Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/1/2007
Publication Date: 7/1/2007
Publication URL: www.nutrition.org
Citation: Quezada-Calvillo, R., Robayo-Torres, C.C., Opekun, A.R., Sen, P., Ao, Z., Hamaker, B.R., Quaroni, A., Brayer, G.D., Wattler, S., Nehls, M.C., Sterchi, E.E., Nichols, B.L. 2007. Contribution of mucosal maltase-glucoamylase activities to mouse small intestinal starch alpha-glucogenesis. Journal of Nutrition. 137(7):1725-1733. Interpretive Summary: The digestion of starch is not controlled by just one enzyme but by six enzymes. The primary enzymes, maltase-glucoamylase, were studied for their role in starch digestion. To investigate the roles of the maltase-glucoamylase we removed the enzymes in mice. To determine the roles of pancreatic amylase in starch digestion we used a synthetic pancreatic amylase enzyme. The removal of maltase-glucoamylase did not halt the starch digesting capacity of the mouse small intestine in test tubes because of persistent sucrase-isomaltase activities. Amylase amplified starch digestion but could not replace maltase-glucoamylase activity in the deficient mice. Using living mice for the breath test for starch digestion and oxidation, we showed that these maltase-glucoamylase deficient mice have a much slower digestion and oxidation of starch to carbon dioxide. We also showed that the capacity for starch digestion into glucose in mice is very sensitive to the plant source of food starches. This principle may be of value in designing dietary treatment of adult onset diabetes.
Technical 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.