Medical Physiology, 3rd Edition

Lipid Absorption

Products of lipolysis enter the bulk water phase of the intestinal lumen as vesicles, mixed micelles, and monomers

After their secretion in pancreatic juice and bile, respectively, the various activated pancreatic lipases and biliary bile salts, phosphatidylcholine, and cholesterol adsorb to the surface of the emulsion droplets arriving from the stomach (Fig. 45-12A). The lipolytic products—MAGs, fatty acids (including long-chain species from gastric lipolysis) that are now ionized at duodenal pH 5.5 to 6.5, lysophosphatidylcholine, and cholesterol—act as additional emulsifiers. As surface TAGs are hydrolyzed, they are replaced by TAGs from the core of the emulsion particle. As the emulsion droplets become progressively smaller, their surface area increases, so that the rate of hydrolysis increases. Initially, a crystalline calcium–fatty-acid soap phase forms near the surface of the TAG droplet, until the local free Ca2+ is depleted. At the same time, a multilamellar liquid crystalline layer of fatty acids, MAGs, lysophosphatidylcholine, cholesterol, and possibly bile salts builds up on the surface of the emulsion particle. This liquid crystalline layer buds off as a multilamellar liquid crystal vesicle (see Fig. 45-12B), which consists of several lipid bilayers. Bile-salt micelles transform these multilamellar vesicles into unilamellar vesicles (see Fig. 45-12C), which are single-lipid bilayers, and then into mixed micelles (see Fig. 45-12D) composed of bile salts and mixed lipids (i.e., fatty acids, MAG, lysophospholipids, and cholesterol).

image

FIGURE 45-12 Breakdown of emulsion droplets to mixed micelles. A, The core of the emulsion droplet contains TAGs, DAGs, and cholesteryl esters. On the surface are fatty acids, MAGs, lysolecithins, and cholesterol. Adsorbed to the surface are pancreatic lipase and possibly bile salts. As the lipases hydrolyze the TAGs at the surface, the TAGs from the core replace them, causing the droplet to shrink. B, A multilamellar liquid-crystalline layer of fatty acids, MAGs, lysolecithins, cholesterol, and bile salts builds up on the surface of the emulsion droplet, causing a small piece to bud off as a multilamellar vesicle. C, The addition of more bile salts to the multilamellar vesicle thins out the lipid coating and converts the multilamellar vesicle to a unilamellar vesicle. D, Further addition of bile salts leads to formation of a mixed micelle, in which hydrophobic lipid tails face inward and polar head groups face outward.

Continued digestion by PLA2 and other esterases can still occur on the mixture of aggregates now present. If intestinal contents taken from humans during fat digestion are centrifuged, three phases separate. An oily layer floats on top and contains fat droplets and lipolytic products that have not been solubilized by bile salts. A middle layer contains lipid vesicles, mixed lipid–bile-salt micelles, simple bile-salt micelles, and lipid monomers. Finally, a pellet contains debris, liquid crystals, and precipitated calcium soaps of fatty acids. Whereas most fat absorption in health is from the micellar phase of digested lipid, in situations in which intraluminal bile-salt concentrations are low (e.g., in newborns and patients with obstructive jaundice), lipid absorption can occur from vesicles.

Lipids diffuse as mixed micelles and monomers through unstirred layers before crossing the jejunal enterocyte brush border

To reach the interior of the enterocyte, lipolytic products must cross several barriers. These include (1) the mucous gel layer that lines the intestinal epithelial surface, (2) the unstirred water layer (disequilibrium zone) contiguous with the enterocyte's apical membrane, and (3) the apical membrane itself. Although the mucous gel that lines the intestine is 95% water, its interstices provide a barrier to the free diffusion—from the bulk phase to the unstirred water layer—of lipid macroaggregates, particularly the various vesicles that exist in equilibrium with the mixed micelles and monomers. Because of this diffusion barrier, the unstirred water lying adjacent to the enterocyte's apical membrane is not in equilibrium with the bulk phase of water in the lumen. According to calculations based on the diffusion of various probes under different experimental conditions and luminal fluid flow rates, it was originally estimated that the unstirred water layer was several hundred micrometers thick and posed a significant barrier to the diffusion of lipid nutrients to the enterocyte brush border. It is now thought that the unstirred water layer is likely only ~40 µm thick and does not constitute a major absorptive barrier.

For SCFAs and MCFAs, imageN45-1 which are readily soluble in water, diffusion of these monomers through the unstirred water layer to the enterocyte is efficient. As fatty-acid chain length increases, the monomer's solubility in water decreases, whereas its partitioning into micelles increases. It is true that the diffusion of a single monomer through the aqueous barriers is speedier than that of a single micelle or vesicle. However, mixed-lipid micelles act as a reservoir to give the aqueous solution such a high effective concentration of fatty acids and other lipid products that the diffusion of these micelles is the most efficient mechanism for bringing lipolytic products to the enterocytes. Calculations suggest that, compared with monomers dissolved in water, vesicular solubilization increases the “concentration” of LCFAs imageN45-1 near the enterocyte's brush-border membrane by a factor of 100,000 and micellar solubilization by a factor of 1,000,000.

When the fatty-acid/bile-salt mixed micelles reach the enterocyte surface, they encounter an acidic microclimate generated by Na-H exchange at the brush-border membrane. It is postulated that fatty acids now become protonated and leave the mixed micelle to enter the enterocyte. The uptake of fatty acids could occur by nonionic diffusion (see pp. 784–785) of the uncharged fatty acid or by collision and incorporation of the fatty acid into the cell membrane. However, it is clear that at least three integral membrane proteins promote the uptake of fatty acids (Fig. 45-13): fatty-acid translocase (FAT or CD36), the plasma-membrane fatty acid–binding protein (FABPpm), and fatty-acid transport proteins (FATPs, members of the SLC27 family; see p. 967). Similarly, unesterified cholesterol and lysophospholipids must leave the micelle carrier to enter the enterocyte as monomers. Investigators have suggested that cholesterol derived from bile is better absorbed than dietary cholesterol. Most of the bile salts do not enter the enterocyte with the dietary lipids, but any that do are pumped back out into the lumen via ABCG5/ABCG8 transporters (see p. 957). The bile salts are absorbed via active transport by the apical, Na+-dependent bile-acid transporter (ASBT) in the distal ileum (see pp. 962–963). As with fatty acids, 2-MAGs, lysophospholipids, and cholesterol traditionally were assumed to enter the enterocyte by simple diffusion across the apical plasma membrane of the brush-border villi. More recently, however, in addition to carriers for fatty acids, membrane proteins have been identified in both enterocytes and hepatocytes that may be responsible for the transfer of fatty acids, phospholipids, and cholesterol across their respective cell membranes. As well as providing a mechanism for facilitated or active absorption of the various products of lipid digestion, such carriers may yet be therapeutic targets for inhibiting lipid absorption. The drug ezetimibe, which lowers plasma cholesterol, impedes cholesterol uptake by inhibiting the internalization, at the enterocyte apical membrane, of a complex consisting of cholesterol and the Niemann-Pick C-1 like 1 (NPC1L1) protein.

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FIGURE 45-13 Micellar transport of lipid breakdown products to the surface of the enterocyte. Mixed micelles carry lipids through the acidic unstirred layer to the surface of the enterocyte. 2-MAG, fatty acids, lysophospholipids, and cholesterol leave the mixed micelle and enter an acidic microenvironment created by an apical Na-H exchanger. The acidity favors the protonation of the fatty acids. The lipids enter the enterocyte by (1) nonionic diffusion, (2) incorporation into the enterocyte membrane (“collision”), or (3) carrier-mediated transport.

The enterocyte re-esterifies lipid components and assembles them into chylomicrons

The assimilation of fats, as described thus far, has been a process of disassembly of energy-dense, water-insoluble lipid macromolecular aggregates into monomers for intestinal absorption. The enterocyte elegantly reverses this process (Fig. 45-14). After absorbing LCFAs, MAGs, lysophospholipids, and cholesterol, the enterocyte re-esterifies them and assembles the products with specific apolipoproteins (or simply “apoproteins”) into emulsion-like particles called chylomicrons. The enterocyte then exports the chylomicrons to the lymph (chyle) for ultimate delivery to other organs via the bloodstream. This process enables the delivery of dietary lipids to their intended target tissues (muscle and adipose tissue) while preventing the disruptive effects on cell membranes, were fatty acids to remain unesterified.

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FIGURE 45-14 Re-esterification of digested lipids by the enterocyte and the formation and secretion of chylomicrons. The enterocyte takes up SCFAs, MCFAs, and glycerol and passes them unchanged into the blood capillaries. The enterocyte also takes up LCFAs and 2-MAG and resynthesizes them into TAG in the SER. The enterocyte also processes cholesterol into cholesteryl esters and lysolecithin into lecithin (phosphatidylcholine). The fate of these substances, and the formation of chylomicrons, is illustrated by steps 1 to 7.

Chylomicrons are the largest of the five lipoprotein particles in the bloodstream. We discuss the other lipoprotein particles—very-low-density lipoproteins (VLDLs), intermediate-density lipoproteins (IDLs), low-density lipoproteins (LDLs), and high-density lipoproteins (HDLs)—on pages 969–970 (see also Table 46-4). With an average diameter of ~250 nm (range, 75 to 1200 nm), chylomicrons consist primarily of TAGs, with smaller amounts of phospholipids, cholesteryl esters, cholesterol, and various apolipoproteins.

The first step in the enterocyte's re-formation of TAGs is the binding of LCFAs to a 14-kDa cytosolic protein called the intestinal-type fatty acid–binding protein (FABP), or FABP2. The intestinal concentration of FABP2 is highest in regions that absorb fats, namely, the villi of the proximal jejunal enterocytes. FABP2 preferentially binds long-chain (rather than medium- or short-chain) fatty acids, thus minimizing both reflux back into the intestinal lumen and toxic damage to the enterocyte. FABP2 also ensures transfer of fatty acids to the enterocyte's smooth endoplasmic reticulum (SER) for re-esterification to form TAGs. After a meal, enterocytes mainly use the MAG pathwayimageN45-11 to re-esterify absorbed fatty acids to absorbed 2-MAG. During fasting, enterocytes mainly use the phosphatidic-acid pathway imageN45-11 to esterify fatty acids that enter from the bloodstream and those derived from biliary phosphatidylcholine. The necessary phosphatidic acid may arise either from glycerol-3-phosphate—itself derived from the metabolism of glucose or amino acids—or from the breakdown of bile phosphatidylcholine that enters from the intestinal lumen. Both the MAG and the phosphatidic-acid pathways depend on the activation of the fatty acid to acyl coenzyme A (acyl CoA), catalyzed by long-chain acyl CoA synthetase. imageN45-12 LCFAs are the preferred substrate for this enzyme. The net effect of this series of reactions is the very rapid formation of TAGs in the SER, which maintains low fatty-acid concentrations in the enterocyte. TAGs and fat droplets may be seen in the cisternae of the SER on electron microscopy. The enterocyte also esterifies both cholesterol and lysophosphatidylcholine.imageN45-13

N45-11

Regeneration of Triacylglycerols Inside of Enterocytes

Contributed by Adrian Reuben

As summarized in eFig. 45-3, pancreatic lipase breaks down TAGs into fatty acids and 2-MAGs in the lumen of the small intestine. These products enter the enterocyte, which uses either the MAG or the phosphatidic-acid pathways to re-esterify them. The resulting TAGs are packaged into chylomicrons, which enter the lymph and, ultimately, the blood.

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EFIGURE 45-3 Digestion and absorption of triacylglycerols.

N45-12

Acyl CoA Synthase

Contributed by Emile Boulpaep, Walter Boron

See Figure 58-10, which shows the enzyme in the cytosol of a hepatocyte.

N45-13

Intestinal Esterification of Cholesterol and Synthesis of Phosphatidylcholine

Contributed by Adrian Reuben

Esterification of cholesterol by enterocytes may involve several enzymes, such as cholesterol esterase and acyl CoA: cholesterol acyltransferase.

The enterocyte may produce phosphatidylcholine (i.e., lecithin) intracellularly by using lysolecithin acyltransferase to esterify absorbed lysolecithin or by de novo synthesis from phosphatidic acid.

Besides the lipids, the other components of the chylomicron are the various apolipoproteins (see Table 46-4), which the enterocyte synthesizes in the rough endoplasmic reticulum (RER). These apolipoproteins, with the exception of apolipoprotein A-I, move to the lumen of the SER, where they associate with newly synthesized TAGs (see Fig. 45-14). Apolipoprotein A-I associates with chylomicrons in the Golgi apparatus. Besides incorporating apolipoproteins, the packaging of nascent chylomicrons involves adding esterified cholesterol and a surface coating of phosphatidylcholine and other phospholipids. It is thought that vesicles (the prechylomicron transport vesicle) derived from the SER carry nascent chylomicrons to the cis face of the Golgi apparatus, where they fuse and deliver their contents internally. Enzymes in the Golgi apparatus glycosylate the apolipoproteins. Vesicles carrying processed chylomicrons bud off from the trans face of the Golgi and move toward the basolateral plasma membrane of the enterocyte. This process is the rate-limiting step in the transit of dietary fat across the enterocyte. There the vesicles fuse with the basolateral membrane and leave the enterocyte.

The foregoing discussion focused on the digestion and absorption of long-chain TAGS. The handling of TAGS with MCFAs is very different, inasmuch as (1) they are water soluble, and (2) the fatty-acid re-esterification enzymes prefer longer-chain fatty acids. Because of their water solubility, fatty acids and MAGs derived from medium-chain TAGS do not require either mixed micelles or bile salts for transport. Further, because the enterocyte does not re-esterify MCFAs, they move from the cell directly into the portal blood. As a result, medium-chain TAGS are suitable fat (caloric) substitutes for feeding patients with fat malabsorption.

The enterocyte secretes chylomicrons into the lymphatics during feeding and secretes VLDLs during fasting

As we have described, vesicles carrying mature chylomicrons discharge their contents from the enterocyte into the lamina propria via exocytosis at the basolateral membrane. Chylomicrons are too large to pass through the fenestrae of blood capillaries, and thus they enter lymph through the larger interendothelial channels of the lymphatic capillaries. In both the fed and fasted states, the intestine also secretes into the lymph VLDLs, which are smaller (30 to 80 nm) than chylomicrons. VLDLs have a protein and lipid composition similar to that of chylomicrons (see Table 46-4) but are synthesized independently and carry mainly endogenous (as opposed to dietary) lipids. The lymph lacteals originate in the tips of the villi and discharge their contents into the cisternae chyli. Lymph flows from the cisternae chyli to the thoracic duct, to enter the blood circulation via the left subclavian vein. The protein and lipid composition of both chylomicrons and VLDLs are modified during their passage through lymph and on entry into the blood.

The process of lipid digestion and absorption has great reserve capacity and many redundancies. For example, the mechanical disruption of food is accomplished in several ways by the mouth, stomach, and proximal intestine. Many of the digestive lipases have overlapping functions, and pancreatic lipase, in particular, is secreted in great excess. Much of the small intestine is not used for fat absorption in healthy individuals because most of the dietary lipid is absorbed within 60 cm from the pylorus. Nonetheless, fat malabsorption does occur in many disease states (Box 45-5). A logical classification for these disorders can be devised based on knowledge of the normal physiology. Thus, fat malabsorption may occur because of impairments in intraluminal digestion, intraluminal dispersion, mucosal penetration, chylomicron assembly, or transport from enterocyte to the blood circulation. Within each major category are subdivisions. Frequently, the pathophysiology of these disorders is mixed (i.e., more than one step in the digestive or absorptive process is deranged), but nonetheless the defective components can be identified, and appropriate therapy given.

Box 45-5

Celiac Disease

Celiac disease (CD) is a not-uncommon disease that over the years has had a variety of names, including celiac sprue, gluten enteropathy, and nontropical sprue. CD was thought to affect primarily children with clinical manifestations of diarrhea and fat malabsorption, the latter leading to excess fecal fat (steatorrhea), and abnormal results on small-intestinal biopsy, all of which respond to a gluten-free diet. However, we now recognize that CD affects a much larger number of children and adults (perhaps nearly 1% of Americans), with a wide spectrum of symptoms or even no symptoms at all. These patients may have diarrhea, steatorrhea, growth retardation, bone disease, or anemia. Duodenal biopsy reveals atrophy of the epithelial villi (see p. 901)—often referred to as a flat biopsy—but with elongated crypts.

CD represents an autoimmune response to a peptide component of gluten called gliadin that is found in wheat, rye, and barley. The development of serological tests—including the detection in the serum of antibodies to tissue transglutaminase (anti-tTG)—has resulted in an increase in the number of individuals diagnosed with CD.

The abnormalities found on biopsy correlate with the symptoms. The absence of villous surface epithelial cells results in decreased nutrient absorption (often accompanied by steatorrhea). The elongation of crypts, which are responsible for electrogenic Cl secretion (see pp. 907–908), accounts for the small-intestinal fluid secretion that occurs in untreated CD.

The need to maintain a gluten-free diet is lifelong. Considerable research focuses on the development both of enzymes that will detoxify gliadin peptides (in vivo or in vitro) and of drugs that will prevent these peptides from being absorbed by the small intestine. Success with either approach would permit patients with CD to consume gluten-containing products with impunity.

 

SMALL INTESTINE

LARGE INTESTINE

Length (m)

6

2.4

Area of apical plasma membrane (m2)

~200

~25

Folds

Yes

Yes

Villi

Yes

No

Crypts or glands

Yes

Yes

Microvilli

Yes

Yes

Nutrient absorption

Yes

No

Active Na+ absorption

Yes

Yes

Active K+ secretion

No

Yes