Armelle Leturque, PhD and Edith Brot-Laroche, PhD∗ TABLE 8-1 Summary of Digestion of Dietary Carbohydrate of starch granules varies with each plant species and within different parts of the plant. Starch digestion begins by the action of secreted α-amylase in the lumen of the gastrointestinal tract and is completed by the action of α-glucosidases that are associated with the apical membrane of the intestinal mucosal cells. Pancreatic amylase is secreted in large quantities into the duodenal lumen. Most of the starch component of grains or legumes establishes contact with the pancreatic α-amylases within the polar bulk phase of the intestinal luminal milieu. The concentration of amylase achieved within the duodenal lumen greatly exceeds that required for cleavage of the bonds joining the glucose components of the starches. Indeed, the cleavage of starches to the final oligosaccharide products normally occurs in the uppermost part of the small intestine and is virtually complete by the time the meal reaches the duodenal–jejunal junction (Fogel and Gray, 1973). Furthermore, refined starches are hydrolyzed efficiently to glucose oligomers, even in patients with exocrine pancreatic insufficiency who have amylase levels that are only about 10% of normal (Fogel and Gray, 1973). Maltase–glucoamylase has a higher affinity (lower Km) and faster rate of substrate turnover (higher kcat) for hydrolysis of glucose oligomers than does sucrase–isomaltase, and thus it will catalyze a more rapid rate of hydrolysis at low concentrations of glucose oligomers (Table 8-2). However, it appears that maltase–glucoamylase contributes a relatively small fraction of the total glucose oligomer-hydrolyzing activity in humans in vivo. Indeed, sucrase–isomaltase activity in humans is about 20 times that of maltase–glucoamylase activity. Moreover, sucrase–isomaltase is not inhibited by lumenal starch-derived oligosaccharides as is the activity of maltase–glucoamylase in assays performed in vitro (Quezada-Calvillo et al., 2007). Thus sucrase–isomaltase probably contributes most of the α-glucosidase activity when high-starch diets are consumed. TABLE 8-2 Kinetic Constants for α-Glucosidases for Oligosaccharide Hydrolysis ∗Km, Michaelis constant; the substrate concentration at which an enzyme or transporter is half-saturated with substrate. †kcat, First-order rate constant corresponding to the slowest step or steps in the overall catalytic reaction; the number of catalytic cycles the active site undergoes per unit time. In addition to digestion of maltose, as described for the end products of starch breakdown, humans have the capacity to hydrolyze unique linkages present in sucrose, lactose, and trehalose. Kinetic properties of these disaccharidase activities are shown in Table 8-3. These disaccharidases are all associated with the luminal surface of the enterocytes, as illustrated in Figure 8-4. TABLE 8-3 ∗Km, Michaelis constant; the substrate concentration at which an enzyme or transporter is half-saturated with substrate. †kcat, First-order rate constant corresponding to the slowest step or steps in the overall catalytic reaction; the number of catalytic cycles the active site undergoes per unit time. Trehalose [α-D-glucopyranosyl-(1,1′)α′-D-glucopyranoside] is a disaccharide present in small amounts in bacteria, fungi, plants, and invertebrates (Elbein et al., 2003; Richards et al., 2002). Significant food sources of trehalose in modern Western diets include baker’s yeast, brewer’s yeast, cultivated and wild mushrooms, honey, sunflower seeds, sea algae, lobster, shrimp, and processed foods to which trehalose has been added. Trehalose may have constituted a larger source of sugar in the diet of ancient man, and some populations eat a much higher proportion of invertebrates (e.g., insects in which trehalose is the major hemolymph sugar) and fungi (e.g., yeasts, molds, mushrooms, and truffles) as part of their diets and therefore ingest more trehalose than is provided by typical Western diets. Reported concentrations of trehalose from natural sources vary because of experimental conditions, analytical methods, and the life stage of the organism assayed. In organisms, trehalose performs diverse functions that include energy storage and transport as well as protection against extreme temperatures or desiccation (Schiraldi et al., 2002). In humans the oligosaccharidases and disaccharidases are present in the gut before birth and are expressed throughout life. In general, levels of these enzymes are reduced in subjects who have fasted or are receiving parenteral nutrition, and levels are increased in subjects fed a carbohydrate-rich diet or in patients with uncontrolled diabetes. In most other mammals the lactase–phlorizin hydrolase is expressed from birth to weaning, and the other oligosaccharidases appear during the transition from maternal milk to starch-based diets. Lactase–phlorizin hydrolase in the intestinal mucosa of humans reaches maximal activity at an early postnatal age and then declines before adulthood in most people (Simon-Assmann et al., 1986). NF-LPH1 is a transcription factor that is present exclusively in intestinal epithelium and is thought to regulate expression of the lactase–phlorizin hydrolase gene. NF-LPH1 declines simultaneously with lactase–phlorizin hydrolase activity during the postweaning period. Thus a reduction in the expression of NF-LPH1 might be the cause of the decline in lactase activity that occurs before adulthood in most humans (Kuokkanen et al., 2003; Troelsen et al., 2003). Expression of oligosaccharidase and disaccharidase activities at the cellular level is tightly linked to enterocyte differentiation (Boudreau et al., 2002; Troelsen et al., 1997). For example, immature enterocytes developing from stem cells in crypts of the intestinal mucosa do not have sucrase–isomaltase activity. As enterocytes traverse to the crypt–villus junction, sucrase–isomaltase messenger RNA (mRNA) and sucrose–isomaltase activity are observed to increase. The level of sucrase–isomaltase mRNA reaches its peak when enterocytes are located in the lower and mid villus region and then progressively decreases as these cells move toward the tip (Traber et al., 1992). Expression of other disaccharidase and oligosaccharidase activities follows a similar pattern. The posttranslational processing of newly synthesized oligosaccharidases and disaccharidases has been the subject of a number of studies. The general pathways, summarized in Figure 8-5, are similar for lactase–phlorizin hydrolase, sucrase–isomaltase, and maltase–glucoamylase, although each has some specific and unique characteristics (Weisz and Rodriguez-Boulan, 2009). Trehalase expression and processing has little similarity to that of the other disaccharidases. All of these oligosaccharidases and disaccharidases are synthesized in the enterocyte where they are processed in the endoplasmic reticulum (ER) and Golgi apparatus with final vectorial transport to the apical membrane. Lactase–phlorizin hydrolase, sucrase–isomaltase, and maltase–glucoamylase all undergo extensive N– and O-glycosylation in the ER and Golgi apparatus, but trehalase has only N-glycosylation sites. The orientation of these enzymes in the membrane is such that the oligosaccharidase domains, including the active catalytic sites, are on the luminal side of the apical membrane and thus available for efficient cleavage of the luminal substrates.
Digestion and Absorption of Carbohydrate
Digestion of Starches
FOOD SOURCE
% OF DIETARY CARBOHYDRATE
PRODUCTS OF LUMINAL HYDROLYSIS
PRODUCTS OF BRUSH BORDER MEMBRANE HYDROLYSIS
Starches (amylose, amylopectin)
60 to 70
Maltose, maltotriose, and α-dextrins
Glucose
Lactose
0 to 10
None
Glucose and galactose
Sucrose
30
None
Glucose and fructose
Luminal Digestion of Starches
Digestion of Starches by Soluble α-Amylase
Contributions of Maltase–Glucoamylase and Sucrase–Isomaltase to Oligosaccharide Hydrolysis
Glucose Residues
Maltase–Glucoamylase
Sucrase–Isomaltase
N
Km∗ (mmol/L)
kcat† (sec−1)
Km (mmol/L)
kcat (sec−1)
2
2.1
56
14
3.2
3
1.1
149
21
3.1
4
0.4
78
24
3.5
5
0.4
63
61
3.4
6
0.7
70
57
2.1
7
1.0
80
120
2.2
Digestion of Dietary Disaccharides
ENZYME
PRINCIPAL SUBSTRATE
Km∗ (mmol/L)
kcat† (sec−1)
α-GLUCOSIDASES
Sucrase
Sucrose
18
120
Trehalase
Trehalose
3
20
β-GALACTOSIDASE
Lactase
Lactose
2
4
Trehalose Digestion by Trehalase
Expression and Processing of the Oligosaccharidases and Disaccharidases
Effect of Developmental Stage and Diet on Saccharidase Expression
Crypt–Villus Expression of Saccharidases
Posttranslational Processing of Saccharidases
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Digestion and Absorption of Carbohydrate
