Zihua Ao

Nutritional property of endosperm starches from maize mutants: a parabolic relationship between slowly digestible starch and amylopectin fine structure.

The relationship between the slow digestion property of cooked maize starch and its molecular fine structure was investigated. Results of the in vitro Englyst assay showed a range of rapidly digestible starch (RDS) (70.1-98.9%), slowly digestible starch (SDS) (0.2-20.3%), and resistant starch (RS) (0.0-13.7%) among the tested maize mutant flour samples. Further analysis showed that amylose content was significantly correlated ( R = 0.763, P < 0.001) with RS amount but not with that of SDS, indicating that amylopectin is the starch molecule associated with SDS. Total starch debranching analysis revealed a parabolic relationship between SDS content and the weight ratio of amylopectin short chains (DP < 13, named SF) to long chains (DP >/= 13, named LF), which means amylopectin with a higher amount of either short chains or long chains can produce relatively high amounts of SDS. Furthermore, debranching analysis of the SDS materials from samples with the highest and lowest weight ratios of SF/LF (both had a high amount SDS) showed significantly different profiles, indicating there is not a uniform molecular structure for SDS. Thus, genetic mutants of maize samples have a good potential to provide raw starch materials of high nutritional quality. An additional finding showed that a simple and comparably high-throughput technique of Rapid Visco-Analyzer (RVA) can be used to screen genetic mutants on the basis of their RVA profiles.

Luminal substrate "brake" on mucosal maltase-glucoamylase activity regulates total rate of starch digestion to glucose.

Background: Starches are the major source of dietary glucose in weaned children and adults. However, small intestine α-glucogenesis by starch digestion is poorly understood due to substrate structural and chemical complexity, as well as the multiplicity of participating enzymes. Our objective was dissection of luminal and mucosal α-glucosidase activities participating in digestion of the soluble starch product maltodextrin (MDx). Patients and Methods: Immunoprecipitated assays were performed on biopsy specimens and isolated enterocytes with MDx substrate. Results: Mucosal sucrase-isomaltase (SI) and maltase-glucoamylase (MGAM) contributed 85% of total in vitro α-glucogenesis. Recombinant human pancreatic α-amylase alone contributed <15% of in vitro α-glucogenesis; however, α-amylase strongly amplified the mucosal α-glucogenic activities by preprocessing of starch to short glucose oligomer substrates. At low glucose oligomer concentrations, MGAM was 10 times more active than SI, but at higher concentrations it experienced substrate inhibition whereas SI was not affected. The in vitro results indicated that MGAM activity is inhibited by α-amylase digested starch product “brake” and contributes only 20% of mucosal α-glucogenic activity. SI contributes most of the α-glucogenic activity at higher oligomer substrate concentrations. Conclusions: MGAM primes and SI activity sustains and constrains prandial α-glucogenesis from starch oligomers at approximately 5% of the uninhibited rate. This coupled mucosal mechanism may contribute to highly efficient glucogenesis from low-starch diets and play a role in meeting the high requirement for glucose during childrens brain maturation. The brake could play a constraining role on rates of glucose production from higher-starch diets consumed by an older population at risk for degenerative metabolic disorders.

Mucosal maltase-glucoamylase plays a crucial role in starch digestion and prandial glucose homeostasis of mice.

Starch is the major source of food glucose and its digestion requires small intestinal alpha-glucosidic activities provided by the 2 soluble amylases and 4 enzymes bound to the mucosal surface of enterocytes. Two of these mucosal activities are associated with sucrase-isomaltase complex, while another 2 are named maltase-glucoamylase (Mgam) in mice. Because the role of Mgam in alpha-glucogenic digestion of starch is not well understood, the Mgam gene was ablated in mice to determine its role in the digestion of diets with a high content of normal corn starch (CS) and resulting glucose homeostasis. Four days of unrestricted ingestion of CS increased intestinal alpha-glucosidic activities in wild-type (WT) mice but did not affect the activities of Mgam-null mice. The blood glucose responses to CS ingestion did not differ between null and WT mice; however, insulinemic responses elicited in WT mice by CS consumption were undetectable in null mice. Studies of the metabolic route followed by glucose derived from intestinal digestion of (13)C-labeled and amylase-predigested algal starch performed by gastric infusion showed that, in null mice, the capacity for starch digestion and its contribution to blood glucose was reduced by 40% compared with WT mice. The reduced alpha-glucogenesis of null mice was most probably compensated for by increased hepatic gluconeogenesis, maintaining prandial glucose concentration and total flux at levels comparable to those of WT mice. In conclusion, mucosal alpha-glucogenic activity of Mgam plays a crucial role in the regulation of prandial glucose homeostasis.

Evidence of native starch degradation with human small intestinal maltase-glucoamylase (recombinant).

Action of human small intestinal brush border carbohydrate digesting enzymes is thought to involve only final hydrolysis reactions of oligosaccharides to monosaccharides. In vitro starch digestibility assays use fungal amyloglucosidase to provide this function. In this study, recombinant N‐terminal subunit enzyme of human small intestinal maltase‐glucoamylase (rhMGAM‐N) was used to explore digestion of native starches from different botanical sources. The susceptibilities to enzyme hydrolysis varied among the starches. The rate and extent of hydrolysis of amylomaize‐5 and amylomaize‐7 into glucose were greater than for other starches. Such was not observed with fungal amyloglucosidase or pancreatic α‐amylase. The degradation of native starch granules showed a surface furrowed pattern in random, radial, or tree‐like arrangements that differed substantially from the erosion patterns of amyloglucosidase or α‐amylase. The evidence of raw starch granule degradation with rhMGAM‐N indicates that pancreatic α‐amylase hydrolysis is not a requirement for native starch digestion in the human small intestine.

Maltase-glucoamylase modulates gluconeogenesis and sucrase-isomaltase dominates starch digestion glucogenesis.

Objectives: Six enzyme activities are needed to digest starch to absorbable free glucose; 2 luminal &agr;-amylases (AMY) and 4 mucosal maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI) subunit activities are involved in the digestion. The AMY activities break down starch to soluble oligomeric dextrins; mucosal MGAM and SI can either directly digest starch to glucose or convert the post-&agr;-amylolytic dextrins to glucose. We hypothesized that MGAM, with higher maltase than SI, drives digestion on ad limitum intakes and SI, with lower activity but more abundant amount, constrains ad libitum starch digestion. Methods: Mgam null and wild-type (WT) mice were fed with starch diets ad libitum and ad limitum. Fractional glucogenesis (fGG) derived from starch was measured and fractional gluconeogenesis and glycogenolysis were calculated. Carbohydrates in small intestine were determined. Results: After ad libitum meals, null and WT had similar increases of blood glucose concentration. At low intakes, null mice had less fGG (P = 0.02) than WT mice, demonstrating the role of Mgam activity in ad limitum feeding; null mice did not reduce fGG responses to ad libitum intakes demonstrating the dominant role of SI activity during full feeding. Although fGG was rising after feeding, fractional gluconeogenesis fell, especially for null mice. Conclusions: The fGNG (endogenous glucogenesis) in null mice complemented the fGG (exogenous glucogenesis) to conserve prandial blood glucose concentrations. The hypotheses that Mgam contributes a high-efficiency activity on ad limitum intakes and SI dominates on ad libitum starch digestion were confirmed.

Branch pattern of starch internal structure influences the glucogenesis by mucosal Nt-maltase-glucoamylase.

To produce sufficient amounts of glucose from food starch, both α-amylase and mucosal α-glucosidases are required. We found previously that the digestion rate of starch is influenced by its susceptibility to mucosal α-glucosidases. In the present study, six starches and one glycogen were pre-hydrolyzed by α-amylase for various time periods, and then further hydrolyzed with the mucosal α-glucosidase, the N-terminal subunit of maltase-glucoamylase (Nt-MGAM), to generate free glucose. Results showed that α-amylase amplified the Nt-MGAM glucogenesis, and that the amplifications differed in various substrates. The amount of branches within α-amylase hydrolysate substrates was highly related to the rate of Nt-MGAM glucogenesis. After de-branching, the hydrolysates showed three fractions, Fraction 1, 2, and 3, in size exclusion chromatographs. We found that the α-amylase hydrolysates with higher quantity of the Fraction 3 (molecules with relatively short chain-length) and shorter average chain-length of this fraction had lower rates of Nt-MGAM glucogenesis. This study revealed that the branch pattern of α-amylase hydrolysates modulates glucose release by Nt-MGAM. It further supported the hypothesis that the internal structure of starch affects its digestibility at the mucosal α-glucosidase level.

The nature of raw starch digestion.

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Controlling the delivery of glucose in foods

Abstract: Glucose is the principal biological fuel for the body, and it has been classified into rapidly available glucose (RAG) and slowly available glucose (SAG) based on glycemic carbohydrate digestion rate that is related to human health. This chapter first reviews the concept of glycemic index (GI), and the digestion and nutritional classification of glycemic carbohydrates. The structural properties, slow digestion mechanism, and the biological functions of slowly digestible carbohydrates (SDC), mainly slowly digestible starch (SDS), are then discussed. Finally, potential formulation and testing of low-GI foods, using SDCs to control the glucose delivery, are described for the purpose of improving human health.

Research progress reported at the 50th Anniversary of the Discovery of Congenital Sucrase-Isomaltase Deficiency Workshop.

C ongenital sucrase-isomaltase deficiency (CSID) is a rare autosomal intestinal disease caused by mutations of the sucrase-isomaltase gene. Patients with CSID have different phenotypes and enzymatic activities associated with sucrase-isomaltase ranging from mild reduction of sucrase activity to complete absence. Low sucrase activity leads to maldigestion of sucrose, resulting in dyspepsia-like symptoms such as chronic diarrhea, abdominal pain, and bloating. The severity of the symptom is related to the amount of sucrase activity and quantity of sucrose ingested. Reduced maltase activity is expected to occur in patients with CSID because both subunits of the SI complex contribute to the total mucosal maltose activity. Indeed, low maltase activity can also lead to maldigestion of starches contributing to the symptoms of dyspepsia. When children are assessed for this gene mutation, duodenal mucosal histology is invariably normal. Because there are no noninvasive methods for specific confirmation of sucrase-isomaltase deficiency, a novel noninvasive C-sucrose labeled substrate has been developed and validated as a sucrase activity breath test for screening and confirmation of CSID. It has been shown that primary sucrase deficiency can be confirmed using this new C breath test, as well as the effectiveness of sucrase replacement therapy. On December 21 and 22, 2011, the 8th Workshop of the Starch Digestion Consortium (SDC) was held at the Children’s Nutrition Research Center at the Baylor College of Medicine and Texas Children’s Hospital. The theme of the workshop was ‘‘50 Years of Progress Since Congenital Sucrase-Isomaltase Deficiency (CSID) Recognition.’’ This was a multidisciplinary workshop that blended clinical medicine with nutritional and food science. The purpose of this workshop was to review progress toward clinical diagnosis and management of sucrase-isomaltase enzyme deficiency during the past 50 years. The nutritional and food technological objectives of the conference were 2-fold: first, to define the role of sucrase-isomaltase enzyme complex in human starch digestion to glucose (a-glucogenesis), and second, by studying sucrase-isomaltase–deficient patients with CSID, envision approaches to botanical and technological alterations in food that will benefit all populations. More than 50 authors and attendees participated in this workshop from 12 different countries. Eighteen original communications were presented. Special thanks go to QOL Medical, the supplier of Sucraid oral enzyme supplement, for sponsoring this event. This supplement to the Journal of Pediatric Gastroenterology and Nutrition reports the proceedings of this workshop. E. Roseland Klein served as the technical editor of the workshop publication. Beth Mays was responsible for all travel arrangements and the workshop organization. Adam Gillum was responsible for graphic and audio arrangements. The group of participants standing among the lobby statuary at the SDC/CSID workshop held at the Children’s Nutrition Research Center, December 21–22, 2011 is shown in Figure 1. Statues are identified by italics; Borden is by Joseph Paderewski (1914–2000) and the others are copies of the work of Elizabet Ney (1833–1907). Research Progress Reported at the 50th Anniversary of the Discovery of Congenital Sucrase-Isomaltase Deficiency Workshop