Contribution of mucosal maltase-glucoamylase activities to mouse small intestinal starch alpha-glucogenesis

Abstract

Digestion of starch requires activities provided by 6 interactive small intestinal enzymes. Two of these are luminal endo-glucosidases named alpha-amylases. Four are exo-glucosidases bound to the luminal surface of enterocytes. These mucosal activities were identified as 4 different maltases. Two maltase activities were associated with sucrase-isomaltase. Two remaining maltases, lacking other identifying activities, were named maltase-glucoamylase. These 4 activities are better described as alpha-glucosidases because they digest all linear starch oligosaccharides to glucose. Because confusion persists about the relative roles of these 6 enzymes, we ablated maltase-glucoamylase gene expression by homologous recombination in Sv/129 mice. We assayed the alpha-glucogenic activities of the jejunal mucosa with and without added recombinant pancreatic alpha-amylase, using a range of food starch substrates. Compared with wild-type mucosa, null mucosa or alpha-amylase alone had little alpha-glucogenic activity. alpha-Amylase amplified wild-type and null mucosal alpha-glucogenesis. alpha-Amylase amplification was most potent against amylose and model resistant starches but was inactive against its final product limit-dextrin and its constituent glucosides. Both sucrase-isomaltase and maltase-glucoamylase were active with limit-dextrin substrate. These mucosal assays were corroborated by a 13C-limit-dextrin breath test. In conclusion, the global effect of maltase-glucoamylase ablation was a slowing of rates of mucosal alpha-glucogenesis. Maltase-glucoamylase determined rates of digestion of starch in normal mice and alpha-amylase served as an amplifier for mucosal starch digestion. Acarbose inhibition was most potent against maltase-glucoamylase activities of the wild-type mouse. The consortium of 6 interactive enzymes appears to be a mechanism for adaptation of alpha-glucogenesis to a wide range of food starches.