digestion & absorption of lipids
TRANSCRIPT
DIGESTION & ABSORPTION OF LIPIDS
بسم الله الرحمن الرحيم
By: Ali M. Osman
IntroductionLipids: Water-insoluble organic compounds, but soluble in polar solvents, e.g.
fats, oils, steroids, waxes, etc. Importance:
o high energy value.o fat-soluble vitamins.o essential FAs.o thermal insulator in adipose tissue.o electrical insulators in nerves.o Phospholipids = constituents of cell membrane & mitochondria.o Lipoproteins transport lipids in blood.o Involved in pathogenesis of: obesity, DM, atherosclerosis, etc.
Classified into: Simple: esters of FAs with alcohols, e.g. fats, oils, waxes. Complex: esters of FAs with alcohols + Group, e.g. Phospholipids, Glycolipids ,
Sulfolipids , Aminolipids, etc. Derived: e.g. steroids, fatty aldehydes, ketone bodies, hydrocarbons, lipid-
soluble vitamins, hormones, etc.
Dietary lipids• TAGs & Phospholipids:–Major ones.–Since hydrophobic, they need to be
hydrolyzed & emulsified to “micelles” before absorption.
• Fat-soluble Vits (A, D, E, & K) & cholesterol absorbed dissolved in lipid micelles.
Digestion & Absorption of TAGs• Initiated by lingual & gastric lipases attacking sn-3
ester bond, forming 1,2- DAGs & FFAs.
• In intestine, further hydrolysis is by pancreatic lipase which:– Needs colipase, for activity.– Specific for sn-1 & sn-3 ester bonds → 2-monoacylglycerols
– major end-products of TAG digestion – & FFAs.• N.B: <25% of ingested TAG is completely hydrolyzed to
glycerol & FFAs.
H
Lipase
H2O
FFA+
BileSalts
Acyl1
Acyl2
Acyl3
TAG 100%
Intestinal Lumen Intestinal Cell Lymphatics
Pan. Lipase
FA
FA
Acyl1
Acyl2
OH1,2-DAG
OH
Acyl2
OH
2-MAG*
Acyl1
OH
OH1-MAG
FA
Pan. Lipase
Isomerase
OH
OH
OHGlycerol
FA
Pan. Lipase
OH
Acyl2
OH
Acyl-CoA
Acyl1
Acyl2
Acyl3
Acyl1
Acyl2
Acyl3
Acyl1
Acyl2
Acyl3
Acyl1
OH
OH
OH
OH
OHGlycerol
FA
Int.Lipase
GlycerolKinase
ATP
ATPAcyl CoASynthetase
OH
OH
PGlycerol-3phosphate
Ente
rohe
patic
circ
ulati
on
Liver
Glycolysis
Monoacylglycerol pathway
Phosphatidic acid pathway
Portal Vein
Glycerol
6%
72%
Acyl CoASynthetase
+ Fat-soluble Vits
& Cholesterol
Chylomicrones
ATP
To ileum
22%
28%
TAG
TAG
1-MAG
Thoracic Duct
2-MAG
• 2-monoacylglycerols, Fat soluble Vits, cholesterol & phospholipids are emulsified by bile salts into micelles and liposomes.
• Water soluble micelles are transported through the intestinal lumen and come into close contact with the brush border of jejunal mucosal cells, allowing cellular uptake of enclosed lipids.
• The released bile salts pass on to the ileum, where absorbed into the enterohepatic circulation returning to the liver.
• Glycerol released in the intestinal lumen is not reutilized but passes via portal vein to the liver; where it is used for esterification of FAs synthesized in lipogenesis.
• Glycerol released within the epithelium is reutilized for TAG synthesis via the normal phosphatidic acid pathway.
• All long-chain fatty acids absorbed are converted to TAG in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct.
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