. Schumann RR, Latz E. Lipopolysaccharide-binding protein. Chem Immunol 2000 · PubMed 10608081
4. Park BS, Lee JO. Recognition of lipopolysaccharide pattern by TLR4 complexes. Exp Mol Med 2013 · PubMed 24310172
5. Munford RS. Endotoxemia — menace, marker, or mistake? J Leukoc Biol 2016 · PubMed 27418356
6. Di Leo R, Cuffaro D, Rossello A, Nuti E. Bacterial Zinc Metalloenzyme Inhibitors: Recent Advances and Future Perspectives. Molecules 2023 · PubMed 37298854
7. Nikaido H. Molecular basis of bacterial outer membrane permeability revisited. Microbiol Mol Biol Rev 2003 · PubMed 14665678

Related pathways

• Defence substances of the gut lining — nf-κb

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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