Section 10Section 10

2. Digestion & absorption 2. Digestion & absorption of lipidsof lipids

1/6/06

Brief review of some basicsBrief review of some basics digestion role: convert foods into absorbable formdigestion role: convert foods into absorbable form

catalysts: hydrolases, usually requiring activationcatalysts: hydrolases, usually requiring activation

absorption role: transfer nutrients from lumen to absorption role: transfer nutrients from lumen to blood/lymphblood/lymph cotransport (symport), facilitated transport, diffusioncotransport (symport), facilitated transport, diffusion

Digestion of fatsDigestion of fats only fatty acids & monoglycerides are absorbableonly fatty acids & monoglycerides are absorbable

products of lipase action on main dietary lipids, triglycerides*products of lipase action on main dietary lipids, triglycerides*

nonpolarity of lipids results in large particles with nonpolarity of lipids results in large particles with most molecules not on surface: most molecules not on surface: oil phaseoil phaseexamplesexamples:: particle size particle size fraction of molecules on surfacefraction of molecules on surface

1 mm 1 mm ~ 4/10 ~ 4/1066 (~ 4 ppm) (~ 4 ppm) 1 1 μμmm ~ 4000 ppm ~ 4000 ppm

result:result: most molecules inaccessible to digestive most molecules inaccessible to digestive enzyme, lipaseenzyme, lipase

effective digestion produced byeffective digestion produced by dispersion of particles: peristalsis, surfactantsdispersion of particles: peristalsis, surfactants anchoring of enzyme to particlesanchoring of enzyme to particles

**akaaka triacylglycerols triacylglycerols

11

OO

OO

RR

O RO slide 9

Lipids & amphiphilesLipids & amphiphiles lipidslipids: organic substances not soluble in H: organic substances not soluble in H22OO structurally largely or completely hydrocarbons,structurally largely or completely hydrocarbons,

therefore mainly nonpolartherefore mainly nonpolar amphiphilesamphiphiles (amphipaths): lipids with both polar & (amphipaths): lipids with both polar &

nonpolar partsnonpolar parts more polar amphiphiles are more polar amphiphiles are surfactantssurfactants ( (detergentsdetergents))

interact with polar (e.g., Hinteract with polar (e.g., H22O) & nonpolar structuresO) & nonpolar structures bind at Hbind at H22O-oil interfaceO-oil interface

results of binding:results of binding: surface of particles (surface of particles (e.g.e.g., oil drops) made more polar, oil drops) made more polar oil phase dispersed into oil phase dispersed into emulsion dropletsemulsion droplets

22

Summary of lipid Summary of lipid digestion & absorptiondigestion & absorption

TGMGFA

MGFA

(>10C)

FA(<12C)

chylomicron

TG

BILESALTS

chylomicron

mixedmicelle

albuminBILE

SALTS

FA

lipase-colipase

lipase-colipase

apolipoproteinsphospholipids

emulsiondroplet

BILESALTS

oildrop

1818

4ATPs/TG

enterocyteenterocyte

MicellesMicelles important type of emulsion dropletimportant type of emulsion droplet essential component:essential component: amphiphilic molecules amphiphilic molecules two molecules thick (bilayer)two molecules thick (bilayer) other dimensions can vary widely in numbers of moleculesother dimensions can vary widely in numbers of molecules

polar

nonpolar

examples:examples: monomer unit:

soluble monomers in soluble monomers in rapid equilibriumrapid equilibriumwith units in micelleswith units in micelles

simple micelle:simple micelle: contains only one substance contains only one substance mixed micelle:mixed micelle: more than one substance more than one substance

↔↔

33

key amphiphilic components of intestinal mixed micelleskey amphiphilic components of intestinal mixed micelles structure: structure: a bile acid (a 24-a bile acid (a 24-CC steroid) linked (amide) to an amino steroid) linked (amide) to an amino

acidacid akaaka conjugated bile acids conjugated bile acids most common bile salt in humans:most common bile salt in humans:

glycocholateglycocholate (cholate + glycine) (cholate + glycine) other bile saltsother bile salts

taurocholate taurocholate glycine replaced by taurine:glycine replaced by taurine:

H H22NCHNCH22CHCH22SOSO33––

glycochendeoxycholateglycochendeoxycholatediffers from glycocholate by having no OH group on differs from glycocholate by having no OH group on C12C12

Bile saltsBile salts

OH OH

OH O

OO

NH

glycocholate

–12

3 7

44

Glycocholate structureGlycocholate structure

polar functional groups polar functional groups grouped on 1 side, grouped on 1 side, forming a forming a polar facepolar face 4-ring part is rigid, 4-ring part is rigid,

directing 3 OH's to directing 3 OH's to point same waypoint same way

side chain is flexible, side chain is flexible, allowing amide & COOallowing amide & COO–– adjustmentadjustment

away from the polar face are away from the polar face are hydrocarbon atoms, forming hydrocarbon atoms, forming nonpolar facesnonpolar faces

–

55

Bile salt functionBile salt function stabilize mixed micellesstabilize mixed micelles

bile salt’s amphiphilic bile salt’s amphiphilic structure suited for structure suited for interfacing interfacing lipid surfaces & Hlipid surfaces & H22OO

form amphiphilic rim on form amphiphilic rim on bilayer sheets & bilayer sheets & disc-shaped micelles disc-shaped micelles so that exposed nonpolar so that exposed nonpolar surfaces aresurfaces areshielded from Hshielded from H22O O (covered up)(covered up)

unable to aggregateunable to aggregateNP

NNP

P

NP

N

NP

N

NP

N

NP

N

NP

P

NP

P

NP

P

bilayer bilayer fragment fragment with “exposed” with “exposed” nonpolar (N) surfacenonpolar (N) surface

bound bile saltsbound bile saltsinterface with interface with nonpolar surface & Hnonpolar surface & H22OO66

Bile salts: Bile salts: synthesis, secretionsynthesis, secretion

synthesized in the liversynthesized in the liver cholesterolcholesterol

bile acidbile acid

bile acyl-CoAbile acyl-CoA+ amino acid+ amino acid

bile saltbile salt secreted as a component of bilesecreted as a component of bile bile secretion stimulated bybile secretion stimulated by

secretinsecretin ( (target: target: liver)liver) CCK CCK ((target: target: liver & gall liver & gall

bladder)bladder) absorbed bile saltsabsorbed bile salts

cylindricalmicelle

bile salt

bilesalt

chol-esterol

fattyacid

monoglyceride

phos-pholipid

nonpolarsurfaces

polar face

OH groups

amide groupcarboxyl group

mixed micellecross-section

Adapted from Adapted from PhysiologyPhysiology(Berne & Levy), Fig. 34(Berne & Levy), Fig. 34 -13-13

77

Enterohepatic circulationEnterohepatic circulation(bile salt recycling)(bile salt recycling) bile salts absorbed bile salts absorbed

toward end of ileumtoward end of ileum absorption by Naabsorption by Na++––

driven cotransportdriven cotransport NaNa++–bile salt symport–bile salt symport

carried in portal blood carried in portal blood bound to bound to albuminalbumin

added to bile again by added to bile again by liver & secreted againliver & secreted again

typically make 3-4 typically make 3-4 roundtrips during roundtrips during average mealaverage meal

cholesterolbile salts

Sherwood, Fig.16-17

88

Digestion of lipids: hydrolysisDigestion of lipids: hydrolysis triglycerides (TG)triglycerides (TG)

TG + HTG + H22O O →→ diglyceride + fatty acid (FA) diglyceride + fatty acid (FA)diglyceride + Hdiglyceride + H22O O →→ monoglyceride (MG) + FA monoglyceride (MG) + FASum:Sum:

cholesterol esters & phospholipids* (PL)cholesterol esters & phospholipids* (PL) ↓ ↓ esteraseesterase ↓ ↓ phospholipasesphospholipases FA + cholesterol (chol) FA + lysoPLFA + cholesterol (chol) FA + lysoPL

in all cases, products are in all cases, products are more polarmore polar than reactants than reactants reactions generate reactions generate surfactantssurfactants

2+2 H2O

lipase

OO

OO

RR

O RO

OHO

OH

RO R

OO–

+ 2 H+

99

OO

OP O

RO R

OO

OR

–

** biliary & dietary biliary & dietary

Digestion of fats: lipase & colipaseDigestion of fats: lipase & colipase lipase: a globular protein unable to penetrate surface lipase: a globular protein unable to penetrate surface

of lipid particles (can only reach surface molecules)of lipid particles (can only reach surface molecules) salivary lipase: active at low pHsalivary lipase: active at low pH pancreatic lipase: active at pH >7pancreatic lipase: active at pH >7

hydrolyzes >80% of dietary fathydrolyzes >80% of dietary fat

colipasecolipase required cofactorrequired cofactor

anchors lipase to lipid particlesanchors lipase to lipid particles nonpolar domain (tail) nonpolar domain (tail)

binds to lipid particlebinds to lipid particle polar domain polar domain

binds & activates lipasebinds & activates lipase

TG particleTG particlecolipasecolipase

lipaselipase

1010

Particle transitions: formation of mixed micellesParticle transitions: formation of mixed micelles initial hydrolysis & dispersioninitial hydrolysis & dispersion

oil phase oil phase →→ emulsion droplets emulsion dropletsaverage particle size: 1000 average particle size: 1000 μμm m →→ 10 10 μμmmdispersion to emulsion droplets due to hydrolysis dispersion to emulsion droplets due to hydrolysis

products (e.g., MG & FA) being products (e.g., MG & FA) being amphiphilesamphiphiles bile salts & phospholipids also contribute to dispersing bile salts & phospholipids also contribute to dispersing

of oil dropsof oil drops additional hydrolysis & mixed micelle formationadditional hydrolysis & mixed micelle formation

emulsion droplets emulsion droplets →→ mixed micelles mixed micellesas [FA] & [MG] as [FA] & [MG] ↑↑ , they leave droplets & combine with , they leave droplets & combine with

bile salts to form bile salts to form mixed micellesmixed micellesaverage particle size reduced from 10 average particle size reduced from 10 μμm to 10m to 10–2–2 μμmm

1111

Lipid particle summaryLipid particle summaryparticleparticle size range, size range, surfacesurface molecular components molecular components typetype μμm m area, cm area, cm22/g/g surface surface inside*inside*

oil dropoil drop 10 102 2 – 10– 1044 1 1 – 100– 100 TGTG TG TG

emulsionemulsion 1 – 10 1 – 1022 10102 2 – 10– 1044 FA, MG, PL,FA, MG, PL, TG TG droplet droplet bile salts, bile salts, TG TG

mixed 10mixed 10––3 3 – 10– 10––11 10105 5 – 10– 1077 bile salts, FA, bile salts, FA, – – micelle micelle MG, PL MG, PL

chylo- 10chylo- 10––1 1 – 1– 1 10104 4 – 10– 1055 apolipo-apolipo- TG TG micron micron proteins, PL proteins, PL

* all contain chol, chol esters & fat-soluble vitamins* all contain chol, chol esters & fat-soluble vitamins

1212

Lipid particlesLipid particles

←TG, DG←MG, FA, etc.

↕

↕MG, FA, etc.

TG ↔

emulsion droplet

mixed micelle

Absorption from lumen Absorption from lumen

brush bordermembrane

microvillus

diffusionof micelles

throughunstirredlayer

well-mixedluminalcontents

monoglycerides

lysophos-pholipids

chol

fattyacids

unstirred layer

cytosol

movement of lipid digestion products (FA, MG, etc) across movement of lipid digestion products (FA, MG, etc) across mucosal plasma membrane by simple diffusion of monomersmucosal plasma membrane by simple diffusion of monomers absorption absorption also occursalso occursvia via fatty acidfatty acidtransfer proteintransfer protein(FATP)(FATP)microvilli microvilli provide very provide very large absorbing large absorbing surface, but surface, but convolutions & convolutions & glycocalyx glycocalyx produce produce unstirred layerunstirred layer

Adapted from Fig. 34 -14 (B & L)

FATP

1313

Absorption: role of micellesAbsorption: role of micellesunstirredunstirred layerlayer200-500 200-500 μμm thickm thickprevents peristaltic prevents peristaltic

mixing from moving mixing from moving luminal contents close luminal contents close to cell surfaceto cell surface

crossed by crossed by micellemicelle diffusion diffusionbecause of very low solubility of lipid molecules & very large because of very low solubility of lipid molecules & very large distance, absorption would be very slow without micellesdistance, absorption would be very slow without micelles

mixed micelles act as: mixed micelles act as: carrierscarriers of lipid monomers (FA, MG, chol, vit. A, D, E, K) of lipid monomers (FA, MG, chol, vit. A, D, E, K)reservoirs:reservoirs: as monomers absorbed, they are rapidly replaced as monomers absorbed, they are rapidly replaced

by dissociation from micellesby dissociation from micelles1414

After absorptionAfter absorption FAs (>10FAs (>10CCs) & MGss) & MGs

converted into TGsconverted into TGsafter diffusion into smooth ER, after diffusion into smooth ER, FAs activated, forming FAs activated, forming acyl-CoAs (acyl-CoAs (2 ATPs used/FA2 ATPs used/FA):):

FA + CoA FA + CoA →→ acyl-CoA acyl-CoAfrom these, TGs synthesized:from these, TGs synthesized:

2 acyl-CoA + MG 2 acyl-CoA + MG →→ TG TG

PL & chol esters (cholE) PL & chol esters (cholE) also resynthesizedalso resynthesized

products packaged into products packaged into chylomicronschylomicrons

MG+2FA

lysoPL +FA

chol+FA

TG PL cholE

chylomicron

lipopro-teincoat

exocytosis

MG+FA lysoPL chol

smoothER

lacteal1515

Packaging for transportPackaging for transport chylomicronschylomicrons

particles for particles for transport of lipids transport of lipids to liver & to liver & adipocytes adipocytes

size: size: 00..1–1 µm1–1 µm averageaverage composition: composition:TG (84%)TG (84%)chol (2%)chol (2%)cholE (4%)cholE (4%)PL (8%) PL (8%) apolipoproteins (2%)apolipoproteins (2%)

apolipo-proteins

cholE, TG

chol PL

PL

Lehninger et al., 3rd ed., Fig. 17-2

1616

Export of chylomicronsExport of chylomicrons exocytosisexocytosis

chylomicrons enclosedchylomicrons enclosed in in secretory vesiclessecretory vesicles in Golgi complex in Golgi complex

vesicles diffuse tovesicles diffuse to basolateral membrane basolateral membrane

via fusion, chylomicronsvia fusion, chylomicrons released into interstitial space released into interstitial space

chylomicrons enterchylomicrons enter lymphatic system (lacteals) by diffusionlymphatic system (lacteals) by diffusion blood at thoracic duct from lymphaticsblood at thoracic duct from lymphatics

FAs with <12 FAs with <12 CCss diffuse through cell & basolateral membrane, into blooddiffuse through cell & basolateral membrane, into blood transported in blood bound to transported in blood bound to albuminalbumin

vesicle membrane

lacteal

1717

Summary of lipid Summary of lipid digestion & absorptiondigestion & absorption

TGMGFA

MGFA

(>10C)

FA(<12C)

chylomicron

TG

BILESALTS

chylomicron

mixedmicelle

albuminBILE

SALTS

FA

lipase-colipase

lipase-colipase

apolipoproteinsphospholipids

emulsiondroplet

BILESALTS

oildrop

1818

4ATPs/TG

enterocyteenterocyte

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3. Distribution of absorbed nutrients3. Distribution of absorbed nutrients