Where is enteropeptidase located and what is its function?

• A. Pancreas; converts trypsinogen to trypsin
• B. Brush border of the duodenum; converts trypsinogen to trypsin
• C. Stomach; activates pepsinogen
• D. Jejunum; activates chymotrypsinogen

Answer: (B)

Enteropeptidase (enterokinase) is a brush-border enzyme on the microvilli of duodenal enterocytes in the small intestine. Its role is to activate pancreatic trypsinogen by cleaving off its activation peptide, producing active trypsin. This kickstarts the pancreatic enzyme cascade, because trypsin then activates other zymogens like chymotrypsinogen and procarboxypeptidases, allowing protein digestion to proceed in the small intestine. The site matters: the duodenum is where chyme mixes with pancreatic juice, so activation starts there. The pancreas produces trypsinogen, not enteropeptidase; pepsinogen is activated in the stomach by acid and pepsin; and chymotrypsinogen is activated downstream by trypsin, not directly by enteropeptidase.