Digestion of starch by amylase is incomplete, resulting in short oligomers of glucose such as maltose and maltotriose, as well as α-limit dextrins. Brush border digestion is necessary for the absorption of monosaccharides in starch. Glucoamylase, sucrase, and isomaltase are hydrolases that can digest straight-chain glucose oligomers at the brush border, yielding free glucose monomers. Isomaltase is critical for cleaving α-1,6 bonds in α-limit dextrins, along with α-1,4 bonds. Monosaccharides produced from digestion need to be transported across the hydrophobic plasma membrane of the enterocyte. The sodium/glucose transporter 1 (SGLT1) is a symporter that transports glucose and galactose against their concentration gradient by coupling their transport to that of Na+. GLUT2 is a transporter that allows glucose and galactose to exit the enterocyte across the basolateral pole. Fructose, on the other hand, is taken up across the apical membrane by GLUT5. However, fructose transport is inefficient and can be overwhelmed if large quantities of food containing fructose are ingested. Malabsorption of fructose can lead to symptoms similar to lactose intolerance. Proteins, unlike carbohydrates, require digestion into smaller constituents before absorption. They contain 20 different amino acids, and enterocytes can transport short oligomers of these amino acids. The liver has the ability to interconvert amino acids based on the body's needs. However, essential amino acids, which cannot be synthesized by the body, must be obtained from the diet. A rare genetic disorder affecting the ability of the intestine to absorb glucose or galactose is caused by mutations in the SGLT1 gene. This results in a faulty or unexpressed protein, or failure of the protein to properly traffic to the apical membrane of enterocytes. In individuals with these mutations, malabsorbed glucose contributes to symptoms such as diarrhea, similar to lactose intolerance. Despite its rarity, this disease provides important insights into the critical process of intestinal epithelial transport.
Fig. 30.8 ). Thus digestion of starch by amylase is of necessity incomplete and results in short oligomers of glucose, including dimers (maltose) and trimers (maltotriose), as well as the simplest branching structures, which are called α-limit dextrins. Thus to allow absorption of its constituent monosaccharides, starch must also undergo brush border digestion.
At the brush border, straight-chain glucose oligomers can be digested by the hydrolases glucoamylase, sucrase, or isomaltase (see
Table 30.1 ). All yield free glucose monomers, which can then be absorbed by the mechanisms discussed later. For α-limit dextrins, on the other hand, isomaltase activity is critical because it is the only enzyme that can cleave α-1,6 bonds that make up the branch points as well as α-1,4 bonds.
Uptake of Carbohydrates
Water-soluble monosaccharides resulting from digestion must next be transported across the hydrophobic plasma membrane of the enterocyte. The sodium/glucose transporter 1 (SGLT1) is a symporter that takes up glucose (and galactose) against its concentration gradient by coupling its transport to that of Na+ (
Fig. 30.9 ). Once inside the cytosol, glucose and galactose can be retained for the epithelium’s metabolic needs or can exit the cell across its basolateral pole via a transporter known as GLUT2. Fructose, in contrast, is taken up across the apical membrane by GLUT5. However, •Fig. 30.9 Absorptionofglucose,galactose,andfructoseinthesmallintestine.GLUT,glucosetransporter;SGLT1,sodium/glucosetransporter1.GlucoseGalactoseFructoseFructoseGLUT2GLUT5 GLTI?K+ATP because fructose transport is not coupled to that of Na+, its uptake is relatively inefficient and can easily be overwhelmed if large quantities of food containing this sugar are ingested. The symptoms that occur from this malabsorption are similar to those experienced by a lactose-intolerant patient who consumes lactose.
Proteins are also water-soluble polymers that must be digested into their smaller constituents before absorption is possible. Their absorption is more complicated than that of carbohydrates because they contain 20 different amino acids and short oligomers of these amino acids (dipeptides, tripeptides, and perhaps even tetrapeptides) can also be transported by enterocytes. The body, particularly the liver (see ), has substantial ability to interconvert various amino acids subject to the body’s needs. However, some amino acids, termed the essential amino acids, cannot be synthesized by the body either de novo or from other amino acids and thus must be obtained from the diet. The amino acids that must be obtained in this way in humans are shown in
Fig. 30.10
Araregeneticdisorderresultsinaninabilityoftheintestinetoabsorbglucoseorgalactose.ThisdiseasehasbeenmappedtoavarietyofmutationsintheSGLT1 genethatresultinafaultyorunexpressedproteinor,morecommonly,failureoftheproteintotrafficappropriatelytotheapicalmembraneofenterocytes.Inpatientscarryingsuchmutations,malabsorbedglucosecontributestodiarrhealandothersymptoms,asdiscussedearlierforlactoseintolerance.Despitetherarityofthedisease,itisimportantintermsoftheinsightitprovidedintoacriticalprocessofintestinalepithelialtransport.