Submitted to: Gastroenterology
Publication Type: Abstract only
Publication Acceptance Date: 3/1/2006
Publication Date: 4/1/2006
Citation: Quezada-Calvillo, R., Robayo-Torres, C.C., Baker, S.S., Hamaker, B., Sterchi, E.E., Nichols, B.L. 2006. Maltotriose, product of alpha-amylase starch hydrolysis, suppresses maltase-glucoamylase activity and slows terminal starch digestion 44.5 fold [abstract]. Gastroenterology. 130(4 Suppl. 2):A604. Interpretive Summary:
Technical Abstract: Starches constitute the main caloric source in the average human diet. The digestion of starches is far more complex than sugars and requires six different enzyme activities to produce free glucose before absorption. Salivary and pancreatic alpha-amylase activities initially hydrolyze internal 1-4 glucose bonds of starch to produce a limited pattern of soluble glucose oligomers, including maltotriose. Terminally, mucosal enzymes maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI) hydrolyze external linear bonds of starch oligosaccharides to free glucose. Our aim was to determine the relative roles of the two mucosal enzymes in terminal starch digestion. These experiments led to the novel discovery that rates of integrated mucosal starch digestion to glucose are regulated by lumenal maltotriose substrate inhibition, termed the maltotriose brake. Methods: Enterocytes were harvested from transplant donor jejunum by scraping. The membranes were solubilized with 10% Nonidet P 40. MGAM and SI activities were immunoprecipitated (IP) with monoclonal antibodies and incubated with dilutions of maltose, maltotriose, and dextromaltose for 60 min. Dextromaltose, partially hydrolyzed amylopectin, substrate was analyzed by Mass Spectrometry. Glucose production was measured by Sigma Infinity reagent. The protocol was approved by BCM IRB as H-1320. Results: Dextromaltose has a median polymer length of 7 glucoses. Enzyme IPs were shown pure and relative concentrations determined by Western blot. Concentration of SI is 20 times MGAM. Under clinical assay conditions, SI accounted for all mucosal maltotriase and 80% of IP maltase and dextromaltase activities. MGAM was inhibited (Ki 1.8 mM) by maltotriose. All activities followed Michaelian kinetics. Both enzymes hydrolyzed dextromaltose, but Km was 1.3 for MGAM and 58 for SI. Conclusion: MGAM and SI accounted for 90% of all mucosal dextromaltase activity. MGAM is 44.5 times more active than SI. MGAM is substrate inhibited by maltotriose, but SI is not. SI is a low activity, high capacity unregulated enzyme for terminal starch digestion. MGAM has higher activity (44.5 fold) and lower capacity (20 fold). MGAM provides rapid glucose production during early and late starch feeding or snacking, but its high activity is suppressed by the maltotriose brake during heavy starch ingestion. MGAM matches Kms of amylase and the glucose transporter, but when the brake is applied, slower SI becomes rate limiting for the digestion of starch to glucose.