Starch Source Influences Dietary Glucose Generation at the Mucosal α-Glucosidase Level

Amy Hui-Mei Lin,Roberto Quezada-Calvillo,Byung-Hoo Lee,Bruce R Hamaker,Buford L Nichols,Hassan Y Naim,David R Rose

doi:10.1074/jbc.m112.378331

Abstract

The quality of starch digestion, related to the rate and extent of release of dietary glucose, is associated with glycemia-related problems such as diabetes and other metabolic syndrome conditions. Here, we found that the rate of glucose generation from starch is unexpectedly associated with mucosal α-glucosidases and not just α-amylase. This understanding could lead to a new approach to regulate the glycemic response and glucose-related physiologic responses in the human body. There are six digestive enzymes for starch: salivary and pancreatic α-amylases and four mucosal α-glucosidases, including N- and C-terminal subunits of both maltase-glucoamylase and sucrase-isomaltase. Only the mucosal α-glucosidases provide the final hydrolytic activities to produce substantial free glucose. We report here the unique and shared roles of the individual α-glucosidases for α-glucans persisting after starch is extensively hydrolyzed by α-amylase (to produce α-limit dextrins (α-LDx)). All four α-glucosidases share digestion of linear regions of α-LDx, and three can hydrolyze branched fractions. The α-LDx, which were derived from different maize cultivars, were not all equally digested, revealing that the starch source influences glucose generation at the mucosal α-glucosidase level. We further discovered a fraction of α-LDx that was resistant to the extensive digestion by the mucosal α-glucosidases. Our study further challenges the conventional view that α-amylase is the only rate-determining enzyme involved in starch digestion and better defines the roles of individual and collective mucosal α-glucosidases. Strategies to control the rate of glucogenesis at the mucosal level could lead to regulation of the glycemic response and improved glucose management in the human body.

Highlights

The quality of starch digestion, related to the rate and extent of release of dietary glucose, is associated with glycemia-related problems such as diabetes and other metabolic syndrome conditions
Glucogenesis was determined by the glucose oxidase/peroxidase assay [11], and the residue structure was studied by high performance anion exchange chromatography (Ref. 12 and data not shown). wx ␣-LDx (25 ␮g) was incubated with four subunits and at three amounts (0.05, 0.1, and 0.2 ␮g of Nt- and Ct-MGAM; 0.25, 0.5, and 1 ␮g of Nt- and Ct-SI), and real-time residue structure change was examined by high performance size exclusion chromatography (HP-1090, Agilent Technologies) equipped with a refractive index detector (RID-10A, Shimadzu Corp., Kyoto, Japan) using two Zorbax PSM 60S columns (50 °C; Agilent Technologies) with a mobile phase of dimethyl sulfoxide containing 50 mM lithium chloride at a flow rate of 0.5 ml/min
The amount of glucose released by Nt-MGAM was near the total anhydroglucose amount of the linear glucans, which was 47% in the ␣-LDx from normal maize

Summary

EXPERIMENTAL PROCEDURES

Gelatinized starch molecules (10 mg/ml in phosphate-buffered saline) were incubated with human recombinant pancreatic ␣-amylase (10 units/5 mg of starch) at 37 °C until the amount of reducing sugar and did not change significantly [6]. Glucogenesis was determined by the glucose oxidase/peroxidase assay [11], and the residue structure was studied by high performance anion exchange chromatography (Ref. 12 and data not shown). Wx ␣-LDx (25 ␮g) was incubated with four subunits and at three amounts (0.05, 0.1, and 0.2 ␮g of Nt- and Ct-MGAM; 0.25, 0.5, and 1 ␮g of Nt- and Ct-SI), and real-time (up to 6 h) residue structure change was examined by high performance size exclusion chromatography (HP-1090, Agilent Technologies) equipped with a refractive index detector (RID-10A, Shimadzu Corp., Kyoto, Japan) using two Zorbax PSM 60S columns (50 °C; Agilent Technologies) with a mobile phase of dimethyl sulfoxide containing 50 mM lithium chloride at a flow rate of 0.5 ml/min. Letters indicate the glucogenesis differences due to the effect of enzymes (combined and individual ␣-glucosidase subunits) on the same substrate.

Normal maize wx aewx duwx

RESULTS

DISCUSSION

Substrates and constants