The digestion of glycemic carbohydrates (predominantly starch and sucrose) to monosaccharides (eg, glucose, fructose) is finally carried out by the mucosal α-glucosidases in the small intestine. They are located in the brush border membrane of the small intestine and are made up of maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI) complexes. These complexes have 2 catalytic α-glucosidases each, at the C- and N-terminal domains (ctMGAM, ntMGAM, ctSI, and ntSI). All 4 mucosal α-glucosidases have hydrolytic activity on the α-1,4 linkage, which is the major linkage of starch molecules, but exhibit different rates of glucogenesis. In addition, ntSI is the major enzyme for hydrolysis of the α-1,6 linkage, whereas ctSI is mainly responsible for the α-1,2 linkage hydrolysis of sucrose (1). Research revealed that ctMGAM also has sucrase activity, although hydrolytic activity was much lower than that of ctSI (2). The monosaccharides released by the action of α-glucosidases are absorbed in the small intestine via the sodium-dependent glucose transporter-1 for glucose and glucose transporter 5 for fructose (3) for use as an energy source through the formation of adenosine triphosphate.
Congenital sucrase-isomaltase deficiency (CSID) is an autosomal recessive disease caused by mutations of the SI gene. Because of the lack of expressed SI to hydrolyze α-glycosidic linkages in the small intestine, patients with CSID have sucrose and starch malabsorption. The undigested starch and sucrose molecules further cause chronic diseases (eg, chronic osmotic diarrhea, abdominal pain) in the colon (4,5).
Starch and sucrose are major glycemic carbohydrates used for energy in the human diet and they are often consumed together in meals (eg, pies, cookies, cakes). Previous research revealed that when certain α-glucosidase substrates exist together, they act as competitive inhibitors (6). In this study, we hypothesized that sucrose, which is undigested because of a lack of SI in patients with CSID, will competitively inhibit the α-1,4 hydrolytic activity of MGAM, and thus hinder starch hydrolysis by the 2 active mucosal α-glucosidases, ntMGAM and ctMGAM. This study of competitive inhibition of MGAM by undigested sucrose may be another factor contributing to starch malabsorption in patients with CSID.
To test the effect of the presence of sucrose molecules on the individual ntMGAM and ctMGAM during starch hydrolysis, different combinations of maltose (applied as an α-1,4 linked substrate instead of starch, 100 mmol/L) and sucrose (10, 50, and 100 mmol/L) solutions were prepared. Concentrations of sucrose were designed based on the approximate amount of sugar (sucrose) contained in a carbonated beverage (approximately 300 mmol/L; that was estimated to be diluted in the gastrointestinal tract to one-third, and then 2 further dilutions were done). Mixtures were reacted with a fixed protein amount (final concentration 30 μg/mL) of recombinant ntMGAM (from human cDNA) and ctMGAM (from mouse cDNA) at 37 °C in 10 mmol/L phosphate-buffered saline (pH 6.9) buffer. The preparation of recombinant enzymes was described in previous studies (7,8). For this experiment, reaction time was controlled to 30 minutes to minimize sucrose hydrolysis by ctMGAM, which occurs at a slow rate. The amount of glucose released from maltose was measured by the glucose oxidase/peroxide method. Statistical analysis of the data was performed using the Tukey's multiple comparison test (P < 0.05) using SAS software (version 9.2, SAS Institute, Cary, NC).
Figure 1 shows inhibition of maltase activity of ntMGAM and ctMGAM in the presence of relatively high amounts of sucrose. Thus, glucose release from maltose was significantly decreased in the presence of higher concentrations of sucrose. For example, when 100 mmol/L sucrose is present in the system (with 100 mmol/L maltose), the glucose release from maltose by ntMGAM and ctMGAM reaction were about 80% and 60% compared with the control (without sucrose), respectively. It is interesting that ntMGAM, which has no sucrase activity, also showed inhibition of maltase activity in the presence of sucrose. Sucrose inhibition of maltase activity on both ctMGAM and ntMGAM may be the result of competitive inhibition as both substrates (maltose and sucrose) compete for the same catalytic site in these mucosal α-glucosidases. This reasoning is supported by previous research that showed reduction of sucrase activity in the presence of isomaltulose (the sucrose isomer with α-1,6 linkage) (9).
CSID is characterized by a lack of SI complex in the small intestine, and starch digestion of patients with CSID is achieved only by nt- and ctMGAM. During starch digestion, the individual ntMGAM and ctMGAM demonstrate unique hydrolysis properties on various maltooligosaccharides; ntMGAM predominantly hydrolyzes short oligosaccharides, whereas ctMGAM has higher hydrolytic activity than ntMGAM both for short and longer maltooligosaccharides (10). Based on the maltose inhibition study, undigested sucrose, at higher concentrations, can inhibit MGAM activity, particularly the highly active ctMGAM. Because ctMGAM seems to play an important role in digesting partially hydrolyzed starch, decreasing the activity of ctMGAM by presence of unhydrolyzed sucrose could affect overall starch digestion rate and extent. Even though patients with CSID normally control dietary sucrose, strict adherence to sucrose-free diets is difficult. In this respect, our research suggests that when a patient with CSID consumes, for instance, a sucrose-containing starchy dessert, sucrose is a factor that contributes to starch malabsorption.
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