organ systems: carbohydrate and protein digestion, pancreatic secretion

[image courtesy of erica newon zelfand]

this unit focused on pancreatic secretions and their role in carbohydrate and protein digestion in the intestine. each day, the pancreas secretes 1L of pancreatic enzymes, among which are enzymes that break down carbohydrates into monosaccharides, enzymes that break down proteins into peptides, lipases which break down triacylglycerols into fatty acids. these digestive enzymes are secreted by the "acinar" cells of the pancreatic ducts, while water and electrolytes such as bicarbonate are secreted by duct cells (a similar strategy to salivary and gastric secretion). the bicarbonate from duct cells comes from carbonic acid, which is formed in duct cells by carbonic anhydrase. bicarbonate is transported into the duct lumen in exchange for a chloride ion by the CFTR transporter. the acinar cells can be stimulated to produce more secretion either directly by vagal stimulation or by different neuropeptides such as CCK, GRP, SubP, VIP. duct cells are stimulated to increase water and bicarbonate production mainly by secretin (which functions to reduce acidity in the intestine and as such also inhibits gastric secretion and emptying).

as with gastric secretion, pancreatic secretions can be divided into three phases (refer to comparison chart), cephalic, gastric, and intestinal. in the cephalic phase, thoughts or sensation of food causes increase of pancreatic secretion via vagal stimulation. in the gastric phase, gastric distention triggers vagal stimulation of pancreatic secretion. the most important phase for pancreatic secretion is the intestinal phase, where intestinal distention, high acid levels, or other digestive products trigger hormonal release that modify pancreatic secretion. CCK is released from I cells in response to fat and proteins, which stimulates acinar cells to increase enzymatic secretion. secretin is released from S cells in response to acid and fat, which stimulates bicarbonate and water secretion from pancreatic duct cells.

carbohydrate digestion, already initiated with salivary alpha amylase, continues with the secretion of pancreatic alpha amylase into the intestinal lumen. polysaccharides are cleaved into smaller di and tri saccharides, and then further digested by the brush border enzymes (see the biochem chapter for a much more detailed description). different sugars are absorbed into the enterocyte and bloodstream by different transporters: for example, glucose and galactose are absorbed into the enterocyte by SGLT1, a sodium co-transporter, while fructose is facilitatively absorbed by GLUT 5. the pancreas also secretes enzymes that digest proteins, all of which are activated by enterokinase, which activates trypsinogen into trypsin, which activates the other zymogens procarboxypeptidase, chymotrypsinogen, proelastase (see the biochem protein digestion chapter).

an interesting note about the role of tight junctions: most absorption of nutrients occurs through the membrane of the enterocyte, or transcellularly. however, in the case of macromolecules that are too large to be absorbed transcellularly, paracellular transport can occur via modulation of the tight junctions between enterocytes, which can occur reversibly via the molecule zonulin. paracellular transport can also occur in "leaky gut", hyperpermeability of the tight junctions, caused by excess glucose levels, alcohol abuse, NSAID/steroidal use,food allergies, crohn's disease. there is a distinct but underexplored relationship between tight junctino permeability and autoimmune disease; people with dysregulation of tight junction permeability have a higher susceptibility for autoimmune diseases.

questions
1. enzymes digest carbohydrates, fats, proteins by what type of reaction?
2. what is SGLT1 and what does it do?

describe the absorption of these substances from the intestinal lumen into the enterocyte, and from there into the blood stream:
3. glucose
4. lactose
5. fructose
6. glycogen
7. sodium

tight junctions...
8. describe the absorptive pathway from the intestinal lumen to the mucosal capillary.
9. what is a function of tight junctions in intestinal epithelium that is not commonly discussed?
10. what is the relationship of tight junctions and the immune system?
11. what are tight junctions made of?
12. what is "leaky gut" and what are some factors that cause it?
13. what is zonulin and what does it do?
14. what is lactulose and how is related to leaky gut?

pancreas...
15. where is the pancreas?
16. how much liquid does the pancreas secrete per day?
17. what do the acinar and duct cells secrete?
18. what are the electrolytes that are secreted by duct cells?
19. describe the secretion of bicarbonate by duct cells.
20. what are the enzymes that are secreted by the pancreas that digest proteins?
21. how are the protein digestion enzymes activated?
22. what does pancreatic amylase do?
23. what are the pancreatic enzymes that digest lipids?
24. what are the different ways in which pancreatic acinar cells are stimulated?
25. what are the actions of secretin?

pancreatic phases...
26. what are the three phases of pancreatic secretion?
27. describe the cephalic phase of pancreatic secretion.
28. describe the gastric phase of pancreatic secretion.
29. describe the intestinal phase of pancreatic secretion.
30. describe the secretion and actions of CCK.
31. describe the secretion and actions of secretin.
32. how does enzymatic secretion adapt to different dietary compositions?

carbohydrate digestion...
33. what are the approximate proportions of the different types of carbohydrates ingested?
34. pancreatic and salivary amylase digests starch into...
35. how are di and trisaccharides digested into monosaccharides?
36. what are the two ways that glucose and galactose can be transported into the intestinal epithelium?
37. compare the absorption of glucose and fructose.
38. describe the negative feedback that can occur in carbohydrate digestion.
39. how can malfunctioning carbohydrate digestion cause diarrhea?
40. what is the hydrogen breath test?
41. what effect does celiac disease has on the intestinal lining?

protein digestion and absorption...
42. how are proteins digested in the stomach?
43. how are proteins digested in the intestine?
44. how are polypeptides digested and absorbed in the intestine?

pancreatic tests...
45. what is the stool chymotrypsin test?
46. what is the pancreatic elastase test and why might it be more reliable than the stool chymotrypsin test?


answers
1. hydrolysis reactions.
2. it is a sodium / glucose co-transporter, facilitating absorption of glucose, galactose

3. glucose is transported into the enterocyte via SGLT1, then into the blood via GLUT2.
4. lactose is broken down into glucose and galactose via the brush border enzyme lactase, and glucose and galactose are absorbed and transported in the same way as question 3.
5. fructose is transported into enterocytes via GLUT5, and absorbed into blood via GLUT2.
6. glycogen is broken down into oligsaccharides and alpha limit dextrins by alpha amylase, and then broken down further into monosaccharides by brush border enzymes and absorbed the same way as in question 3.
7. sodium is transported along with glucose or galactose via the SGLT1 co-transporter.

8. intestinal lumen -> unstirred layer of fluid -> glycocalxces on microvilli of enterocytes -> cell membranes and cytoplasm -> basement membrane -> capillary
9. tight junctions can facilitate absorption of macromolecules, nutrients that are too big to be absorbed directly into enterocytes.
10. when tight junction regulation of macromolecule absorption is dysregulated, this can sometimes lead to intestinal and extraintestinal autoimmune disorders.
11. occludins, claudin family of proteins, junctional adhesion molecules.
12. hyperpermeability of the intestinal epithelium. caused by alcohol abuse, high sugar intake, food allergies, NSAIDS/steroid drugs, celiac disease and crohn's disease.
13. a molecules that facilitates the permeability of tight junctions and as such is involved in the absorption of fluid and macromolecules across the intestinal barrier, and also protect the intestine from being colonized by microorganisms.
14. a macromolecule that can only be absorbed paracellularly; its presence in the blood as compared to mannose (which is absorbed transcellularly) is a good indicator of leaky gut.

15. the body of the pancreas lies deep to the stomach and the tail extends to the spleen.
16. 1L per day
17. acinar cells secrete enzymes, duct cells secrete water and electrolytes.
18. bicarbonate and sodium.
19. bicarbonate is formed from carbonic acid which is formed by carbonic anhydrase and CO2. H+ is absorbed into blood, and bicarbonate is transported into the lumen by the transmembrane regulator CFTR (cystic fibrosis transmembrane regulator) which exchanges a chloride ion for a bicarbonate ion.
20. trypsinogen, proelastase, chymotrypsinogen, procarboxypeptidase.
21. trypsinogen is activated by the brush border protease enterokinase, forming trypsin, which activates the other enzymes.
22. an endoglucosidase that hydrolyzes carbohydrates into di and tri-saccharides.
23. pancreatic lipase, cholesterol esterase, phospholipase.
24. directly via the vagus nerve, or indirectly via stimulation of the vagus nerve by CCK, VIP, GRP, SubP.
25. stimulates duct cells to release bicarbonate, inhibits gastric secretion and emptying- overall effect is to raise pH.

26. cephalic, gastric, intestinal
27. stimulated by thoughts or sensation of food, vagus nerve stimulates pancreas secretion.
28. gastric distention causes vagal stimulation, which causes pancreas secretion.
29. the most important phase for pancreatic secretion; digestive products or low pH trigger release of hormones that control secretion.
30. CCK is secreted from I cells in the intestinal epithelium in response to proteins or fats. chief among its many effects is to stimulate pancreatic acinar secretion of enzymes.
31. secretin is released from S cells in the intestine in response to acid or fat-- its main effect is to increase water and bicarbonate secretion by pancreatic duct cells, thereby raising the pH of the intestinal lumen. it also decreases gastric motility and emptying.
32. CCK can up or downregulate the protein digesting proteases and the carbohydrate digesting amylases depending on the ratio of protein to carbohydrates in the diet.

33. 50% starch, 20% sucrose, 6% lactose, 1-2% maltose.
34. di and trisaccharides.
35. by the brush border enzymes.
36. transcellularly via the SGLT1 Na+ cotransporter or paracellularly with water with high glucose concentrations (see section on tight junctions)
37. fructose absorption is slower and uses a facilitated transporter rather than a co-transporter. (see question 5)
38. chemo and osmotic receptors sense high glucose levels in the duodenum and jejunum and trigger the vasovagal reflex, which decreases gastric motility and emptying.
39. in the case of lactose intolerance, lactose remains undigested and cause a hyperosmolar intestinal lumen from excess sugar as well as products from bacterial processing of these sugars; causing water to accumulate in the lumen and cause diarrhea.
40. a test which measures amount of hydrogen in breath, which is produced from bacteria that metabolize lactose in the intestine in lactose intolerant people.
41. destroys villi.

42. gastric pepsinogen is secreted and converted to pepsin by the low pH environment of the stomach. it cleaves proteins into smaller polypeptides.
43. in the intestine, proteins are digested by different pancreatic enzymes: trypsin, chymotrypsin, carboxypeptidase, elastase. trypsin activates the other three enzymes, which cleave large polypeptides into small polypeptides.
44. small polypeptides are cleaved further into smaller polypeptides and amino acids, which are then transported into the enterocyte and then absorbed into the blood by carrier proteins.

45. a marker for pancreatic output; normal is >9U/g stool, low is <>200 mcg/g stool, low is <100>
46. another test for pancreatic output, which might be more accurate because elastase is only produced by human pancreatic cells. normal is >200 mcg/g stool, low is <100>

biochem: mark's medical biochem chapter 27: digestion and transport of carbohydrates

this chapter covers several aspects of carbohydrate digestion. first it goes over some basics about carbohydrates and digestion in general, then talks about the specific enzymes at work in the brush border of the small intestine, then a brief section about lactose intolerance, and finally, a section about the transport of glucose from the intestine into the blood.

the normal american diet is made of 40-50% carbohydrates, and of this, 50-60% is made of the starch molecules (10,000 to 1 million glucosyl units long) amylose and amylopectin. amylose consists of glucose molecules bonded with alpha 1-4 bonds, while amylopectin is the same, but with alpha 1-6 bonds (and therefore branches) as well. digestion of these starches begins in the mouth with alpha amylase, which is an example of an endoglucosidase-- an enzyme which cleaves alpha 1-4 bonds at random intervals. this initial digestion leaves chunks of polysaccharides called alpha-dextrins. stomach acid deactivates the amylase, and carbohydrate digestion continues in the small intestine.

in the small intestine, carbohydrate digestion takes place mainly on the "brush border" of the intestinal mucosa, which have enzymes that are embedded in the intestinal membrane that poke into the intestinal lumen. the four major "glucosidases" are glucoamylase, trehalase, beta-glucosidase, and sucrase-isomaltase. each of these enzymes has a specific structure and catalytic sites that are specific to certain types of carbohydrates. glucoamylase breaks down alpha 1-4 bonds from the non reducing end ("tail end") of the sugar, until isomaltase remains-- isomaltase is basically just two sugar units branched together in an alpha 1-6 bond. the action of glucoamylase on polysaccharides seems similar to the action of glycogen phosphorylase in the breakdown of glycogen in that both are removing glucose residues one at a time from the tail end, and both can not remove the last glucose unit (or in glucose phosphorylase's case, the last 4).

sucrase-isomaltase is another brush border glucosidase with two catalytic sites. one is specific to isomaltose and maltose (and thus can break down the isomaltose from glucoamylase's activity) and one is specific to sucrose and maltose. beta glucosidase also has two catalytic sites. one is specific to breaking the beta 1-4 bond between glucose and galactose in lactose, and the other is specific to cleaving beta 1-4 bonds in glycolipids. the last glucosidase in the brush border is trehalase, which has only one catalytic site which is specific to trehalose, a sugar found only in some insects, mushrooms, and algae. these four enzymes work in tandem to break down the different types of carbohydrates that are dumped into the duodenum. the relative concentration of these enzymes change depending on the location in the gut. for example, pancreatic alpha amylase is secreted mainly in the duodenum, sucrase-maltase and beta-glucosidase is mainly in the jejunum, and glucoamylases is most common in the ileum.

a couple other notes about carbohydrate digestion. lactose intolerance occurs with either a lactase deficiency or intestinal damage. most adults only have 10% of the lactase activity that they had as children. lactose intolerance can ultimately lead to malabsorption of nutrients: lactose is undigested in the gut, and is instead metabolized by the bacterial flora in the colon, producing gases and lactic acid. the lactic acid can increase the intestinal lumen's osmolarity and cause water to distend the abdomen, which increases peristalsis and potentially causes diarrhea and malabsorption of other nutrients.


the last section of the chapter focused on the membrane channels that transport glucose across the intestinal lining into the blood. there are two types of such transport proteins, regular facilitated glucose channels and Na+ facilitated channels. Na+ facilitated channels use an ATPase Na+/K+ pump to establish a low concentration of Na+ inside the intestinal cell. the concentration gradient that is formed from the higher Na+ concentration in the intestinal lumen is then coupled with glucose transport in these membrane proteins. facilitative glucose transporters (also called "GLUT" transporters) do not require ATP and simply allow glucose to flow down its concentration gradient from the intestinal lumen, into the epithelium, and out into the serosa side ("blood side").

a few interesting notes relating to these glucose transporting membrane proteins. the liver's GLUT transporters have a higher than usual Km (the concentration at which the substrate is half-saturated with enzyme-- generally representing the enzyme's affinity for substrate and in this case the GLUT for glucose) for glucose, because it will only accept glucose into its cells (to be converted into glycogen for storage) when the blood glucose concentration is very high, like after a high carb meal. also, in muscle and fat cells, insulin stimulates glucose absorption by means of recruiting intracellular vesicles of glucose transport proteins to the cell membrane, where they can facilitate glucose transport into the cell.

questions
1. what percentage of a normal american diet consists of carbohydrates?
2. what percentage of the carbohydrate calories consists of amylose and amylopectin?
3. how many glucosyl units do amylose and amylopectin have?
4. what types of bonds do amylose and amylopectin have?
5. what are the major natural sweeteners found in fruit, honey, and vegetables?
6. what is the major dietary carb found from animal sources?
7. how much liquid do the salivary glands secrete per day?
8. what is an "endoglucosidase" and what is an example of one?
9. what is salivary amylase inactivated by?
10. how much digestive enzyme is secreted by the pancreas per day?
11. what are in the pancreatic secretions?
12. what are oligosaccharides?

13. what are the glucosidases found in the brush border of the small intestine?
14. describe the digestion of lactose and sucrose in the small intestine.
15. describe the structure and activity of glucoamylase?
16. what are alpha-dextrins vs. limit-dextrins?
17. describe the structure and activity of the sucrase-isomaltase complex.
18. what percentage of maltase activity can be attributed to the sucrose-isomaltose complex?
19. describe the structure and activity of trehalase.
20. describe the structure and activity of the beta-glucosidase complex
21. pancreatic alpha-amylase activity is highest in...
22. sucrase-isomaltase activity is highest in...
23. beta-glucosidase activity is highest in...
24. glucoamylase activity is highest in...

25. what type of carbohydrates enter the colon?
26. what are the fatty acids that result from bacterial starch digestion in the colon?
27. what are the gases that result from bacterial starch digestion in the colon?

28. lactose intolerance can be caused by...
29. what are normal lactase levels of an adult as compared to a child?
30. what happens when lactose is ingested by a lactose intolerant person?

31. what does the glycemic index represent?
32. which sugars have the highest glycemic index?
33. what are the two types of glucose transport proteins?
34. describe the mechanism of the Na+ dependent glucose transporter.
35. what are facilitative glucose transporters?
36. compare the digestion of glucose with that of galactose and fructose.
37. in body tissues, why is glucose transport across membranes not the rate limiting step of glucose metabolism?
38. how does the high Km of glucose transport proteins in the liver relate to the liver's blood glucose regulation?
39. how is insulin related to glucose transport proteins in the liver?


answers
1. 40-45%
2. 50-60%
3. 10,000 to 1 million
4. amylose has alpha 1-4 bonds between glucosyl residues. amylopectin has alpha 1-4 bonds between glucosyl units as well as alpha 1-6 bonds between branches.
5. fructose, sucrose, glucose.
6. lactose, which is made of glucose and galactose.
7. ~1 liter a day
8. an enzyme that breaks internal alpha 1-4 bonds in a polysaccharide at random intervals, such as amylase.
9. acidity of the stomach
10. ~1.5 liters a day
11. trypsinogen, chymotrypsinogen, carboxypeptidase (for digestion of proteins), alpha-amylase (for carbohydrates), lipase (fat), and bicarbonate (neutralizing gastric acidity)
12. 4-9 glucosyl units long, contain one or more alpha-1,6 branches.

13. beta-glucoamylase, sucrase-isomaltase, beta-glycosidase, trehalase.
14. converted to monosaccharides by glucosidases attached to the brush border lining.
15. it has two domains that have different substrate specificity, and acts as an exoglucosidase by breaking alpha 1-4 bonds on the non reducing ("tail end") of the saccharides, releasing glucose units until only isomaltose remains.
16. alpha dextrins are the pieces of polysaccharides that result from salivary alpha-amylase's endoglucosidase activity. limit dextrins are oligosaccharides that have been formed from the further breakdown of polysaccharides by pancreatic alpha-amylase.
17. made of two subunits: sucrose-maltose subunit cleaves alpha 1-4 bonds in sucrose, maltose. isomaltose-maltose unit cleaves alpha 1-6 bonds in isomaltose, and also breaks down maltose.
18. 80%
19. trehalose is a smaller dissaccharidase that only has one catalytic site with specificity for trehalose, which is a relatively rare source of carbohydrate found in some insects, algae, and mushrooms.
20. has two catalytic sites: glucosyl-ceramidase site, which cleaves beta bonds in glycolipids, and lactase site, which breaks beta 1-4 bonds between glucose and galactose in lactose.
21. duodenum
22. jejunum
23. jejunum
24. ileum

25. any undigested starches: starches high in amylose, poorly hydrated starches (like in dried beans), dietary fiber.
26. acetic acid, propionic acid, butyric acid. 2,3,4 carbon.
27. hydrogen gas, CO2, methane

28. low lactase levels or intestinal injury
29. 10% of the level of a child
30. lactose is undigested by the lactase in the small intestine and is therefore metabolized by the colonic bacteria, which produces lactic acid, methane, and H2. the increased lactic acid increases osmolarity of the intestinal lumen, causing more water to be dumped into the lumen, causing excess peristalsis, causing malabsorption of other nutrients.

31. how quickly blood glucose levels rise after consumption of a food.
32. glucose and maltose.
33. sodium dependent glucose transporters and facilitative glucose transporters
34. a Na+ / K+ ATPase pump pumps Na+ out of the intestinal epithelium cells so that there is a low Na+ concentration within. the Na+ dependent transporter channels then use the resulting Na+ concentration gradient to power movement of glucose within the cell.
35. these are glucose channels that exist on both the luminal and serosal side of the intestinal epithelium that allow glucose to move down its concentration gradient without expenditure of energy.
36. galactose passes through the intestinal membrane in a similar way to glucose- via both Na+ facilitated channels and facilitative glucose transporters. fructose passes through by facilitated diffusion only.
37. because the transport proteins have a high affinity for glucose (a low Km) or are present in high numbers.
38. the liver will only transport glucose into its cells (and therefore convert glucose to glycogen for storage) when the blood glucose level is high, such as right after a high carb meal.
39. binding of insulin recruits GLUT proteins from intracellular vescicles onto the membrane.