2.LIPIDS Digestion

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2. Digestion, absorption and transport

Lipids2


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The major dietary lipids are:

Triglicerides

Phospholipids

Cholesterol ant its esters

Digestion of lipids takes place:

1. In mouth very little, due to the presence of linguallipase

2. In stomach also very little, due to the presence ofgastric lipase

3. In duodenum and small intestine the main placeof lipid digestion due to the presence of bile acids,pancreatic juice with pancreatic lipases.


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Digestion in duodenum and smallDigestion in duodenum and smallintestine includes:intestine includes:

1. Emulsification;

2. Hydrolysis;

3. Mixed micelle formation

Absorption of lipids includesAbsorption of lipids includes::

4. transfer of mixed micelles into enterocytes5. re-synthesis of triglycerides in enterocytes6. formation of chylomicrons7. transport of chylomicrons to the tissues.


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1. Emulsification - aprocess when largeaggregates of dietarytriglyceride and other

lipids, which areinsoluble in an aqueousenvironment, is brokendown physically and

held in suspension.The key players in thistransformation are

bile acids.

Emulsification of LipidsEmulsification of Lipids


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Lipids (fats and oils) are insoluble in water (hydrophobic). Lipids tend to form larger droplets which reduces thesurface area for digestion. The hydrophobic lipid is only accessible to the water soluble

lipases at the interface between lipid and water.

To increase the access (increased surface area) and rateof lipid digestion the lipid droplet must be broken up.

Bile salts secreted from the liver (via gallbladder) havemolecules with a combination of hydrophobic and(lipophilic) hydrophilic regions.Bile salts break up the lipid droplet into many smallerdroplets thereby increasing the surface area of lipid-water

access.

Emulsification of LipidsEmulsification of Lipids


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Bile acidsBile acids--(bile salts) are polar derivatives of cholesterol, formed in(bile salts) are polar derivatives of cholesterol, formed in

liver and secreted into the gall bladder.liver and secreted into the gall bladder.They pass via the bile duct into the intestine, where theyThey pass via the bile duct into the intestine, where theyaid digestion of fats & fataid digestion of fats & fat--soluble vitamins.soluble vitamins.

Bile acids have detergent properties.Bile acids have detergent properties.

Bile acidsBile acids emulsifyemulsifyfat globules intofat globules intosmaller micelles,smaller micelles,increasing the surfaceincreasing the surfacearea accessible toarea accessible tolipidlipid--hydrolyzinghydrolyzingenzymes.enzymes. They alsoThey also activateactivatethe lipolytic enzymes.the lipolytic enzymes.


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Hydrolysis - triglyceride molecules must beenzymatically digested to yield monoglyceride andfatty acids, both of which can efficiently diffuse or betransported into the enterocyte

2. Hydrolysis of Lipids2. Hydrolysis of Lipids

The key players in this transformation are:

Pancreatic lipasePancreatic lipase (secreted into the intestine(secreted into the intestine

by pancreas), catalyzes hydrolysis ofby pancreas), catalyzes hydrolysis oftriacylglycerols at positions 1 & 3, forming 1,2triacylglycerols at positions 1 & 3, forming 1,2--diacylglycerols, & then 2diacylglycerols, & then 2--monoacylglycerols.monoacylglycerols.

AproteinAprotein colipasecolipase is required to aid binding ofis required to aid binding of

the enzyme at the lipidthe enzyme at the lipid--water interface.water interface.


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Action of Pancreatic Lipase:Action of Pancreatic Lipase:

Pancreatic lipase hydrolyses theesteric bonds between glycerol and

fatty acids, resulting free glyceroland three fatty acids, but theenzyme can hydrolyze the bondsonly at positions 1 and 3 of thetriglyceride, leaving two free fatty

acids and a 2-monoglyceride.


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Digestion of GlycerophospholipidsDigestion of Glycerophospholipids

Phospholipase APhospholipase A22

is secreted by the pancreasis secreted by the pancreasinto the intestine.into the intestine.

It hydrolyzes the ester linkage between the fattyIt hydrolyzes the ester linkage between the fattyacid and the hydroxyl on C2 of phospholipidsacid and the hydroxyl on C2 of phospholipids

There are alsoThere are alsophospholipasephospholipaseA1 , C and D,A1 , C and D,

which hydrolyzeswhich hydrolyzesother bonds ofother bonds ofphospholipidsphospholipids


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Digestion of cholesterol estersDigestion of cholesterol esters

cholesterolesterase

Cholesterol andfatty acid


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In the result oflipolytic enzymesaction a large numberof different hydrolysisproducts are obtained:glycerol, fatty acids,2-monoacilglycerols,

lysophospholipids.

Glycerol and small chain fatty acids aredirectly absorbed from intestinal lumen into

enterocytes by simple diffusion , than to portalvein and are taken to liver.


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3. Micelles formation:3. Micelles formation:

Mixed micelles are a mediator for gastrointestinalabsorption of lipid-soluble ingredients

Other product long chainfatty acids, colesterol, 2-monoacilglycerols,lysophospholipids are

incorporated together withbile acids into molecularaggregates called mixedmicelles


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Absorption of lipids includesAbsorption of lipids includes::

4. transfer of mixed micelles into enterocytes5. re-synthesis of triglycerides in enterocytes6. formation of chylomicrons7. transport of chylomicrons to the tissues.


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7. Transport ofchylomicrons to the tissues-

Exocytosis of the chylomicronsreleases the lipoprotein fromthe cell. The lipoprotein is takenup in the lymphatic system andthen enter the general blood

circulation

Absorption:4. Transfer of mixed micellesinto enterocytes. The structureof the micelles allows them tofuse with the cell membrane andthe fatty acid molecules to passinto the epithelial cells of smallintestine villus.

5. Re-synthesis oftriglycerides: The fatty acidsand glycerol recombine in the

endoplasmic reticulum to formlipid.

6. Formation of chylomicronsProtein is added to the lipid to

form lipoproteins calledchylomicrons.


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Lipiddigestion

andabsorbtion


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Anatomy of a LipoproteinAnatomy of a Lipoprotein

The principal lipidThe principal lipidcomponents arecomponents aretriglycerides,triglycerides,cholesterol, cholesterylcholesterol, cholesteryl

esters andesters andphospholipids. Thephospholipids. Thehydrophobic core of thehydrophobic core of the

particle is formed byparticle is formed bythe triglycerides andthe triglycerides and

cholesteryl esters. Allcholesteryl esters. Allthe apoproteins arethe apoproteins areamphipathic andamphipathic andinteract with both lipidsinteract with both lipids

and water.and water.


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LipoproteinsLipoproteins

Various combinations of lipid and protein produceVarious combinations of lipid and protein produceparticles of different densities. They are:particles of different densities. They are:

chylomicrons,chylomicrons,

very lowvery low--density lipoproteins (VL

DL),density lipoproteins (VL

DL),

lowlow--density lipoproteins (LDL),density lipoproteins (LDL),

highhigh--density lipoproteins (HDL).density lipoproteins (HDL).

The most distinguishing feature of eachThe most distinguishing feature of eachclass is the density and relative amountsclass is the density and relative amountsof lipid and protein calledof lipid and protein called apoproteins.apoproteins.


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Human plasma lipoproteins

chylomicron VLDL IDL LDL HDL

Density (g/ml)


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ChylomicronsChylomicrons

In lipid uptake from theIn lipid uptake from theintestineintestine ,,chylomicronschylomicrons, which, which

contain apoprotein Ccontain apoprotein C--IIII(apoC(apoC--II), move from theII), move from theintestinal mucosa into theintestinal mucosa into thelymphatic system, fromlymphatic system, fromwhich they enter thewhich they enter theblood and are carried toblood and are carried to

muscle and adiposemuscle and adiposetissuetissue


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In the capillaries of these tissues, the extracellularIn the capillaries of these tissues, the extracellularenzyme lipoprotein lipase (LPL) is activated byenzyme lipoprotein lipase (LPL) is activated byapoCapoC--II. This enzyme hydrolyzes triacylglycerols toII. This enzyme hydrolyzes triacylglycerols to

fatty acids and glycerol, which are taken up byfatty acids and glycerol, which are taken up bycells in the target tissues. In muscle, the fattycells in the target tissues. In muscle, the fattyacids are oxidized for energy; in adipose tissueacids are oxidized for energy; in adipose tissuethey are stored as triglycerides.they are stored as triglycerides.


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2.LIPIDS Digestion

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