Digestion & Absorption

Question 0

Uptake of primary and secondary bile acids into liver via enterohepatic circulation

Answer 0

• reabsorption of about 95% of bile salts
• transported from ileum to liver by portal vein, conjugated to albumin
• liver converts both primary and secondary bile acids to bile salts and secretes them into bile

Question 1

Cholestyramine & bile acid sequestrants

Answer 1

• bind bile acids in gut and prevent their reabsorption by ileum
• increased excretion of bile salts in feces & disruption of enterohepatic circulation, forcing the liver to divert more cholesterol to bile acid synthesis
• decrease the amount of cholic acid present in liver, removing inhibition on 7-alpha-hydroxylase and allowing more cholesterol to be converted to bile acids
• these bile acids sequestrants used in rx of hypercholesterolemia

Question 2

Intestinal a.a. absorption

Answer 2

Absorption rapid in duodenum & jejunum but slow in ileum

• Dipeptides: transported into enterocytes by H+ co-transport
• Free aa: absorbed across intestinal mucosa by
• Absored aa transported into portal vein and metabolized by liver or enter general circulation

Question 3

Intestinal monosaccharide absorption

Answer 3

• does not require insulin
• Transport from lumen to enterocyte:
• Glucose & Galactose: Na+ co-transport (SGLT-1 & SGLT-2)
• Fructose: GLUT5 (facilitated diffusion), independent of sodium
• GLUT2 transport from enterocyte to portal circulation for all monosaccharides

Question 4

Intestinal fat absorption

Answer 4

• end products of lipid digestion
• free fatty acids
• 2 monoacyglycerols
• free cholesterol

Question 5

Micelles

Answer 5

• bile salts combine with fat digestates to form micelles
• micelles are small spherical cylindrical globules composed of about 20-40 molecules of bile salts
• develop as each bile salt molecule contains a highly fat soluble sterol nucleus and a highly water soluble polar group
• sterol nucleus takes up fatty acids, monoglycerides, and cholesterol, forming a small fat globule in middle of micelle
• once at brush border, the lipids diffuse out of micelles and are taken up by mucosal cells
• bile salts are not absorbed and remain in intestinal lumen

Question 6

Resynthesis of TAG & cholesteryl ester

Answer 6

After being absorbed by intestinal mucosal cell, mixture of lipids taken up by sER where resynthesis of TAG and re-esterification of cholesterol takes place. Main enzymes involved in this process are as follow:

• Fatty acyl CoA synthetase (thiokinase): FA -> Fatty Acyl CoA
• TAG Synthase: Monoacylglycerols -> TAG
• Acyltransferases: Lysophospholipids -> Phospholipids
• Acyl CoA: Cholesterol acyltransferase -> Re-esterification of cholesterol

Question 7

Formation of chylomicrons in intestinal mucosal cells

Answer 7

• assembled in intestinal mucosal cells
• carry the digested and absorbed lipids to peripheral tissues
• chylomicrons released into lymph and then enter circulation
• rapidly modified to ApoE and ApoC-II in plasma
• ApoC-II is activator for lipoprotein lipase which hydrolyses the TAG w/in the chylomicron to FA and glycerol

Question 8

Absorption of glucose into intestinal mucosal cells

Answer 8

Transport from lumen to enterocyte: Glucose & Galactose: Na+ co-transport (SGLT-1 & SGLT-1)