Sucrose isomaltase enzyme

Dextrins are hydrolyzed by a membrane-bound enzyme isomaltase, which occurs in the same polypeptide chain as sucrase, the enzyme that hydrolyzes sucrose, Two active sites (catalytic sites) reside on one polypeptide chain. The entire protein is called sucrase-isomaltase. Enzymes containing more than one active site on one polypeptide chain are called multi functional. The orientation of sucrase-isomaltase in the gut cell, or enterocyte, is shown in Figure 2.43. Both active sites are situated in the lumen of the gut the N-terminal region is anchored in the membrane. Each of the active sites of sucrase-isomaltase is capable of hydrolyzing maltose. Perhaps a better, although cumbersome, name for the enzyme would be sucrase/maltase-isomaltase/maliase. The isomaltase catalytic site is closest to the membrane, whereas the sucrasc site is the C-terminal portion of the enzyme. 

Isomaltase-sucrase deficiency This enzyme deficiency results in an intolerance of ingested sucrose. This disorder is found in about ten percent of Greenland s Eskimos, whereas two percent of North Americans are heterozygous for the deficiency. Treatment is to withhold dietary sucrose. 

The brush border enzymes with disaccharidase and ohgosaccharidase activity are listed in Table 48-1. The sucrase-isomaltase complex comprises most of the sucrase, isomaltase, and maltase (80%) activity of the small intestine. It hydrolyzes sucrose to its constituent monosaccharides, cleaves glucose from a-limit dextrins with 1,6 bonds, and hydrolyzes maltose. The activity of the complex is fourfold to fivefold greater in the jejunum than in the ileum. Changes in diet have a marked effect on the expression of the complex starvation leads to a rapid decline in activity, which is rapidly restored on refeeding. AH small intestinal saccha-ridases may decrease with infection or inflammation of the small bowel to the extent that carbohydrate malabsorption... 

Sucrase-isomaltase complex An enzyme complex comprised of two enzyme units. Both units have high a-l,4-glucosidase activity and will hydrolyze maltose and maltotriose to glucose. The sucrase unit will also hydrolyze sucrose to fructose and glucose, whereas the isomaltase unit will hydrolyze a-1,6 bonds found in isomaltose and the limit dextrins of starch. 

Oligo-1,6-D-glucosidase-sucrose a-D-glucohydrolase ( sucrase-isomaltase ) is an intestinal membrane enzyme consisting of two active moieties, each with its hydrolytic site available for nutrient digestion at the luminal-cell interface. 

Studies on the enzyme complex oligo-l,6-D-glucosidase-sucrose a-D-glucohydro-lase ( sucrase-isomaltase ) of the AB intestinal brush border membrane have shown that a hydrophobic section of the oligo-l,6-D-glucosidase is responsible for binding of the complex to the membrane (see p. 522). ... 

Sucrase—isomaltase is a bifunctional enzyme that catalyses the hydrolysis of sucrose to glucose and fructose, and of isomaltose to two molecules of glucose. 

An intestinal disaccharidase which catalyses the hydrolysis of sucrose into glucose and fructose. A deficiency of the enzyme occurs, along with a deficiency of the other intestinal dis-accharidases, in conditions where there is generalised disease of the intestinal wall. A congenital deficiency of sucrase, usually co-existing with isomaltase deficiency, can occur. Acquired sucrase-isomaltase deficiency has also been described but is much rarer. 

This is a rare inborn error in the metabolism of carbohydrates. The most common symptom is diarrhea. In this disease the enzymes invertase and isomaltase are missing Both enzymes are disaccharidases. Invertase splits sucrose (sugar) and isomaltase splits isomaltose, a product of starch digestion. Therefore, sources of sucrose and isomaltose—wheat and potatoes—should be omitted from the diet to control the disease. 

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