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Biochemical, Molecular, and Genetic Mechanisms

Contribution of Mucosal Maltase-Glucoamylase Activities to Mouse Small Intestinal Starch -Glucogenesis^1–3,

⁴ CIEP-Facultad de Ciencias Quimicas, Universidad Autonoma de San Luis Potosi, Zona Universitaria, San Luis Potosí, S.L.P., Mexico 78360; ⁵ USDA, Agricultural Research Service, Children's Nutrition Research Center and Department of Pediatrics, Baylor College of Medicine, Houston, TX, 77030-2600; ⁶ Whistler Center for Carbohydrate Research and Department of Food Science, Purdue University, West Lafayette, Indiana 47907-2009; ⁷ Division of Biological Sciences, Section of Physiology, Cornell University, Ithaca, New York 14853 ⁸ Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver V6T 1Z3, Canada; ⁹ Ingenium Pharmaceuticals AG, 82152 Martinsried, Germany; and ¹⁰ Institute of Biochemistry and Molecular Medicine, University of Berne, Berne CH-3012, Switzerland

^* To whom correspondence should be addressed. E-mail: bnichols{at}bcm.tmc.edu.

Digestion of starch requires activities provided by 6 interactive small intestinal enzymes. Two of these are luminal endo-glucosidases named -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 -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 -glucogenic activities of the jejunal mucosa with and without added recombinant pancreatic -amylase, using a range of food starch substrates. Compared with wild-type mucosa, null mucosa or -amylase alone had little -glucogenic activity. -Amylase amplified wild-type and null mucosal -glucogenesis. -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 -glucogenesis. Maltase-glucoamylase determined rates of digestion of starch in normal mice and -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 -glucogenesis to a wide range of food starches.

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				R. Quezada-Calvillo, L. Sim, Z. Ao, B. R. Hamaker, A. Quaroni, G. D. Brayer, E. E. Sterchi, C. C. Robayo-Torres, D. R. Rose, and B. L. Nichols Luminal Starch Substrate "Brake" on Maltase-Glucoamylase Activity Is Located within the Glucoamylase Subunit J. Nutr., April 1, 2008; 138(4): 685 - 692. [Abstract] [Full Text] [PDF]