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Luminal Events in Gastrointestinal Lipid Digestion

Bengt Borgström, John S. Patton

10.1002/cphy.cp060422

Source: Supplement 19: Handbook of Physiology, The Gastrointestinal System, Intestinal Absorption and Secretion

Originally published: 1991

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Enzymes Related to Intraluminal Digestion of Dietary Fat
1.1 Lingual Lipase and Gastric Lipase
1.2 Pancreatic Lipase and Colipase
1.3 Carboxyl Ester Lipase
1.4 Bile Salts as Cofactors of Carboxyl Ester Lipase
1.5 Phospholipase A2
2 Bile
3 Product Phases of Lipid Digestion: Experience From in vitro Studies
4 Lipid‐Phase Products Found in Luminal Contents During Digestion
5 Cooperative Effects in Lipid Digestion
6 Fate of Fat‐Soluble Molecules During Fat Digestion
7 Conclusions
Figure 1. Figure 1.

Differing positional specificity of triglyceride lipases. Lingual and gastric lipases initiate lipolysis in the stomach by removing 1 fatty acid from the triglyceride molecule. In vitro in the presence of albumin, acid lipases can completely hydrolyze a triglyceride molecule; however, in stomach content the reaction produces mainly diglyceride and fatty acid 100. Pancreatic lipase and colipase have an absolute specificity for the outside ester bonds of the triglyceride molecule. Lipolysis is stepwise: first 1 fatty acid is removed to produce a molecule of diglyceride and then remaining outer fatty acid is removed from the diglyceride to produce 2‐monoglyceride 206.

[From Patton and Hofmann 225.]
Figure 2. Figure 2.

Hypothetical drawing of lingual lipase (acid lipase) molecules as they penetrate a milk fat globule membrane and initiate hydrolysis of triglyceride molecules in fat droplet core. Darker figures, right, fatty acids and diglyceride produced by lipolysis. Milk fat droplets are covered with inner phospholipid monolayer and outer phospholipid bilayer that originate from apical cell membrane during milk fat secretion.

[From Patton et al. 166.]
Figure 3. Figure 3.

Relative importance of different lipases for triglyceride digestion changes with age. At birth both pancreatic lipase and carboxyl ester lipase are secreted by the pancreas in relatively low amounts (2%‐5% adult level). Acid lipases appear to be present at adult levels in newborns. Human milk lipase (carboxyl ester lipase) partially compensates for low levels of pancreatic lipases. Pasteurization of milk decreases fat absorption in preterm infants by 1/3 82, 105.

[From Patton and Hofmann 225.]
Figure 4. Figure 4.

Relative importance of acid lipases in fat digestion in cases of pancreatic insufficiency, where total fat absorption may be reduced 50%. Acid lipase function and output are not usually impaired in these cases and are thus responsible for a much greater proportion of fat digestion than in normal subjects. It is uncertain whether acid lipase output increases with pancreatic insufficiency. Because pancreatic output of bicarbonate is also impaired with pancreatic insufficiency, the pH of duodenal contents is lower and conditions for acid lipolysis in the small intestine are more favorable than in normal subjects 81, 182.

[From Patton and Hofmann 225.]
Figure 5. Figure 5.

Effect of bile salts and colipase on pancreatic lipase activity. In the absence of bile salts, pancreatic lipase (Mr ∼52,000) by itself can hydrolyze long‐chain triglyceride (panel 1). With addition of micellar bile salts to lipase–fat droplet system, lipase is swept off the surface of fat droplets into the aqueous phase (panel 2). Pancreatic colipase (Mr 10,000) binds to fat droplets in presence of micellar bile salts and serves as an anchor for lipase at fat droplet surface (panel 3). Colipase has no known catalytic activity. Colipase is secreted in pancreatic juice as procolipase. In intestinal content, trypsin cleaves a pentapeptide from the NH2 terminus of procolipase to form colipase. This “activated” colipase is now capable of serving as an anchor for lipase on phospholipid‐coated fat droplets in the presence of bile salts. Procolipase does not possess this function but can still reverse inhibition of lipase by bile salts in the absence of phospholipid 34.

[From Patton and Hofmann 225.]
Figure 6. Figure 6.

Schematic illustration of regulation effects exerted by “interfacial quality” and colipase on lipase activity toward a phosphatidylglycerol film.

[Adapted from Verger 206.]
Figure 7. Figure 7.

Mechanism for hydrolysis of fat in the intestine by lipase and colipase. A: lipase and procolipase are secreted as separate proteins from pancreas. Lipid‐binding sites of colipase, Ile7–9 and Tyr55–56, are hidden by NH2− and COOH‐terminal chains of colipase; potential lipid‐binding site in lipase is buried within the molecule. B: in the intestine the 2 proteins are bound in a complex that is not fully active. Binding is favored by mixed long‐chain fatty acids–bile salt micelles. C: trypsin attacks procolipase, with removal of peptide chains in NH2 and COOH‐terminal ends, which leads to exposure of lipid‐binding regions in colipase. At the same time lipase conformation is changed, leading to exposure of a tryptophan residue in lipase. This combined lipid‐binding region drives the complex to the mixed substrate interface, where the triacylglycerol is hydrolyzed by lipase. L, lipase; PL, phospholipid; TG, triglyceride.

[From Borgström and Erlanson‐Albertsson 40.]
Figure 8. Figure 8.

Pancreatic phospholipase A2 hydrolyzes membrane phospholipids. Dietary phospholipids are hydrolyzed at position 2 by pancreatic phospholipase A2 to yield 1 molecule of lysophospholipid and 1 molecule of free fatty acid. More is known about the chemistry of phospholipase A2 than the other lipolytic enzymes because it is small (Mr 14,000) and quite stable (7 disulfide bridges) and therefore easy to work with. Phospholipase A2 is secreted in pancreatic juice as an inactive proenzyme. Tryptic removal of 7 amino acids from the NH2 terminus makes it active against aggregated substrates if bile salts and calcium are present. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cardiolipin are all substrates for phospholipase A2.

[From Patton and Hofmann 225.]
Figure 9. Figure 9.

Schematic diagram of cholesterol (Ch) solubilization in bile salt (BS)‐lecithin (L) “mixed‐disk” micelles. Top, equilibrium between monomers and micellar aggregates in unsaturated systems. In supersaturated systems (bottom), monomers interact to form precipitated phases. During fat digestion concentrated product phases are dispersed from liquid crystals to vesicles to mixed‐disk micelles.

[From Mazer and Carey 146.]
Figure 10. Figure 10.

Bile is a source of endogenous lipid and detergent (bile salt). Bile salts are natural detergent molecules derived from hepatic catabolism of cholesterol. In addition to bile salts, bile contains phospholipids (principally phosphatidylcholine), unesterified cholesterol, bile pigments, and a mixture of proteins. In addition many other endogenous substances may be secreted in bile in small amounts and undergo enterohepatic cycling, including bacterial reduction products of bilirubin, lipovitamins (particularly biologically active forms of vitamin D2), water‐soluble vitamins (particularly vitamin B12), folic acid, pyridoxine, and many estrogenic steroids. Several exogenous substances also are secreted in bile and can undergo enterohepatic circulation, including commonly used drugs such as indomethacin, cardiac glycosides, chlorpromazine, antibiotics, cholophilic dyes, and radiocontrast media 46.

[From Patton and Hofmann 225.]
Figure 11. Figure 11.

Models of bile salt (BS)‐phosphatidylcholine (PC) mixed micelles: PCDe and PCDi, external and internal PC of the mixed disk, respectively.

[From Hauton et al. 109.]
Figure 12. Figure 12.

Average daily input of lipids to gastrointestinal tract of western human. Lipids enter gastrointestinal tract from 3 major sources: diet, bile, and desquamated cells. In addition an unknown amount of membrane lipid is synthesized in the lumen by microbes. Dietary lipid includes 70–160 g/day triglyceride, 2–4 g/day phospholipid, 0.5 g/day sterols, and 0.5 g/day trace lipids. Biliary input comprises 7–22 g/day lipid in the form of phospholipids and sterols 48.

[From Patton and Hofmann 225.]
Figure 13. Figure 13.

Fatty acids produced during gastric lipolysis help emulsify fat in the stomach. Products of acid lipases, protonated fatty acid and diacylglycerol, remain dissolved in the oil phase and at water‐oil interphase of fat droplets. Fatty acids produced by acid lipases then stabilize the fat emulsion that occurs during muscular activity of the stomach 10, 164.

[From Patton and Hofmann 225.]
Figure 14. Figure 14.

Unstirred digestion of a fat droplet by pancreatic lipase and colipase at pH 6.9. In the presence of bile salts visible product phases can be seen on a microscope slide when micellar solution becomes saturated. Rigid crystalline calcium fatty acid–soap phase forms first if unbound calcium is present. Depletion of calcium and accumulation of monoglyceride act to stop calcium soap formation. Remaining undigested fat droplet is often extruded out of the calcium soap shell. Digestion continues until only a tiny remnant of the original fat droplet is present within a pool of isotropic (liquid‐crystalline) product phase. Stirring disperses the liquid‐crystalline phase into vesicles 163, 166.
Figure 15. Figure 15.

Freeze‐fracture electron micrograph of isotropic product phase produced by pancreatic lipase at pH 8.3 in the absence of bile salts. At this pH many of the fatty acids are ionized and product phases form swollen lamellae and vesicles, which can be seen (arrows) at periphery of lamellae. Isotropic appearance may be caused by variable thickness of water layers.

[From Rigler and Patton 178.]
Figure 16. Figure 16.

Freeze‐fracture electron micrographs of morphology of lipolytic product phases in killifish gut contents 30 min after feeding a high‐fat meal. Crystalline calcium soap phase was not observed. A: masses of product phases of rough lamellar product lipid (LA) fill the gut lumen. B: higher magnification of rough lamellae seen in A. Note irregular surface with bumps (arrows).

[From Rigler et al. 177.]
Figure 17. Figure 17.

A: phase diagram of ternary sunflower oil monoglyceride–soybean oil–water system at 40°C. Liquid‐crystalline (LC) phases are lamellar (L‐LC), cubic (C‐LC), and reversed hexagonal (H‐LC). B: phase diagram of ternary monoolein/oleic acid–oleate/sodium taurodeoxycholate (NaTDC) system in 10 mM Trismaleate buffer (pH 6.5) + 150 mM NaCl + 0.02% NaN, at 25°C. Total amount of 3 compounds is held constant at 1% (weight). Phases formed in excess of aqueous phase are a liquid‐crystalline phase (LC; a viscous isotropic phase) and an L2 phase (inverted micelles or microemulsion). With increasing amounts of NaTDC, lipid phases are dispersed (D) in the aqueous phase above the critical micellar concentration. In area where NaTDC dominates a clear isotropic mixed micellar (M) solution exists.

[A from Lindström et al. 135; B from Borgstrom 33.]
Figure 18. Figure 18.

Lipid phases formed by different proportions of monoolein and oleic acid coexisting with excess of aqueous buffer phase at pH 3.0 (A) and pH 6.5 (B). Total amount of lipid was held constant at 1% (wt/wt). Observed lipid phases are cubic (C‐LC), reversed hexagonal (H‐LC), and an L2 phase. Between hexagonal and L2 phases is a viscous isotropic phase of unknown structure. C: proposed model for cubic phase. Hexagon bilayer units are connected into polyhydra, which are connected along the cubic cell axis, resulting in a continuous lipid bilayer separating 2 continuous water regions (shaded areas). If the bilayer is curved, another possible structural arrangement with the same general characteristics is obtained. D: structure of reversed hexagonal phase, which shows birefringence in the polarizing microscope. Infinite water channels (shaded areas) arranged parallel in a continuous lipid matrix. E: proposed structure of L2 phase. Units of water layers (shaded areas) are separated by lipid bilayers.

[From Lindström et al. 135.]
Figure 19. Figure 19.

Hydrolysis of long‐chain triglyceride (olive oil) by porcine pancreatic lipase and colipase in the presence of 4 mM bile salt mixture of cholate and taurocholate (1:1). Electron micrograph shows product lamellae and vesicles (arrows) at surface of a large oil droplet and vesicles in aqueous phase 10 min after addition of enzyme. Inset, lipolytic product–bile salt ratio and number of liposomes (vesicles) produced over 30 min. In early minutes of reaction, no product phases can be seen by electron microscopy because unsaturated micellar solution solubilizes lipid faster than it can accumulate.

[From Rigler et al. 177.]
Figure 20. Figure 20.

Size distribution of vesicles seen in Figure 19 and extreme magnification of smallest visible vesicle with scale drawing of mixed‐lipid micelle (15 nm diam). Arrows, spherical vesicles, which project out of the plane of the page. Granular background is made up of platinum grains (2–4 nm diam).

[From Rigler et al. 177.]
Figure 21. Figure 21.

Lipolytic product structure in a system containing pancreatic lipase and colipase, triolein, and 17 mM taurodeoxycholate in which lipolysis was allowed to go to completion (2 h). Upper left, apparent layers of product lamellae (LA); lower right, aqueous phase. Middle, vesiculation of product lamellae (VE) is occurring.

[From Rigler et al. 177.]
Figure 22. Figure 22.

Freeze‐fracture electron microscopy of fat droplets undergoing digestion in intestinal lumen of killifish. A: 60 min after feeding a fat meal product, lamellae can be seen vesiculating (arrows) at surface of fat droplets. B: oil droplet after 90 min, with lamellar and vesicular products

[From Rigler et al. 177.]
Figure 23. Figure 23.

Schematic “zipper model” of fat digestion and dispersion. Right, oil phase (fat droplet) contains triglyceride, diglyceride, and dissolved nonpolar lipid (which includes lipophilic xenobiotics). As digestion proceeds and triglyceride and diglyceride are hydrolyzed to fatty acids and monoglcyerides, nonpolar lipids dissolve in the oil flow directly into product phases (center, viscous isotropic phase). Because lipid hydrolysis is a water‐dependent process, aqueous channels must be present to supply water molecules to active site of pancreatic lipase. Bile salts disperse product phases produced by lipase into mixed disk micelles and/or liposomes and vesicles. Monomeric bile salts and lipids are in equilibrium with micellar aggregates. Nonpolar lipids may remain with lipolytic products in micelles or may precipitate as crystalline aggregates (not shown) in micellar phase.

[From Patton 159.]


Figure 1.

Differing positional specificity of triglyceride lipases. Lingual and gastric lipases initiate lipolysis in the stomach by removing 1 fatty acid from the triglyceride molecule. In vitro in the presence of albumin, acid lipases can completely hydrolyze a triglyceride molecule; however, in stomach content the reaction produces mainly diglyceride and fatty acid 100. Pancreatic lipase and colipase have an absolute specificity for the outside ester bonds of the triglyceride molecule. Lipolysis is stepwise: first 1 fatty acid is removed to produce a molecule of diglyceride and then remaining outer fatty acid is removed from the diglyceride to produce 2‐monoglyceride 206.

[From Patton and Hofmann 225.]


Figure 2.

Hypothetical drawing of lingual lipase (acid lipase) molecules as they penetrate a milk fat globule membrane and initiate hydrolysis of triglyceride molecules in fat droplet core. Darker figures, right, fatty acids and diglyceride produced by lipolysis. Milk fat droplets are covered with inner phospholipid monolayer and outer phospholipid bilayer that originate from apical cell membrane during milk fat secretion.

[From Patton et al. 166.]


Figure 3.

Relative importance of different lipases for triglyceride digestion changes with age. At birth both pancreatic lipase and carboxyl ester lipase are secreted by the pancreas in relatively low amounts (2%‐5% adult level). Acid lipases appear to be present at adult levels in newborns. Human milk lipase (carboxyl ester lipase) partially compensates for low levels of pancreatic lipases. Pasteurization of milk decreases fat absorption in preterm infants by 1/3 82, 105.

[From Patton and Hofmann 225.]


Figure 4.

Relative importance of acid lipases in fat digestion in cases of pancreatic insufficiency, where total fat absorption may be reduced 50%. Acid lipase function and output are not usually impaired in these cases and are thus responsible for a much greater proportion of fat digestion than in normal subjects. It is uncertain whether acid lipase output increases with pancreatic insufficiency. Because pancreatic output of bicarbonate is also impaired with pancreatic insufficiency, the pH of duodenal contents is lower and conditions for acid lipolysis in the small intestine are more favorable than in normal subjects 81, 182.

[From Patton and Hofmann 225.]


Figure 5.

Effect of bile salts and colipase on pancreatic lipase activity. In the absence of bile salts, pancreatic lipase (Mr ∼52,000) by itself can hydrolyze long‐chain triglyceride (panel 1). With addition of micellar bile salts to lipase–fat droplet system, lipase is swept off the surface of fat droplets into the aqueous phase (panel 2). Pancreatic colipase (Mr 10,000) binds to fat droplets in presence of micellar bile salts and serves as an anchor for lipase at fat droplet surface (panel 3). Colipase has no known catalytic activity. Colipase is secreted in pancreatic juice as procolipase. In intestinal content, trypsin cleaves a pentapeptide from the NH2 terminus of procolipase to form colipase. This “activated” colipase is now capable of serving as an anchor for lipase on phospholipid‐coated fat droplets in the presence of bile salts. Procolipase does not possess this function but can still reverse inhibition of lipase by bile salts in the absence of phospholipid 34.

[From Patton and Hofmann 225.]


Figure 6.

Schematic illustration of regulation effects exerted by “interfacial quality” and colipase on lipase activity toward a phosphatidylglycerol film.

[Adapted from Verger 206.]


Figure 7.

Mechanism for hydrolysis of fat in the intestine by lipase and colipase. A: lipase and procolipase are secreted as separate proteins from pancreas. Lipid‐binding sites of colipase, Ile7–9 and Tyr55–56, are hidden by NH2− and COOH‐terminal chains of colipase; potential lipid‐binding site in lipase is buried within the molecule. B: in the intestine the 2 proteins are bound in a complex that is not fully active. Binding is favored by mixed long‐chain fatty acids–bile salt micelles. C: trypsin attacks procolipase, with removal of peptide chains in NH2 and COOH‐terminal ends, which leads to exposure of lipid‐binding regions in colipase. At the same time lipase conformation is changed, leading to exposure of a tryptophan residue in lipase. This combined lipid‐binding region drives the complex to the mixed substrate interface, where the triacylglycerol is hydrolyzed by lipase. L, lipase; PL, phospholipid; TG, triglyceride.

[From Borgström and Erlanson‐Albertsson 40.]


Figure 8.

Pancreatic phospholipase A2 hydrolyzes membrane phospholipids. Dietary phospholipids are hydrolyzed at position 2 by pancreatic phospholipase A2 to yield 1 molecule of lysophospholipid and 1 molecule of free fatty acid. More is known about the chemistry of phospholipase A2 than the other lipolytic enzymes because it is small (Mr 14,000) and quite stable (7 disulfide bridges) and therefore easy to work with. Phospholipase A2 is secreted in pancreatic juice as an inactive proenzyme. Tryptic removal of 7 amino acids from the NH2 terminus makes it active against aggregated substrates if bile salts and calcium are present. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cardiolipin are all substrates for phospholipase A2.

[From Patton and Hofmann 225.]


Figure 9.

Schematic diagram of cholesterol (Ch) solubilization in bile salt (BS)‐lecithin (L) “mixed‐disk” micelles. Top, equilibrium between monomers and micellar aggregates in unsaturated systems. In supersaturated systems (bottom), monomers interact to form precipitated phases. During fat digestion concentrated product phases are dispersed from liquid crystals to vesicles to mixed‐disk micelles.

[From Mazer and Carey 146.]


Figure 10.

Bile is a source of endogenous lipid and detergent (bile salt). Bile salts are natural detergent molecules derived from hepatic catabolism of cholesterol. In addition to bile salts, bile contains phospholipids (principally phosphatidylcholine), unesterified cholesterol, bile pigments, and a mixture of proteins. In addition many other endogenous substances may be secreted in bile in small amounts and undergo enterohepatic cycling, including bacterial reduction products of bilirubin, lipovitamins (particularly biologically active forms of vitamin D2), water‐soluble vitamins (particularly vitamin B12), folic acid, pyridoxine, and many estrogenic steroids. Several exogenous substances also are secreted in bile and can undergo enterohepatic circulation, including commonly used drugs such as indomethacin, cardiac glycosides, chlorpromazine, antibiotics, cholophilic dyes, and radiocontrast media 46.

[From Patton and Hofmann 225.]


Figure 11.

Models of bile salt (BS)‐phosphatidylcholine (PC) mixed micelles: PCDe and PCDi, external and internal PC of the mixed disk, respectively.

[From Hauton et al. 109.]


Figure 12.

Average daily input of lipids to gastrointestinal tract of western human. Lipids enter gastrointestinal tract from 3 major sources: diet, bile, and desquamated cells. In addition an unknown amount of membrane lipid is synthesized in the lumen by microbes. Dietary lipid includes 70–160 g/day triglyceride, 2–4 g/day phospholipid, 0.5 g/day sterols, and 0.5 g/day trace lipids. Biliary input comprises 7–22 g/day lipid in the form of phospholipids and sterols 48.

[From Patton and Hofmann 225.]


Figure 13.

Fatty acids produced during gastric lipolysis help emulsify fat in the stomach. Products of acid lipases, protonated fatty acid and diacylglycerol, remain dissolved in the oil phase and at water‐oil interphase of fat droplets. Fatty acids produced by acid lipases then stabilize the fat emulsion that occurs during muscular activity of the stomach 10, 164.

[From Patton and Hofmann 225.]


Figure 14.

Unstirred digestion of a fat droplet by pancreatic lipase and colipase at pH 6.9. In the presence of bile salts visible product phases can be seen on a microscope slide when micellar solution becomes saturated. Rigid crystalline calcium fatty acid–soap phase forms first if unbound calcium is present. Depletion of calcium and accumulation of monoglyceride act to stop calcium soap formation. Remaining undigested fat droplet is often extruded out of the calcium soap shell. Digestion continues until only a tiny remnant of the original fat droplet is present within a pool of isotropic (liquid‐crystalline) product phase. Stirring disperses the liquid‐crystalline phase into vesicles 163, 166.



Figure 15.

Freeze‐fracture electron micrograph of isotropic product phase produced by pancreatic lipase at pH 8.3 in the absence of bile salts. At this pH many of the fatty acids are ionized and product phases form swollen lamellae and vesicles, which can be seen (arrows) at periphery of lamellae. Isotropic appearance may be caused by variable thickness of water layers.

[From Rigler and Patton 178.]


Figure 16.

Freeze‐fracture electron micrographs of morphology of lipolytic product phases in killifish gut contents 30 min after feeding a high‐fat meal. Crystalline calcium soap phase was not observed. A: masses of product phases of rough lamellar product lipid (LA) fill the gut lumen. B: higher magnification of rough lamellae seen in A. Note irregular surface with bumps (arrows).

[From Rigler et al. 177.]


Figure 17.

A: phase diagram of ternary sunflower oil monoglyceride–soybean oil–water system at 40°C. Liquid‐crystalline (LC) phases are lamellar (L‐LC), cubic (C‐LC), and reversed hexagonal (H‐LC). B: phase diagram of ternary monoolein/oleic acid–oleate/sodium taurodeoxycholate (NaTDC) system in 10 mM Trismaleate buffer (pH 6.5) + 150 mM NaCl + 0.02% NaN, at 25°C. Total amount of 3 compounds is held constant at 1% (weight). Phases formed in excess of aqueous phase are a liquid‐crystalline phase (LC; a viscous isotropic phase) and an L2 phase (inverted micelles or microemulsion). With increasing amounts of NaTDC, lipid phases are dispersed (D) in the aqueous phase above the critical micellar concentration. In area where NaTDC dominates a clear isotropic mixed micellar (M) solution exists.

[A from Lindström et al. 135; B from Borgstrom 33.]


Figure 18.

Lipid phases formed by different proportions of monoolein and oleic acid coexisting with excess of aqueous buffer phase at pH 3.0 (A) and pH 6.5 (B). Total amount of lipid was held constant at 1% (wt/wt). Observed lipid phases are cubic (C‐LC), reversed hexagonal (H‐LC), and an L2 phase. Between hexagonal and L2 phases is a viscous isotropic phase of unknown structure. C: proposed model for cubic phase. Hexagon bilayer units are connected into polyhydra, which are connected along the cubic cell axis, resulting in a continuous lipid bilayer separating 2 continuous water regions (shaded areas). If the bilayer is curved, another possible structural arrangement with the same general characteristics is obtained. D: structure of reversed hexagonal phase, which shows birefringence in the polarizing microscope. Infinite water channels (shaded areas) arranged parallel in a continuous lipid matrix. E: proposed structure of L2 phase. Units of water layers (shaded areas) are separated by lipid bilayers.

[From Lindström et al. 135.]


Figure 19.

Hydrolysis of long‐chain triglyceride (olive oil) by porcine pancreatic lipase and colipase in the presence of 4 mM bile salt mixture of cholate and taurocholate (1:1). Electron micrograph shows product lamellae and vesicles (arrows) at surface of a large oil droplet and vesicles in aqueous phase 10 min after addition of enzyme. Inset, lipolytic product–bile salt ratio and number of liposomes (vesicles) produced over 30 min. In early minutes of reaction, no product phases can be seen by electron microscopy because unsaturated micellar solution solubilizes lipid faster than it can accumulate.

[From Rigler et al. 177.]


Figure 20.

Size distribution of vesicles seen in Figure 19 and extreme magnification of smallest visible vesicle with scale drawing of mixed‐lipid micelle (15 nm diam). Arrows, spherical vesicles, which project out of the plane of the page. Granular background is made up of platinum grains (2–4 nm diam).

[From Rigler et al. 177.]


Figure 21.

Lipolytic product structure in a system containing pancreatic lipase and colipase, triolein, and 17 mM taurodeoxycholate in which lipolysis was allowed to go to completion (2 h). Upper left, apparent layers of product lamellae (LA); lower right, aqueous phase. Middle, vesiculation of product lamellae (VE) is occurring.

[From Rigler et al. 177.]


Figure 22.

Freeze‐fracture electron microscopy of fat droplets undergoing digestion in intestinal lumen of killifish. A: 60 min after feeding a fat meal product, lamellae can be seen vesiculating (arrows) at surface of fat droplets. B: oil droplet after 90 min, with lamellar and vesicular products

[From Rigler et al. 177.]


Figure 23.

Schematic “zipper model” of fat digestion and dispersion. Right, oil phase (fat droplet) contains triglyceride, diglyceride, and dissolved nonpolar lipid (which includes lipophilic xenobiotics). As digestion proceeds and triglyceride and diglyceride are hydrolyzed to fatty acids and monoglcyerides, nonpolar lipids dissolve in the oil flow directly into product phases (center, viscous isotropic phase). Because lipid hydrolysis is a water‐dependent process, aqueous channels must be present to supply water molecules to active site of pancreatic lipase. Bile salts disperse product phases produced by lipase into mixed disk micelles and/or liposomes and vesicles. Monomeric bile salts and lipids are in equilibrium with micellar aggregates. Nonpolar lipids may remain with lipolytic products in micelles or may precipitate as crystalline aggregates (not shown) in micellar phase.

[From Patton 159.]
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Article Title: Luminal Events in Gastrointestinal Lipid Digestion
 

How to Cite

Bengt Borgström, John S. Patton. Luminal Events in Gastrointestinal Lipid Digestion. Compr Physiol 2011, Supplement 19: Handbook of Physiology, The Gastrointestinal System, Intestinal Absorption and Secretion: 475-504. First published in print 1991. doi: 10.1002/cphy.cp060422