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Short answer
Trypsin is a digestive protease made by the pancreas. It breaks proteins into peptides by cutting peptide bonds on the carboxyl side of the amino acids lysine and arginine, and it also switches on other digestive enzymes made in the pancreas. The same cleavage rule is why labs use purified trypsin to detach cells and to prepare proteins for mass spectrometry.
What trypsin is: from trypsinogen to active enzyme
The pancreas produces trypsin as an inactive precursor, trypsinogen, which is secreted into the small intestine. According to MedlinePlus Genetics, “when the enzyme is needed, trypsinogen is cleaved again into its working (active) form called trypsin.” The human gene PRSS1 encodes cationic trypsinogen (UniProt P07477, recommended name serine protease 1, alternative name cationic trypsinogen; EC 3.4.21.4), and its expression is enriched in the pancreas.
Active trypsin breaks down proteins, and MedlinePlus Genetics adds that it also activates other digestive enzymes produced in the pancreas. Keeping the enzyme inactive until it reaches the intestine matters: MedlinePlus notes that trypsin activity inside the pancreas can damage pancreatic tissue and trigger an immune response. More than 40 PRSS1 variants are linked to hereditary pancreatitis, and UniProt lists susceptibility to the condition among the gene’s disease associations.
How trypsin cleaves: the lysine and arginine rule
UniProt records trypsin’s catalytic activity as preferential cleavage of the bond after arginine or lysine (Arg-|-Xaa and Lys-|-Xaa). That is the answer to what trypsin breaks down and what it digests: not every protein bond equally, but the bonds following K and R residues.
Supplier documentation for sequencing-grade trypsin (Promega, product V5111) states two refinements that matter at the bench:
- Cleavage is blocked when proline follows the lysine or arginine (Lys-Pro, Arg-Pro).
- Acidic residues (aspartate, glutamate) flanking the site reduce activity.
So the rule predicts where trypsin usually cuts, not every cut. Software that predicts tryptic peptides, such as those described in the mass spectrometry database search guide, builds on this specificity. For the cell-culture use of the same rule, see the trypsinization guide.
One enzyme, three settings
The table compares the roles of trypsin; each row rests on the sources named in the last column.
| Setting | What trypsin does | Why the K/R rule matters | Source |
|---|---|---|---|
| Digestion in the body | Breaks proteins down and activates other pancreatic digestive enzymes | Cuts after lysine and arginine, producing peptides for further digestion | MedlinePlus Genetics (PRSS1); UniProt P07477 |
| Cell culture | Detaches adherent cells from the surface by cutting proteins that hold them there | The same protein-cutting activity, applied to cell attachments; see trypsinization and dissociation reagent comparison | CASRAI trypsinization guide |
| Proteomics | Digests protein samples into peptides before mass spectrometry | Cleaving C-terminal to Lys and Arg gives predictable peptides; see proteomics sample preparation | Promega V5111 product documentation (cleavage rule); CASRAI proteomics guides |
What to check on a research-grade trypsin
Because the enzyme digests itself and often arrives with other proteases, the grade on the label changes the result. The points below come from Promega’s product documentation for sequencing-grade modified trypsin.
- Autolysis. Native trypsin digests itself, forming pseudotrypsin, which cleaves at hydrophobic residues and yields unexpected peptides. The documentation says reductive methylation is used to block autolysis, so “modified” and unmodified trypsin are not interchangeable.
- Chymotrypsin contamination. Trypsin prepared from porcine pancreas can carry chymotrypsin. TPCK treatment inactivates it, and the documentation states the product has no detectable chymotryptic activity.
- Intended application. Promega states it cannot guarantee mass spectrometry performance for this particular grade and points to other grades of its own for that use. Read the stated application on the specification rather than assuming a sequencing grade suits every method.
Check the certificate or specification for these points before substituting one trypsin for another, since a different grade can change which peptides appear on a gel or a spectrum (see the SDS-PAGE guide for how digested samples are visualised).
Common misunderstandings
Trypsin is not the only digestive protease
MedlinePlus describes trypsin as one of several pancreatic digestive enzymes, and notes it activates others. Treating it as the single enzyme that digests protein oversimplifies digestion.
Trypsin does not digest all proteins at the same sites
Its preference is for K/R bonds, with the proline and acidic-residue exceptions above.
Lab trypsin is not the same preparation as the digestive enzyme in the body
Laboratory trypsin is a purified or modified preparation; grade, treatment and activity vary, so the label matters.
FAQ
What does trypsin break down?
Proteins. It cleaves peptide bonds after lysine or arginine, unless proline follows (UniProt P07477; Promega V5111).
What does trypsin digest?
The same: proteins in the small intestine, and in the lab, proteins in cell attachments or protein samples prepared for mass spectrometry.
What is the role of trypsin?
In the body it breaks down proteins and activates other pancreatic digestive enzymes (MedlinePlus Genetics). In the lab it is a reagent for detaching cells and for protein digestion before analysis.
What are the different trypsin enzymes?
In humans, PRSS1 encodes cationic trypsinogen (UniProt P07477), the pancreatic form described on this page. Commercial products also differ by source and treatment, as covered in the checklist above.
Sources
All links checked 2026-10-29.
- UniProt, P07477 (PRSS1, serine protease 1 / cationic trypsinogen): https://rest.uniprot.org/uniprotkb/P07477.txt
- MedlinePlus Genetics, PRSS1 gene: https://medlineplus.gov/genetics/gene/prss1/
- Promega, Sequencing Grade Modified Trypsin (V5111): https://www.promega.com/products/mass-spectrometry/proteases-and-surfactants/sequencing-grade-modified-trypsin/?catNum=V5111
