Location: Children's Nutrition Research CenterTitle: Individual mammalian mucosal glucosidase subunits digest various starch structures differently Author
Submitted to: Gastroenterology
Publication Type: Abstract Only
Publication Acceptance Date: 5/1/2010
Publication Date: 5/1/2010
Citation: Lin, A.H., Nichols, B.L., Quezada-Calvillo, R., Sim, L., Rose, D., Hamaker, B.R. 2010. Individual mammalian mucosal glucosidase subunits digest various starch structures differently [abstract]. Gastroenterology. 138(Suppl.1):S-566-S-567. Interpretive Summary:
Technical Abstract: Starch digestion in the human body requires two luminal enzymes,salivary and pancreatic alpha-amylase (AMY), and four small intestinal mucosal enzyme activities related to the maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI) complexes. Starch consists of two polysaccharides, amylose (AM) and amylopectin (AMP). AM has long linear chains of 1,4-linked alpha-D-glucose residues with few branches. AMP has a large molecular weight with shorter linear chains of alpha-1,4 linked glucose units and is highly branched by alpha-1,6 linkages. Chain length, branch density and other structural factors affect starch digestion. AMY is not an absolute requirement for starch digestion to glucose but dramatically amplifies glucogenesis by producing malto-oligomers as favored substrates for the glucosidases. In this study, we examined the individual mucosal enzyme subunit digestion of various starch structures to test the hypothesis that certain structures are digested slowly. Recombinant human and mouse enzymes were applied in this study. One normal and three maize starch mutants (wx, aewx, duwx) were used that vary in amylose content, chain length and branch density. Starches were hydrolyzed by AMY to alpha-limit dextrins (LDx) that cannot be further hydrolyzed by AMY. Four alpha-LDx were incubated with individual or combined mucosal enzymes to reach maximum glucogenesis. Our results show individual subunits digest various starch structures differently at the alpha-LDx level. N-terminal MGAM mainly digests short linear oligomers released by AMY. C-terminal MGAM (C-MGAM) and SI binds longer glucose units and digests both long and short chains. N-terminal SI (N-SI) digests both long and short chains as well by cleaving alpha-1,6 linkages and is the only subunit with this activity. A combination of four subunits reached highest glucogenesis indicating a broader spectrum of glucosidase activities than any individual subunit alone. Some alpha-LDx was not completely available for enzyme hydrolysis, and residues are considered either slowly digestible or resistant to human enzyme digestion. This study for the first time shows the direct role of individual mucosal glucosidase subunits in starch digestion and reveals the potential of producing slow glucose release dextrins.