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2. Digestion & absorption of lipids

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1 2. Digestion & absorption of lipids
Section 10 2. Digestion & absorption of lipids 1/6/06

2 Brief review of some basics
digestion role: convert foods into absorbable form catalysts: hydrolases, usually requiring activation absorption role: transfer nutrients from lumen to blood/lymph cotransport (symport), facilitated transport, diffusion Main points of what we’ve done so far. What digestion is there for, what the chemistry of breaking down these food molecules is, is to get them in a form which is absorbable by the enterocytes of the small intestine. What does this, at the catalytic level, are these hydrolases , peptidases in the case of proteins. Generally activation is almost an invariable part of getting them going, need to be synthesized in the pancreas in an inactive precursor form or else they would digest the proteins that are needed for pancreatic structure and function. Pancreatitis is a serious illness that results from premature activation of digestive enzymes, self digestion of tissue and organs takes place. Pancreas also secretes trypsin inhibitor, protein that inhibits trypsin when its present in only small quantities. When its activated in the GI lumen, this inhibitory effect is overwhelmed. Trypsin inhibitor has the effect of counteracting small of trypsin that might be activated prematurely. Function of absorption is to take these molecules from digestion and take them from the GI lumen and get them into the fluid which enables their distribution to be consumers of these materials. These take place by two main fluids, the blood or the two nutrient types, carbohydrates and amino acids, and lymph is involved in initial distribution of fats. Cotransport and symport process, facilitated transport, and in some cases simple diffusion is all that’s needed to get things out of the GI lumen and into cells.

3 Digestion of fats only fatty acids & monoglycerides are absorbable
products of lipase action on main dietary lipids, triglycerides* nonpolarity of lipids results in large particles with most molecules not on surface: oil phase examples: particle size fraction of molecules on surface 1 mm ~ 4/106 (~ 4 ppm) 1 μm ~ 4000 ppm result: most molecules inaccessible to digestive enzyme, lipase effective digestion produced by dispersion of particles: peristalsis, surfactants anchoring of enzyme to particles *aka triacylglycerols Problem with fats is they are not water soluble, lipid material. Fat produces some properties digestion and absorption, and distribution affected. Point out what stage to get to, to get absorption, what does digestion have to do. FA and monoglycerides are forms of fats, digestive products of fats, that can be absorbed. The enzyme is lipase, pancreatic lipase in this context, and triglycerides are what fats are, synonym for triglyceride, consist of alcohol with three hydroxyl groups, two acyl groups attached. Three carbons of the glycerol, these would be the three hydroxyl group, esterified to three fatty acids. Carbonyl groups of each of these fatty acids. Digestive reaction, hydrolysis is what will take place across the ester linkage. One survives, not a substrate for the enzyme and is why one of the products is a monoglyceride. Glycerol with one acyl group still attached. The one in the middle that is attached. Particles have most of the molecules in subsurface locations, oil phase. If you imagine a particle inside, 1 mm, small on macroscopic scale, fraction of molecules on the surface if it’s a sphere is tiny, four out of every million molecules is on the surface. The enzyme that is going to digest these substrates, triglyceride has to meet the substrate in the aqueous solution or at the interphase, only the surface molecules are susceptible to digestion, vast majority of this particle of 1 mm size, all but four out of a million would be not subject to digestion. If digestion were to happen only at particles of this size, would take an infinite amount of time, cannot have such large particles and have digestion take place. If the particles are broken up and that’s part of what happens, something 1000 the size, see larger fraction of the molecules are on the surface. Still only a small fraction but increase by a factor of A process that enables particles to be dispersed, broken into smaller size is what's needed for digestion to take place. Disperse the particles, two processes that do that, first is physical, peristalsis that takes place in GI, breaks particles large ones into many smaller, and at molecular level, are the actions of surfactants, detergents with dispersive effect, enable particle surfaces to be more compatible with aqueous environment. Some of the particles we will deal with. Oil drops are the largest microns to 10 mm. Broken into smaller particles to emulsion droplets, some sort of emulsion has been formed, then get down to smaller size, as we go down in size, increase in surface area for a given amount of material. Square centimeters per gram of material. Decrease by 100 is increase in surface area by similar number, inverse proportionality. Also, smaller, molecular dimensions, mixed micelle, and fourth particle not involved directly in digestion but in absorption and distribution, chylomicron. Particles are the story of digestion, absorption and distribution of lipids. Enormous difference in sizes. The circle compared to partial circle, differ by factor of 100. Surfactants play an important role, but another factor is that the process is accelerated by the fact hat the enzyme, lipase, is going to be anchored to the particles, attached so it wont diffuse away after doing one hydrolysis and stays in the right place, as a result it works much faster. slide 9 1

4 Lipids & amphiphiles lipids: organic substances not soluble in H2O
structurally largely or completely hydrocarbons, therefore mainly nonpolar amphiphiles (amphipaths): lipids with both polar & nonpolar parts more polar amphiphiles are surfactants (detergents) interact with polar (e.g., H2O) & nonpolar structures bind at H2O-oil interface results of binding: surface of particles (e.g., oil drops) made more polar oil phase dispersed into emulsion droplets Lipid, any organic molecule or substance that is not water soluble. Hydrocarbons are the main feature of these lipids materials that make them non polar, no bases for interaction with the polar solvent water. Amphiphilic substances have hydrocarbon parts, non polar and polar parts. These play a role in digestion and absorption of the fats. Sometimes these go by names like surfactants, tend to bind to surfaces and act as interfacial molecules, and detergents to deter substances from aggregating and forming large insoluble particles. Idea is they interact with the solvent, water, and the non polar structures and they bind at the interphase between these two dissimilar materials. The binding that results from this means that the particles, oil drops, are made more polar, now have surface that is polar, and these particles will not as readily aggregate into much larger particles, oil phase is dispersed into emulsion droplets, smaller particles than oil drops. 2

5 Summary of lipid digestion & absorption
oil drop enterocyte lipase- colipase MG FA (>10C) 4ATPs/TG TG emulsion droplet apolipoproteins phospholipids adapted from Devlin, 5th ed. pp TG MG FA BILE SALTS Summary of lipid digestion and absorption. Pics not proportional. Begin with oil drop, and the early stages of digestion produce smaller particles, oil drop contains the lypitic material, triglycerides cholesterol esters in small amounts. The early digestive process, produces the smaller particles, emulsion droplets, consisting of triglycerides, early digestion products, and later on these small particles are broken down into still smaller ones, known as mixed micelles, consequence of bile salts combining with these materials, further digestion is also part of this process, need to get to the stage of mixed micelles that contain the monoglycerides and fatty acids that enables them to get at the surface of the absorbing cell, enterocyte, and then these arrows indicate the components that are absorbed, bile salts are resorbed and recycled, fatty acids of small size, 12 carbons, go through these cells directly, and they are polar enough, water soluble enough that they can get directly into the blood and carried by protein albumin. Have a simple short circuit way of getting through the absorbing cell and on their way. Vast majority of fatty acids are longer, have more carbons than this. Milk has short chain hydrocarbons, special fluid designed to provide nutrition for young mammals, role is to get some nutrition there fast, and other material to show up at later time, spread out the nutrient value. These short chain hydrocarbons, role of fuel into circulation at a rapid rate. Long chain fatty acids get absorbed, undergo further chemistry in enterocyte, go back to triglycerides, still being non polar substances need a lot of help to be carried around. Be big fat globules stuck inside enterocytes. They are packaged into chylomicrons, these chylomicrons are the form that these triglycerides get out of enterocyte and ultimately into blood and lymph. Idea is to start with big insoluble particles and getting down to the stage of absorption. lipase- colipase chylomicron chylomicron FA (<12C) FA mixed micelle BILE SALTS albumin BILE SALTS 18

6 Micelles ↔ important type of emulsion droplet
essential component: amphiphilic molecules two molecules thick (bilayer) other dimensions can vary widely in numbers of molecules examples: monomer unit: polar nonpolar soluble monomers in rapid equilibrium with units in micelles Micelles are a form of emulsion droplets, need amphiphilic molecules to form them, all the molecules in micelles are amphiphilic, triglyceride are not a component. Molecular dimension, just two molecules thick, bilayer structure, resemble the bilayer structure of cell membranes, but different, because those tend to be closed compartments, these do not form such a complex structure. Here is a cross section of spherical simple micelle, basic arrangement, polar towards the outside, there is a monomer unit of micelle and there is an elongated version of the same thing, these things can stretch even further to form disk shaped particles. Just two molecules in thickness, every molecules has a part of it at the interphase of it and the aqueous solution. Important property of these micelles is that the monomers are not just stuck in the structure, for a cell membrane lipid material stays there, forms a barrier. In these micelles these monomers are in rapid equilibrium with the units inside of cells. This sort of process would be going on, idea is every one o those monomers would be rapidly dissociating and being replaced by nearby monomer. Dynamic structures, useful as carriers of these monomers, not able to be in aqueous solution on their own, but in particles, they can move around much better. The micelles are simple micelles if each one of these monomers was the same chemical structure. Mixed are the ones that are common in the digestion absorption process, several types of substances. FA, monoglycerides and bile salts. simple micelle: contains only one substance mixed micelle: more than one substance 3

7 Bile salts key amphiphilic components of intestinal mixed micelles
structure: a bile acid (a 24-C steroid) linked (amide) to an amino acid aka conjugated bile acids most common bile salt in humans: glycocholate (cholate + glycine) other bile salts taurocholate glycine replaced by taurine: H2NCH2CH2SO3– glycochendeoxycholate differs from glycocholate by having no OH group on C12 Crucial component, bile salts, amphiphilic consisting of steroid part in yellow, yellow is cholic acid, and the green part is glycine. Two linked by amide bond, like all steroids, cholic acid is derived from cholesterol in the liver. Among steroids, like cholesterol, just hydrocarbon and one hydroxyl group, very lipophilic or non polar substance, here is one that is more polar than typical steroid, three hydroxyl groups, add glycine, this part is polar. Amphiphilic molecule, but it consists of structure that would appear that these hydroxyl groups are in different directions, in fact those hydroxyls are on the same face of the structure. Also mention that glycocholate is prototype for most common human bile salts, but there are others, taurocholate , similar amino acid instead of glycine, a sulfonate, and another one listed, is essentially with just one of the hydroxyl groups missing. Consider glycocholate as the prototype of bile salts, remembering its not the only one. Bile salt means bile acid, cholate plus glycine. Sometimes referred to as conjugated bile acids. 4

8 Glycocholate structure
polar functional groups grouped on 1 side, forming a polar face 4-ring part is rigid, directing 3 OH's to point same way side chain is flexible, allowing amide & COO– adjustment away from the polar face are hydrocarbon atoms, forming nonpolar faces Shows that in 3D, the three hydroxyl groups are all on the same side of the molecule, face the same way, in addition, other polar part of the structure, with the negative charge at glycine carboxyl group, this is actually a more flexible part of the structure, three hydroxyls are rigidly stuck facing this direction, the other part is more flexible, can adjust to specific needs of the role of the material, which is to have the left surface cover up, expose the hydrocarbons, and the right surface interface the solvent. Good at dispersing lipid particles into smaller ones, acting like detergents. One surface that is polar, other surfaces are pure hydrocarbon and can interface with the hydrocarbon surface and the molecule as whole acts as a dispersant, bridging agent between the non polar faces in these particles and the aqueous solution. 5

9 Bile salt function stabilize mixed micelles
bile salt’s amphiphilic structure suited for interfacing lipid surfaces & H2O form amphiphilic rim on bilayer sheets & disc-shaped micelles so that exposed nonpolar surfaces are shielded from H2O (covered up) unable to aggregate bilayer fragment with “exposed” nonpolar (N) surface N P N P N P N P N P Bilayer part of fragment exposing hydrocarbons, not stable, surface would aggregate with other non polar surfaces, to form large aggregates. Having a material that acts as an interfacial agent, detergent, N is the non polar surfaces of a bile salt, and the P is the polar surface with those hydroxyl groups and that would be the place for aqueous solution. Non polar surfaces of each of these is interfacing with one another, covering up the surface, this non polar surface. Adding them one at a time, idea that what is happening as a result is that a non polar surface is converted into a polar surface, this is then stable in aqueous solution whereas a hydrocarbon surface exposed is not a stable situation. This when continued for this particle would enable it to diffuse around and deliver its contents to an absorbing cell. N P N P N P N P bound bile salts interface with nonpolar surface & H2O 6

10 Bile salts: synthesis, secretion
Adapted from Physiology (Berne & Levy), Fig Bile salts: synthesis, secretion bile salt polar face synthesized in the liver cholesterol bile acid bile acyl-CoA + amino acid bile salt secreted as a component of bile bile secretion stimulated by secretin (target: liver) CCK (target: liver & gall bladder) absorbed bile salts nonpolar surfaces OH groups amide group carboxyl group cylindrical micelle mixed micelle cross-section Where these bile salts come from, all made in the liver, cholesterol is the starting point ,precursor of all steroids. Cholesterol in multistep pathway get first bile acid, carboxyl group is activated by linking with coenzyme A, those details are omitted. Then linked with amino acid. Bile in addition to coming from the liver is stored and concentrated by gall bladder, stimulus for secretion from these two places are the two hormones that have been previously mentioned, secretin and CCK are both secreted by duodenum in response to materials that enter that part of the GI tract, secretion stimulates the liver to produce more bile and the CCK causes the gall bladder to contract and force the bile into the GI tract. In addition bile slats themselves are stimulants for further secretion of bile salts. Mixed micelle in cross section, interior are FA monoglycerides, vitamin A D, glycerol, at the edges or rims are where bile salts bind and in effect, cover up the other wise exposed hydrocarbon surfaces that are part of these structures. bile salt chol- esterol phos- pholipid fatty acid 7 monoglyceride

11 Enterohepatic circulation (bile salt recycling)
bile salts absorbed toward end of ileum absorption by Na+– driven cotransport Na+–bile salt symport carried in portal blood bound to albumin added to bile again by liver & secreted again typically make 3-4 roundtrips during average meal bile salts cholesterol Bile salts undergo recycling process in digestion, enterohepatic circulation, ere shows the secretion, they do their job in GI lumen, towards the end of small intestine they get reabsorbed and that reabsorption involves a mechanism, sodium gradient driven symport process, sodium ion drives the active absorption of bile salt so they get into portal circulation, get back to the liver, and are repackaged in bile and get secreted again This appearance of bile salt back in the liver is stimulator, signal for the liver to make and secrete more of them. Several round trips occur of bile slats during a meal of significant size. Sherwood, Fig.16-17 8

12 Digestion of lipids: hydrolysis
triglycerides (TG) TG + H2O → diglyceride + fatty acid (FA) diglyceride + H2O → monoglyceride (MG) + FA Sum: cholesterol esters & phospholipids* (PL) ↓ esterase ↓ phospholipases FA + cholesterol (chol) FA + lysoPL in all cases, products are more polar than reactants reactions generate surfactants + 2 H+ Simple chemistry involved in getting the process going. Here is the substrate, triglyceride, two step process to go from triglycerides to monoglycerides. Two steps, in essence, lipase is hydrolyzing two ester bonds, so that here are the two FA that results, this ester linkage is not hydrolyzed by the lipase. An important feature of this is that it converts something that is not absorbable, triglyceride into substances that are absorbable. But also result of this reaction is that it converts something which is very non polar into products that are amphiphilic. Carbonyl groups are partially polar, make them into carboxylic acids, this functional group is much more polar, also oxygen without hydrogen, added hydrogens to them, two hydrogen bond forming groups, hydroxyl groups that are more polar than these bridge oxygens, what is happening in the process is we are generating amphiphilic substances that will be useful for dispersion of products and substrates themselves. In addition cholesterol esters are acted upon by an esterase and phospholipases convert these phospholipids to removing one of the fatty acids, location, arrow is site of action, removing one of the fatty acids forming lysophospholipid, more polar, amphiphilic , contributes to dispersion of the substrates, fats themselves. Digestion producing materials that make digestion go faster. Digestion starts slowly but accelerates as a consequence of product starting to participate as facilitators. 9 * biliary & dietary

13 Digestion of fats: lipase & colipase
lipase: a globular protein unable to penetrate surface of lipid particles (can only reach surface molecules) salivary lipase: active at low pH pancreatic lipase: active at pH >7 hydrolyzes >80% of dietary fat colipase required cofactor anchors lipase to lipid particles nonpolar domain (tail) binds to lipid particle polar domain binds & activates lipase TG particle colipase lipase Some of it comes from salivary glands, its active at low pH, fat digestion gets a start in the stomach, taken over by the lipase from the pancreas that requires high pH, or at least neutral. Colipase is an essential activator. TG particle anchors the lipase by this colipase with a non polar tail group immersed into the lipid material and a globular head group that interacts with lipase enzyme, anchoring it there and also activating its catalytic activity. Lipase, instead of diffusing after brief action, is held there and does its job more rapidly as a consequence of this anchoring effect. 10

14 Particle transitions: formation of mixed micelles
initial hydrolysis & dispersion oil phase → emulsion droplets average particle size: 1000 μm → 10 μm dispersion to emulsion droplets due to hydrolysis products (e.g., MG & FA) being amphiphiles bile salts & phospholipids also contribute to dispersing of oil drops additional hydrolysis & mixed micelle formation emulsion droplets → mixed micelles as [FA] & [MG] ↑ , they leave droplets & combine with bile salts to form mixed micelles average particle size reduced from 10 μm to 10–2 μm How the particles undergo changes. The first part of this process is very slow. Idea is to convert oil phase, big particles into the emulsion droplets, go from a 1 mm particle, 1000 microns to one hundredth of that, in the 10 micron range. Get this dispersion as a result of the products, monoglycerides and fatty acids, amphiphilic, dispersion of big particles into smaller ones. Additional hydrolysis enables the formation of mixed micelles. Emlusion droplet to mixed micelle, from 10 micron to .01 microns. This is a consequence of more fatty acids, more monoglycerides and the bile salts themselves. Bile salts are crucial for formation of mixed micelles, two molecule thick particles that are the product of the digestion. 11

15 Lipid particle summary
particle size range, surface molecular components type μ m area, cm2/g surface inside* oil drop 102 – – 100 TG TG emulsion – – 104 FA, MG, PL, TG droplet bile salts, TG mixed –3 – 10–1 105 – 107 bile salts, FA, – micelle MG, PL chylo –1 – – 105 apolipo- TG micron proteins, PL * all contain chol, chol esters & fat-soluble vitamins relative diameters of small circle & large (part) circle is 100 Summary of some of the particles so far. Oil drop originally only fat, on surface and inside. Emulsion droplets have TG still on the inside, and on surface they have FA, monoglycerides, all allow larger fraction of surface susceptible to digestion. Mixed micelle ands bile salts to this, no triglycerides. Do not have separate molecules on interior, they are two molecules thick, interior is the hydrocarbon part of all of these surface molecules. Emulsion droplet which follows upon dispersion or emulsification of the oil drop, which in its purest form is just the TG, as indicated here, it has TG on the surface and in its interior. The inside is defined as location that doesn’t have surface contact, right below the surface. Emulsion droplet has a lot of surfactants on its surface, that’s why its emulsified, consists of some digestion products, FA MG, phospholipids, and also some triglycerides. The lipase that’s doing the digestion only does that on the surface of these particles so some of the triglyceride, most is in the interior, but at least some has to be on the surface, transiently, in order fo the lipase to get at the material to digest it. The emulsion droplets give way to mixed micelles. Most important feature of which is they have no molecules on the interior. All of the components have a surface exposure and these are all amphiphilic, polar parts interface with the solvent and hydrocarbon parts are in the interior. To clarify this, table doesn’t indicate how they go form one to the other. 12

16 Lipid particles ↕ ↕ emulsion droplet TG ↔ ←TG, DG ←MG, FA, etc.
Here, representation of emulsion droplet that has triglycerides inside, constitutes the bulk of the particle, and on the surface are surfactants consisting of monoglycerides FA, so on. In addition on the surface will be some of the substrates that targets for the lipase, triglycerides and diglycerides (intermediate on the way to monoglyceride). Other point is that these droplets would be in rapid equilibrium with the monomeric form of these molecules. Amphiphilics substances are not very soluble, but not zero solubility, constantly dissociating and rebinding. Small fraction of these materials are in monomeric free form in te aqueous solution and they are also in rapid equilibrium with the mixed micelles. Mixed micelle is a bilayered structure where the edges or rims are occupied by the bile salts, and in the center section of these are all of these amphiphilic substances, monoglycerides, FA< with polar head groups interphasing with the solvent and hydrocarbon in the interior. Includes cholesterol with hydroxyl sticking out but the rest of it immersed in the interior. During the process start out with only oil drop, get increasing fraction of the lipid in the form of emulsion droplet, give way to mixed micelles which is the stage at which absorption can take place. mixed micelle

17 Absorption from lumen movement of lipid digestion products (FA, MG, etc) across mucosal plasma membrane by simple diffusion of monomers absorption also occurs via fatty acid transfer protein (FATP) microvilli provide very large absorbing surface, but convolutions & glycocalyx produce unstirred layer well-mixed luminal contents unstirred layer brush border membrane microvillus cytosol FATP diffusion of micelles fatty acids through unstirred layer lysophos- pholipids Absorption process, from GI lumen into enterocyte. Fatty acid monomer molecule that is diffusing across the phospholipid bilayer membrane but getting into the interior of the cell by simple diffusion. In addition FA are carried by transfer protein, fatty acid transfer protein, FATP. The other monomers, monoglyceride, cholesterol so on, get across by simple diffusion, first partition into the phospholipid membrane then get into the cytosol. Reason there is net flow is that once these get into the cell they are further processed, concentrations are diminished, net transfer down the concentration gradient. Problem with these molecules getting to the surface where they can diffuse across is because of the low solubility, concentration at the surface is low. In addition don’t get across this layer, unstirred layer because peristalsis which is important in moving things along in digestive process does not affect a layer near the surface, the unstirred layer. Material that is part of the cell surface is partly accountable, glycocalyx, protecting the cell surfaces and inhibits the mixing up that would enable the substances to get at the surface. This is where the role of micelles is applied. Micelle starts out in the well mixed contents and then acts as a carrier to carry these materials across this unstirred layers. chol monoglycerides Adapted from Fig (B & L) 13

18 Absorption: role of micelles
unstirred layer μm thick prevents peristaltic mixing from moving luminal contents close to cell surface crossed by micelle diffusion because of very low solubility of lipid molecules & very large distance, absorption would be very slow without micelles mixed micelles act as: carriers of lipid monomers (FA, MG, chol, vit. A, D, E, K) reservoirs: as monomers absorbed, they are rapidly replaced by dissociation from micelles The mixed micelle diffuses and gets close enough to dissociate its contents, indicated by the double headed arrows, dynamic structures with monomers constantly dissociating and rebinding. Result is that adequate concentrations of these monomers are at the surface to allow the absorption process to take place. Micelles are carriers of these materials, act as reservoirs too. In the absence of these micelles, very little absorption of lipid material takes place. One strategy to limit the uptake of fats, calories, is to knock out the affect of these micelles. 14

19 After absorption FAs (>10Cs) & MGs converted into TGs
Adapted from Fig (B & L) After absorption MG+FA lysoPL chol FAs (>10Cs) & MGs converted into TGs after diffusion into smooth ER, FAs activated, forming acyl-CoAs (2 ATPs used/FA): FA + CoA → acyl-CoA from these, TGs synthesized: 2 acyl-CoA + MG → TG PL & chol esters (cholE) also resynthesized products packaged into chylomicrons MG+ 2FA lysoPL +FA chol +FA smooth ER for WebPage TG PL cholE exocytosis lipopro- tein coat Next thing that happens to absorbed materials is they undergo a chemical conversion back into triglycerides and related materials. Go outside, get into the cell, and this takes lace in the smooth ER lumen where they are resynthesized. First FA are activated to acyl CoA’s, 2 ATP for each coenzyme A linked up, then activated fatty acids are relinkied to monoglycerides, get back triglycerides. In a similar way phospholipids are resynthesized and cholesterol esters. Carboxyl groups must undergo activation, become activated precursors before being linked up. Some energy is used to resynthesize these materials. What is happened is that we have back these non polar substances and in order to be transported elsewhere, they have to be packaged into a particle which will not aggregate and form an insoluble mess. Particle that is reduced in the cells is the chylomicron. These are non polar cores containing these products with a coating that consists of phospholipid and protein. Once those are formed by exocytosis, escape the cell and get into the lymphatic system. chylomicron 15 lacteal

20 Packaging for transport
Lehninger et al., 3rd ed., Fig. 17-2 chylomicrons particles for transport of lipids to liver & adipocytes size: 0.1–1 µm average composition: TG (84%) chol (2%) cholE (4%) PL (8%) apolipoproteins (2%) apolipo- proteins Lehninger et al., 2nd ed., Fig. 16-2 PL Versions of lipoproteins where the core is the non polar material, triglyceride predominantly. 84% on average, with smaller amounts of other lipids and phospholipid primarily being at the surface acting as surfactant along with the proteins. Several proteins that have these code names, collectively known as apolipoproteins, protein part of a particle known as lipoproteins. Protein constitutes only a small fraction chol PL 16 cholE, TG

21 Export of chylomicrons
exocytosis chylomicrons enclosed in secretory vesicles in Golgi complex vesicles diffuse to basolateral membrane via fusion, chylomicrons released into interstitial space chylomicrons enter lymphatic system (lacteals) by diffusion blood at thoracic duct from lymphatics FAs with <12 Cs diffuse through cell & basolateral membrane, into blood transported in blood bound to albumin vesicle membrane lacteal Process of getting these chylomicrons first into lymphatics, eventually get into the blood. FA that are short chained, about 10 or fewer carbons bypass this complex pathway and get directly into the blood by way of the enterocytes and are carried in the blood by protein albumin, carrier of amphiphilic and non polar materials in the blood. 17

22 Summary of lipid digestion & absorption
oil drop enterocyte lipase- colipase MG FA (>10C) 4ATPs/TG TG emulsion droplet apolipoproteins phospholipids adapted from Devlin, 5th ed. pp TG MG FA BILE SALTS Another reminder: component of the micelle enables them to form bile salts, are also reabsorbed far down in small intestine, these bile salts get back to the liver, carried by albumin, and the bile salts typically will undergo recycling, several times during a meal of any average amount of fat in them. The chylomicrons are the major carriers of absorbed fat away from the GI tract. lipase- colipase chylomicron chylomicron FA (<12C) FA mixed micelle BILE SALTS albumin BILE SALTS 18

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