KINETIC ANALYSIS OF STARCH DIGESTION REVEALS MAJOR ROLE OF SUCRASE‐ISOMALTASE AND INHIBITION OF MALTASE‐GLUCOAMYLASE BY MALTOTRIOSE

Abstract

Food starch digestion is a complex process. Glucose oligomers (GO) resulting from digestion by amylases, are terminally digested to free glucose by intestinal Maltase-Glucoamylase (MGAM) and Sucrase-Isomaltase (SI). Aims: 1. To define the relative roles of MGAM and SI in terminal starch digestion using kinetic models with maltose, maltotriose, and mixed GO (Polycose) as substrates; 2. To compare the model to experimental data using immunoisolated human MGAM and SI activities. MGAM and SI activities from pooled Nonidet P-40/ DOC solubilized human duodenal biopsy homogenates were immunoisolated with monoclonal antibodies on protein A beads. Protein purification was assessed by Western blots. Enzyme activities against maltose, maltotriose and GO (Polycose) were assayed on the beads by the TGO method. Mathematical models were constructed using Michaelis-Menten equations. MGAM and SI account for >90% of biopsy maltase, maltotriase and GOase activities. MGAM contributed only 36 % of maltase and 22% of GOase. Km for maltose was higher for SI (40 mM) than for MGAM (3.7 mM); however, apparent Vmax for SI was >2 X MGAM. This difference was visualized on Western blots. Maltotriose caused substrate inhibition of MGAM with Km of 20 and Ki of 1.8 mM, while SI had a Km of 22 mM and no inhibition. MGAM activities for GO resembled those observed with Maltotriose with respect to substrate inhibition. Kinetic modeling of the activities with maltose and maltotriose correlated with experimental data from isolated MGAM and SI. Modeling of GO hydrolysis correlated only at low concentrations because other inhibitions occur at high concentrations. Conclusions: 1. >90% of GO digestion is by MGAM and SI. 2. Kinetic models prove that SI accounts for >66% of GO digestion at concentrations >10 mM. 3. Maltotriose at concentration >6 mM causes substrate inhibition of MGAM.