Medical Physiology, 3rd Edition

Lipid Digestion

Natural lipids of biological origin are sparingly soluble in water

Lipids in the diet are derived from animals or plants and are composed of carbon, hydrogen, and a smaller amount of oxygen. Some lipids also contain small but functionally important amounts of nitrogen and phosphorus (Fig. 45-11). Lipids are typified by their preferential solubility in organic solvents compared with water. A widely used indicator of the lipidic nature of a compound is its octanol-water partition coefficient, which for most lipids is between 104 and 107.

image

FIGURE 45-11 Chemical formulas of some common lipids. The example in A is stearic acid, a fully saturated fatty acid with 18 carbon atoms. B shows glycerol, a trihydroxy alcohol, with hydroxyl groups in positions sn1, sn2, and sn3. In C, the left sn1– and center sn2–fatty acids are palmitic acid, a fully saturated fatty acid with 16 carbon atoms. The rightmost sn3–fatty acid is palmitoleic acid, which is also a 16-carbon structure, but with a double bond between carbons 9 and 10. In F, the left sn1–fatty acid is palmitic acid, and the right sn2–fatty acid is palmitoleic acid. In I, the example is the result of esterifying cholesterol and palmitic acid.

The biological fate of lipids depends critically on their chemical structure as well as on their interactions with water and other lipids in aqueous body fluids (e.g., intestinal contents and bile). Thus, lipids have been classified according to their physicochemical interactions with water. Lipids may be either nonpolar and thus very insoluble in water (e.g., cholesteryl esters and carotene) or polar and thus interacting with water to some degree. Even polar lipids are only amphiphilic; that is, they have both polar (hydrophilic) and nonpolar (hydrophobic) groups. Polar lipids range from the insoluble, nonswelling amphiphiles (e.g., triacylglycerols) to the soluble amphiphiles (e.g., bile acids). Added in small amounts, insoluble polar lipids form stable monolayers on the surface of water (see Fig. 2-1C), whereas the soluble amphiphiles do not. The physicochemical behavior in bulk solution varies from insolubility—as is the case with triacylglycerols (TAGs) and cholesterol—to the formation of various macroaggregates, such as liquid crystals and micelles. Less-soluble lipids may be incorporated into the macroaggregates of the more polar lipids and are thus stably maintained in aqueous solutions.

Dietary lipids are predominantly TAGs

The term fat is generally used to refer to TAGs—formerly called triglycerides—but it is also used loosely to refer to lipids in general. Of the fat in an adult diet, >90% is TAGs, which are commonly long-chain fatty acyl esters of glycerol, a trihydroxyl alcohol. The three esterification (i.e., acylation) positions on the glycerol backbone that are occupied by hydroxyl groups are designated sn1, sn2, and sn3, according to a stereochemical numbering system adopted by an international committee on biochemical nomenclature (see Fig. 45-11C–E). At body temperature, fats are usually liquid droplets. Dietary fat is the body's only source of essential fatty acids, and its hydrolytic products promote the absorption of fat-soluble vitamins (the handling of which is discussed on p. 933). Fat is also the major nutrient responsible for postprandial satiety.

Typical adult Western diets contain ~140 g of fat per day (providing ~60% of the energy), which is more than the recommended intake of less than ~70 g of fat per day (<30% of total dietary calories). The ratio of saturated to unsaturated fatty acids in TAGs is high in animal fats and low in plant fats. Current guidelines suggest that only one third of lipid intake should be saturated fats. Cholesterol intake should be limited to <300 mg/day, and trans fatty acids avoided. Of the fats ingested, those that are liquid at room temperature should be emphasized, especially those that contain omega-3 (or n-3) fatty acids imageN45-7 found especially in fish. Because of the high fat content of milk, newborn infants consume three to five times more lipid than adults, relative to body weight.

N45-7

Omega-3 Fatty Acids

Contributed by Emile Boulpaep, Walter Boron

eFigure 45-2 shows the structure of α-linolenic acid (ALA), which is an 18-carbon fatty acid with three double bonds.

If we use the numbering system of chemists—which, as indicated by the blue numerals in the figure, assigns the carboxyl carbon as the No. 1 position—the three double bonds begin at positions 9, 12, and 15. Thus, this fatty acid can be referred to as 18 : 3Δ9c,12c,15c.

Alternatively, physiologists count from the opposite or methyl (–CH3) end of the carbon chain—also known as the n end—starting with the terminal or omega (ω) carbon. In eFigure 45-2, we indicate this numbering system by the red numerals. The rationale for the n numbering system is that the methyl end of the molecule is less susceptible to biochemical modification. In this nomenclature, the double bond closest to the methyl carbon is termed the omega-3 or n-3 double bond, and fatty acids containing such a double bond are termed omega-3 or n-3 fatty acids. We would refer to α-linolenic acid as 18 : 3(n-3). Here the first double bond starts at the n-3 position and the other two double bonds each begin 3 carbons further down the chain (i.e., at n-6 and n-9).

image

EFIGURE 45-2 Structure of ALA.

Reference

Wikipedia. s.v. Omega-3 fatty acid. [Last modified May 11]  http://en.wikipedia.org/wiki/Omega-3_fatty_acid; 2015 [Accessed May 15, 2015].

Approximately 5% (4 to 6 g/day) of dietary lipids come from cell membranes and are phospholipids. Most phospholipids are glycerophospholipids. They consist of a glycerol backbone that is esterified at the first two positions to fatty acids and at the third position to a phosphate that in turn is esterified to a head group (see Fig. 2-2A–C). One of the glycerophospholipids, phosphatidylcholine (lecithin), is the predominant phospholipid (see Fig. 45-11F). The sphingolipids (see Fig. 2-2D, E), which have a serine rather than a glycerol backbone, form the other major class of membrane phospholipids.

The typical Western diet contains ~0.5 g of unesterified cholesterol (also derived from animal cell membranes; see Fig. 45-11H), whereas esterified cholesterol (see Fig. 45-11I) is usually found only in liver or food made from blood products. Traces of lipovitamins and provitamins (e.g., carotene) are present in dietary fat, which may also contain lipid-soluble toxins and carcinogens from the environment. These undesirable lipid-soluble chemicals—which include nitrosamines, aflatoxins, and polycyclic hydrocarbons such as benzo(a)pyrene—are all found in small amounts in margarine, vegetable oils, and other dietary fats. The diet also may contain skin lipids, which are chemically diverse and complex and are difficult to digest.

Endogenous lipids are phospholipids and cholesterol from bile and membrane lipids from desquamated intestinal epithelial cells

The bile secreted into the intestine (see p. 957) plays a key role in the assimilation of dietary lipids, as we explain below. This bile contains phospholipid (10 to 15 g/day)—also predominantly phosphatidylcholine—and unesterified cholesterol (1 to 2 g/day). Quantitatively, these biliary lipids exceed those present in the diet by 2- to 4-fold. Membrane lipids from desquamated intestinal cells account for a further 2 to 6 g of lipid for digestion. Dead bacteria contribute ~10 g/day of lipids, mainly in the colon.

The mechanical disruption of dietary lipids in the mouth and stomach produces an emulsion of lipid particles

The central process in the digestion of lipids is their hydrolysis in the aqueous milieu of the intestinal lumen. Lipid hydrolysis is catalyzed by lipases secreted by the glands and cells of the upper gastrointestinal tract. The products of lipolysis diffuse through the aqueous content of the intestinal lumen, traverse the so-called unstirred water layer and mucus barrier that line the intestinal epithelial surface, and enter the enterocyte for further processing. Because dietary lipids are insoluble in water, digestive lipases have evolved to act more efficiently at oil-water interfaces than on water-soluble substrates.

A key step preliminary to lipid digestion is the transformation of ingested solid fat and oil masses into an emulsion of fine oil droplets in water. The emulsification of dietary fats begins with food preparation (grinding, marinating, blending, and cooking), followed by chewing and gastric churning (see pp. 876–877) caused by antral peristalsis against a closed pylorus. Emulsification of ingested lipids is enhanced when muscular movements of the stomach intermittently squirt the gastric contents into the duodenum and, conversely, when peristalsis of the duodenum propels the duodenal contents in retrograde fashion into the stomach through the narrow orifice of a contracted pylorus. The grinding action of the antrum also mixes food with the various digestive enzymes derived from the mouth and stomach. Intestinal peristalsis mixes luminal contents with pancreatic and biliary secretions. Together, these mechanical processes that reduce the size of the lipid droplets also dramatically increase their ratio of surface area to volume, thereby increasing the area of the oil-water interface.

The fine emulsion produced by the mechanical processes just outlined is stabilized by coating the emulsion droplets with membrane lipids, denatured protein, dietary polysaccharides, certain products of digestion—including fatty acids released by gastric lipase as well as fatty acids and monoacylglycerols (MAGs) from intestinal and pancreatic digestion—and biliary phospholipids and cholesterol. This coating prevents the lipid droplets from coalescing. Phospholipids and cholesterol are well suited as emulsion stabilizers because they are attracted to oil-water interfaces by virtue of their amphiphilic properties. Thus, they form a surface monomolecular layer on emulsion particles. The polar groups of the phospholipids and hydroxyl groups of cholesterol project into the water; the charges on the polar groups and their high degree of hydration prevent coalescence of the emulsion particles. The core of the emulsion particle is composed of TAG, which also contains cholesteryl esters and other nonpolar or weakly polar lipids. A very small fraction of the TAG in the lipid particle localizes to the particle surface. The fat in breast milk is already emulsified by proteins and phospholipids incorporated into the surface of fat droplets during lactation. Foods such as sauces, ice cream, and puddings are stably emulsified during their preparation.

Lingual and gastric (acid) lipase initiate lipid digestion

In some species, but not in humans, a small amount of lipid digestion begins in the mouth, mediated by lingual lipase. In the stomach, both lingual lipase that is swallowed and a gastric lipase secreted by gastric chief cells digest substantial amounts of lipid. Human gastric lipase is a 42-kDa glycoprotein whose secretion is stimulated by gastrin. Gastric lipase secretion is already well established in the neonatal period (unlike pancreatic lipase secretion), and thus gastric lipolysis is important for fat digestion in newborn infants. Gastric lipase is also important in the digestion of dietary fat in patients with pancreatic insufficiency, where it hydrolyzes approximately one third of the fat due to its preference for fatty acids at the sn3 position, leaving behind intact sn1,2-diacylglycerols (DAGs; see Fig. 45-11D). Gastric and lingual—as well as pharyngeal—lipases belong to a family of serine hydrolases that have acidic pH optima (pH 4), are stable in the acidic environment of the stomach, are resistant to digestion by pepsin, and are not inhibited by the surface layer of membrane lipids (emulsifiers) that envelopes TAG droplets. However, acid lipases are inactive at neutral pH and also are readily inactivated by pancreatic proteases (especially in the presence of bile salts) once they reach the small intestine. imageN45-8

N45-8

Gastric Lipase

Contributed by Henry Binder

In healthy adult humans, ~15% of fat digestion occurs in the stomach. In patients with pancreatic insufficiency, however, the lack of pancreatic proteases and image in the duodenal lumen may permit the continued action of gastric lipase after the gastric contents leave the stomach and enter the duodenum. Extended gastric lipase activity partly alleviates fat malabsorption resulting from pancreatic disease and pancreatic lipase deficiency.

The carboxyl groups of long-chain fatty acids (LCFAs),imageN45-1 released from TAGs in the stomach, are protonated and insoluble at the acidic pH prevailing in the stomach. LCFAs are not absorbed in the stomach, but rather they remain in the core of the TAG droplets. In the more alkaline environment of the small intestine, the fatty acids become ionized and participate in emulsification of the lipid droplets. Medium-chain fatty acids (MCFAs), imageN45-1 and SCFAs are mainly protonated in gastric juice during feeding and passively move across the gastric mucosa into portal blood. imageN45-9

N45-9

Gastric Absorption of Medium- and Short-Chain Fatty Acids

Contributed by Emile Boulpaep, Walter Boron

In aqueous solutions, fatty acids exist in both anionic (A) and neutral/protonated (HA) forms, according to the following equilibrium:

image (NE 45-1)

All of the fatty acids, regardless of the length of the alkyl chain, are relatively water soluble when the fatty acids are in their anionic form. However, the solubility of the protonated forms of the fatty acids depends markedly on the length of the alkyl chain. Thus, as stated in the text, the protonated forms of the LCFAs are relatively water insoluble. On the other hand, the protonated forms of the MCFAs and SCFAs become progressively more water soluble.

The pK values for the equilibrium in Equation NE 45-1 are rather independent of the length of the alkyl chain. Indeed, all saturated fatty acids of physiological importance have pK values between those of the 2-carbon acetic acid (pK = 4.76) and the 3-carbon propionic acid (pK = 4.87). Only the 1-carbon formic acid has an appreciably lower pK value (~3.75). Thus, at the lowest gastric pH values, all fatty acids—long-chain, medium-chain, and short-chain—exist almost exclusively in the neutral/protonated (HA) forms. The major difference, as noted in the text, is that the protonated forms of the LCFAs, which are insoluble in water and thus largely partitioned into triacylglycerol droplets, are unavailable for uptake in the stomach. The protonated forms of the MCFAs and SCFAs are more water soluble and thus more available for passive absorption by nonionic diffusion (see p. 784) in the stomach.

Pancreatic (alkaline) lipase, colipase, milk lipase, and other esterases—aided by bile salts—complete lipid hydrolysis in the duodenum and jejunum

The process of fat digestion that begins in the stomach is completed in the proximal small intestine, predominantly by enzymes synthesized and secreted by pancreatic acinar cells (see p. 882) and carried into the duodenum in the pancreatic juice. In humans, a lipase found in human milk—the so-called bile salt–stimulated milk lipase—also digests fat. A similar lipase is found in the milk of relatively few other animal species. Milk lipase is stable during passage through the acid environment of the stomach, yet it is active at the alkaline pH of the duodenum and jejunum, where it hydrolyzes DAGs, MAGs, cholesteryl esters, and fat-soluble vitamin esters, as well as TAGs. Bile salts not only stimulate milk lipase activity but also protect the enzyme from proteolysis in the small intestine. Like gastric lipase, milk lipase is important for fat digestion in breast-fed infants.

Once the fatty acids generated in the stomach reach the duodenum, they trigger the release of CCK and gastric inhibitory polypeptide (GIP) from the duodenal mucosa. CCK stimulates the flow of bile into the duodenum by causing the gallbladder to contract and the sphincter of Oddi to relax (see pp. 961–962). CCK also stimulates the secretion of pancreatic enzymes, including lipases and esterases (see pp. 882–883). As we discuss below, LCFAs also facilitate the lipolytic action of pancreatic lipase.

The major lipolytic enzyme of pancreatic juice is a 48-kDa carboxylic esterase known as pancreatic (TAG) lipase, sometimes referred to as colipase-dependent pancreatic lipase. In adults but not in infants, the active form of this enzyme is secreted into the duodenum at a level 1000-fold higher than that necessary to digest all dietary TAGs not hydrolyzed in the stomach. imageN45-10 Full lipolytic activity of pancreatic lipase requires the presence of the small (10-kDa) protein cofactor called colipase, as well as an alkaline pH, Ca2+, bile salts, and fatty acids. The pancreas secretes colipase in the pro form (i.e., procolipase), which has no intrinsic lipolytic activity. Trypsin cleaves procolipase into colipase and an N-terminal pentapeptide, enterostatin. This cleavage is important because the newly formed colipase is a cofactor of pancreatic lipase, as we discuss below. In addition, the N-terminal pentapeptide may partially control satiety.

N45-10

Pancreatic Lipase

Contributed by Emile Boulpaep, Walter Boron

The crystal structure of human pancreatic lipase, a 48-kDa protein, has been elucidated, and models of enzyme action have been devised.

References

van Tilbeurgh H, Egloff MP, Martinez C, et al. Interfacial activation of the lipase–procolipase complex by mixed micelles revealed by X-ray crystallography. Nature. 1993;362:814–820.

Winkler FK, D'Arcy A, Hunziker W. Structure of human pancreatic lipase. Nature. 1990;343:771–774.

Pancreatic lipase is active only at the oil-water interface of a TAG droplet. However, surface emulsifier components (e.g., phospholipids, proteins, fatty acids) present at that interface inhibit lipase action. Bile-salt micelles also inhibit lipolysis by displacing the lipase from the oil-droplet surface. Colipase reverses this inhibition through interfacial binding, either by attaching first to the interface and serving as an anchor for the binding of the lipase, or by first forming a colipase–pancreatic lipase complex that then binds to the lipid interface. Colipase also can penetrate the phospholipid coating of the TAG emulsion. Bile-salt micelles bring the colipase closer to the interface. Because bile salts are nearby, they can participate in the solubilization and removal of the products of lipolysis released from the emulsion droplet. Fatty acids have a biphasic effect. Increasing levels at first enhance emulsification and augment lipolysis (probably by enhancing the binding of the colipase-lipase complex to the lipid interface). Further increases in fatty-acid levels cause product inhibition of lipase.

Studies of the crystal structure of pancreatic lipase have shown that when the enzyme is free in solution, the catalytic site of the lipase—located in a cleft in the molecule—is partly covered by a lid formed by a loop of its peptide chain. The interaction of the colipase and lipase with the interface causes a conformational change in the lipase molecule that opens the lid, thereby allowing lipid substrate to diffuse to the now-exposed catalytic site of the enzyme.

Pancreatic lipase mainly hydrolyzes the ester bonds of TAGs at the first and third positions of the glycerol backbone. The end products of such reactions are two fatty acids and a single sn2-MAG (2-MAG) (see Fig. 45-11E).

The pancreas secretes other enzymes that hydrolyze lipid esters. Carboxyl ester hydrolase is a pancreatic enzyme that is the same protein as bile salt–stimulated milk lipase. Like milk lipase, carboxyl ester hydrolase lacks substrate specificity and is active against a wide range of esters. Carboxyl ester hydrolase is probably the same enzyme as “pancreatic esterase,” “cholesterol esterase,” “lysophospholipase,” and others. Among the many products of reactions catalyzed by this enzyme are free cholesterol and free glycerol.

The pancreas also secretes phospholipase A2 (PLA2), which is active against glycerophospholipids (but not sphingolipids), releasing a single sn2–fatty acid, which is usually an unsaturated fatty acid, to yield sn1-lysophospholipids (see Fig. 45-11G). Pancreatic PLA2, secreted as a trypsin-activatable proenzyme, is effective at alkaline pH and requires bile salts and Ca2+ for activity. A separate PLA2 in the small intestine, with a preference for phosphatidylglycerol, a common phospholipid in bacteria, is derived from Paneth cells, whereas phospholipase found in the colon probably comes from anaerobic flora there. In contrast to the human intestinal lipases, bacterial lipases are nonspecific with respect to substrates, have neutral or slightly acidic pH optima, are not inhibited by bile acids, and do not require cofactors. In the human colon, both TAGs and phospholipids are totally hydrolyzed by bacteria. Fecal fat is thus generally present as fatty-acid soaps and sterols. Even in cases of severe fat malabsorption, intact acylglycerols are rarely found in the stools unless the patient has exocrine pancreatic insufficiency. The chemical test for stool fat, Sudan III staining, must be done in an acid environment because it depends on the property of Sudan III dye to partition into “oils” that contain protonated fatty acids.

 

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