Final answer:
Trypsinogen, secreted by the pancreas, is activated by the enzyme enteropeptidase present in the intestinal brush border, leading to a cascade of activation for other digestive enzymes.
Step-by-step explanation:
Trypsinogen is secreted from the pancreas and is activated in the duodenum, which is the first part of the small intestine. The activation of trypsinogen is initiated by an enzyme known as enteropeptidase, also previously called enterokinase, present in the intestinal brush border.
Once activated, trypsin itself activates other pancreatic zymogens, including procarboxypeptidase and chymotrypsinogen, converting them into their active forms, carboxypeptidase and chymotrypsin respectively.
This cascade of activation is crucial because it allows the protein-digesting enzymes to be transported in inactive forms, preventing them from digesting the pancreas itself, a process that can lead to pancreatitis.