Sucrase-isomaltase, small intestinal

V al-Met-Ser-Asp-T rp-Ala-Ala-His-His-Ala-Gly-Val-)S-D-Glucosidase A (bitter almonds) lle-Thr-Glx-Glx-Val-Phe-Gly-Asp-Ser-(Ala, Asxj, Glx, Pro)-Lys ff-D-Glucosidase (human placenta, lysosomal) -Val-Ala-Ser-Gln-Lys-Asn-Asp-Leu-Asp-Ala-Val-Ala-a-D-Glucosidase (sucrase, rabbit small intestine) -lle-Asp-Met-Asn-Glu-Pro-Asn-a-D-Glucosidase (isomaltase, rabbit small intestine) -Gly-Gly-Gln-lle-Asp-Met-)ff-D-Galactosidase (Escherichia co//) -Ser-Leu-Gly-Asn-Glu-Ser-Gly-His-Gly-Ala-... 

The intestinal oligo- and disaccharidases are fixed components of the cell membrane of the brush border region of the wall of the small intestine. These enzymes digest dietary carbohydrate to monosaccharides which are absorbed through the intestinal wall. They include sucrase, maltase, isomaltase, lactase, trehalase, and hetero-p-glucosidase. In the late 1970s, it was realized that inhibition of all or some of these activities could regulate the absorption of carbohydrate, and that these inhibitors could... 

It can be seen from Table 2 that a large number of the naturally-occurring polyhydroxylated alkaloids are potent inhibitors of the various a-glucosidase-specific disaccharidases involved in mammalian digestion (e.g. sucrase, maltase, isomaltase, etc.,). These enzymes are expressed at the surface of the epithelial cells of the brush border in the small intestine. In the late 1970s it was realised that inhibitors of these enzymes, such as DNJ, could be used therapeutically in the oral treatment of the non-insulin-dependent (type n) diabetes mellitus. ... 

A variety of membrane-bound proteins are of vital interest to the medical and nutritional ientist, because defects or changes in these proteins can cause such problems as lactose intolerance, cardiovascular disease, cystic fibrosis, and diabetes. Sucrase-isomaltase, an enzyme of the small intestine, is a membrane-bound protein, bound to the plasma membrane of the cnterocyte (gut cell). Part of the production of this enzyme is depicted in Figure 1,26. in Step 1, the polypeptide chain is polymerized on the ribosome (shown in black). In Step 2, part of the amino add chain near the N terminus crosses the membrane of the ER into the lumen but some of the amino acids at the N terminus remain outside. Step 3 shows the protein assuming a three-dimensional shape within the lumen both the C and N... 

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... 

The specificity of conduritol B epoxide is such that -glucosidases from widely differing sources have been found to react with loss of enzymic activity from various Aspergillus species, > yeast, snail Helix pomatia) sweet almonds, and mammals. The only exceptions have been the / -glucosidases from garbanzo plants Cicer arietum L.) and from Alocasia macrorrhiza The only other enzymes that have been found to be covalently inhibited are a-glucosidase from yeast Saccharo-myces cerevisiae) and the sucrase-isomaltase complex from rabbit small intestine. ... 

Amino-acid sequences around the essential carboxylic acid group in the active sites of the sucrose a-D-glucohydrolase oligo-l,6-glucosidase (sucrase-isomaltase) complex of rabbit small intestines have been determined by selective labelling with [ H]conduritol-B epoxide. ... 

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