Unit VI- Intestinal Transport II

Question 1

Digestive enzymes

Answer 1

-salivary: amylase (CHO), lingual lipase (fat)

-stomach: gastric chief cells secrete pepsinogen which forms pepsin at low pH. Pepsin is an endopeptidase for aromatic L-amino acids; has a pH optimum of 1-3; inactive when denatured at pH > 5 in alkaline pancreatic juice in the intestine
gastric lipase (fat)

-pancreas: amylase (CHO); endopeptidase (Trypsin, Chymotrysin, Elastase); Exopeptidase (Carboxypeptidase); Lipase/Colipase- fat; Phospholipase A2- phosopholipid; Cholesterol esterase

-Intestinal extoenzymes: membrane-bound in brush border; catalytic site faces lumen
enterokinase- activates trypsin; disaccharidases (maltase, sucrase, lactase, trehalase, isomaltase); peptidases (aminooligopeptidase, Dipeptidase)

Question 2

Absorption of amino acids

Answer 2

-proteins are digested by pancreatic endo-peptidases (trypsin and chymotrypsin) and by exopeptidases (carboxypeptidases) and by several other proteases
-enterokinase is the trigger for intestinal protein digestion; enterokinase converts trypsinogen released by the pancreas into trypsin
-enterokinase activates trypsinogen to trypsin
-trypsin then activates endo- and exo- peptidases, yielding amino acids, dipeptides, and tripeptides:
trypsinogen -> trypsin
chymotrypsinogen -> chymotrypsin
proelastase -> elastase
procarboxypeptidase A -> carboxypeptidase A
procarboxypeptidase B -> carboxypeptidase B

Question 3

Protein Digestion and Absorption of Peptides and Amino Acids

Answer 3

• luminal digestion yields 40% free amino acids and 60% peptides consisting of primarly of 2 to 6 amino acid residues
• usually only amino acids or di, tri peptides are absorbed via carrier mediated transporters
• exceptions include newborn infants absorbing globulins and other whole proteins, and patients with certain food allergies in which whole proteins can also be absorbed, possibly by pinocytosis at the base of microvilli
• in addition the major absorptive processes shown above, small amounts of certain small peptides can be absorbed

Question 4

Action of luminal, brush border, and cytosolic peptidases

Answer 4

• pepsin from the stomach and the five pancreatic proteases hydrolyze proteins- both dietary and endogenous to single amino acids or to oligopeptides
• these reactions occur in the lumen of the stomach or small intestine
• various peptidases at the brush borders of enterocytes then progressively hydrolyze oligopeptides to amino acids
• the amino acids are directly taken up by any of several transporters
• the enterocyte directly absorbs some of the small oligopeptides through the action of the H+/oligopeptide cotransporter
• these small peptides are digested to amino acids by peptidases in the cytoplasm of the enterocyte
• several Na+ independent amino acid transporter move amino acids out of the cell across the basolateral membrane

Question 5

Sterospecific Absorption of L-Amino Acids

Answer 5

• amino acid transport is sterospecific with L-isomers preferentially absorbed over D- isomers
• transcellular concentrative uptake of L-amino acids against their concentration gradients

Question 6

Na-Coupled Amino Acid Transport

Answer 6

• must amino acids requires an inward sodium concentration gradient for concentrative uptake
• Na drives amino acids in, like a coupled reaction 100 out and 10 in when aa out is 1 but still goes in even though there 10 aa in already

Question 7

Transport Kinetics

Answer 7

• transport is determined by two parameter- VMAX and KM
• VMAX is determined by the numbers of transporters in the membrane and by turnover time of a single transporter
• saturation kinetics implies a limited number of transport sites
• free diffusion would be linear, and would not show a plateau

Question 8

Carrier Systems for Amino Acid Transporter

Answer 8

Brush Border Membrane:
All neutral aromatic and aliphatic AA- Na gradient dependent
Phenylalanine and methionine- Na gradient dependent
Glutamate, asparate- Na gradient
Proline, hydroxyproline- Na gradient
Basic AA- not Na gradient
Neutral AA with hydrophobic side chain- not Na

Basolateral Membrane
Small neutral AA- Na gradient
Three and four carbon neutral AA- Na gradient
Neutral AA with hydrophobic side chains- No
Basic AA- no

Question 9

Absorption of oligopeptides

Answer 9

• H+/oligopeptide cotransporter PepT1 moves dipeptides, tripeptides, and tetrapeptides into the enterocyte across the apical membrane
• peptidases in the cytoplasm hydrolyze the oligopeptides into their constituent amino acids, which then exit across the basolateral membrane through one of three Na+-independent amino acid transporters
• if glycine is present in the lumen only as a free amino acid then the enterocyte absorbs it only through amino acid transporters. However if the same amount of glycine is present in the lumen in the form of the dipeptide glycylglycine, the rate of appearance of glycine in the blood is about twice as high
• PepT1 which moves several amino acid monomers for each turnover of the transporter is an effective mechanism for absorbing amino acids

Question 10

Hartnup disease

Answer 10

• the system B apical membrane amino acid transporter is defective
• as a reslt the absorption of neutral amino acids such as L-phenylanine is reduce
• the defective transporter controls the absorption of neutral amino acids not only in the intestine but also in the kidneys
• excessive amounts of tryptophan are excreted in the urine
• tryptophan is a precursor for serotonin, melatonin, and niacin

Question 11

Cystinuria

Answer 11

• the system B apical membrane amino acid transporter is defective
• as a result, the absorption of L-cystine and basic amino acids is reduced
• in the kidney, the affected patient has inadequate reabsorption of cystine which form kidney stones

Question 12

Digestion and Absorption of Sugars

Answer 12

-brush border ectoenzymes convert maltose (Glc-Glc), lactose (Glc-Gal), and sucrose (Glc-frc) to monosaccharides

carbohydrates are absorbed as simple sugars

• Glc and Gal compete for the same Na+ coupled carrier SGLT1
• Frc is independent of Na+ and cannot be concentrated in the cell GLUT5
• once the monosaccharides are inside the enterocyte- GLUT2 mediates their efflux across the basolateral membrane into the intestinal space

brush border enzymes: lactase specifically breaks lactose into glucose and galactose. Maltase, sucrase and isomaltase are 3 addition enzymes that participate in breaking up oligosaccharides into monosaccharides

Question 13

Digestion of Starch

Answer 13

• ingested carbohydrate is 60% starch, 30% sucrose and 10% lactose
• salivary amylase (Ptyalin) begins the conversion of starch to sugars
• Ptyalin has a pH optimum of 6.7 and is inactivated in the stomach
• most starch is broken down in the intestine by pancreatic amylase

Question 14

Undigested Fiber

Answer 14

• human GI tract has no cellulase for digesting cellulose or hemi-cellose, thus accounting for undigested fiber in the diet
• fiber maintains the consistency of the stool

Question 15

Lactase Deficiency

Answer 15

• lack of lactase in the adult in cause of lactose intolerance
• lactose is osmotically active, resulting in osmotic diarrhea
• lactaid is milk containing lactas
• lactose tolerance test- give oral load of lactose, measure blood glucose over time
• without lactase, the lactose in milk remains un-hydrolyzed and unabsorbed
• bacteria in the gut adapt to the relative abundance of lactose and switch to metobolizing lactose and produce copious amounts of gas by fermentation-stomach cramps and bloating
• lactose raises the osmotic pressure of the colon contents, preventing the colon from reabsorbing water and hence causing a laxative effect

Question 16

Effects of lactase deficiency on glucose in plasma and H2 in breath

Answer 16

• in an individual with normal lactase activity, blood glucose levels rise after ingestion of either glucose or lactose- H2 in breath is low
• in an adult with low lactase activity, the rise in blood levels is less pronounced after ingesting lactose
• because the rise is normal after ingesting glucose we can tell that lactase is the problem
• lactase deficiency has large amounts of H2 in breath- product of catabolism by colonic bacteria

Question 17

Glucose-Galactose Malabsorption

Answer 17

• rare genetic disease in which Glc/Gal carriers (SGLT1) are missing are defective (2 copies)
• accumulation of Glc in intestine causes diarrhea, dehydration and death
• parient dies soon after birth unless diarrhea is diagnosed properly
• restricted diet of fructose which is the only monosaccharide that can be absorbed

Question 18

Fats

Answer 18

-include neutral fat (triglycerides)
phospholipids
cholesterol
fatty acids
waxes of ingested plant cell walls

Question 19

Presence of fats in duodedum

Answer 19

• release of GIP which decreases gastric acid secretion
• CCK
• slows gastric motility and emptying
• stimulate pancreatic enzyme secretion
• stimulates intestinal fluid secretion
• stimulates gallbladder contraction
• relaxes sphincter of Oddi

Question 20

Digestion of Fats

Answer 20

• starts with lingual lipase-water soluble
• continues with gastric lipase and food bearing lipase with acidic pH optima
• digestion occurs mostly in the jejunum and is completed by the mid jejunum via pancreatic phospholipase A2, cholesterol esterase, pancreastic lipase
• colipase anchors lipase to micelles and provides access to neutral triglycerides inside micells
• pancreatic lipase hydrolyzes only at 1 and 3 position of triglycerides: triglyceride -> 2-monoglyceride + FFA
• some 2-monoglycerides isomerize to 1- or 3- monoglycerides which are then converted by lipase to glycerol
• glycerol is absorbed by free diffusion, and some nonsterified glycerol passes across the basolateral membrane into the portal blood

Question 21

Fat Globules and Micelles

Answer 21

• micelles include: long chain fatty acids, cholesterol, monoglycerides, phospholipids, bile salts, fat soluble vitamins
• fats separate from aqueous phase of chyme as globules about 1 micron in diameter
• amphipathic bile salts (and fatty acids and phospholipids) emulsify fat globules into smaller micells 30-100 A in diameter
• micelles have negative surface charge,mutual charge repulsion stabilizes the emulsion
• large surface area of micells make digestion more efficient

Question 22

The breakdown of emulsion droplets to mixed micelles

Answer 22

• the core of the emulsion droplet contains TAGs, diacylglycerols, and cholesteryl esters
• on the surface are fatty acids, MAGs, lysolecithins and cholesterol
• absorbed to the surface are pancreatic lipase and possibly bile salts
• as the lipases hydrolyze the TAGs at the surface, the TAGs from the core replace them causing the droplet to shrink
• a multilamellar liquid crystalline layer of fatty acids, MAGs, lysolectithins, cholesterol, and bile salts builds up on the surface of the emulsion droplet and causes a small piece to bud off as a vesicle
• addition of more bile salts to the multilamellar vesicles thins out the lipid coating and converts the multilamellar vesicle to a unilamellar vesicle
• further addition of bile salts leads to formation of a mixed micelle, in which hydrophobic lipid tails face inward and polar head groups face outward

Question 23

Absorption of Neutral Fat

Answer 23

• fatty acid tranport proteins facilitate uptake of long-chain fatty acids
• Niemann_Pick C1 like 1 (NPC1L1) is required for cholesterol absorption
• mixed micelles carry the major part of all lipids that are absorbed in intestine
• when the lipidds of the mixed micelle have diffused into the enterocyte, emulsifying more hydrolysed lipids recycles the empty bile micelle
• neither bile salts micelles nore bile salt molecule diffuse into the enterocyte
• short chain fatty acids hydrophilic-enterocyte transfers them directly to portal blood

Question 24

Long chain fatty acids

Answer 24

• fatty acid transport proteins FATP such as FATP4 are present at brush border
• enter the enterocyte, bind to cytosolic protein called fatty acid binding protein
• fatty acid binding protein minimizes reflux back into the intestinal lumen and ensures transfer of fatty acids up to the smooth endoplasmic reticulum of enterocytes for reesterfication
• in addition the enterocyte re-estefies absorbed monoglycerides, lisophospholipids, and cholesterol and assembles the products with specific apoproteins into emulsion-like particles called chylomicrons

Question 25

Absorption of cholesterol

Answer 25

• Niemann-Pick C1-like NPC1L1 serves as a transporter and is required for cholesterol uptake by the enterocyte
• the cholesterol-lowering drug Ezetimibe works by inhibiting this transporter

Question 26

Bile salt absorption

Answer 26

• bile salts are not absorbed in the jejunum but rather in the ileum where they enter to enterohepatic circulation and travel back to the liver where they are re-secreted
• the bile salts can be re-used several times in the course of a single meal

Question 27

Formation of Chylomicrons

Answer 27

• long chain fatty acids and other products of lipid digestion are converted back to triglycerides, phospholipids and esters of cholesterol in the SER
• fat droplets form in the cisternae of the SER
• apoproteins are synthesized in the RER and then transfered to the SER, where the apoproteins associate with lipid droplets
• nascent chylomicrons and VLDLs arrive at the cis face of the Golgi apparatus. Here apoproteins are glycosylated
• vesicles carrying chylomicrons or VLDLs bud off from the trans Golgi apparatus and move to the basolateral membrane in transport vesicles
• transport vesicles fuse with the basolateral membrane, release chylomicrons or VDLDs
• chlyomicrons and VLDLs pass through large interendothelial channels of lymphatic capillaries and enter the lymph
• plasma is milky following a fatty meal because of the chylomicrons

Question 28

Re-esterification of digested lipids by the enterocyte and the formation and secretion of chylomicrons

Answer 28

• the enterocyte takes up short and medium chain fatty acids and glycerol and passes the unchanged into the blood capillaries
• the enterocyte also takes up long chain fatty acids and 2-MAG and resynthesizes them into TAG in the SER
• the enterocyte also processes cholesterol into cholesteryl esters and lysolecithin into lecithin