The final digestive processes occur primarily at the mucosal lining of the duodenum and upper jejunum and include the action of several disaccharidases (see Fig. 7.9). For example, isomaltase cleaves the α(1→6) bond in isomaltose, and maltase cleaves the α(1→4) bond in maltose and maltotriose, each producing glucose. Sucrase cleaves the α(1→2) bond in sucrose, producing glucose and fructose, and lactase (β-galactosidase) cleaves the β(1→4) bond in lactose, producing galactose and glucose. [Note: The substrates for isomaltase are broader than its name suggests, and it hydrolyzes the majority of maltose.] Trehalose, an α(1→1) disaccharide of glucose found in mushrooms and other fungi, is cleaved by trehalase. These enzymes are transmembrane proteins of the brush border on the luminal (apical) surface of the enterocytes.
Sucrase and isomaltase are enzymic activities of a single protein that is cleaved into two functional subunits, which remain associated in the cell membrane and form the sucrase-isomaltase (SI) complex. In contrast, maltase is one of two enzymic activities of the single membrane protein maltase-glucoamylase (MGA) that does not get cleaved. Its second enzymic activity, glucoamylase, cleaves α(1→4) glycosidic bonds in dextrins.
D. Intestinal absorption of monosaccharides
The upper jejunum absorbs the bulk of the monosaccharide products of digestion. However, different sugars have different mechanisms of absorption (Fig. 7.10). For example, galactose and glucose are taken into enterocytes by secondary active transport that requires a concurrent uptake (symport) of sodium (Na+) ions. The transport protein is the sodium-dependent glucose cotransporter 1 (SGLT-1). [Note: Sugar transport is driven by the Na+ gradient created by the Na+-potassium (K+) ATPase that moves Na+ out of the enterocyte and K+ in (see Fig. 7.10).] Fructose absorption utilizes an energy-and Na+-independent monosaccharide transporter (GLUT-5). All three monosaccharides are transported from the enterocytes into the portal circulation by yet another transporter, GLUT-2. [Note: See p. 97 for a discussion of these transporters.]
E. Abnormal degradation of disaccharides
The overall process of carbohydrate digestion and absorption is so efficient in healthy individuals that ordinarily all digestible dietary carbohydrate is absorbed by the time the ingested material reaches the lower jejunum. However, because only monosaccharides are absorbed, any deficiency (genetic or acquired) in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine. As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea. This is reinforced by the bacterial fermentation of the remaining carbohydrate to two-and three-carbon compounds (which are also osmotically active) plus large volumes of carbon dioxide and hydrogen gas (H2), causing abdominal cramps, diarrhea, and flatulence.
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