Proceedings of a meeting of the American College of Nutrition focus on important nutrition issues in infant and child health. A discussion of malnutrition reviews the biochemical, physiological and ...morphological effects of diarrhea on the intestine. Discussion of the opposite problem of overnutrition covers the energetic basis of obesity, serum lipids and their relation to diet, and body composition in adolescence. A history of vitamin D begins with the identification of vitamin D deficiency as a cause of rickets and continues to the discovery of its metabolic pathways. A comprehensive review of human milk includes its nutritional composition and its role in preventing infection. The use of stable isotopes in the study of protein metabolism is described. (ds)
This collection of papers present a scientific view of nutrition during pregnancy. A wide range of topics are addressed by 33 contributors in 24 papers. Topics include: determining the nutritional ...requirements of normal infants; physiology of lactation; infant feeding practices; nutritional anemia of infancy; nutrient content of infant formulas; and special methods of feeding the preterm infant
Abstract only
Starch requires 6 enzymes for digestion to free glucose: 2 amylases (salivary and pancreatic) and 4 mucosal maltase activities; Sucrase‐isomaltase (Si) and Maltase‐glucoamylase ...complexes (Mgam). All 6 activities are deficient in suckling rodents.
Objective
: Test
13
C‐starch digestion before weaning (at 21d) from mother's milk by measuring enrichment of blood
13
C‐glucose in Mgam Null and Wild Type mice.
Methods
Mgam gene was ablated at the N‐terminal expressing the membrane binding domain. Dams were fed low
13
C‐diet and litters kept on low
13
C‐diet. Pups were weaned at 21d. Digestion was tested at 13d and 25d by intragastric feeding of amylase predigested UL
13
C‐α‐limit dextrins (LDx). Blood
13
C‐glucose enrichment was measured by GCRMS using pentaacetate derivatives.
Results
4 h after feeding, blood
13
C‐glucose MPE% was enriched by 26×10
3
in null and 18×10
3
foldat 13d and 0.3×10
3
and 0.2×103 fold at 25d (vs. fasting p = 0.045 and p = 0.045). By jejunal enzyme assay, imunohistochemistry or Western blots there was no maltase activity or brush border staining with anti‐Si or Mgam antibodies at 13d but these were fully developed in WT by 25d. In 13d null mice lumenal contents were stained by both specific antibodies.
Conclusion
1.
13
C‐LDx was rapidly digested to
13
C‐glucose in 13d mice independent of age, Mgam genotype or mucosal maltase activity. 2. This novel observation demonstrates that a maltase activity is secreted which enables suckling pup starch digestion well before brush border enzyme development. 3. The origin of this secreted starch digesting activity is presently uncertain.
Abstract only
All starch digesting activities are reported absent in suckling mice by jejunal mucosal enzyme assay, imunohistochemistry or Western blots. We confirmed that there was no maltase ...activity or brush border staining with Mgam antibodies at 13d and that these were detected at 25d only in WT. Last year we reported that at 13 d both Mgam null and WT suckling mice can digest amylase treated
13
C‐starch derived α‐limit dextrins (LDx) to increase blood
13
C glucose paradoxically in 13d Mgam null suckling mice lumenal contents were stained by specific Mgam antibodies.
Objective
Test lactating mouse mammary glands for Mgam and Si gene expression.
Methods
RNA was extracted from the mammary gland of 10 different strains of WT 30d pp lactating mice. After RT, the cDNA was amplified for Mouse Si (amplicons for exons 1 to 6 or exons 16 to 18) and Mgam (amplicons for exons 1 to 7 or exons 3 to 7).
Results
Only the Mgam Exon 3 to 7 primers produced 516 bp bands on the Gel.
Conclusion
we suggest that maltase activity is secreted in mouse milk which enables unweaned pup starch digestion well before brush border bound enzyme development. Splicoforms of the Mgam message, lacking the membrane binding exon 2 coding region, in lactating mouse mammary tissue suggest synthesis of a soluble enzyme activity. This was confirmed by the expression of the exon 2 deleted Mgam message in lactating mammary glands of 10 different mouse strains.
For digestion of starch in humans, α-amylase first hydrolyzes starch molecules to produce α-limit dextrins, followed by complete hydrolysis to glucose by the mucosal α-glucosidases in the small ...intestine. It is known that α-1,6 linkages in starch are hydrolyzed at a lower rate than are α-1,4 linkages. Here, to create designed slowly digestible carbohydrates, the structure of waxy corn starch (WCS) was modified using a known branching enzyme alone (BE) and an in combination with β-amylase (BA) to increase further the α-1,6 branching ratio. The digestibility of the enzymatically synthesized products was investigated using α-amylase and four recombinant mammalian mucosal α-glucosidases. Enzyme-modified products (BE-WCS and BEBA-WCS) had increased percentage of α-1,6 linkages (WCS: 5.3%, BE-WCS: 7.1%, and BEBA-WCS: 12.9%), decreased weight-average molecular weight (WCS: 1.73×10 8 Da, BE-WCS: 2.76×10 5 Da, and BEBA-WCS 1.62×10 5 Da), and changes in linear chain distributions (WCS: 21.6, BE-WCS: 16.9, BEBA-WCS: 12.2 DP w). Hydrolysis by human pancreatic α-amylase resulted in an increase in the amount of branched α-limit dextrin from 26.8% (WCS) to 56.8% (BEBA-WCS). The α-amylolyzed samples were hydrolyzed by the individual α-glucosidases (100 U) and glucogenesis decreased with all as the branching ratio increased. This is the first report showing that hydrolysis rate of the mammalian mucosal α-glucosidases is limited by the amount of branched α-limit dextrin. When enzyme-treated materials were gavaged to rats, the level of postprandial blood glucose at 60 min from BEBA-WCS was significantly higher than for WCS or BE-WCS. Thus, highly branched glucan structures modified by BE and BA had a comparably slow digesting property both in vitro and in vivo . Such highly branched α-glucans show promise as a food ingredient to control postprandial glucose levels and to attain extended glucose release.
The mammalian mucosal α-glucosidase complexes, maltase–glucoamylase (MGAM) and sucrase–isomaltase (SI), have two catalytic subunits (N- and C-termini). Concurrent with the desire to modulate glycemic ...response, there has been a focus on di-/oligosaccharides with unusual α-linkages that are digested to glucose slowly by these enzymes. Here, we look at disaccharides with various possible α-linkages and their hydrolysis. Hydrolytic properties of the maltose and sucrose isomers were determined using rat intestinal and individual recombinant α-glucosidases. The individual α-glucosidases had moderate to low hydrolytic activities on all α-linked disaccharides, except trehalose. Maltase (N-terminal MGAM) showed a higher ability to digest α-1,2 and α-1,3 disaccharides, as well as α-1,4, making it the most versatile in α-hydrolytic activity. These findings apply to the development of new glycemic oligosaccharides based on unusual α-linkages for extended glycemic response. It also emphasizes that mammalian mucosal α-glucosidases must be used in in vitro assessment of digestion of such carbohydrates.
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.
Background: α-Amylase was thought to be the sole enzyme that determines starch digestion rate. Mucosal α-glucosidases were considered to simply convert post-α-amylase dextrins to glucose.
Results: The mucosal α-glucosidases can digest a wide range of α-limit dextrin molecules and digest starches from various sources differently.
Conclusion: Starch chemical structure drives the digestion difference at the brush-border area.
Significance: The findings provide new insight into controlling the glycemic response.
Starch digestion involves the breakdown by α-amylase to small linear and branched malto-oligosaccharides, which are in turn hydrolyzed to glucose by the mucosal α-glucosidases, maltase-glucoamylase ...(MGAM) and sucrase-isomaltase (SI). MGAM and SI are anchored to the small intestinal brush-border epithelial cells, and each contains a catalytic N- and C-terminal subunit. All four subunits have α-1,4-exohydrolytic glucosidase activity, and the SI N-terminal subunit has an additional exo-debranching activity on the α-1,6-linkage. Inhibition of α-amylase and/or α-glucosidases is a strategy for treatment of type 2 diabetes. We illustrate here the concept of “toggling”: differential inhibition of subunits to examine more refined control of glucogenesis of the α-amylolyzed starch malto-oligosaccharides with the aim of slow glucose delivery. Recombinant MGAM and SI subunits were individually assayed with α-amylolyzed waxy corn starch, consisting mainly of maltose, maltotriose, and branched α-limit dextrins, as substrate in the presence of four different inhibitors: acarbose and three sulfonium ion compounds. The IC50 values show that the four α-glucosidase subunits could be differentially inhibited. The results support the prospect of controlling starch digestion rates to induce slow glucose release through the toggling of activities of the mucosal α-glucosidases by selective enzyme inhibition. This approach could also be used to probe associated metabolic diseases.
Background: Proper breakdown of starch by hydrolytic enzymes to yield glucose has profound implications for avoiding type 2 diabetes and obesity.
Results: Starch digestion by the different human enzymes is controlled using a panel of compounds.
Conclusion: Inhibitors can be used to switch off selectively the different enzyme activities.
Significance: More refined control of starch hydrolysis with the aim of slow glucose delivery is possible.
PDF
