Trypsin is a digestive protease produced in the pancreas (it is a component of pancreatic juice, which is released in the duodenum during digestion). Its function is to cleave dietary proteins, and for this it recognizes amino acids with basic side chains (lysine and arginine) and cleave peptide bonds in which they are involved. In order to be inactive in the cells that produce it (if this were not the case it would begin to degrade our own proteins), it is synthesized as trypsinogen. Trypsinogen is the zymogen of trypsin, an inactive form of an enzyme, characterized by having more amino acids than those that are required for the enzyme to be in its functional form. The idea is that these additional amino acids block the catalytic activity (e.g., preventing access of substrate to the active site).
Once in the intestine, trypsinogen is proteolytically cleaved by the action of an enzyme called intestinal enteropeptidase. From the moment that a molecule becomes active trypsin, itself can begin to activate (by proteolytical cleavage) all other zymogens, not only trypsin but also chymotrypsin, carboxypeptidases and aminopeptidases. Thus, trypsin has a central role in the activation of digestive proteases, so it is necessary to ensure that any molecule, under normal conditions, will not become catalytically active inside the cells. The first line of protection is the synthesis of the enzyme in the form of trypsinogen. Additionally, cells which produce pancreatic trypsin also have a second line of defense, which involves the production of an inhibitory protein called pancreatic trypsin inhibitor. Therefore, even if spontaneously one trypsinogen acquires activity inside the cells, the presence of this inhibitor will prevent it to exerts its function and consequently, avoiding the cell to begin to cleave and activate other zymogens proteins.