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Digestive enzyme secretion is coordinated by neural (vagal) and hormonal (cholecystokinin, secretin, motilin) signals. The stomach secretes pepsinogen (activated to pepsin by HCl) for protein digestion; the pancreas secretes amylase, lipase, and protease precursors (zymogens: trypsinogen, chymotrypsinogen, proelastase) into the duodenum, where enterokinase activates them. The intestinal brush border secretes dipeptidases, disaccharidases, and other enzymes. Secretion is stimulated by nutrient presence (especially fat and protein) and pH, and is inhibited by parasympathetic antagonists and by nutrient absence. Enzyme deficiency (lactase, amylase, lipase) reduces digestion and absorption of specific nutrients.
Map hormone release (CCK, secretin, gastrin, motilin) in response to meal composition; predict enzyme secretion patterns and predict digestion efficiency given specific enzyme deficiencies.
From your study of digestive glands, you know that the stomach, pancreas, and intestinal lining all secrete substances into the gut lumen. From enzyme structure and function, you know that enzymes are proteins that lower the activation energy of specific reactions and are sensitive to pH and substrate availability. Digestive enzyme secretion is where these two ideas meet: the body must coordinate enzyme release so that the right enzymes are present in the right place at the right time, in quantities matched to what was actually eaten.
The stomach's contribution is pepsinogen, secreted by chief cells in response to gastric distension and the hormone gastrin. Pepsinogen is an inactive zymogen — a precursor that becomes active only when cleaved. In the acidic stomach environment (pH ~2), HCl denatures food proteins and autocatalytically activates pepsinogen into pepsin, which begins protein hydrolysis. The reason pepsinogen is stored as a zymogen is protective: if pepsin were always active in the cells that make it, it would digest the cell itself. Mucus and bicarbonate from goblet cells and surface epithelium protect the stomach lining from both acid and pepsin.
The pancreas secretes the majority of digestive enzymes into the duodenum. When fat and protein reach the duodenum, enteroendocrine cells (I-cells) release cholecystokinin (CCK). CCK acts on the pancreas to stimulate secretion of lipase (fat digestion), amylase (starch digestion), and a suite of protease zymogens: trypsinogen, chymotrypsinogen, and proelastase. Simultaneously, acid entering the duodenum from the stomach triggers S-cells to release secretin, which stimulates the pancreatic ductal cells to secrete bicarbonate-rich fluid — neutralizing the acid and raising luminal pH to ~7, the optimal range for pancreatic enzymes. This pH shift is also essential for bile salts to function in fat emulsification.
The activation cascade in the duodenum is a masterpiece of sequential zymogen activation. Enterokinase (enteropeptidase), a brush border enzyme, cleaves trypsinogen into trypsin. Active trypsin then cleaves all the other zymogens — chymotrypsinogen into chymotrypsin, proelastase into elastase, and more trypsinogen into trypsin (autocatalysis). This cascade amplifies a small initial signal into a large burst of protease activity, and its location in the duodenal lumen (rather than in the pancreatic cells themselves) protects the pancreas. When this protection fails — as in acute pancreatitis, often triggered by gallstones or alcohol — zymogens are activated prematurely inside the pancreas, causing autodigestion and severe inflammation.
The intestinal brush border adds the final layer: disaccharidases (lactase, sucrase, maltase) break disaccharides into monosaccharides, and dipeptidases cleave small peptides into amino acids. These are membrane-bound, not secreted into the lumen, which means their capacity is limited by intestinal surface area. Lactase deficiency — the most common enzyme deficiency worldwide — illustrates the clinical consequence: undigested lactose reaches the colon, where bacteria ferment it, producing gas and osmotic diarrhea. The pattern is the same for any enzyme deficiency: undigested substrate persists, changes the osmotic environment, and feeds colonic bacteria instead of being absorbed. Understanding the normal secretion and activation sequence makes every enzyme deficiency syndrome immediately interpretable.