Dietary carbohydrates influence the structure and function of the intestinal alpha-glucosidases.

摘要

As the primary products of starch digestion by pancreatic alpha-amylase, maltooligosaccharides (including maltose) are the main substrates for the alpha-glucosidases at the intestinal brush border. Here, maltose was shown to induce the formation of a higher molecular weight (HMW) sucrase-isomaltase (SI) species in Caco-2 cells that sorts more quickly to the enterocyte surface to act as a digestive enzyme. As this finding suggested a maltose sensing ability of small intestinal enterocytes, molecular mechanisms associated with the maturation and trafficking of HMW SI were further investigated. A pulse-chase experiment using [ 35S]-methionine revealed a higher rate of early trafficking and maturation of the HMW SI species in cells treated with maltose. Endoglycosidase treatment of immunoprecipitated SI showed that increased molecular weight is a consequence of additional N- and O-glycosylation of the enzyme. In comparison to the control, the HMW SI was found to be more associated with lipid rafts at the membrane surface which was related to the higher apical sorting of these species. Thus, maltose sensing of small intestinal enterocytes triggers the intracellular processing of HMW SI, speculatively to enhance its digestive property. Study on the sweet taste receptor subunits (T1R2 and T1R3) by qRT-PCR showed that the expression of T1R2 and T1R3 increased in presence of maltose compared to glucose. It appeared also that maltooligosaccharides may signal other events in small intestine enterocytes. Culture-related conditions (e.g. glucose concentration) are known to alter physical barrier properties of Caco-2 cell monolayers, which affect transepithelial transport of solutes permeating the cell monolayer barrier. A transepithelial electrical resistance experiment was conducted to measure the integrity of the Caco-2 monolayer in response to maltooligosaccharides. Maltose promoted higher tight junction formation and permeability compared to glucose and sucrose at 12 hours. Contrary to this finding, paracellular permeability increased with maltose. In addition to barrier function of small intestinal enterocytes, a metabolomics study was conducted using high-resolution 1H NMR. Results showed that concentration of metabolites (taurine, phosphorycholine, and glycerophosphocholine) which are known to be markers for cell differentiation increased in Caco-2 cells treated with different types of maltooligosaccharides compared to glucose and sucrose. Overall, these findings are indicative of a maltooligosaccharide sensing ability by enterocytes that trigger a number of important events in the cell including a higher level of SI processing and enzyme activation for digestion purpose, and an increased cell differentiation and tight junction barrier function. From the broader perspective of a desire to control of postprandial glycemic response, as well as to elicit the gut-brain axis and ileal brake mechanisms for appetite control, this work suggests a new potential point of controlling glucose release and absorption in the small intestine (i.e., a putative maltooligosaccharide enterocyte receptor).