Digestion

Digestion is the chemical breakdown of ingested food into absorbable molecules and is due to enzymes secreted into the lumen or bound to the apical membranes of the enterocytes. This suggests two major types of enzymes and digestion: Luminal or cavital digestion takes place in the lumen of the gastrointestinal tract and is carried out by enzymes secreted by the salivary glands, stomach, and pancreas. Membrane digestion, also referred to as contact digestion, is carried out by hydrolytic enzymes synthesized by the enterocytes and inserted in their brush borders as integral parts of the microvillar membrane. Enterocytes continuously synthesize these enzymes to replace those that are broken down or lost into the lumen. As the enterocyte migrates up the villus, the total number of enzymes present and, hence, its capacity to hydrolyze nutrients increase. The important digestive enzymes and their sources are shown in Table 1.

A significant surplus of digestive enzymes is normally present. The enzymes of the salivary glands and stomach are not essential. All luminal digestion can easily be accomplished by the pancreatic enzymes. The pancreatic

TABLE 1 Digestive Enzymes According to Their Sources

Source

Enzyme

Salivary glands

Stomach Pancreas

Intestinal mucosa

Amylase Lingual lipase Pepsin Amylase Trypsin Chymotrypsin Carboxypeptidase Elastase Lipase-colipase Phospholipase A2 Cholesterol esterase (nonspecific) Enterokinase Disaccharidases Sucrase Maltase Trehalase a-Dextrinase (isomaltase) Peptidases

Aminooligopeptidase Dipeptidase enzymes are also present in excess, and maldigestion and malabsorption do not occur unless secretion is reduced to a small fraction of normal. Steatorrhea (dietary fat in the stool), for example, does not occur unless lipase secretion is reduced by at least 80%. Hydrolysis by membrane-bound enzymes is also extremely rapid. Digestion and absorption of essentially all major dietary components are complete before the ingesta enter the ileum.

Developmental Changes

In some species, notably the rat (in which most studies have been performed), levels of disaccharidases except lactase are extremely low at birth. At the time of weaning, the third week of life, activities of sucrase, maltase, trehalase, and isomaltase increase dramatically, and lactase activity decreases to near zero. This shift in the pattern of enzyme activities to accommodate the change in diet is genetically programmed and is not dependent on the change in diet. The enzyme shift can be triggered by corticosterone, but it also occurs in adrenalectomized rats.

Humans do not undergo these changes in enzyme activities. All disaccharidases are present from birth at adult levels. Lactase levels are actually higher in the newborn than in the adult. In fact, in many adults— blacks and Asians in particular—lactase levels are sufficiently depressed so that these individuals cannot digest milk and other lactose-containing dairy products. This is not surprising if one stops to think that man is the only mammal that consumes milk after weaning. Interestingly, adult human populations showing the smallest degree of lactose intolerance are those originating in northern Europe, where herd animals were first domesticated.

Dietary Regulation

The pattern of enzymes produced by the pancreas shifts in response to chronic changes in the diet. This type of change on the part of an organism that allows it to cope with an alteration in its environment is called an adaptation. Substitution of protein for a significant portion of the carbohydrates present in the diet results in a decrease in the amount of amylase secreted and an increase in the amounts of proteases. Similarly, the total amount of lipase secreted is increased, compared with the other enzymes, after exposure to a high-fat diet. These changes are produced at the level of enzyme synthesis, for, in general, the enzymes are secreted in the same proportion as they are synthesized. The mechanism of this feedback regulation involves effects of cholecystokinin (CCK), secretin, and insulin on the expression of messenger RNAs (mRNAs) for the various enzymes (see Chapter 34).

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