Chemical Digestion in the Small Intestine: Brush Border Enzymes and Pancreatic Enzymes

Two Sources of Digestive Enzymes

The small intestine is the primary site of chemical digestion. Enzymes come from:

1. Brush Border (Intestinal Mucosa): Disaccharidases (lactase, sucrase-isomaltase, maltase-glucoamylase), peptidases (aminopeptidases, dipeptidases), nucleosidases.
2. Pancreas: Amylase (starch), lipase (triglycerides), proteases (trypsinogen, chymotrypsinogen, procarboxypeptidases) activated in lumen.

Coordination

• Pancreatic amylase → maltose/maltotriose → brush border maltase/sucrase-isomaltase → glucose.
• Pancreatic lipase + colipase + bile salts → 2-MAG + FFA → absorbed.
• Pancreatic proteases → oligopeptides → brush border peptidases → amino acids/di/tripeptides.

Clinical Correlate

Pancreatic insufficiency (cystic fibrosis, chronic pancreatitis) → maldigestion → steatorrhea, creatorrhea; treated with pancreatic enzyme replacement therapy (PERT).