Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Lipid Metabolism
Digestion and Absorption of Lipids

Triglyceride Digestion IN THE digestive tract. Saliva does not contain fat-splitting Enzymes; consequently, fats undergo no Changes in the Oral Cavity. In adults, fats pass through The Stomach largely unchanged as well. Gastric juice contains a lipase known as gastric lipase, yet its role in the Hydrolysis of dietary triglycerides in adults is minor. First, the content of gastric lipase in the gastric juice of adults and other mammals is extremely low. Second, the pH of gastric juice is far from the optimum for this enzyme's activity (the optimal pH value for gastric lipase ranges from 5.5 to 7.5). Recall that the pH of gastric juice is around 1.5. Third, the stomach lacks the conditions necessary for triglyceride emulsification, whereas lipase can actively act only on triglycerides present in an emulsion form. Therefore, in adults, unemulsified triglycerides, which constitute the bulk of dietary fat, pass through the stomach without significant alteration. At the same time, triglyceride breakdown in the stomach plays an important role in digestion in infants, especially newborns. The mucous membrane of the ROOT of the Tongue and the adjacent pharyngeal area in infants secretes its own lipase in response to sucking and swallowing movements during breastfeeding. This enzyme is called lingual lipase. The activity of lingual lipase does not have time to manifest in the oral cavity, and its primary Site of Action is the stomach. The pH optimum of lingual lipase is within 4.0–4.5, which is close to the pH of gastric juice in such infants. Lingual lipase acts most actively on triglycerides containing short- and medium-chain Fatty acids, which is characteristic of milk triglycerides. In other words, milk fat is the most suitable substrate for this enzyme. In adults, lingual lipase activity is extremely low.

The breakdown of triglycerides in the adult stomach is minimal, but it to some extent facilitates their subsequent digestion in the intestine. Even a negligible amount of triglyceride Cleavage in the stomach leads to the appearance of free fatty acids which, without being absorbed in the stomach, enter the intestine and promote fat emulsification there, thereby facilitating the action of pancreatic juice lipase.

Once the chyme enters the duodenum, the Hydrochloric acid of the gastric juice brought into the intestine with food is first neutralized by bicarbonates contained in the Pancreatic and Intestinal juices. The carbon dioxide bubbles released during the decomposition of bicarbonates contribute to thorough mixing of the food chyme with digestive juices. Simultaneously, fat emulsification begins. The most powerful emulsifying effect on fats is exerted by Bile acid salts, which enter the duodenum with bile in the form of sodium salts. Most bile acids are conjugated with Glycine or taurine. Chemically, bile acids are derivatives of cholanic acid:

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Bile acids represent the primary end product of Cholesterol METABOLISM.

Human bile mainly contains cholic (3,7,12-trioxycholanic), deoxycholic (3,12-dioxycholanic), and chenodeoxycholic (3,7-dioxycholanic) acids (all hydroxyl groups have the α-configuration and are therefore indicated by a dashed line):

In addition, human bile contains small amounts of lithocholic (3α-oxycholanic) acid, as well as allocholic and ursodeoxycholic acids, which are stereoisomers of cholic and chenodeoxycholic acids.

As noted, bile acids are present in bile in a conjugated form, i.e., as glycocholic, glycodeoxycholic, glycochenodeoxycholic (about ²/₃ to ⁴/₅ of all bile acids) or taurocholic, taurodeoxycholic, and taurochenodeoxycholic (about ¹/₅ to ¹/₃ of all bile acids) acids. These compounds are sometimes called paired bile acids because they consist of two components: a bile acid and glycine or taurine. The ratios between the Two Types of conjugates can vary depending on The Nature of the diet: if CARBOHYDRATES predominate, the relative content of glycine conjugates increases, whereas a high-protein diet increases taurine conjugates. The Structure of paired bile acids can be represented as follows:

It is believed that only the combination of bile acid salt + unsaturated fatty acid + monoglyceride provides the necessary degree of fat emulsification. Bile acid salts dramatically reduce surface tension at the fat/Water interface, whereby they not only facilitate emulsification but also stabilize the already formed emulsion.

It is known that the bulk of dietary glycerides undergo cleavage in the upper sections of the Small Intestine under the action of pancreatic juice lipase. This enzyme was first discovered by the renowned French physiologist C. Bernard in the middle of the last century.

Pancreatic lipase (EC 3.1.1.3) is a glycoprotein with a Molecular Weight of 48,000 (in humans) and a pH optimum of 8–9. This enzyme cleaves triglycerides present in an emulsified state (the enzyme's action on dissolved substrates is significantly weaker). Like other digestive enzymes (Pepsin, Trypsin, Chymotrypsin), pancreatic lipase enters the upper small intestine as an inactive prolipase.

The conversion of prolipase into active lipase occurs with the participation of bile acids and another protein of pancreatic juice, colipase (molecular weight 10,000). The latter binds to prolipase in a 2:1 molecular ratio. This renders the lipase active and resistant to trypsin.

It has been established that the main products of triglyceride cleavage by pancreatic lipase are ß(2)-monoglyceride and fatty acids. The enzyme catalyzes the hydrolysis of ester bonds at the α(1) and α'(3) positions, resulting in The formation of ß(2)-monoglyceride and two fatty acid molecules. The rate of lipase-catalyzed triglyceride hydrolysis is not significantly affected by either the degree of unsaturation of the fatty acids or their chain length (from C₁₂ to C₁₈).

The hydrolysis of pancreatic lipase-mediated triglycerides can be depicted by the following scheme:

Along with lipase, pancreatic juice contains monoglyceride isomerase, an enzyme that catalyzes the intramolecular transfer of An acyl group from the ß(2) position of a monoglyceride to the α(1) position. During the Digestion of dietary fats with the help of this enzyme, approximately one-third of the ß-monoglyceride is converted into α-monoglyceride. Since the ester bond at the α-position is sensitive to pancreatic lipase, the latter cleaves most of the α-monoglycerides down to the end products: glycerol and fatty acid. A smaller fraction of α-monoglycerides manages to be absorbed into the small intestinal wall, bypassing the action of lipase.

Absorption of triglycerides and their breakdown products. Absorption takes place in the proximal section of the small intestine. Finely emulsified fats (the size of emulsion fat droplets must not exceed 0.5 µm) can be partially absorbed through the intestinal walls without prior hydrolysis. The bulk of fat is absorbed only after being split by pancreatic lipase into fatty acids, monoglycerides, and glycerol. Fatty acids with a short carbon chain (fewer than 10 carbon atoms) and glycerol, being highly water-soluble, are readily absorbed in the intestine and enter the portal vein Blood, and from there the Liver, without undergoing any transformations in the intestinal wall.

The absorption of long-chain Fatty Acids and monoglycerides is more complex. This process is carried out with the participation of bile and, primarily, the bile acids it contains. In bile, bile acid salts, Phospholipids, and cholesterol are present in a ratio of 12.5:2.5:1.0. Long-chain fatty acids and monoglycerides in the intestinal lumen form micelles with these compounds that are stable in an aqueous environment. The structure of micelles is such that their Hydrophobic core (fatty acids, monoglycerides, etc.) is surrounded externally by a hydrophilic shell made of bile acids and phospholipids. Micelles are approximately 100 times smaller than the smallest emulsified fat droplets. Within micelles, Higher Fatty Acids and monoglycerides are transported from the site of fat hydrolysis to the absorptive surface of the intestinal epithelium. There is no consensus regarding The Mechanism of fat micelle absorption. Some researchers believe that As a result of so-called micellar diffusion, and possibly pinocytosis, entire micelles penetrate into the epithelial Cells of the villi, where the breakdown of fat micelles occurs. In this process, bile acids immediately enter the bloodstream and, via the portal system, reach the liver first and then return to bile. Other researchers allow for the possibility that only the lipid component of fat micelles transfers into the villous cells. Bile acid salts, having fulfilled their physiological role, remain in the intestinal lumen; later, the bulk of them is absorbed into the blood (in the ileum), reaches the liver, and is subsequently excreted with bile. Thus, all researchers recognize that a continuous Circulation of bile acids takes place between The Liver and the intestine. This process is called enterohepatic circulation.

Using the radiolabeled tracer method, it was shown that bile contains only a small fraction of bile acids (10–15% of the total amount) newly synthesized by the liver. Thus, the bulk of bile acids (85–90%) consists of bile acids reabsorbed in the intestine and re-secreted as part of bile. It has been established that the total bile acid pool in humans is approximately 2.8–3.5 g, undergoing 6–8 cycles per day.

DIGESTION AND ABSORPTION of phospholipids and cholesterol. The overwhelming majority of phospholipids in the small intestinal contents is represented by phosphatidylcholine (lecithin), the bulk of which enters the intestine with bile (11–12 g/day) and a smaller portion (1–2 g/day) with food.

There are two viewpoints regarding The Fate of exogenous and endogenous phospholipids entering the small intestine. According to one, both types of phospholipids undergo attack in the intestine by phospholipase A₂, which catalyzes the hydrolysis of the ester bond at the ß-position. As a result of the phospholipase A₂-catalyzed reaction, Glycerophospholipids are cleaved to form a lysophospholipid and a fatty acid. Lysophospholipid can be further degraded by another pancreatic juice enzyme, lysophospholipase. Consequently, the last fatty acid molecule is released from lysolecithin, forming glycerophosphocholine, which is highly soluble in aqueous media and is absorbed from the intestine into the blood.

Proponents of another view believe that "bile-derived" (more precisely, hepatic) phospholipids, unlike dietary phospholipids, are not subjected to the action of phospholipase A₂. Consequently, the function of "biliary" phospholipids is exclusively associated with the enterohepatic circulation of bile: they enter the intestine with bile, participate in the micellar solubilization of Lipids alongside bile acids, and return to the liver with them. Thus, there appear to be two pools of phospholipids in the intestine: a "biliary" pool, protected from phospholipase A₂ action, and a "dietary" pool, susceptible to it. It is currently difficult to explain the reason for the existence of two phospholipid pools and their differing behavior toward phospholipase A₂.

Depending on the diet, the adult human body receives 300–500 mg of cholesterol daily, contained in food products partly in a free (unesterified) form and partly as fatty acid esters. Cholesterol esters are cleaved into cholesterol and fatty acids by a specific enzyme of pancreatic and intestinal juices known as cholesterol ester hydrolase, or cholesterol esterase (EC 3.1.1.3). The absorption of cholesterol takes place in the small intestine, originating from sources such as:

— dietary cholesterol (0.3–0.5 g/day; significantly less in vegetarians);

— biliary cholesterol (1–2 g of endogenous unesterified cholesterol is secreted daily with bile);

— cholesterol contained in the desquamated epithelium of the digestive tract and in intestinal juices (up to 0.5 g/day).

A total of 1.8–2.5 g of endogenous and exogenous cholesterol enters the intestine. Of this amount, about 0.5 g is excreted in the feces as a reduced product, coprostanol, and a very small portion as oxidized products, such as cholestanone and others. Both the reduction and oxidation of cholesterol occur in the Large Intestine under the action of enzymes from the microbial flora. The bulk of unesterified cholesterol is absorbed in the small intestine as part of mixed fat micelles composed of bile acids, fatty acids, monoglycerides, phospholipids, and lysophospholipids.

Resynthesis of lipids in the intestinal wall. Triglycerides. According to modern concepts, the resynthesis of triglycerides occurs in epithelial cells (enterocytes of the small intestinal villi mucosa) via two pathways. The first is the ß-monoglyceride pathway. For a long time, this was considered the sole pathway. Its essence is that ß-monoglycerides and fatty acids penetrating the epithelial cells of the intestinal wall during absorption are retained in the smooth Endoplasmic reticulum of the cells. Here, Fatty acids are converted into their active form, acyl-CoA, followed by the acylation of ß-monoglycerides, yielding first diglycerides and then triglycerides:

ß-Monoglyceride + R—CO—S-CoA -> Diglyceride + HS-CoA ;

Diglyceride + R₁—CO—S-CoA -> Triglyceride + HS-CoA .

All reactions are catalyzed by an enzyme complex, triglyceride synthetase, which includes acyl-CoA synthetase, monoglyceride acyltransferase, and diglyceride acyltransferase.

The second pathway of triglyceride resynthesis takes place in the rough endoplasmic reticulum of epithelial cells and involves the following reactions:

1) Formation of the active form of fatty acid, acyl-CoA, involving acyl-CoA synthetase;

2) formation of a-glycerophosphate involving glycerol kinase;

3) conversion of a-glycerophosphate into phosphatidic acid involving glycerophosphate acyltransferase;

4) conversion of phosphatidic acid into diglyceride involving phosphatidate phosphohydrolase;

5) acylation of the diglyceride to form a triglyceride involving diglyceride acyltransferase.

As can be seen, the first and last reactions mirror those of the ß-monoglyceride pathway. It has been established that the a-glycerophosphate pathway of fat (triglyceride) resynthesis becomes prominent when predominantly fatty acids enter the epithelial cells of the small intestinal mucosa. If fatty acids enter the intestinal wall together with ß-monoglycerides, the ß-monoglyceride pathway is triggered. As a rule, an excess of ß-monoglycerides in epithelial cells inhibits the a-glycerophosphate pathway.

Resynthesis of phospholipids in the intestinal wall. Along with triglyceride resynthesis, enterocytes also resynthesize phospholipids. Resynthesized diglyceride participates in the formation of phosphatidylcholines and phosphatidylethanolamines, whereas resynthesized Phosphatidic acid is involved in the formation of phosphatidylinositols. The participation of these substrates in phospholipid formation within the intestinal wall follows the same patterns as in other Tissues (see pp. 396, 397).

It must be emphasized that the intestinal wall synthesizes fats that are largely species-specific and differ in structure from dietary fat. To a certain extent, this is ensured by the fact that both exogenous and endogenous fatty acids take part in the synthesis of triglycerides (as well as phospholipids) in the intestinal wall. However, the capacity of the intestinal wall to synthesize fat specific to a given animal species is still limited. It has been shown that when an animal (e.g., a dog), especially one that has been previously starved, is fed large amounts of foreign fat (such as linseed oil or camel fat), part of it is found unchanged in the animal's adipose tissues. Adipose tissue is most likely the only tissue where foreign fats can be deposited. Lipids that are part of The Cell protoplasm in other Organs and tissues exhibit high Specificity, and their COMPOSITION AND PROPERTIES depend very little on dietary fats.

Chylomicron formation and lipid transport. Triglycerides and phospholipids resynthesized in the intestinal epithelial cells, as well as cholesterol entering these cells from the intestinal lumen (where it may be partially esterified), combine with a small amount of protein to form relatively stable complex particles called chylomicrons (CM). The latter contain about 2% protein, 7% phospholipids, 8% cholesterol and its esters, and over 80% triglycerides. The diameter of CM ranges from 0.1 to 5 μm. Due to their large size, CM are unable to penetrate from the intestinal endothelial cells into blood capillaries and instead diffuse into the intestinal Lymphatic system, and from there into the Thoracic duct. Subsequently, CM pass from the thoracic duct into the bloodstream; thus, they mediate The transport of exogenous triglycerides, cholesterol, and partially phospholipids from the intestine via The Lymphatic System into the blood. Within 1–2 hours after a fat-containing meal, alimentary hyperlipemia is observed. This is a physiological phenomenon characterized primarily by an elevated blood triglyceride concentration and the appearance of CM in the blood. The peak of alimentary hyperlipemia occurs 4–6 hours after a fatty meal. Typically, 10–12 hours postprandially, triglyceride levels return to normal, and CM completely disappear from the bloodstream.

It is known that the liver and adipose tissue play the most crucial role in the subsequent fate of CM. The latter readily diffuse from Blood Plasma into the intercellular spaces of the liver (sinusoids). It is assumed that the hydrolysis of CM triglycerides occurs both inside and On the surface of liver cells. CM are unable (due to their size) to penetrate adipose tissue cells. Consequently, CM triglycerides undergo hydrolysis on The surface of capillary endothelium in adipose tissue through the action of the enzyme lipoprotein lipase.