Where Does Pepsin Come From and What Does It Do?

Pepsin is a digestive enzyme produced by specialized cells in the lining of your stomach, and its primary job is breaking down the proteins you eat into smaller fragments that your intestines can absorb. It holds the distinction of being one of the first enzymes ever discovered, and it remains the dominant protein-digesting enzyme in the human stomach. But the story of pepsin is more layered than a simple “stomach acid dissolves food” narrative, because the enzyme isn’t secreted in its active form, it can cause real damage when it ends up in the wrong place, and researchers now use it as a diagnostic marker for conditions well beyond simple indigestion.

Chief Cells and the Birth of Pepsinogen

Pepsin originates in cells called chief cells (also known as peptic cells or zymogenic cells), which sit near the base of the gastric glands deep in your stomach lining. These cells do not release pepsin directly. Instead, they secrete an inactive precursor called pepsinogen, a slightly larger molecule with an extra segment that blocks its active site. This design is a safety mechanism: if chief cells made active pepsin on the spot, the enzyme would start digesting the very cells that produced it.

The release of pepsinogen is tightly controlled by chemical signals. Nerve impulses carried by the vagus nerve trigger pepsinogen secretion through specific receptor types on the chief cell surface. Research on mouse gastric tissue has shown that two muscarinic receptor subtypes, M1 and M3, handle the cholinergic (nerve-driven) side of this process, with no other muscarinic receptor types involved.¹ Beyond nerve signals, chief cells also respond to hormones and other chemical messengers, including secretin, gastrin, cholecystokinin, and vasoactive intestinal peptide, all of which fine-tune how much pepsinogen gets released depending on what and when you’ve eaten.²

How Pepsinogen Becomes Pepsin

Once pepsinogen leaves the chief cell and enters the stomach’s interior, it encounters hydrochloric acid secreted by neighboring parietal cells. At a pH between roughly 1 and 3, the acidic environment triggers pepsinogen to clip off its own inhibitory segment, a process called intramolecular activation. In other words, the molecule essentially unfolds itself into the active enzyme without needing any outside enzyme to do the cutting. Kinetic studies have confirmed that at these very low pH values, this self-activation follows a predictable first-order pattern, meaning the rate depends only on the amount of pepsinogen present, not on how much active pepsin already exists.³

At a slightly higher pH, around 4, things get more interesting. Existing pepsin molecules can accelerate the conversion of remaining pepsinogen in a chain-reaction style, where each newly activated molecule helps activate more. This mixed mechanism means that once digestion gets going and pepsin accumulates, the conversion speeds up on its own.³ It also means that acid secretion is the true gatekeeper: without a sufficiently acidic environment, pepsinogen simply sits there, inert.

What Pepsin Does to the Proteins You Eat

Pepsin’s function is to cleave large, tangled protein molecules into shorter peptide chains. It belongs to a family of enzymes called aspartic proteases, named for two aspartic acid residues that sit at the core of its active site and work together to break the bonds holding amino acids in a protein chain.⁴ Pepsin has a broad appetite: unlike some enzymes that only cut at very specific amino acid sequences, pepsin attacks many types of bonds, especially those near large, hydrophobic amino acids like phenylalanine, tyrosine, and leucine. This broad specificity makes it well-suited for the messy opening stage of protein digestion.

The gastric phase of digestion, where pepsin operates, is the first real step in breaking down dietary protein.⁵ Pepsin doesn’t finish the job. It reduces big proteins into medium-sized fragments, and once those fragments pass into the duodenum (the first stretch of the small intestine), a fresh set of enzymes from the pancreas takes over to break them down further into individual amino acids your body can absorb. Pepsin itself becomes irrelevant once it leaves the stomach, because the near-neutral pH of the duodenum inactivates it.

How the Stomach Protects Itself

It sounds paradoxical: the stomach produces a potent protein-digesting enzyme, yet it’s made of protein itself. The explanation lies in a carefully maintained physical barrier. The stomach lining is coated in a continuous layer of mucus gel, secreted by surface epithelial cells. Embedded within this gel, bicarbonate ions create a pH gradient. The innermost surface of the gel, right against the stomach wall, sits at a near-neutral pH, while the outer surface facing the stomach cavity remains highly acidic. This design means that acid and active pepsin are kept away from the vulnerable cells underneath.⁶

The mucus layer also serves as a physical barrier to pepsin specifically. Pepsin is a relatively large molecule, and the dense mucus gel slows its diffusion, preventing it from reaching the epithelial surface in damaging concentrations. Under normal conditions, this mucus-bicarbonate barrier is sufficient on its own to protect the stomach lining from both acid and pepsin.⁶ When this barrier breaks down, whether from chronic use of anti-inflammatory drugs, bacterial infection, or other causes, the result is often a gastric ulcer.

Why Cooking Temperature Matters for Pepsin Digestion

How you cook your food changes how efficiently pepsin can break it down. Heating meat to moderate temperatures, around 70°C, unfolds (denatures) the proteins and exposes more of their bonds to pepsin, actually speeding up digestion.⁷ But pushing the temperature past about 100°C triggers chemical reactions, including protein oxidation and Maillard browning, that cause proteins to clump together in aggregates. These aggregates are harder for pepsin to penetrate, slowing the initial rate of digestion.

Research on pork proteins tells a similar story: meat cooked at a gentle 58°C had a significantly higher rate of pepsin digestion than meat cooked at 80°C or 160°C, with Maillard reaction products and structural changes inversely related to how quickly proteolysis proceeded.⁸ None of this means you should eat undercooked meat for nutritional reasons, since food safety concerns obviously outweigh marginal differences in protein digestibility. But it does explain why different cooking methods can affect how much work your stomach has to do.

The food matrix itself also matters. Plant proteins that have been cross-linked, for example through enzymatic treatments used in food manufacturing, respond to pepsin differently depending on how tightly the protein network is packed. Looser networks let pepsin and acid diffuse in easily, while denser structures slow things down.⁹

How Heat Affects Pepsin Itself

Separate from what heat does to food, pepsin itself is surprisingly heat-resistant at moderate temperatures. Five minutes at 65°C barely dents it, leaving it with about 95% of its activity. Even after 15 minutes at that temperature, nearly half its activity remained. To fully and irreversibly inactivate pepsin, you need to reach at least 75°C and hold it there for five minutes or more.¹⁰ This matters for food science and for anyone studying digestion in the lab, because experimental protocols that don’t heat samples enough can leave residual pepsin activity that skews results.

When Pepsin Escapes the Stomach

Pepsin was once thought of as a strictly gastric enzyme, relevant only inside the stomach. That picture has changed. In gastroesophageal reflux, stomach contents wash upward into the esophagus and sometimes reach the throat, airways, and even the middle ear. Research now shows that in these “extra-esophageal” locations, pepsin rather than acid is the primary agent of tissue damage.¹¹

The mechanism is particularly insidious in the larynx and throat. Unlike the esophagus, which has some built-in acid defenses, the laryngeal lining has very little protection. Pepsin sticks to the surface of laryngeal cells, depletes their protective defenses, and can even be taken up inside the cells through a process called endocytosis, where it causes internal damage.¹¹ This helps explain why people with laryngopharyngeal reflux (the kind that reaches the throat) can have significant symptoms like chronic hoarseness, cough, and throat clearing even when standard acid measurements look relatively normal. The damage isn’t coming from acid alone.

Research into pepsin inhibition has shown that irreversibly blocking pepsin’s activity prevents the cellular changes that reflux causes, while merely reducing acid (the approach taken by most reflux medications) is not always sufficient.¹² This finding has prompted interest in developing drugs that directly target pepsin, though no widely available pepsin-specific therapy has reached the market yet.

Pepsin as a Diagnostic Tool

Because pepsin should only be present in the stomach under normal circumstances, finding it in other body fluids can serve as a signal that reflux is occurring. Researchers have explored salivary pepsin testing as a non-invasive way to diagnose gastroesophageal reflux disease without requiring pH monitoring probes or endoscopy.

Studies comparing salivary pepsin levels in patients with confirmed reflux disease versus healthy controls have found that people with erosive esophagitis, non-erosive reflux disease, and Barrett’s esophagus all had significantly higher levels of pepsin in their saliva.¹³ As a standalone diagnostic, though, salivary pepsin has modest accuracy. One study using formal impedance-pH monitoring as the gold standard found that salivary pepsin had a sensitivity of about 76% and specificity of about 63% for confirmed reflux disease.¹⁴ Another assessment described the performance as “modest,” with the threshold you choose for a positive result determining whether you favor catching more true cases or avoiding false positives.¹⁵

Salivary pepsin testing is best understood as a screening tool rather than a definitive diagnosis. It can flag patients who deserve further workup, but it isn’t accurate enough on its own to replace traditional testing.

Blood Pepsinogen for Stomach Cancer Screening

Pepsinogen levels in the blood serve a completely different diagnostic role. The stomach releases small amounts of pepsinogen into the bloodstream, and the ratio between two subtypes (pepsinogen I and pepsinogen II) changes when the stomach lining becomes atrophied, a condition that raises gastric cancer risk. A meta-analysis of this approach found that combining pepsinogen I concentration with the pepsinogen I-to-II ratio yielded a sensitivity of about 79% and specificity of about 89% for detecting atrophic gastritis.¹⁶ This blood test is widely used in Japan as part of population-level gastric cancer screening programs, though it hasn’t been adopted as broadly in Western countries, where stomach cancer rates are lower.

Pepsin’s Evolutionary Roots

Pepsin isn’t unique to humans or even to mammals. The genes encoding pepsinogens appear to date back to the common ancestor of all jawed vertebrates, a lineage that includes sharks, bony fish, amphibians, reptiles, birds, and mammals. Researchers recently identified pepsinogen genes in the genomes of cartilaginous fish like whale sharks, catsharks, and skates for the first time, confirming that this enzyme family is ancient.¹⁷

Perhaps more interesting is what happens when species lose their stomachs. Several fish lineages, including pufferfish, seahorses, and lungfish, have independently evolved to live without a stomach. In every case examined, the loss of the stomach is matched by the complete loss of functional pepsinogen genes and the genes for the acid-producing proton pump. These genes don’t just go quiet; they become pseudogenes, broken remnants that can no longer produce a working protein.¹⁸ This tight correlation between losing the stomach and losing pepsinogen genes reinforces how central pepsin is to the stomach’s purpose. A stomach without pepsin, from an evolutionary standpoint, apparently isn’t worth keeping.

Among species that do have stomachs, the number of pepsinogen genes varies wildly. Some fish have just two, while certain mammals carry dozens of copies.¹⁸ Why some species need so many variants isn’t fully understood, though it may relate to dietary diversity or the pH ranges their stomachs maintain.

Pepsinogen in the Lungs

One of the more unexpected findings in pepsin research is the detection of pepsinogen C (one of the pepsinogen subtypes) in human lung tissue, specifically in the lungs of newborns. Immunohistochemistry studies found pepsinogen C in a specific cell type called type II pneumocytes in the majority of neonatal lung samples examined.¹⁹ This is distinct from finding pepsin in the lungs of adults with reflux, where the enzyme arrives via aspiration of gastric contents. In the neonatal case, the pepsinogen appears to be produced locally by the lung cells themselves.

Why neonatal lungs would make pepsinogen remains an open question. One possibility is that it plays a role in processing or remodeling the airway lining during the transition from fluid-filled fetal lungs to air-breathing ones. Separately, when pepsin is found in adult lung fluid, it serves as a marker of gastric aspiration rather than local production. A study examining bronchoalveolar fluid from adults found pepsin protein but no pepsinogen mRNA, confirming the enzyme had arrived from the stomach rather than being manufactured on site.²⁰

Industrial Uses of Pepsin

Pepsin has commercial applications well beyond the human body. Porcine pepsin, extracted from pig stomachs, is one of the most commonly used enzymes in food processing and biomedical manufacturing. One major application is the production of pharmaceutical-grade collagen. Pepsin cleaves the non-helical end regions (telopeptides) of collagen molecules while leaving the main triple-helix structure intact, yielding a purified collagen product used in medical devices, wound dressings, and cosmetic fillers.²¹ Removing the telopeptides also reduces the immune response the collagen triggers when implanted in human tissue, making the final product safer for medical use.

In food science labs, pepsin is a standard component of in vitro digestion models, the bench-top simulations researchers use to predict how well a food will be digested in a real stomach. These models typically expose food samples to pepsin at an acidic pH to mimic gastric conditions, then follow up with pancreatic enzymes at a neutral pH to simulate the intestinal phase. The results help food scientists compare the digestibility of different protein sources, evaluate new food products, and study how processing techniques change nutritional availability. Pepsin’s broad specificity makes it especially useful for these simulations, because it mimics the indiscriminate first pass that food proteins undergo in a real stomach.