Intestinal Enzymes

In addition to providing a large surface area for absorption, the plasma membranes of the microvilli contain digestive enzymes that hydrolyze disaccharides, polypeptides, and other substrates (table 18.1). These brush border enzymes are not secreted into the lumen, but instead remain attached to the cell membrane with their active sites exposed to the chyme. One brush border enzyme, enterokinase, is required for activation of the protein-digesting enzyme trypsin, which enters the small intestine in pancreatic juice.

Physiology Boarder

■ Figure 18.13 Electron micrographs of microvilli. Microvilli are evident at the apical surface of the columnar epithelial cells in the small intestine. These are seen here (a) at lower magnification and (b) at higher magnification. Microvilli increase the surface area for absorption and also have the brush border digestive enzymes embedded in their plasma membranes.

From Keith R. Porter, D. H. Alpers, and D. Seetharan, "Pathophysiology of Diseases Involving Intestinal Brush-Border Proteins" in New England Journal of Medicine, Vol. 296, 1977, p. 1047, fig. 1. Copyright © 1977 Massachusetts Medical Society. All rights reserved.

■ Figure 18.13 Electron micrographs of microvilli. Microvilli are evident at the apical surface of the columnar epithelial cells in the small intestine. These are seen here (a) at lower magnification and (b) at higher magnification. Microvilli increase the surface area for absorption and also have the brush border digestive enzymes embedded in their plasma membranes.

From Keith R. Porter, D. H. Alpers, and D. Seetharan, "Pathophysiology of Diseases Involving Intestinal Brush-Border Proteins" in New England Journal of Medicine, Vol. 296, 1977, p. 1047, fig. 1. Copyright © 1977 Massachusetts Medical Society. All rights reserved.

Table 18.1 Brush Border Enzymes Attached to the Cell Membrane of Microvilli in the Small Intestine

Category Enzyme Comments

Disaccharidase

Sucrase

Digests sucrose to glucose and fructose; deficiency produces gastrointestinal disturbances

Maltase

Digests maltose to glucose

Lactase

Digests lactose to glucose and galactose; deficiency produces gastrointestinal disturbances

(lactose intolerance)

Peptidase

Aminopeptidase

Produces free amino acids, dipeptides, and tripeptides

Enterokinase

Activates trypsin (and indirectly other pancreatic juice enzymes); deficiency results in protein malnutrition

Phosphatase

Ca2+, Mg2+-ATPase

Needed for absorption of dietary calcium; enzyme activity regulated by vitamin D

Alkaline phosphatase

Removes phosphate groups from organic molecules; enzyme activity may be regulated by vitamin D

Slow waves

Small Intestine Segmentation

Slow waves

■ Figure 18.14 Segmentation of the small intestine. Simultaneous contractions of numerous segments of the intestine help to mix the chyme with digestive enzymes and mucus.

The ability to digest milk sugar, or lactose, depends on the presence of a brush border enzyme called lactase. q This enzyme is present in all children under the age of 4 but becomes inactive to some degree in most adults (people of Asian or African heritage are more often lactase deficient than Caucasians). A deficiency of lactase can result in lactose intolerance, a condition in which too much undigested lactose in the intestine causes diarrhea, gas, cramps, and other unpleasant symptoms. Yogurt is better tolerated than milk because it contains lactase (produced by the yogurt bacteria), which becomes activated in the duodenum and digests lactose.

Food Allergies

Food Allergies

Peanuts can leave you breathless. Cat dander can lead to itchy eyes, a stuffy nose, coughing and sneezing. And most of us have suffered through those seasonal allergies with horrible pollen counts. Learn more...

Get My Free Ebook


Responses

  • ADELMIO
    Do microvilli contain digestive enzymes?
    7 years ago
  • jonathan
    What are intestinal enzymes?
    7 years ago
  • brendan
    Which enzymes are present in the microvilli brush border of the small intestine?
    7 years ago
  • Gina
    What enzymes are produced by the brush border of the small intestines?
    7 years ago
  • piera
    What are the brush border enzymes called?
    7 years ago
  • morgan
    Which microville contains digestive enzymes?
    7 years ago
  • Helen
    What cells in the small intestines have digestive enzymes attached to their surfaces?
    7 years ago
  • Lisa
    Why do enzymes have to be activated by brush border?
    3 years ago
  • iggi
    Which enzyme are present in the microvilli brush border of the small intestine?
    2 years ago
  • ricardo
    Which cells in thew small intestine have the digestive enzymes attached to their surfaces?
    2 years ago

Post a comment