Introduction to Metabolismand Glycolysis 8

I. METABOLISM OVERVIEW

In Chapter 5, individual enzyme reactions were analyzed toexplain the mechanisms of catalysis. However, in cells,these reactions rarely occur in isolation. Instead, they areorganized into multistep sequences called pathways, suchas that of glycolysis (Fig. 8.1). In a pathway, the product ofone reaction serves as the substrate of the subsequentreaction. Most pathways can be classified as eithercatabolic (degradative) or anabolic (synthetic). Catabolicpathways break down complex molecules, such as proteins,polysaccharides, and lipids, to a few simple molecules (e.g.,carbon dioxide, ammonia, and water). Anabolic pathwaysform complex end products from simple precursors, forexample, the synthesis of the polysaccharide glycogen fromglucose. Different pathways can intersect, forming anintegrated and purposeful network of chemical reactions.Metabolism is the sum of all the chemical changes occurringin a cell, a tissue, or the body. Metabolites are intermediateproducts of metabolism. The next several chapters focus onthe central metabolic pathways that are involved insynthesizing and degrading carbohydrates, lipids, andamino acids.

Introduction toCarbohydrates 7

I. OVERVIEW

Carbohydrates are the most abundant organic molecules innature. They have a wide range of functions, includingproviding a significant fraction of the dietary calories formost organisms, acting as a storage form of energy in thebody, and serving as cell membrane components thatmediate some forms of intercellular communication.Carbohydrates also serve as a structural component ofmany organisms, including the cell walls of bacteria, theexoskeleton of insects, and the fibrous cellulose of plants.The empiric formula for many of the simpler carbohydratesis (CH2O)n, where n ≥ 3, hence the name hydrate ofcarbon.

II. CLASSIFICATION AND STRUCTURE

Monosaccharides or simple sugars can be classifiedaccording to the number of carbon atoms they contain.Examples of some monosaccharides commonly found inhumans are listed in Figure 7.1. They can also be classifiedby the type of carbonyl group they contain. Carbohydrateswith an aldehyde as their carbonyl group are called aldoses,whereas those with a keto as their carbonyl group are calledketoses (Fig. 7.2). For example, glyceraldehyde is an aldose,whereas dihydroxyacetone is a ketose. Carbohydrates thathave a free carbonyl group have the suffix -ose. (Note:

Ketoses have an additional ul in their suffix such asxylulose. There are exceptions, such as fructose, to thisrule.) Monosaccharides can be linked by glycosidic bonds tocreate larger structures (Fig. 7.3). Disaccharides contain twomonosaccharide units, oligosaccharides contain 3 to 10monosaccharide units, and polysaccharides contain morethan 10 monosaccharide units and can be hundreds of sugarunits in length.

Figure 7.1Examples of monosaccharides found in humans, classified accordingto the number of carbons they contain.

Figure 7.2Examples of an aldose (A) and a ketose (B) sugar.

A. Isomers and epimers

Compounds that have the same chemical formula butdifferent structures are isomers of each other. Forexample, fructose, glucose, mannose, and galactose allhave the same chemical formula, C6H12O6, with differentstructures. Carbohydrate isomers that differ inconfiguration around only one specific carbon atom (withthe exception of the carbonyl carbon, see C. 1. below)are defined as epimers of each other. For example,glucose and galactose are C-4 epimers because theirstructures differ only in the position of the –OH(hydroxyl) group at carbon 4. (Note: The carbons insugars are numbered beginning at the end that containsthe carbonyl carbon [i.e., the aldehyde or keto group], asshown in Fig. 7.4.) Glucose and mannose are C-2epimers. However, because galactose and mannosediffer in the position of –OH groups at two carbons(carbons 2 and 4), they are isomers rather than epimers(see Fig. 7.4).

Figure 7.3A glycosidic bond between two hexoses producing adisaccharide.

B. EnantiomersA special type of isomerism is found in the pairs ofstructures that are mirror images of each other. Thesemirror images are called enantiomers, and the twomembers of the pair are designated as a D- and an L-sugar (Fig. 7.5). The vast majority of the sugars inhumans are D-isomers. In the D-isomeric form, the –OHgroup on the asymmetric carbon (a carbon linked to fourdifferent atoms or groups) farthest from the carbonylcarbon is on the right, whereas in the L-isomer, it is onthe left. Most enzymes are specific for either the D or theL form, but enzymes known as isomerases are able tointerconvert D- and L-isomers.

C. Monosaccharide cyclizationLess than 1% of each of the monosaccharides with fiveor more carbons exists in the open-chain (acyclic) formin solution. Rather, they are predominantly found in aring or cyclic form, in which the aldehyde (or keto) grouphas reacted with a hydroxyl group on the same sugar,making the carbonyl carbon (carbon 1 for an aldose,carbon 2 for a ketose) asymmetric. This asymmetriccarbon is referred to as the anomeric carbon.1. Anomers: Creation of an anomeric carbon (the former

carbonyl carbon) generates a new pair of isomers,the α and β configurations of the sugar (e.g., α-D-glucopyranose and β-D-glucopyranose), as shown inFigure 7.6, that are anomers of each other. (Note: Inthe α configuration, the –OH group on the anomericcarbon projects to the same side as the ring in amodified Fischer projection formula [see Fig. 7.6A]and is trans to the CH2OH group in a Haworthprojection formula [see Fig. 7.6B]. The α and β forms

are not mirror images, and they are referred to asdiastereomers.) Enzymes are able to distinguishbetween these two structures and use one or theother preferentially. For example, glycogen issynthesized from α-D-glucopyranose, whereascellulose is synthesized from β-D-glucopyranose. Thecyclic α and β anomers of a sugar in solutionspontaneously (but slowly) form an equilibriummixture, a process known as mutarotation (see Fig.7.6). (Note: For glucose, the α form makes up 36% ofthe mixture.)

2. Reducing sugars: If the hydroxyl group on theanomeric carbon of a cyclized sugar is not linked toanother compound by a glycosidic bond (see E.below), the ring can open. The sugar can act as areducing agent and is termed a reducing sugar. Suchsugars can react with chromogenic agents (e.g., theBenedict reagent) causing the reagent to be reducedand colored as the aldehyde group of the acyclicsugar is oxidized to a carboxyl group. Allmonosaccharides, but not all disaccharides, arereducing sugars. (Note: Fructose, a ketose, is areducing sugar because it can be isomerized to analdose.)

Figure 7.4

Carbon-2 (C-2) and C-4 epimers and an isomer of glucose.

A colorimetric test can detect a reducing sugar in urine. Apositive result is indicative of an underlying pathology(because sugars are not normally present in urine) and canbe followed up by more specific tests to identify thereducing sugar.

D. Monosaccharide j oiningMonosaccharides can be joined to form disaccharides,oligosaccharides, and polysaccharides. Importantdisaccharides include lactose (galactose + glucose),sucrose (glucose + fructose), and maltose (glucose +glucose). Important polysaccharides include branchedglycogen (from animal sources) and starch (plantsources) and unbranched cellulose (plant sources). Eachis a polymer of glucose.

Figure 7.5Enantiomers (mirror images) of glucose. Designation of D and Lis by comparison to a triose, glyceraldehyde. (Note: Theasymmetric carbons are shown in green.)

E. Glycosidic bondsThe bonds that link sugars are called glycosidic bonds.They are formed by enzymes known asglycosyltransferases that use nucleotide sugars(activated sugars) such as uridine diphosphate glucoseas substrates. Glycosidic bonds between sugars arenamed according to the numbers of the connectedcarbons and with regard to the position of the anomerichydroxyl group of the first sugar involved in the bond. Ifthis anomeric hydroxyl is in the α configuration, then thelinkage is an α-bond. If it is in the β configuration, thenthe linkage is a β-bond. Lactose, for example, issynthesized by forming a glycosidic bond betweencarbon 1 of β-galactose and carbon 4 of glucose.Therefore, the linkage is a β(1→4) glycosidic bond (seeFig. 7.3). (Note: Because the anomeric end of theglucose residue is not involved in the glycosidic linkage,it [and, therefore, lactose] remains a reducing sugar.)

Figure 7.6A: The interconversion (mutarotation) of the α and βanomeric forms of glucose shown as modified Fischerproj ection formulas. B: The interconversion shown asHaworth proj ection formulas. (Note: A sugar with a six-membered ring [ 5 C + 1 O] is termed a pyranose, whereasone with a five-membered ring [ 4 C + 1 O] is a furanose.Virtually all glucose in solution is in the pyranose form.)

F. Carbohydrate linkage to noncarbohydratesCarbohydrates can be attached by glycosidic bonds tononcarbohydrate structures, including purine and

pyrimidine bases in nucleic acids, aromatic rings such asthose found in steroids, proteins, and lipids. If the groupon the noncarbohydrate molecule to which the sugar isattached is an –NH2 group, then the bond is called an N-glycosidic link. If the group is an –OH, then the bond isan O-glycosidic link (Fig. 7.7). (Note: All sugar-sugarglycosidic bonds are O-type linkages.)

III. DIETARY CARBOHYDRATE DIGESTION

The principal sites of dietary carbohydrate digestion are themouth and intestinal lumen. This digestion is rapid and iscatalyzed by enzymes known as glycoside hydrolases(glycosidases) that hydrolyze glycosidic bonds (Fig. 7.8).Because little monosaccharide is present in diets of mixedanimal and plant origin, the enzymes are primarilyendoglycosidases that hydrolyze polysaccharides andoligosaccharides and disaccharidases that hydrolyze tri- anddisaccharides into their reducing sugar components.Glycosidases are usually specific for the structure andconfiguration of the glycosyl residue to be removed as wellas for the type of bond to be broken. The final products ofcarbohydrate digestion are the monosaccharides glucose,galactose, and fructose that are absorbed by cells(enterocytes) of the small intestine.

Figure 7.7Examples of N- and O-glycosidic bonds in glycoproteins.

A. Salivary α-amylaseThe major dietary polysaccharides consumed by humansare of plant (starch, composed of amylose andamylopectin) and animal (glycogen) origin. Duringmastication or chewing, salivary α-amylase acts brieflyon dietary starch and glycogen, hydrolyzing randomα(1→4) bonds. (Note: There are both α[1→4]- andβ[1→4]-endoglucosidases in nature, but humans do notproduce the latter. Therefore, we are unable to digest

cellulose, a carbohydrate of plant origin containingβ[1→4] glycosidic bonds between glucose residues.)Because branched amylopectin and glycogen alsocontain α(1→6) bonds, which α-amylase cannothydrolyze, the digest resulting from its action contains amixture of short, branched and unbranchedoligosaccharides known as dextrins (Fig. 7.9). (Note:Disaccharides are also present as they, too, are resistantto amylase.) Carbohydrate digestion halts temporarily inthe stomach, because the high acidity inactivatessalivary α-amylase.

B. Pancreatic α-amylaseWhen the acidic stomach contents reach the smallintestine, they are neutralized by bicarbonate secretedby the pancreas, and pancreatic α-amylase continuesthe process of starch digestion.

Figure 7.8Hydrolysis of a glycosidic bond.

C. Intestinal disaccharidases

The final digestive processes occur primarily at themucosal lining of the duodenum and upper jejunum andinclude the action of several disaccharidases (see Fig.7.9). For example, isomaltase cleaves the α(1→6) bondin isomaltose, and maltase cleaves the α(1→4) bond inmaltose and maltotriose, each producing glucose.Sucrase cleaves the α(1→2) bond in sucrose, producingglucose and fructose, and lactase (β-galactosidase)cleaves the β(1→4) bond in lactose, producing galactoseand glucose. (Note: The substrates for isomaltase arebroader than its name suggests, and it hydrolyzes themajority of maltose.) Trehalose, an α(1→1) disaccharideof glucose found in mushrooms and other fungi, iscleaved by trehalase. These enzymes aretransmembrane proteins of the brush border on theluminal (apical) surface of the enterocytes.

Sucrase and isomaltase are enzymatic activities of a singleprotein cleaved into two functional subunits that remainassociated in the cell membrane and form the sucrase–isomaltase (SI) complex. In contrast, maltase is one of twoenzymic activities of the single membrane protein maltase—glucoamylase (MGA) that does not get cleaved. Its secondenzymic activity, glucoamylase, cleaves α(1→4) glycosidicbonds in dextrins.

D. Intestinal absorption of monosaccharidesThe upper jejunum absorbs the bulk of themonosaccharide products of digestion. However,different sugars have different mechanisms ofabsorption (Fig. 7.10). For example, galactose andglucose are taken into enterocytes by secondary activetransport that requires a concurrent uptake (symport) ofsodium (Na+) ions. The transport protein is the sodium-dependent glucose cotransporter 1 (SGLT-1). (Note:

exoglycosido.se/glucoamyasdad-sononthe membrane x1→4 glucoamylose

Sugar transport is driven by the Na+ gradient created bythe Na+-potassium [K+] ATPase that moves Na+ out ofthe enterocyte and K+ in [see Fig. 7.10].) Fructoseabsorption utilizes an energy- and Na+-independentmonosaccharide transporter (GLUT-5). All threemonosaccharides are transported from the enterocytesinto the portal circulation by yet another transporter,GLUT-2. (Note: See pp. 106 and 107 for a discussion ofthese transporters.)

exoglucosidasealso has maltase

activity

Figure 7.9Digestion of carbohydrates. (Note: Indigestible celluloseenters the colon and is excreted.)

E. Abnormal degradation of disaccharidesThe overall process of carbohydrate digestion andabsorption is so efficient in healthy individuals thatordinarily all digestible dietary carbohydrate is absorbed

cellulose is not broken downinto its monosaccharides asthere are no enzymes

that canact on its bonds

cellulose can bind to watermolecules in the intestinedueto its polarity, givingthe membrane proteins protrudingfeelingof satiety outside the intestinal cellscellulose helps in peristalsis andreduce the amount of caloriestaken into thebodycellulose alsobinds to differentmolecules suchas toxins

,givingit a protectivefeatures

by the time the ingested material reaches the lowerjejunum. However, because only monosaccharides areabsorbed, any deficiency (genetic or acquired) in aspecific disaccharidase activity of the intestinal mucosacauses the passage of undigested carbohydrate into thelarge intestine. As a consequence of the presence of thisosmotically active material, water is drawn from themucosa into the large intestine, causing osmoticdiarrhea. This is reinforced by the bacterial fermentationof the remaining carbohydrate to two- and three-carboncompounds (which are also osmotically active) plus largevolumes of carbon dioxide and hydrogen gas (H2),causing abdominal cramps, diarrhea, and flatulence.1. Digestive enzyme deficiencies: Genetic deficiencies

of the individual disaccharidases result indisaccharide intolerance. Alterations in disaccharidedegradation can also be caused by a variety ofintestinal diseases, malnutrition, and drugs thatinjure the mucosa of the small intestine. Forexample, brush border enzymes are rapidly lost innormal individuals with severe diarrhea, causing atemporary, acquired enzyme deficiency. Therefore,patients suffering or recovering from such a disordercannot drink or eat significant amounts of dairyproducts or sucrose without exacerbating thediarrhea.

Figure 7.10Absorption by enterocytes of the monosaccharide productsof carbohydrate digestion. GLUT = glucose transporter;SGLT-1 = sodium (Na+)-dependent glucose cotransporter. K+= potassium.

2. Lactose intolerance: Over 60% of the world’s adultsexperience lactose malabsorption because they lackthe enzyme lactase (Fig. 7.11). Individuals ofNorthern European heritage are most likely tomaintain the ability to digest lactose into adulthood.Up to 90% of adults of African or Asian descent arelactase deficient. Consequently, they are less able tometabolize lactose than are individuals of NorthernEuropean origin. The age-dependent loss of lactaseactivity starting at approximately age 2 yearsrepresents a reduction in the amount of enzymeproduced. It is thought to be caused by smallvariations in the DNA sequence of a region on

SGLT:

these proteins areNat- dependent and actbyactive transport to transport

GLUTI GLUT14 : glucose and galactoseagainst their concentrationthese transporters are gradientsNat- independent and actbythe

facilitated diffusion mechanism theyact inthe smallintestinebyactively

glucose is transported byGLUT uptakingmolecules fromthelumen of the intestinewhen thereis alargeconcentration and in thekidneybyof glucose inthelumen compared with reabsorbingglucoseinthethe concentration intheintestinal cells proximal tubuleandbySALTwhen thereis a

low

concentration in thelumen

chromosome 2 that controls expression of the genefor lactase, also on chromosome 2. Treatment for thisdisorder is to reduce consumption of milk; eat yogurtand some cheeses (bacterial action and agingprocess decrease lactose content) as well as greenvegetables, such as broccoli, to ensure adequatecalcium intake; use lactase-treated products; or takelactase in pill form prior to eating. Rare cases ofcongenital lactase deficiency are known.

Figure 7.11Abnormal lactose metabolism. CO2 = carbon dioxide; H2 =hydrogen gas.

vitaminD is better taken

lactase reaches its maximal from the sunlight as we

activityat theage of 1 monthneed to consume hugeamounts of milk 110 litres

its activitydeclines at 5to7 daily/totake sulhicientyears of age

amounts of vitaminD

1 cupof milk 19 grams

of lactosecan cause the loss of 1 litre of

extracellular fluid

3. Sucrase–isomaltase deficiency: SI deficiency resultsin intolerance of ingested sucrose. This condition wasconsidered quite rare, more common in the Inuitpeople of Alaska and Greenland; now, up to 9% ofAmericans of European descent are estimated to beaffected by a form of SI deficiency.Initially considered to be exclusively an autosomal-recessive disorder, those with one mutation (carriers)sometimes express disease manifestations. Now, 25different mutations in the human sucrose gene areknown. Individuals homozygous for mutationsexpress congenital SI deficiency and experienceosmotic diarrhea, mild steatorrhea, irritability, andvomiting after consuming sucrose. Heterozygouscarriers often have symptoms including chronicdiarrhea, abdominal pain, and bloating. Treatmentincludes the dietary restriction of sucrose andenzyme replacement therapy.

4. Diagnosis of enzyme deficiencies: Identification of aspecific enzyme deficiency can be obtained byperforming oral tolerance tests with the individualdisaccharides. Measurement of H2 in the breath is areliable test for determining the amount of ingestedcarbohydrate not absorbed by the body, but which ismetabolized instead by the intestinal flora (see Fig.7.11).

IV. Chapter Summary

Monosaccharides (Fig. 7.12) containing an aldehyde group are calledaldoses, and those with a keto group are called ketoses.Disaccharides, oligosaccharides, and polysaccharides consist ofmonosaccharides linked by glycosidic bonds.Compounds with the same chemical formula but different structures arecalled isomers.Two monosaccharide isomers differing in configuration around onespecific carbon atom (not the carbonyl carbon) are defined as epimers.In enantiomers (mirror images), the members of the sugar pair aredesignated as D- and L-isomers. When the aldehyde group on anacyclic sugar is oxidized as a chromogenic agent is reduced, that sugaris a reducing sugar.When a sugar cyclizes, an anomeric carbon is created from thecarbonyl carbon of the aldehyde or keto group. The sugar can have twoconfigurations, forming α or β anomers.A sugar can have its anomeric carbon linked to an –NH2 or an –OHgroup on another structure through N- and O-glycosidic bonds,respectively.Salivary α-amylase initiates digestion of dietary polysaccharides(e.g., starch or glycogen), producing oligosaccharides. Pancreatic α-amylase continues the process. The final digestive processes occur atthe mucosal lining of the small intestine.Several disaccharidases (e.g., lactase [β-galactosidase], sucrase,isomaltase, and maltase) produce monosaccharides (glucose,galactose, and fructose). These enzymes are transmembraneproteins of the luminal brush border of intestinal mucosal cells(enterocytes).Absorption of the monosaccharides requires specific transporters. Ifcarbohydrate degradation is deficient (as a result of heredity, disease,or drugs that injure the intestinal mucosa), undigested carbohydratewill pass into the large intestine, where it can cause osmotic diarrhea.Bacterial fermentation of the material produces large volumes of carbondioxide and hydrogen gas, causing abdominal cramps, diarrhea, andflatulence. Lactose intolerance, primarily caused by the age-dependent loss of lactase (adult-type hypolactasia), is by far themost common of these deficiencies.

as it is specificfor glucose

asit is

nonspecific

thebrain dependsmainlyon glucoseas a source of energy

IV. Chapter Summary

Monosaccharides (Fig. 7.12) containing an aldehyde group are calledaldoses, and those with a keto group are called ketoses.Disaccharides, oligosaccharides, and polysaccharides consist ofmonosaccharides linked by glycosidic bonds.Compounds with the same chemical formula but different structures arecalled isomers.Two monosaccharide isomers differing in configuration around onespecific carbon atom (not the carbonyl carbon) are defined as epimers.In enantiomers (mirror images), the members of the sugar pair aredesignated as D- and L-isomers. When the aldehyde group on anacyclic sugar is oxidized as a chromogenic agent is reduced, that sugaris a reducing sugar.When a sugar cyclizes, an anomeric carbon is created from thecarbonyl carbon of the aldehyde or keto group. The sugar can have twoconfigurations, forming α or β anomers.A sugar can have its anomeric carbon linked to an –NH2 or an –OHgroup on another structure through N- and O-glycosidic bonds,respectively.Salivary α-amylase initiates digestion of dietary polysaccharides(e.g., starch or glycogen), producing oligosaccharides. Pancreatic α-amylase continues the process. The final digestive processes occur atthe mucosal lining of the small intestine.Several disaccharidases (e.g., lactase [β-galactosidase], sucrase,isomaltase, and maltase) produce monosaccharides (glucose,galactose, and fructose). These enzymes are transmembraneproteins of the luminal brush border of intestinal mucosal cells(enterocytes).Absorption of the monosaccharides requires specific transporters. Ifcarbohydrate degradation is deficient (as a result of heredity, disease,or drugs that injure the intestinal mucosa), undigested carbohydratewill pass into the large intestine, where it can cause osmotic diarrhea.Bacterial fermentation of the material produces large volumes of carbondioxide and hydrogen gas, causing abdominal cramps, diarrhea, andflatulence. Lactose intolerance, primarily caused by the age-dependent loss of lactase (adult-type hypolactasia), is by far themost common of these deficiencies.

IV. Chapter Summary

Monosaccharides (Fig. 7.12) containing an aldehyde group are calledaldoses, and those with a keto group are called ketoses.Disaccharides, oligosaccharides, and polysaccharides consist ofmonosaccharides linked by glycosidic bonds.Compounds with the same chemical formula but different structures arecalled isomers.Two monosaccharide isomers differing in configuration around onespecific carbon atom (not the carbonyl carbon) are defined as epimers.In enantiomers (mirror images), the members of the sugar pair aredesignated as D- and L-isomers. When the aldehyde group on anacyclic sugar is oxidized as a chromogenic agent is reduced, that sugaris a reducing sugar.When a sugar cyclizes, an anomeric carbon is created from thecarbonyl carbon of the aldehyde or keto group. The sugar can have twoconfigurations, forming α or β anomers.A sugar can have its anomeric carbon linked to an –NH2 or an –OHgroup on another structure through N- and O-glycosidic bonds,respectively.Salivary α-amylase initiates digestion of dietary polysaccharides(e.g., starch or glycogen), producing oligosaccharides. Pancreatic α-amylase continues the process. The final digestive processes occur atthe mucosal lining of the small intestine.Several disaccharidases (e.g., lactase [β-galactosidase], sucrase,isomaltase, and maltase) produce monosaccharides (glucose,galactose, and fructose). These enzymes are transmembraneproteins of the luminal brush border of intestinal mucosal cells(enterocytes).Absorption of the monosaccharides requires specific transporters. Ifcarbohydrate degradation is deficient (as a result of heredity, disease,or drugs that injure the intestinal mucosa), undigested carbohydratewill pass into the large intestine, where it can cause osmotic diarrhea.Bacterial fermentation of the material produces large volumes of carbondioxide and hydrogen gas, causing abdominal cramps, diarrhea, andflatulence. Lactose intolerance, primarily caused by the age-dependent loss of lactase (adult-type hypolactasia), is by far themost common of these deficiencies.

Figure 7.12Key concept map for the classification and structure ofmonosaccharides and the digestion of dietary carbohydrates.

Study Questions

Choose the ONE best answer.

7.1 Glucose isA. a C-4 epimer of galactose.B. a ketose and usually exists as a furanose ring in solution.C. produced from dietary starch by the action of α-amylase.D. utilized in biologic systems only in the L-isomeric form.

Correct answer = A. Because glucose and galactose differ only inconfiguration around carbon 4, they are C-4 epimers that are interconvertibleby the action of an epimerase. Glucose is an aldose sugar that typicallyexists as a pyranose ring in solution. Fructose, however, is a ketose with afuranose ring. α-Amylase does not produce monosaccharides. The D-isomericform of carbohydrates is the form typically found in biologic systems, incontrast to amino acids that typically are found in the L-isomeric form.

7.2 A 28-year-old male presents in the office with a chief complaint of recurrentbloating and diarrhea. His eyes are sunken, and the physician notesadditional signs of dehydration. The patient’s temperature is normal. Heexplains that the most recent episode occurred last night soon after he hadice cream for dessert. This clinical picture is most likely caused by adeficiency in the activity of:A. isomaltase.B. lactase.C. pancreatic α-amylase.D. salivary α-amylase.E. sucrase.

Correct answer = B. The physical symptoms suggest a deficiency in anenzyme responsible for carbohydrate degradation. The symptoms observedfollowing the ingestion of dairy products suggest that the patient is deficientin lactase as a result of the age-dependent reduction in expression of theenzyme.

7.3 Routine examination of the urine of an asymptomatic pediatric patientshowed a positive reaction with Clinitest (a copper reduction method ofdetecting reducing sugars) but a negative reaction with the glucose oxidasetest for detecting glucose. Using these data, show on the chart below whichof the sugars could (YES) or could not (NO) be present in the urine of thisindividual.

Sugar Yes NoFructoseGalactoseGlucoseLactoseSucroseXylulose

Each of the listed sugars, except for sucrose and glucose, could be present inthe urine of this individual. Clinitest is a nonspecific test that produces achange in color if urine is positive for reducing substances such as reducingsugars (fructose, galactose, glucose, lactose, xylulose). Because sucrose isnot a reducing sugar, it is not detected by Clinitest. The glucose oxidase testwill detect only glucose, and it cannot detect other sugars. The negativeglucose oxidase test coupled with a positive reducing sugar test means thatglucose cannot be the reducing sugar in the patient’s urine.

7.4 Explain why α-glucosidase inhibitors such as acarbose and miglitol, whichare taken with meals, can be used in the treatment of some patients withdiabetes mellitus. What effect should these drugs have on the digestion oflactose?

α-Glucosidase inhibitors slow the production of glucose from dietarycarbohydrates, thereby reducing the postprandial rise in blood glucose andfacilitating better blood glucose control in diabetic patients. These drugshave no effect on lactose digestion because the disaccharide lactosecontains a β-glycosidic bond, not an α-glycosidic bond.

7.4 Explain why α-glucosidase inhibitors such as acarbose and miglitol, whichare taken with meals, can be used in the treatment of some patients withdiabetes mellitus. What effect should these drugs have on the digestion oflactose?

α-Glucosidase inhibitors slow the production of glucose from dietarycarbohydrates, thereby reducing the postprandial rise in blood glucose andfacilitating better blood glucose control in diabetic patients. These drugshave no effect on lactose digestion because the disaccharide lactosecontains a β-glycosidic bond, not an α-glycosidic bond.

لاسه هلتجع ام لاإ لسه لا ملهلالاسه تئش اذإ نلحزا لعتج نتأو