Submitted to: Pediatric Research
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2006
Publication Date: 5/1/2006
Citation: Robayo-Torres, C.C., Quezada-Calvillo, R., Baker, S.S., Sterchi, E.E., Nichols, B.L. 2006. Maltotriose brake: alpha-amylase hydrolysis product maltotriose regulates maltase-glucoamylase activity and controls total rates of starch digestion to glucose [abstract]. Pediatric Research. E-PAS2006. 59:4136.8. Interpretive Summary:
Technical Abstract: BACKGROUND: Food starches provide 75% of meal-derived glucose required for normal brain energy supply. The digestion of cereals to glucose thus may be critically important in a weaning infant's diet. Human starch digestion is a multienzyme process involving six different enzymes. Salivary and pancreatic alpha-amylase activities initially cleave internal glucose bonds of starch producing soluble oligosaccharides, including maltotriose. Finally, two activities each of mucosal alpha-glucosidases maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI) hydrolyze external bonds of oligosaccharides to produce glucose. OBJECTIVE: Determination of the roles of the mucosal alpha-glucosidase MGAM and SI complexes in starch digestion to glucose. DESIGN/METHODS: Enterocytes were harvested from transplant-donor jejunum. Membranes were solubilized with 10% Nonidet P 40. MGAM and SI activities were immunoprecipitated (IP) and incubated with maltose, maltotriose, and maltodextrins (alpha-amylase partially hydrolyzed starch) for 60 min. Glucose production was measured with Sigma Infinity reagent. RESULTS: The IP activities were pure by Western blots. SI-IP accounted for 80% of mucosal maltase and maltodextrinase but all maltotriase at clinical assay substrate concentrations. MGAM was substrate inhibited (Ki 1.8 mM) by maltotriose concentrations above 5 mM. Both enzymes hydrolyzed starch oligomers (maltodextrins); Km was 1.3 mM for MGAM-IP and 58 mM for SI-IP. Combined, both accounted for 90% of all mucosal maltodextrinase activity. CONCLUSIONS: MGAM has higher activity (44.5 X SI) and lower capacity (1/20 of SI). Maltotriose contributes about 35% of post-amylase lumenal starch oligomers. MGAM provides rapid glucose production during snacking but its activity is inhibited by the maltotriose brake during meal-time starch ingestion. SI is the default low activity and high capacity enzyme for terminal starch digestion during meals. MGAM matches the Kms of alpha-amylase and Na/glucose transporter, but when the brake is applied, SI becomes a limiting step, a 44.5 fold slower rate of digestion of starch to glucose.