Abstract
Starch digestion involves the breakdown by α-amylase to small linear and branched malto-oligosaccharides, which are in turn hydrolyzed to glucose by the mucosal α-glucosidases, maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI). MGAM and SI are anchored to the small intestinal brush-border epithelial cells, and each contains a catalytic N- and C-terminal subunit. All four subunits have α-1,4-exohydrolytic glucosidase activity, and the SI N-terminal subunit has an additional exo-debranching activity on the α-1,6-linkage. Inhibition of α-amylase and/or α-glucosidases is a strategy for treatment of type 2 diabetes. We illustrate here the concept of "toggling": differential inhibition of subunits to examine more refined control of glucogenesis of the α-amylolyzed starch malto-oligosaccharides with the aim of slow glucose delivery. Recombinant MGAM and SI subunits were individually assayed with α-amylolyzed waxy corn starch, consisting mainly of maltose, maltotriose, and branched α-limit dextrins, as substrate in the presence of four different inhibitors: acarbose and three sulfonium ion compounds. The IC(50) values show that the four α-glucosidase subunits could be differentially inhibited. The results support the prospect of controlling starch digestion rates to induce slow glucose release through the toggling of activities of the mucosal α-glucosidases by selective enzyme inhibition. This approach could also be used to probe associated metabolic diseases.
Highlights
Proper breakdown of starch by hydrolytic enzymes to yield glucose has profound implications for avoiding type 2 diabetes and obesity
As representative inhibitors, acarbose [1], de-O-sulfonated kotalanol [6], C-3Ј-maltose-extended de-O-sulfonated ponkoranol analog [10], and C-5Ј--maltose-extended de-O-sulfonated ponkoranol [11], and we report here their inhibitory effects on recombinant human MGAM and SI using a mixture of the ␣-amylase degradation products (LM/␣LDx) as a substrate
One unit of enzyme activity was defined as 1 M glucose released from 1% (w/v) maltose or linear maltooligosaccharides and branched ␣-limit dextrins (LM/␣LDx) in 1 min. ctMGAM had ϳ1.8 –3.8 and 2–3.6 times higher hydrolytic activity with maltose and LM/␣LDx, respectively, compared with the other ␣-glucosidases. ntSI had the lowest specific activity for both maltose and LM/␣LDx hydrolysis among the four mucosal ␣-glucosidases
Summary
Proper breakdown of starch by hydrolytic enzymes to yield glucose has profound implications for avoiding type 2 diabetes and obesity. The results support the prospect of controlling starch digestion rates to induce slow glucose release through the toggling of activities of the mucosal ␣-glucosidases by selective enzyme inhibition.
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