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Digestion/Absorption: Lipids (classes of dietary lipids (phospholipids =…
Digestion/Absorption: Lipids
classes of dietary lipids
triacylglycerols (TAG)
= 90%
must be degraded into FFAs & MAGs to be absorbed
phospholipids
= 5%
phosphatidylcholine (lecithin)
sphingolipid
cholesterol
= 0.5%
unesterified cholesterol only;
esterified only found in liver & blood foods
breakdown
emulsification
formation of oil droplets in water
d/t chemical (
lingual lipase
) & mechanical processes (chewing, gastric churning, peristalsis) >> dec. oil droplet size & inc. surface area
dietary & secreted lipids >> monolayer at interface >> stabilizes emulsions
lypolysis
(digestion)
lipases: present in aq. lumen at oil-water interfaces
gastric lipase
active & stable at pH ~4
inactivated by pancreatic proteases in bile salts ...
pancreatic deficiency >> extending activity into duo
action: cleaves FA from TAGs
results:
1 protonated FFA (medium & short chain move into portal blood thru gastric mucosa)
1 diacylglycerol
secreted by chief cells; resistant to pepsin
pancreatic lipase
secreted in huge XS into duo by acinar cells (exocrine pancreas)
requires:
presence of colipase
alkaline pH
Ca
bile salts
FA substrates
action:
at oil-water interface:
colipase
reduces inhibition from phospholipids or proteins on micelle surface
hydrolyzes all TAGs
results:
2 FFA
1 MAG
Xenical & Alli (Orlistat)
: obesity Rx inhibit pancreatic lipase >> fatty stool
phospholipase A2
pancreatic proenzyme
action: cleaves FFA from glycerophospholipid
results: lysophospholipid (no middle FA)
requires: bile salts & alkaline pH
in SI & LI (all from bacteria)
carboxyl ester hydrolase
not substrate specific; hydrolyzes all esters
releases free cholesterol & glycerol
same action as
Bile-salt Stimulated Milk Lipase
in human milk for infant fat digestion
stable but inactive thru gastric acidity
active at alkaline pH of duo and jeju
action: hydrolyzes DAGs, MAGs, TAGs, fatty esters
absorption
CCK
stimulus: FFAs in duo
actions: (+) bile flow into duo & secretion of pancreatic enzymes (lipase & esterase)
FAs
MCFAs
directly transferred into blood from enterocytes
important fat substitute for pts w/ related malabs.
abs. independently of micelles or bile salts
LCFAs
chylomicrons >> lipase >> fat >> liver
SCFAs
easily absorbed
emulsion droplets
abs products & components of lipid digestion
surface components:
cholesterol
MAGs
lecithins
pancreatic lipases
(built up; assist in formation of unilamellar vesicle) bile salts
liquid crystalline layer (bile salts, surface components & Ca-rich fatty soaps)
core lipids: DAGs, TAGs, cholesterol esters
micelle formation
MAGs hydrolyzed by lipases >> FFAs released >> core DAGs & TAGs replace surface MAGs >> dec. droplet size & inc. surface area >> more hydrolytic digestion
inc. deposition of liquid crystalline layer >> budding of
multilamellar
vesicles
more bile salts convert multilamellar >>
unilamellar
vesicles
more bile salts >> mixed
micelles
absorption across enterocyte brush border occurs in
microclimates
of the intestine
bulk water phase of lumen
hydrolysis
micelle formation
mucous gel layer
lines epi surface
mucin: barrier to diffusion & pathogens
unstirred water layer
juxtaposed to cell membranes
in disequlibrium w/ bulk water phase d/t mucous gel
direct diffusion of SCFAs & MCFAs
larger FAs partitioned back into micelles present;
maintain high [lipids] in layer bc they diffuse from micelles d/t protonated environment maintained by Na-H exchangers
enterocyte apical membrane
micelles can't diffuse across
lipids must leave micelle behind
FA translocases & FA binding proteins
enhance translocation
preferentially bind LCFAs (don't easily diffuse)
mixed micelles >> acidic unstirred layer & surface of enterocyte >>
2-MAG, FAs, lysophospholipids & cholesterol leave mixed micelle >>
enter acidic microenviro (favors protonation of FAs) >> enter enterocyte by:
(1) nonionic diffusion
(2) incorporation into enterocyte membrane (collison)
(3) carrier-mediated transport
once inside enterocytes...
re-esterification
SER: LCFAs >> TAGs & phospholipids
packaging
RER: apolipoproteins synthesized >> SER & Golgi
SER: apolipoproteins + TAGs, phospholipids & cholesterol esters
chylomicrons
largest lipoprotein
primarily TAGs
surface coated w/ lecithin & phospholipids
VLDLs
carry mainly endogenous lipids
during both fed & fasting states
trafficking
SER: vesicles carry chylomicrons & VLDLs to Golgi cis face
Golgi: assoc. w/ apolipoprotein A-I >> glycosylation >> vesicles carry chylomicrons & VLDLs from trans face to basolateral membrane
Lymph: enter thru lacteals (lymphatic capillaries) >> cisternae chyli >> thoracic duct >> L. subclavian a. >>>> liver >> body
steatorrhea
intact acylglycerols rarely found, even in severe malabsorption (d/t bacterial digestion in colon)
Sudan III Staining
: chemical test for fat in stool
lipids in stool