Physiology - GI - DIGESTION AND ABSORPTION - Carbohydrates

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  1. Carbohydrates
    • sucrose
    • lactose
    • plant starch
  2. The most important digestive enzymes for carbohydrate digestion are ________. The resulting monosaccharides are then absorbed by _______ and enter the blood.
    components of the brush border membrane of the small intestine

    specialized transporters
  3. Discuss lumenal and mucosal carbohydrate digestion in all GI segments. List the source of these digestive enzymes and the three resulting monosaccharides.
    • mouth - lumenal
    • salivary α-amylase: partially degrade starch, ultimately maltose, maltosetriose, α-dextrin
    • stomach - lumenal
    • same as mouth in bolus, deactivated in acid
    • small intestine - lumenal
    • pancreatic α-amylase: degrade starch to maltose, maltosetriose, α-dextrin
    • sucrose and lactose arrive, too
    • small intestine - mucosal
    • maltase: maltose/maltosetriose -> glucose
    • isomaltase: α-dextrin -> glucose
    • sucrase: sucrose -> glucose + fructose
    • lactase: lactose -> glucose + galactose
  4. Identify the primary intestinal segments absorbing monosaccharides. Give an example of a nondigestable carbohydrate and describe its fate.
    • jejunum>ileum>duodenum
    • fiber, cellulose -> bulk facilitates the motility, cleaned out by MMC -> -> stool
  5. Describe the role of SGLT1, GLUT2, GLUT5 and the Na+-K+ pump in the absorption of monosaccharides across the apical and basolateral membranes. Discuss the specificity of each nutrient transporter for ions and monosaccharides.
    SGLT1: apical, cotransporter, transfer Na and glucose (and galactose) across membrane. Specific for Na. Glucose > galactose

    GLUT2: basolateral, facilitated diffusion, transfer glucose, galactose, fructose out of the cells. Glucose has the highest affinity. 6x higher than fructose.

    GLUT5: apical, facilitated diffusion, transfer fructose across membrane. Fructose>glucose

    Na/K pump: maintain the concentration gradient of Na, which is the driving force of SGLT1
  6. Discuss the general clinical symptoms of monosaccharide malabsorption.
    • sugar tolerance test: some sugar won't show glucose spile
    • brush border enzyme/transporter deficiency
    • osmatic diarrhea
    • abdominal pain
  7. Describe the causes and symptoms of the following disorders: lactose intolerance, fructose intolerance, glucose-galactose malabsorption syndrome. Relate their distribution among ethnic groups.
    • lactose intolerance
    • - lactase low or none
    • - lactose tolerance test has problem; abdominal pain; diarrhea; colicky

    • fructose intolerance
    • - excessive uptake of fructose
    • - may mimic other disorder; leaky gut, inflammation, type II diabetes, obesity

    • glucose-galactose malabsorption syndrome
    • - SGLT1 defect
    • - severe diarrhea and dehydration in infant; no glucosuria
  8. Carbohydrate Polymers: subcategories
    • Non Digestible
    • - fiber, cellulose - fruits and vegetable
    •     - bulk facilitates gut motility
    •     - bind bile acids - lowers chlosterol
    • - undigestable β-1,4 linkages - no intestinal enzymes to digest linkages
    •     - bacteria digest small amount

    • - Digestible
    •     - Starch polymers - plants
    •         - amylose - straight chain α-1,4 linkages
    •         - amylopectin - branched chain α-1,4 and α-1,6 linkages
    •         - digested two stages
  9. Starch: mixture of ________

    Salivary amylase breaks _______ to release ________.

    The enzyme activity is optimal at _____ pH and its activity is ______ by the acid pH of the stomach.
    amylase and amylopectin

    • the α-1,4 glycosidic linkages
    • maltose, maltotrioses and α-limit dextrins

    • neutral
    • inactivated
  10. α-amylase
    • - saliva and pancreas
    •     - 94% structurally identical
    • - hydrolyze only internal α-1,4 linkages
    •     - not terminal α-1,4 linkages nor α-1,6 linkages
    •     - further action at brush border

    • - mouth - 5% of total carbohydrates - short time exposure
    • - stomach - inside bolus 1hr, acid inactivated
    • - intestine - pancreatic enzymes - remaining
  11. Carbohydrates - Lumenal Digestion Products and Brush Border Enzymes (Hydrolases)
    1) α limit dextrins
    2) Maltose and Maltotrioses
    3) Disaccharides
    - Enzyme Activity Gradient
    - Final products
    • - contain α-1,6 branched chain
    • - α dextrinase (isomaltase) -> glucose

    - maltase (glucoamylase) -> glucose

    • - lactase: lactose (milk sugar) -> glucose and galactose
    • - sucrase: sucrose (cane sugar) -> glucose and fructose

    - upper jejunum > ileum >> colon (none)

    - glucose (80%), fructose, galactose
  12. Glucose Absorption
    - Na+ - dependent

    - SGLT (sodium glucose transporter) family: SGLT-1, the specific intestinal form in human; species conserved

    • - Transporter Distribution
    •     - localized to apical membrane - IDed w/ Ab
    •     - density gradient within small intestine: Jejunum (primary) > ileum > duodenum > colon (none)

    • - site density correlated with
    •     - enzyme activity
    •     - rate of Na absorption
    •     - both higher in jejunum
  13. Monosaccharide Absorption - Apical (Mucosal) Transporter
    • SGLT1
    • - Two negatively charged sites - highly specific for Na
    •     - substituting Li+, K+, choline+ -> lower glucose affinity and transport to 5%

    • - One uncharged receptor site - binds monosaccharide
    •     - highest affinity glucose
    •     - D-hexose preferred
    •     - L-hexose low affinity
    •     - competitively binds other monosaccharides (galactose) with lower affinity

    • - Inhibitors
    •     - ouabain: inhibits Na-K ATPase; reduces Na+ gradient and driving force for SGLT1
    •     - phlorhizin: large molecule - laboratory inhibitor
    •         - phloretin + glucose -> too large, does not translocate
  14. Monosaccharide Absorption - Basolateral Transport
    • - Facilitated diffusion - GLUT-2 - major pathway
    •     - confined to basolateral membrane
    •     - Na+-independent transport
    •     - Km glucose - 10mM (faster than w/ SGLT1)
    •     - Inhibitors
    •          - phloretin: non-competitive; mechanism unknown

    - Passive diffusion – minor pathway
  15. Fructose Transport - jejunum
    • Apical Membrane Absorption
    • - facilitated diffusion
    •     - GLUT5 - jejunum
    •         - 41% identical to GLUT2
    •         - NOT Na+-dependent
    •         - low glucose affinity

    • Basolateral Transport
    • - GLUT2
    •     - glucose > fructose (6-fold lower; Km 10mM vs 67mM)
  16. Abnormalities of Monosaccharide Transport
    • Common Characteristics
    • - abnormal diagnosis of oral sugar tolerance test: multiple sugars provided; sugar w/ problematic pathway won't show in blood
    • - brush border enzyme defects
    •     - confirmed by abnormal biopsy
  17. Lactose malabsorption
    - Normal conditions
    lactose absorption accompanied by ______.

    - Lactase deficient _______
    Lactose remains in intestinal lumen as an _______ that produces an _______.

    - Symptoms aggravated by bacteria which ________.
    osmotic absorption of water

    • brush border
    • osmotically active particle
    • osmotic diarrhea (water retains and moved to colon)

    ferment the sugar and produce irritating gases (CO2, H2), distention and pain

    H2 can be detected from the breath and can be used to diagnose.
  18. Lactose Intolerance
    - cause
    - test
    - congenital
    - age related
    • - brush border lactase absent or at low levels
    •     - Asians, Africans, American Indians
    •     - brush border lactase declines with age
    •     - more osmotically active molecules

    • - results of sugar tolerance test
    •     - glucose appears in blood
    •         - SGLT1 and GLUT2 - functional
    •     - no glucose in blood: lactase deficient

    • - congenital lactase deficiency: diagnosed in infancy
    •     - colicky babies
    •     - substitute special formulas with sucrose or fructose for milk

    • - age related intolerance
    •     - correlated with decrease in brush border lactase
    •     - avoiding milk -> calcium deficiency
    •         - substitute yogurt - intrinsic lactase activity
    •         - take oral enzymes
  19. Fructose Intolerance
    - cause
    - symptoms/misdiagose
    - neonates
    - corticosteroids
    • - high dietary fructose - sodas and other foods
    •     - excess fructose intake - osmotic force

    • - symptoms mimic other sugar disorders
    •     - misdiagnosed as irritable bowel syndrome

    • - neonates - GLUT-5 expression low but increases with age
    •     - considerable early exposure
    •         - sweetner in baby foods
    •         - aggressive marketing
    •     - early high exposure leads to
    •         - leaky gut - inflammation
    •         - Type 2 Diabetes and obesity

    • - corticosteriods - prescribed during preganncy to prevent premature delivery
    •     - may be contraindicated in some - can affect GLUT-5 expression
  20. Glucose-Galactose Malabsorption Syndrome
    • - rare, congenital mutation SGLT-1
    •     - cannot absorb glucose or galactose - water absorption reduced
    •     - osmotic secretion of water
    •     - no glycosuria (kidney SGLT2 – functional)

    • - infants
    •     - develop severe diarrhea and dehydartion - fatal
    •     - sugar substitution - fructose absorptiion normal
Author:
akhan
ID:
315904
Card Set:
Physiology - GI - DIGESTION AND ABSORPTION - Carbohydrates
Updated:
2016-02-14 20:28:30
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Description:
Physiology - GI - DIGESTION AND ABSORPTION - Carbohydrates
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