PMN-Elastase: the pathway in the body

PMN-Elastase is part of the pathway “Granulocytes in the gut wall”. This page shows the whole pathway; the station of PMN-Elastase is highlighted.

Where this laboratory value sits: PMN elastase — protein-splitting enzyme. PMN elastase is a protein-splitting enzyme (a serine protease) from the azurophil granules of neutrophils. It is not the same as the elastase of the pancreas. Source 3

In brief

Granulocytes are white blood cells that store ready-made defence substances in granules: neutrophils hold lactoferrin and elastase, eosinophils hold EPX. Once in the gut wall they release them; alpha-1 antitrypsin from the blood restrains the elastase there.

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. Neutrophils → Into the gut wall Signalling molecules from irritated tissue make neutrophils stick to the vessel wall. The cells squeeze between the vessel cells and migrate into the gut lining. Source 6↑ supplies Migration carries the defence substances to where signalling molecules report irritation. The stronger the signal from the tissue, the more cells leave the vessel. established physiology Source 6
   ⚖ When the balance tips

   too much — If the signal persists, more and more neutrophils cross over and gather in the lining; some of them reach the gut contents.

   too little — If neutrophils barely stick to the vessel wall, they stay in the blood, and only a few of their defence substances reach the gut wall.

   established physiology · Source 6

2. Into the gut wall → Granules emptied · Calcium At the site of irritation the granules fuse with the cell membrane and release their contents. Lactoferrin comes from the specific granules, elastase from the azurophil granules. Source 1↑ supplies Emptying releases the stored defence substances. The granules open in a fixed order; the azurophil granules with elastase come last, and part of the elastase acts inside the cell itself. established physiology Source 1
   ⚖ When the balance tips

   too much — If many granules are emptied at once, larger amounts of elastase and lactoferrin reach the surrounding tissue and the gut contents.

   too little — If the granules stay closed, their substances act only inside the cell, and little of them is found outside or in the stool.

   established physiology · Source 1

3. Granules emptied → Lactoferrin Lactoferrin is a protein of the transferrin family. It occurs in breast milk and mucosal secretions and in neutrophil granules. It stays intact in the stool for a long time and can be measured there. Source 2, 9↓ depletes Lactoferrin binds iron very tightly and withholds it from bacteria that need it to grow. It can also attack the envelope of some bacteria; this part comes mainly from laboratory experiments. observed in studies Source 2
   ⚖ When the balance tips

   too much — If many neutrophils enter the gut wall, the amount of lactoferrin in the gut contents rises, and more free iron is bound there.

   too little — If hardly any neutrophils migrate in, lactoferrin in the gut comes mainly from secretions or, in breastfed infants, from breast milk.

   established physiology · Source 2, 9

   Field of research — Lactoferrin in stool is being studied as a marker in inflammatory bowel conditions. Source 9

4. Granules emptied → PMN elastase PMN elastase is a protein-splitting enzyme (a serine protease) from the azurophil granules of neutrophils. It is not the same as the elastase of the pancreas. Source 3↕ both, depending on amount Inside the cell, elastase breaks down proteins of engulfed bacteria. Outside, it also splits elastin and other building blocks of connective tissue; how far depends on how much inhibitor is available. established physiology Source 3
   ⚖ When the balance tips

   too much — If free elastase outweighs its inhibitors, it also breaks down proteins of the surrounding connective tissue, and fragments of these attract further defence cells.

   too little — If little elastase is released, neutrophils break down proteins of engulfed bacteria more slowly; in animal models without the enzyme, defence against certain bacteria is weaker.

   observed in studies · Source 3

   Field of research — PMN elastase in stool is being studied as a marker in inflammatory bowel conditions. Source 9

5. Lactoferrin → Iron held tight · Carbonate Each lactoferrin molecule holds two iron ions, each together with a carbonate ion. The bond holds even in acidic surroundings, where transferrin already releases its iron. Source 2↓ depletes Tightly bound iron is no longer available to microbes. Bacteria that need a lot of iron then grow more slowly; species with their own iron scavengers cope better. observed in studies Source 2
   ⚖ When the balance tips

   too much — If plenty of iron-free lactoferrin is present, it binds free iron quickly, and little is left for microbes nearby.

   too little — If there is little lactoferrin, or if it is already saturated with iron, more iron stays free and available to microbes.

   observed in studies · Source 2

6. PMN elastase → Proteins broken down Elastase cuts protein chains at particular sites. The fragments are broken down further; the elastase itself passes with the gut contents into the stool, where it is measured. Source 3, 9↓ depletes Breaking down proteins clears away bacterial remains and damaged tissue. Elastin fragments and modified signalling molecules in turn act as a signal for further defence cells. observed in studies Source 3
   ⚖ When the balance tips

   too much — If elastase keeps breaking down protein for long, more fragments arise that attract further defence cells; the response thus lasts longer.

   too little — If little is broken down, bacterial remains and dead cell parts stay longer and are cleared by other routes.

   observed in studies · Source 3

7. Alpha-1 antitrypsin → A1AT in stool When blood protein leaks through the gut lining, alpha-1 antitrypsin comes with it. Digestive enzymes barely break it down, so it appears in the stool largely unchanged. Source 7↓ depletes In the stool, alpha-1 antitrypsin shows how much protein from the blood is lost across the gut wall. Because it withstands digestive enzymes, it stands in for other blood proteins. established physiology Source 7
   ⚖ When the balance tips

   too much — If the lining is more permeable or irritated, more blood proteins leak out and more alpha-1 antitrypsin reaches the stool; blood in the gut contents also raises the amount.

   too little — If the lining is tight, hardly any blood protein crosses over, and little alpha-1 antitrypsin is found in the stool. The stool level says nothing about production in the liver.

   established physiology · Source 7

8. Eosinophils → EPX EPX, also called EDN (eosinophil-derived neurotoxin), is a protein from eosinophil granules. Activated cells release it; it passes with the gut contents into the stool and can be measured there. Source 5, 10↕ both, depending on amount EPX breaks down RNA and thus acts against some viruses; it also attracts dendritic cells. In larger amounts it also attacks surrounding cells. Much of this comes from laboratory experiments. observed in studies Source 5
   ⚖ When the balance tips

   too much — If many eosinophils are activated, EPX accumulates in the tissue and gut contents; in laboratory experiments it then also damages nearby cells.

   too little — If little EPX is released, its breakdown of viral RNA stays low, and hardly any is found in the stool.

   observed in studies · Source 5, 10

   Field of research — Eosinophil proteins in stool are being studied in inflammatory bowel conditions. Source 10

9. EPX → RNA broken down EPX belongs to the ribonuclease family (RNase 2). It cuts RNA, that is, the genetic material of some viruses and the transcripts from which cells build their proteins. Source 5↓ depletes By breaking down RNA, viruses with an RNA genome, such as respiratory syncytial virus, can lose their ability to infect cells; this was shown in laboratory experiments. observed in studies Source 5
   ⚖ When the balance tips

   too much — If a lot of EPX is active, RNA in nearby cells is broken down as well; in laboratory experiments it is thus toxic to nerve cells, hence the name neurotoxin.

   too little — If little EPX is present, RNA in the surroundings stays largely untouched, and the contribution of eosinophils to antiviral defence is smaller.

   observed in studies · Source 5

Further stations

• Neutrophils — most common white blood cells Neutrophil granulocytes are the most common white blood cells. In their granules they store ready-made defence substances, including lactoferrin and PMN elastase. Source 1↑ supplies Neutrophils arrive with a ready-made stock of defence substances. The granules are filled while the cell matures in the bone marrow, so it does not have to make anything new at the site of action. established physiology Source 1
  ⚖ When the balance tips

  too much — If many neutrophils circulate in the blood, more cells are ready to enter the tissue during irritation and empty their granules there.

  too little — If there are few neutrophils in the blood, fewer cells reach the gut wall, and less lactoferrin and elastase arrive there with them.

  established physiology · Source 1, 6

• Alpha-1 antitrypsin — inhibitor from the liver Alpha-1 antitrypsin is a protein made mainly by the liver and released into the blood. In tissue it is the main inhibitor of neutrophil elastase. Source 4↓ depletes Alpha-1 antitrypsin binds elastase and inactivates it permanently. It thereby limits how far elastase breaks down tissue beyond the site of defence. established physiology Source 4, 3
  ⚖ When the balance tips

  too much — If the liver makes more alpha-1 antitrypsin, for example as part of the acute-phase response, more free elastase is captured.

  too little — If there is little alpha-1 antitrypsin in the tissue, or its binding site has been oxidised by oxygen radicals, more elastase stays active and breaks down connective tissue for longer.

  established physiology · Source 4

• Eosinophils — white blood cells Eosinophil granulocytes are white blood cells that also reside in the healthy gut lining. Their granules hold basic proteins, including EPX. Source 8, 5↑ supplies Eosinophils are involved in defence against worms and in reactions to environmental substances. They reside permanently in the gut and release their granule proteins when activated. established physiology Source 8
  ⚖ When the balance tips

  too much — If signalling molecules such as eotaxin and IL-5 draw many eosinophils into the gut wall, they accumulate there and release more granule proteins.

  too little — If there are few eosinophils in the gut wall, correspondingly little EPX is released on activation, and hardly any is found in the stool.

  established physiology · Source 8

Cofactors in this pathway

• Iron — Held very tightly by lactoferrin at two binding sites and thus withheld from bacteria Source 2In the ORY catalogue as a laboratory value: Eisen
• Carbonate — Binds alongside iron at each site of lactoferrin and is needed for tight iron binding Source 2
• Calcium — Rises inside the cell and triggers the emptying of the granules in a fixed order Source 1In the ORY catalogue as a laboratory value: Calcium (intrazellulär)
• Methionine — Sits in the reactive centre of alpha-1 antitrypsin that traps elastase; oxidation inactivates it Source 4In the ORY catalogue as a laboratory value: Methionin

Sources

1. Borregaard N, Sørensen OE, Theilgaard-Mönch K. Neutrophil granules: a library of innate immunity proteins. Trends Immunol 2007 · PubMed 17627888
2. González-Chávez SA, Arévalo-Gallegos S, Rascón-Cruz Q. Lactoferrin: structure, function and applications. Int J Antimicrob Agents 2009 · PubMed 18842395
3. Korkmaz B, Horwitz MS, Jenne DE et al. Neutrophil elastase, proteinase 3, and cathepsin G as therapeutic targets in human diseases. Pharmacol Rev 2010 · PubMed 21079042
4. Janciauskiene SM, Bals R, Koczulla R et al. The discovery of α1-antitrypsin and its role in health and disease. Respir Med 2011 · PubMed 21367592
5. Acharya KR, Ackerman SJ. Eosinophil granule proteins: form and function. J Biol Chem 2014 · PubMed 24802755
6. Kolaczkowska E, Kubes P. Neutrophil recruitment and function in health and inflammation. Nat Rev Immunol 2013 · PubMed 23435331
7. Umar SB, DiBaise JK. Protein-losing enteropathy: case illustrations and clinical review. Am J Gastroenterol 2010 · PubMed 19789526
8. Rothenberg ME, Mishra A, Brandt EB et al. Gastrointestinal eosinophils. Immunol Rev 2001 · PubMed 11292017
9. Siddiqui I, Majid H, Abid S. Update on clinical and research application of fecal biomarkers for gastrointestinal diseases. World J Gastrointest Pharmacol Ther 2017 · PubMed 28217373
10. Peterson CG, Sangfelt P, Wagner M et al. Fecal levels of leukocyte markers reflect disease activity in patients with ulcerative colitis. Scand J Clin Lab Invest 2007 · PubMed 18034391

Whole pathway: Granulocytes in the gut wall

Related pathways

• ANCA against MPO and PR3 — granules emptied
• ASCA and ANCA — neutrophils
• Clostridioides difficile — neutrophils
• Calprotectin — neutrophils
• Candida — neutrophils

As of 2026-10-05. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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