Digestion and Absorption in the Mammalian Gastrointestinal Tract - Peptidases

Hydrolysis of disaccharides into monosaccharides by enzymes in the small intestine: The enzymes sucrase, lactase, and maltase catalyze the hydrolysis of sucrose, lactose, and maltose into their constituent monosaccharides. These enzymes are built into the membranes of the microvilli brush border of the epithelial cells lining the villi in the small intestine. The disaccharides are hydrolyzed as they come in contact with the enzymes of the membranes. Only monosaccharides are absorbed into the epithelial cells and from there into the blood capillaries of the villi.

Hydrolysis of small polypeptides in the small intestine: Protein digestion is completed in the small intestine by enzymes called exopeptidases which are also built into the membranes of the microvilli brush border. These enzymes protrude through the membranes into the exterior where they come in contact with the intestinal fluids. Small polypeptides reaching the small intestine are hydrolyzed by endopeptidases and exopeptidases into tripeptides, dipeptides, and free amino acids. All three of these substances are easily transported through the microvillar membrane to the interior of the epithelial cell. Once inside the cell other exopeptidases catalyze the hydrolysis of the di- and tripeptides into individual amino acids, which can then pass out of the cell and enter the blood capillaries of the villi.

Note that proteases can be classified as exo- or endopeptidases. Exopeptidases cleave single amino acids off one or the other end of the polypeptide. They digest the protein directly into amino acids, working inward from the ends. In contrast, endopeptidases cleave bonds adjacent to specific amino acids, so they break up the protein into smaller chunks, from middle, working outwards.

Primary Structure
of a Polypeptide: Lys-Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala

Action of
Exopeptidase:     Lys + Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala

Action of
Endopeptidase:    Lys-Val-Phe-Gly + Arg-Cys-Glu-Leu-Ala

Hydrolysis and absorption of fat in the small intestine: Because fat (and partially digested fat) is nonpolar, it is highly soluble in the lipid portion of the epithelial cell membrane, and can be absorbed without digestion. Much of the fat (triglyceride) is partially digested by the enzyme lipase, which is secreted by the pancreas. Lipase catalyzes the hydrolysis of fat into free fatty acids and monoglycerides (see diagram), which are lipid soluble and diffuse through the microvillar membrane into the interior of the cell. Once inside the cell, the free fatty acids and monoglycerides are taken up into the smooth endoplasmic reticulum where they are recombined to form new triglyceride (fat) molecules. The triglyceride molecules aggregate and combine with other lipid-soluble substances, such as cholesterol and phospholipids, to form globules. The globules are then coated with protein to form lipoprotein complexes (called chylomicrons). They are expelled from the cell by exocytosis. From there they enter a lymph vessel found in the center of each villus and move through the lymphatic system, eventually emptying into the blood circulatory system in the great veins of the neck.

Linings of the Stomach and Intestine

Cross section of mucosa in the stomach body shows arrangement of gastric cells.

Epithelial cells, which secrete mucus, cover the surface of the stomach and also line the gastric pits. The oxyntic (or parietal) cells produce hydrochloric acid; the chief (or zymogenic) cells produce pepsinogen. These cells lie in deep tubules; their secretions reach the surface through the gastric pits.

Tight junctions formed by fused membranes near the tips of the epithelial cells provide a barrier to diffusion (see text page 860).

Structure of a villus. Villi cover the surface of the intestinal mucosa. Each villus consists of a layer of columnar epithelial cells and a central core of connective tissue. Microvilli project from the epithelial cells, forming an absorptive bush border. Embedded in the connective tissue core are the lymphatic lacteals and blood capillaries. Fatty acids diffuse across the membranes of the microvilli and into the epithelial cells. Short-chain fatty acids are transported directly to the blood capillaries embedded in the core of the villus. Long-chain fatty acids are packaged into chylomicrons and transported to the lymphatic lacteals.