Final answer:
Trypsinogen is converted into active trypsin in the stomach by the enzyme enterokinase, secreted by the intestinal cells.
Step-by-step explanation:
The proteolytic enzyme trypsinogen is converted into the active enzyme trypsin in the stomach by an enzyme called enterokinase, which is secreted by the intestinal cells. Enterokinase removes a hexapeptide from the N-terminal end of trypsinogen, resulting in the formation of trypsin. Trypsin is then responsible for activating the remaining trypsinogen as well as other enzymes involved in protein digestion.