Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting long chains of amino acids into smaller pieces.

It is a serine protease from the PA clan superfamily, found in the digestive system of many vertebrates, where it hydrolyzes proteins.

[5] Although many sources say that Kühne named trypsin from the Ancient Greek word for rubbing, 'tripsis', because the enzyme was first isolated by rubbing the pancreas with glass powder and alcohol, in fact Kühne named trypsin from the Ancient Greek word 'thrýpto' which means 'I break' or 'I break apart'.

Once in the small intestine, the enzyme enterokinase (also called enteropeptidase) activates trypsinogen into trypsin by proteolytic cleavage.

[9][page needed] By these means, the nucleophilicity of the active site serine is increased, facilitating its attack on the amide carbon during proteolysis.

The enzymatic reaction that trypsin catalyzes is thermodynamically favorable, but requires significant activation energy (it is "kinetically unfavorable").

The activity of trypsin is not affected by the enzyme inhibitor tosyl phenylalanyl chloromethyl ketone, TPCK, which deactivates chymotrypsin.

One consequence of the autosomal recessive disease cystic fibrosis is a deficiency in transport of trypsin and other digestive enzymes from the pancreas.

Trypsin is particularly suited for this, since it has a very well defined specificity, as it hydrolyzes only the peptide bonds in which the carbonyl group is contributed either by an arginine or lysine residue.

[20] In contrast with nearly all known protein assemblies, some complexes of trypsin bound by its inhibitors do not readily dissociate after treatment with 8M urea.

[22] It is of public health policy interest to explore various ways to mitigate this occurrence including use of trypsin inhibitors.

These inhibitors have capabilities of reducing colon, breast, skin, and prostate cancer by way of radioprotective and anticarcinogenic activity.

Trypsin inhibitors can act as regulatory mechanisms to control release of neutrophil proteases and avoid significant tissue damage.

[23] Trypsin inhibitors from amphibian skin showed bacterial death promotion that affected the cell wall and membrane of Staphylococcus aureus.

[27] ProAlanase is an Aspergillus niger fungus protease that can achieve high proteolytic activity and specificity for digestion under the correct conditions.

[27] ProAnalase performed optimally in LC-MS applications with short digestion times and highly acidic pH.