OH group on another structure through N-and O-glycosidic bonds, respectively. Salivary α-amylase initiates digestion of dietary polysaccharides (for example, starch or glycogen), producing oligosaccharides. Pancreatic α-amylase continues the process. The final digestive processes occur at the mucosal lining of the small intestine. Several disaccharidases (for example, lactase [β-galactosidase], sucrase, isomaltase, and maltase) produce monosaccharides (glucose, galactose, and fructose). These enzymes are transmembrane proteins of the luminal brush border of intestinal mucosal cells (enterocytes). Absorption of the monosaccharides requires specific transporters. If carbohydrate degradation is deficient (as a result of heredity, disease, or drugs that injure the intestinal mucosa), undigested carbohydrate will pass into the large intestine, where it can cause osmotic diarrhea. Bacterial fermentation of the material produces large volumes of carbon dioxide and hydrogen gas, causing abdominal cramps, diarrhea, and flatulence. Lactose intolerance, primarily caused by the age-dependent loss of lactase (adult-type hypolactasia), is by far the most common of these deficiencies.
Choose the ONE best answer.
.1. Which of the following statements best describes glucose?
A. It is a C-4 epimer of galactose.
B. It is a ketose and usually exists as a furanose ring in solution.
C. It is produced from dietary starch by the action of α-amylase.
D. It is utilized in biological systems only in the L-isomeric form.
Correct answer = A. Because glucose and galactose differ only in configuration around carbon 4, they are C-4 epimers that are interconvertible by the action of an epimerase. Glucose is an aldose sugar that typically exists as a pyranose ring in solution. Fructose, however, is a ketose with a furanose ring. α-Amylase does not produce monosaccharides. The D-isomeric form of carbohydrates is the form typically found in biologic systems, in contrast to amino acids that typically are found in the L-isomeric form.
.2. A young man entered his physician’s office complaining of bloating and diarrhea. His eyes were sunken, and the physician noted additional signs of dehydration. The patient’s temperature was normal. He explained that the episode had occurred following a birthday party at which he had participated in an ice cream–eating contest. The patient reported prior episodes of a similar nature following ingestion of a significant amount of dairy products. This clinical picture is most probably due to a deficiency 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 an enzyme responsible for carbohydrate degradation. The symptoms observed following the ingestion of dairy products suggest that the patient is deficient in lactase as a result of the age-dependent reduction in expression of the enzyme.
.3. Routine examination of the urine of an asymptomatic pediatric patient showed a positive reaction with Clinitest (a copper reduction method of detecting reducing sugars) but a negative reaction with the glucose oxidase test for detecting glucose. Using these data, show on the chart below which of the sugars could (YES) or could not (NO) be present in the urine of this individual.