Digestion and absorption of carbohydrates

Question 1

salivary alpha amylase

Answer 1

endoglucosidase cleaves alpha 1,4 glucosidic bonds. Acts on dietary glycogen, amylose, amylopectin

Question 2

secretin

Answer 2

lumen of duodenum. enzymes release pancreatic juice (high in HCO3-) and neutralizes acidic chyme from stomach

Question 3

CCK

Answer 3

stimulates release of pancreatic alpha amylase, which cleaces alpha 1,4 glucosidic bonds and digests oligosacharrides.
products: maltose, glucose, limit dextrin

Question 4

dissacharidases

Answer 4

mucosal brush border as integral membrane proteins, active site in lumen of intestine, they are embedded in glycocalyx
products: 2 glucose from maltose and isomaltose, fructose and glucose from sucrose and galactose and glucose from lactose
isomaltose and sucrose are both hydrolyzed from same two headed enzyme

Question 5

goblet cells

Answer 5

secret mucin to protect dissacharridases

Question 6

dextrinase

Answer 6

embedded in glycocalyx, cleaves alpha 1,6 bonds of limit dextrin

Question 7

describe absorption in upper jejunum.

Answer 7

passive diffusion of pentoses

three intestinal sugar transporters: SGLT1, GLUT 5 and GLUT2

Question 8

SGLT1

Answer 8

transports glucose and galactose from lumen into intestinal mucosal cell
brush border of intestine and kidney
high affinity for glucose
cotransport Na+ and D-glucose or D galactose with energy from Na+ gradient
intestinal absorption and renal reabsorption
inhibited by phlorizin and ouabain

Question 9

GLUT5

Answer 9

fructose into intestinal cell
on brush border
not inhibited by phlorizin or cytochalasin B

Question 10

GLUT2

Answer 10

3 hexoses (glucose, galactose, fructose) from intestinal cytosol into portal circulation
basolateral membrane
high capacity and low affinity for glucose
not saturated and physiological levels
inhibited by cytochalasin B

enriched in hepatocytes, pancreatic beta cells, kidney and basolateral membrane of intestinal cells
low affinity and high capacity for glucose transport
not stimulated by insulin
not dependent on sodium
rate depends on gradient of free glucose, allowing pancreatic beta cells to sense changes in blood glucose levels
liver takes up glucose in fed state and releases in fasted state

Question 11

GLUT 4

Answer 11

enriched in fat cells, skeletal muscle, heart muscle
stimulated by insulin
surplus of transporters -> trans-Golgi
insulin stimulates recruitment of spare transporters from ER and Golgi into plasma membrane. Glucose rapidly enters tissues in periods of elevated blood glucose

Question 12

GLUT1

Answer 12

enriched in red blood cells and endothelial cells of blood brain barrier, not stimulated by insulin
has a high affinity and high capacity for glucose transport
saturated at physiological glucose concentrations, ensuring constant supply of glucose to those tissues

Question 13

GLUT 3

Answer 13

enriched in neurons
highest affinity and rate of transport for glucose of all GLUT isozymes, ensuring supply of energy to brain at low blood glucose levels
not stimulated by insulin

Question 14

Lactose intolerance

Answer 14

lactase deficiency common, treatment: dietary limitation

consequence: bacterial enzymes hydrolyze the disaccharide to LMW acids and hydrogen gas -> painful intestinal distention, watery diarrhea, impaired intestinal absorption
diagnosis: direct assay, lactose load the blood glucose test, hydrogen breath test, look at symptoms

Question 15

Sprue

Answer 15

diarrhea, anorexia and weight loss, intestinal morphology abnormal, absorption of nutrients impaired. some form of sprue induced by gluten (wheat, rye, oats, and barley). Treatment: removal from diet

Question 16

Aldol reductase

Answer 16

reduces glucose to sorbitol (many tissues, including retina, lens, kidneys and nerves) high Km for glucose, production of sorbitol most significant when glucose concentration increases above normal as in diabetes. GLUT2 expressed in these tissues. Cellular uptake is glucose dependent on plasma glucose concetration. Hyperglycemia in untreated diabetics -> increased glucose uptake into thses cells -> increased sorbital -> strong osmotic effects -> tissue damage (cataracts, blindness, nephropathy, neuropathy)