Digestive enzyme deficiencies: Genetic deficiencies of the individual disaccharidases result in disaccharide intolerance. Alterations in disaccharide degradation can also be caused by a variety of intestinal diseases, malnutrition, and drugs that injure the mucosa of the small intestine. For example, brush border enzymes are rapidly lost in normal individuals with severe diarrhea, causing a temporary, acquired enzyme deficiency. Therefore, patients suffering or recovering from such a disorder cannot drink or eat significant amounts of dairy products or sucrose without exacerbating the diarrhea.
2.
Lactose intolerance: Over 60% of the world’s adults are lactose intolerant (Fig. 7.11). This is particularly manifested in certain populations. For example, up to 90% of adults of African or Asian descent are lactase deficient. Consequently, they are less able to metabolize lactose than are individuals of Northern European origin. The age-dependent loss of lactase activity starting at approximately age 2 years represents a reduction in the amount of enzyme produced. It is thought to be caused by small variations in the DNA sequence of a region on chromosome 2 that controls expression of the gene for lactase, also on chromosome 2. Treatment for this disorder is to reduce consumption of milk; eat yogurts and some cheeses (bacterial action and aging process decrease lactose content) as well as green vegetables, such as broccoli, to ensure adequate calcium intake; use lactase-treated products; or take lactase in pill form prior to eating. [Note: Because the loss of lactase is the norm for most of the world’s adults, use of the terms adult-type hypolactasia or lactase nonpersistence rather than lactose intolerance is becoming more common.] Rare cases of congenital lactase deficiency are known.
3.
Congenital sucrase-isomaltase deficiency: This autosomal-recessive disorder results in an intolerance of ingested sucrose. Congenital SI deficiency has a prevalence of 1:5,000 in individuals of European descent and appears to be much more common (up to 1:20) in the Inuit people of Greenland and Canada. Treatment includes the dietary restriction of sucrose and enzyme replacement therapy.
4.
Diagnosis: Identification of a specific enzyme deficiency can be obtained by performing oral tolerance tests with the individual disaccharides. Measurement of H2 in the breath is a reliable test for determining the amount of ingested carbohydrate not absorbed by the body, but which is metabolized instead by the intestinal flora (see Fig. 7.11).
IV. CHAPTER SUMMARY
Monosaccharides (Fig. 7.12) containing an aldehyde group are called aldoses, and those with a keto group are called ketoses. Disaccharides, oligosaccharides, and polysaccharides consist of monosaccharides linked by glycosidic bonds. Compounds with the same chemical formula but different structures are called isomers. Two monosaccharide isomers differing in configuration around one specific carbon atom (not the carbonyl carbon) are defined as epimers. In enantiomers (mirror images), the members of the sugar pair are designated as D-and L-isomers. When the aldehyde group on an acyclic sugar gets oxidized as a chromogenic agent gets reduced, that sugar is a reducing sugar. When a sugar cyclizes, an anomeric carbon is created from the carbonyl carbon of the aldehyde or keto group. The sugar can have two configurations, forming α or β anomers. A sugar can have its anomeric carbon linked to an –NH2 or an –