Digestion of fats in the gastrointestinal tract. Digestion of lipids in the gastrointestinal tract

The daily diet usually contains 80-100 g of fat. Saliva does not contain fat-breaking enzymes. Consequently, fats do not undergo any changes in the oral cavity. In adults, fats also pass through the stomach without any special changes. Gastric juice contains lipase, called gastric, but its role in the hydrolysis of dietary triglycerides in adults is small. Firstly, the content of lipase in the gastric juice of an adult human and other mammals is extremely low. Secondly, the pH of gastric juice is far from the optimal action of this enzyme (the optimal pH value for gastric lipase is 5.5–7.5). Recall that the pH value of gastric juice is about 1.5. Thirdly, there are no conditions in the stomach for the emulsification of triglycerides, and lipase can only actively act on triglycerides that are in the form of an emulsion.

Digestion of fat in the human body occurs in the small intestine. Fats are first converted into an emulsion with the help of bile acids. During the emulsification process, large fat droplets turn into small ones, which significantly increases their total surface area. Pancreatic juice enzymes - lipases, being proteins, cannot penetrate into fat droplets and only break down fat molecules located on the surface. Therefore, increasing the total surface area of fat droplets due to emulsification significantly increases the efficiency of this enzyme. Under the action of lipase, fat is broken down by hydrolysis into glycerol and fatty acids.

CH -~ OH + R 2 - COOH I

CH 2 - O - C - R 1 CH 2 OH R 1 - COOH

CH - O - C - R 2 CH - OH + R 2 - COOH

CH 2 - O - C - R 3 CH 2 OH R 3 - COOH

Fat Glycerin

Since there are a variety of fats in food, as a result of their digestion, a large number of varieties of fatty acids are formed.

Fat breakdown products are absorbed by the mucous membrane of the small intestine. Glycerin is soluble in water, so it is easily absorbed. Fatty acids that are insoluble in water are absorbed in the form of complexes with bile acids (complexes consisting of fatty and bile acids are called choleic acids). In the cells of the small intestine, choleic acids are broken down into fatty and bile acids. Bile acids from the wall of the small intestine enter the liver and are then released again into the cavity of the small intestine.

The released fatty acids in the cells of the wall of the small intestine recombine with glycerol, resulting in the formation of a fat molecule again. But only fatty acids that are part of human fat enter into this process. Thus, human fat is synthesized. This conversion of dietary fatty acids into their own fats is called fat resynthesis.

Resynthesized fats through the lymphatic vessels, bypassing the liver, enter the systemic circulation and are stored in fat depots. The main fat depots of the body are located in the subcutaneous fatty tissue, the greater and lesser omentum, and the perinephric capsule.

Changes in fats during storage. The nature and extent of changes in fats during storage depend on their exposure to air and water, temperature and duration of storage, as well as on the presence of substances that can interact chemically with fats. Fats can undergo various changes - from inactivation of the biologically active substances they contain to the formation of toxic compounds.

During storage, a distinction is made between hydrolytic and oxidative spoilage of fats; often both types of spoilage occur simultaneously.

Hydrolytic breakdown of fats occurs during the production and storage of fats and fat-containing products. Fats, under certain conditions, react with... water, forming glycerol and fatty acids.

The degree of fat hydrolysis is characterized by the content of free fatty acids, which impair the taste and smell of the product. The hydrolysis reaction can be reversible and depends on the water content in the reaction medium. Hydrolysis proceeds stepwise in 3 stages. At the first stage One fatty acid molecule is split off from a triglyceride molecule to form a diglyceride. Then at the second stage a second fatty acid molecule is split off from the diglyceride to form a monoglyceride. And finally, at the third stage As a result of the separation of the last fatty acid molecule from the monoglyceride, free glycerol is formed. Di- and monoglycerides formed at intermediate stages help accelerate hydrolysis. With complete hydrolytic cleavage of the triglyceride molecule, one molecule of glycerol and three molecules of free fatty acids are formed.

3. Fat catabolism.

The use of fat as an energy source begins with its release from fat depots into the bloodstream. This process is called fat mobilization. Fat mobilization is accelerated by the action of the sympathetic nervous system and the hormone adrenaline.

Instructions

The digestion process usually begins in the mouth with the help of enzymes contained in saliva. However, this does not apply to fats. There are no enzymes in saliva that can break them down. Next, the food enters the stomach, but even here the fats are not amenable to local digestive enzymes. Only a small proportion is decomposed by the enzyme lipase, very insignificant. The main process of fat digestion occurs in the small intestine.

Fats cannot dissolve in water, but they need to be mixed with water first. Only in this case can they be exposed to enzymes dissolved in water. The process of mixing fats with water is called emulsification, and it occurs with the participation of bile salts. These acids are then secreted into the gallbladder. After fatty foods enter the body, cells in the small intestine begin to produce a hormone that causes contractions of the gallbladder.

The gallbladder releases bile into the duodenum. Bile acids are located on the surface of fat droplets, which leads to a decrease in surface tension. Drops of fat break down into small ones; this process is also helped by contractions of the intestinal walls. As a result, the surface area between the fat and water phases increases. After emulsification, hydrolysis of fats occurs under the influence of pancreatic enzymes. Hydrolysis refers to the decomposition of a substance when it interacts with water.

Next, fat molecules are broken down by the pancreatic enzyme lipase. It is secreted into the cavity of the small intestine and acts on emulsified fat together with the protein colipase. This protein binds to euulsified fat, which significantly speeds up the process. As a result of cleavage by lipase, glycerol and fatty acids are formed.

Fatty acids combine with bile acids and penetrate the intestinal walls. There they combine with glycerol to form a fat triglyceride. Triglyceride, in combination with a small amount of protein, forms special substances, chylomicrons, which penetrate into the lymph. From lymph to blood, then to lungs. These substances contain absorbed fat. Thus, the products of fat breakdown enter the lungs.

The lungs contain cells that can trap fat. They protect the blood from excess fat. Fatty acids are also partially oxidized in the lungs, and the heat released warms the air entering the lungs. From the lungs, chylomicrons enter the blood, from where some move to the liver. A lot of fat accumulates in the liver when it is consumed in excess.

In the oral cavity, lipids are subjected only to mechanical processing. The stomach contains a small amount of lipase, which hydrolyzes fats. Low activity of gastric juice lipase is associated with the acidic reaction of the stomach contents. In addition, lipase can only affect emulsified fats; there are no conditions in the stomach for the formation of a fat emulsion. Only in children and monogastric animals does gastric juice lipase play an important role in lipid digestion.

The intestine is the main site of lipid digestion. In the duodenum, lipids are affected by liver bile and pancreatic juice, and at the same time neutralization of intestinal contents (chyme) occurs. Emulsification of fats occurs under the influence of bile acids. The composition of bile includes: cholic acid, deoxycholic (3.12 dihydroxycholanic), chenodeoxycholic (3.7 dihydroxycholanic) acids, sodium salts of paired bile acids: glycocholic, glycodeoxycholic, taurocholic, taurodeoxycholic. They consist of two components: cholic and deoxycholic acids, as well as glycine and taurine.

deoxycholic acid chenodeoxycholic acid

glycocholic acid

taurocholic acid

Bile salts emulsify fats well. This increases the area of contact between enzymes and fats and increases the effect of the enzyme. Insufficient synthesis of bile acids or delayed intake impairs the effectiveness of enzyme action. Fats, as a rule, are absorbed after hydrolysis, but some of the finely emulsified fats are absorbed through the intestinal wall and pass into the lymph without hydrolysis.

Esterases break the ester bond in fats between the alcohol group and the carboxyl group of carboxylic acids and inorganic acids (lipase, phosphatases).

Under the action of lipase, fats are hydrolyzed into glycerol and higher fatty acids. Lipase activity increases under the influence of bile, i.e. bile directly activates lipase. In addition, the activity of lipase is increased by Ca ++ ions due to the fact that Ca ++ ions form insoluble salts (soaps) with the released fatty acids and prevent their inhibitory effect on lipase activity.

Under the action of lipase, the ester bonds at the α and α 1 (side) carbon atoms of glycerol are first hydrolyzed, then at the β-carbon atom:

Under the action of lipase, up to 40% of triacylglycerides are broken down to glycerol and fatty acids, 50-55% are hydrolyzed to 2-monoacylglycerols and 3-10% are not hydrolyzed and are absorbed in the form of triacylglycerols.

Feed sterides are broken down by the enzyme cholesterol esterase into cholesterol and higher fatty acids. Phosphatides are hydrolyzed under the influence of phospholipases A, A 2 , C and D. Each enzyme acts on a specific ester bond of the lipid. The points of application of phospholipases are presented in the diagram:

Pancreatic phospholipases, tissue phospholipases, are produced in the form of proenzymes and are activated by trypsin. Snake venom phospholipase A 2 catalyzes the cleavage of the unsaturated fatty acid at position 2 of phosphoglycerides. In this case, lysolecithins with a hemolytic effect are formed.

phosphotidylcholine lysolecithin

Therefore, when this poison enters the blood, severe hemolysis occurs. In the intestine, this danger is eliminated by the action of phospholipase A 1, which quickly inactivates lysophosphatide as a result of the cleavage of a saturated fatty acid residue from it, converting it into inactive glycerophosphocholine.

Lysolecithins in low concentrations stimulate the differentiation of lymphoid cells, the activity of protein kinase C, and enhance cell proliferation.

Colamine phosphatides and serine phosphatides are cleaved by phospholipase A to lysocolamine phosphatides, lysoserine phosphatides, which are further cleaved by phospholipase A 2 . Phospholipases C and D hydrolyze choline bonds; colamine and serine with phosphoric acid and the remainder of phosphoric acid with glycerol.

Absorption of lipids occurs in the small intestine. Fatty acids with a chain length of less than 10 carbon atoms are absorbed in non-esterified form. Absorption requires the presence of emulsifying substances - bile acids and bile.

Resynthesis of fat characteristic of a given organism occurs in the intestinal wall. The concentration of lipids in the blood is high within 3-5 hours after eating food. Chylomicrons– small particles of fat formed after absorption in the intestinal wall are lipoproteins, surrounded by phospholipids and a protein shell, containing molecules of fat and bile acids inside. They enter the liver, where lipids undergo intermediate metabolism, and bile acids pass into the gallbladder and then back to the intestines (see Fig. 9.3 on p. 192). As a result of this circulation, a small amount of bile acids is lost. It is believed that a molecule of bile acid completes 4 cycles per day.

The digestive glands play a major role in the chemical transformation of food taken by humans. Namely, their secretion. This process is strictly coordinated. In the gastrointestinal tract, food is exposed to different digestive glands. Thanks to the entry of pancreatic enzymes into the small intestine, proper absorption of nutrients and a normal digestion process occur. In this whole scheme, enzymes necessary for the breakdown of fat play an important role.

Reactions and splitting

Digestive enzymes have the narrowly focused task of breaking down complex substances that enter the gastrointestinal tract with food. These substances are broken down into simple ones that are easy for the body to absorb. In the mechanism of food processing, enzymes, or enzymes that break down fat, play a special role (there are three types). They are produced by the salivary glands and the stomach, in which enzymes break down a fairly large volume of organic substances. These substances include fats, proteins, and carbohydrates. As a result of the influence of such enzymes, the body qualitatively assimilates the incoming food. Enzymes are needed for accelerated reactions. Each type of enzyme is suitable for a specific reaction, acting on the corresponding type of bond.

Assimilation

For better absorption of fats, gastric juice containing lipase works in the body. This enzyme, which breaks down fat, is produced by the pancreas. Carbohydrates are broken down by amylase. After disintegration, they are quickly absorbed and enter the bloodstream. Salivary amylase, maltase, and lactase also contribute to breakdown. Proteins are broken down thanks to proteases, which are also involved in the normalization of the microflora of the gastrointestinal tract. These include pepsin, chymosin, trypsin, erepsin and pancreatic carboxypeptidase.

What is the name of the main enzyme that breaks down fat in the human body?

Lipase is an enzyme whose main task is to dissolve, fractionate and digest fats in the human digestive tract. Fats entering the intestines are not able to be absorbed into the blood. To be absorbed, they must be broken down into fatty acids and glycerol. Lipase helps in this process. If there is a case where the enzyme that breaks down fat (lipase) is reduced, it is necessary to carefully examine the person for oncology.

Pancreatic lipase in the form of an inactive proenzyme of prolipase is excreted into the duodenum. Prolipase is activated under the influence of colipase, another enzyme from pancreatic juice. Lingual lipase is produced in infants by the oral glands. It is involved in the digestion of breast milk.

Hepatic lipase is secreted into the blood, where it binds to the vascular walls of the liver. Most fats from food are broken down in the small intestine by lipase from the pancreas.

Knowing which enzyme breaks down fats and what exactly the body cannot cope with, doctors can prescribe the necessary treatment.

The chemical nature of almost all enzymes is protein. at the same time it is also an endocrine system. The pancreas itself is actively involved in the digestion process, and the main gastric enzyme is pepsin.

How do pancreatic enzymes break down fat into simpler substances?

Amylase breaks down starch into oligosaccharides. The oligosaccharides are then broken down into glucose by other digestive enzymes. Glucose is absorbed into the blood. For the human body it is a source of energy.

All human organs and tissues are built from proteins. The pancreas is no exception, which activates enzymes only after they enter the lumen of the small intestine. When the normal functioning of this organ is disrupted, pancreatitis occurs. This is a fairly common disease. A disease in which there is no enzyme that breaks down fats is called intrasecretory.

Deficiency problems

Exocrine insufficiency reduces the production of digestive enzymes. In this case, a person cannot eat large amounts of food, since the function of breaking down triglycerides is impaired. Such patients, after eating fatty foods, experience symptoms of nausea, heaviness, and abdominal pain.

With intrasecretory insufficiency, the hormone insulin, which helps absorb glucose, is not produced. A serious disease occurs, which is called diabetes mellitus. Another name is sugar diabetes. This name is associated with an increase in urine production by the body, as a result of which it loses water and the person feels constant thirst. Carbohydrates almost do not enter the cells from the blood and therefore are practically not used for the body’s energy needs. The level of glucose in the blood increases sharply, and it begins to be excreted through the urine. As a result of such processes, the use of fats and proteins for energy purposes increases greatly, and products of incomplete oxidation accumulate in the body. Ultimately, the acidity in the blood also increases, which can even lead to a diabetic coma. In this case, the patient experiences respiratory distress, including loss of consciousness and death.

This example clearly shows how important enzymes are that break down fats in the human body so that all organs work harmoniously.

Glucagon

If any problems arise, you definitely need to solve them and help the body with the help of various treatment methods and medications.

Glucagon has the opposite effect of insulin. This hormone affects the breakdown of glycogen in the liver and the conversion of fats into carbohydrates, thereby increasing the concentration of glucose in the blood. And the hormone somatostatin inhibits the secretion of glucagon.

Self-medication

In medicine, enzymes that break down fats in the human body can be obtained with the help of medications. There are many of them - from the most famous brands to little-known and less expensive, but just as effective. The main thing is not to self-medicate. After all, only a doctor, using the necessary diagnostic methods, can select the right drug to normalize the functioning of the gastrointestinal tract.

However, often we only help the body with enzymes. The hardest part is getting it to work correctly. Especially if the person is already elderly. It is only at first glance that it seems that you bought the necessary tablets - and the problem is solved. In reality, everything is completely different. The human body is a perfect mechanism, which nevertheless ages and wears out. If a person wants it to serve him as long as possible, it is necessary to support it, diagnose and treat it on time.

Of course, after reading and finding out which enzyme breaks down fats during human digestion, you can go to the pharmacy and ask the pharmacist to recommend a drug with the desired composition. But this can only be done in exceptional cases, when for some compelling reason it is not possible to visit a doctor or invite him to your home. You need to understand that you can be very wrong and the symptoms of different diseases can be similar. And in order to make a correct diagnosis, you definitely need medical help. Self-medication can cause serious harm.

Digestion in the stomach

Gastric juice contains pepsin, hydrochloric acid and lipase. Pepsin acts only in and breaks down proteins into peptides. Lipase in gastric juice breaks down only emulsified (milk) fat. The fat-digesting enzyme becomes active only in the alkaline environment of the small intestine. It comes along with the composition of the food semi-liquid gruel, pushed out by the contracting smooth muscles of the stomach. It is pushed into the duodenum in separate portions. Some small part of the substances is absorbed in the stomach (sugar, dissolved salt, alcohol, pharmaceuticals). The digestion process itself mainly ends in the small intestine.

Food advanced into the duodenum receives bile, intestinal and pancreatic juices. Food moves from the stomach to the lower sections at different speeds. Fatty ones linger, but dairy ones pass quickly.

Lipase

Pancreatic juice is an alkaline liquid that is colorless and contains trypsin and other enzymes that break down peptides into amino acids. Amylase, lactase and maltase convert carbohydrates into glucose, fructose and lactose. Lipase is an enzyme that breaks down fats into fatty acids and glycerol. Digestion time and juice release depend on the type and quality of food.

The small intestine performs parietal and cavity digestion. After mechanical and enzymatic treatment, the breakdown products are absorbed into the blood and lymph. This is a complex physiological process that is carried out by villi and directed strictly in one direction, villi from the intestine.

Suction

Amino acids, vitamins, glucose, and mineral salts in the aqueous solution are absorbed into the capillary blood of the villi. Glycerol and fatty acids do not dissolve and cannot be absorbed by the villi. They move into epithelial cells, where fat molecules are formed that enter the lymph. Having passed the barrier of the lymph nodes, they enter the blood.

Bile plays a very important role in the absorption of fats. Fatty acids, combining with bile and alkalis, are saponified. In this way, soaps (soluble salts of fatty acids) are formed that easily pass through the walls of the villi. The glands in the large intestine primarily secrete mucus. The large intestine absorbs water up to 4 liters per day. A very large number of bacteria live here, participating in the breakdown of fiber and the synthesis of vitamins B and K.

The role of lipids in nutrition

Lipids are an essential part of a balanced human diet. It is generally accepted that with a balanced diet, the ratio of proteins, lipids and carbohydrates in the diet is approximately 1: 1: 4. On average, about 80 g of fats of animal and plant origin enter the body of an adult with food every day. In old age, as well as with little physical activity, the need for fat decreases; in cold climates and with heavy physical work, it increases.

The value of fats as a food product is very diverse. First of all, fats in human nutrition have an important energy value. The high calorie content of fats compared to proteins and carbohydrates gives them special nutritional value when the body expends large amounts of energy. It is known that 1 g of fats, when oxidized in the body, gives 38.9 kJ (9.3 kcal), while 1 g of protein or carbohydrates - 17.2 kJ (4.1 kcal). It should also be remembered that fats are solvents for vitamins A, D, E, etc., and therefore the body’s supply of these vitamins largely depends on the intake of fats in food. In addition, some polyunsaturated acids (linoleic, linolenic, arachidonic) are introduced into the body with fats, which are classified as essential fatty acids, because human tissues and a number of animals have lost the ability to synthesize them. These acids are conventionally combined into a group called “vitamin F”.

Finally, with fats the body receives a complex of biologically active substances, such as phospholipids, sterols, etc., which play an important role in metabolism.

Digestion and absorption of lipids

Breakdown of fats in the gastrointestinal tract. Saliva does not contain fat-breaking enzymes. Consequently, fats do not undergo any changes in the oral cavity. In adults, fats also pass through the stomach without any special changes, since the lipase contained in small quantities in the gastric juice of adults and mammals is inactive. The pH value of gastric juice is about 1.5, and the optimal pH value for gastric lipase is in the range of 5.5-7.5. In addition, lipase can actively hydrolyze only pre-emulsified fats; in the stomach, there are no conditions for emulsifying fats.

Digestion of fats in the stomach cavity plays an important role in the digestion process in children, especially infants. It is known that the pH of gastric juice in infants is about 5.0, which facilitates the digestion of emulsified milk fat by gastric lipase. In addition, there is reason to believe that with long-term consumption of milk as the main food product in infants, an adaptive increase in the synthesis of gastric lipase is observed.

Although no significant digestion of food fats occurs in the stomach of an adult, partial destruction of the lipoprotein complexes of food cell membranes is still observed in the stomach, which makes fats more accessible for the subsequent action of pancreatic juice lipase on them. In addition, a slight breakdown of fats in the stomach leads to the appearance of free fatty acids, which, when entering the intestines, contribute to the emulsification of fats there.

The breakdown of fats that make up food occurs in humans and mammals mainly in the upper parts of the small intestine, where there are very favorable conditions for the emulsification of fats.

After the chyme enters the duodenum, here, first of all, the hydrochloric acid of the gastric juice that enters the intestine with food, bicarbonates contained in the pancreatic and intestinal juices is neutralized. The bubbles of carbon dioxide released during the decomposition of bicarbonates contribute to good mixing of the food gruel with digestive juices. At the same time, fat emulsification begins. The most powerful emulsifying effect on fats, undoubtedly, is exerted by bile salts, which enter the duodenum with bile in the form of sodium salts, most of which are conjugated with glycine or taurine. Bile acids are the main end product of cholesterol metabolism.

The main stages of the formation of bile acids, in particular cholic acid, from cholesterol can be represented as follows. The process begins with the hydroxylation of cholesterol at the 7th α-position, i.e., with the inclusion of a hydroxyl group at position 7 and the formation of 7-hydroxycholesterol. Then, through a series of steps, 3,7,12-trihydroxycoprostanoic acid is formed, the side chain of which undergoes β-oxidation. In the final stage, propionic acid (in the form of propionyl-CoA) is separated and the side chain is shortened. A large number of liver enzymes and coenzymes take part in all these reactions.

By their chemical nature, bile acids are derivatives of cholanic acid. Human bile mainly contains cholic (3,7,12-trioxycholanic), deoxycholic (3,12-dihydroxycholanic) and chenodeoxycholic (3,7-dihydroxycholanic) acids.

In addition, human bile contains lithocholic (3-hydroxycholanic) acid in small (trace) quantities, as well as allocholic and ureodeoxycholic acids - stereoisomers of cholic and chenodeoxycholic acids.

As already noted, bile acids are present in bile in conjugated form, i.e. in the form of glycocholic, glycodeoxycholic, glycochenodeoxycholic (about 2/3-4/3 of all bile acids) or taurocholic, taurodeoxycholic and taurochenodeoxycholic (about 1/5-1 /3 of all bile acids). These compounds are sometimes called paired compounds, since they consist of two components - bile acid and glycine, or bile acid and taurine.

Note that the ratios between the conjugates of these two types can vary depending on the nature of the food: if carbohydrates predominate in it, the relative content of glycine conjugates increases, and with a high-protein diet, the content of taurine conjugates increases. The structure of these conjugates can be presented as follows:

It is believed that only the combination: bile salt + unsaturated fatty acid + monoglyceride can provide the required degree of fat emulsification. Bile salts dramatically reduce the surface tension at the fat/water interface, due to which they not only facilitate emulsification, but also stabilize the already formed emulsion.

Bile acids also play an important role as a kind of activator of pancreatic lipase 1, under the influence of which fat is broken down in the intestine. Lipase produced in the pancreas breaks down triglycerides that are in an emulsified state. It is believed that the activating effect of bile acids on lipase is expressed in a shift in the optimum action of this enzyme from pH 8.0 to 6.0, i.e., to the pH value that is more constantly maintained in the duodenum during the digestion of fatty foods. The specific mechanism of lipase activation by bile acids is still unclear.

1 However, there is an opinion that lipase activation does not occur under the influence of bile acids. Pancreatic juice contains a lipase precursor, which is activated in the intestinal lumen by forming a complex with colipase (cofactor) in a molar ratio of 2: 1. This helps to shift the pH optimum from 9.0 to 6.0 and prevent denaturation of the enzyme. It has also been established that the rate of hydrolysis catalyzed by lipase is not significantly affected by either the degree of unsaturation of fatty acids or the length of the hydrocarbon chain (from C 12 to C 18). Calcium ions accelerate hydrolysis mainly because they form insoluble soaps with the liberated fatty acids, i.e., they practically shift the reaction in the direction of hydrolysis.

There is reason to believe that there are two types of pancreatic lipase: one of them is specific for the ester bonds in positions 1 and 3 of the triglyceride, and the other hydrolyzes the bonds in position 2. Complete hydrolysis of triglycerides occurs in stages: first, bonds 1 and 3 are quickly hydrolyzed, and then hydrolysis of the 2-monoglyceride occurs slowly (scheme).

It should be noted that intestinal lipase is also involved in the breakdown of fats, but its activity is low. In addition, this lipase catalyzes the hydrolytic breakdown of monoglycerides and does not act on di- and triglycerides. Thus, practically the main products formed in the intestines during the breakdown of dietary fats are fatty acids, monoglycerides and glycerol.

Absorption of fats in the intestines. Absorption occurs in the proximal small intestine. Thinly emulsified fats (the size of fat droplets of the emulsion should not exceed 0.5 microns) can be partially absorbed through the intestinal wall without prior hydrolysis. However, the bulk of the fat is absorbed only after it is broken down by pancreatic lipase into fatty acids, monoglycerides and glycerol. Fatty acids with a short carbon chain (less than 10 C atoms) and glycerol, being highly soluble in water, are freely absorbed in the intestine and enter the blood of the portal vein, from there to the liver, bypassing any transformations in the intestinal wall. The situation is more complicated with long-carbon chain fatty acids and monoglycerides. The absorption of these compounds occurs with the participation of bile and mainly the bile acids included in its composition. Bile contains bile salts, phospholipids and cholesterol in a ratio of 12.5:2.5:1.0. Long-chain fatty acids and monoglycerides in the intestinal lumen form micelles (micellar solution) that are stable in an aqueous environment with these compounds. The structure of these micelles is such that their hydrophobic core (fatty acids, glycerides, etc.) is surrounded on the outside by a hydrophilic shell of bile acids and phospholipids. Micelles are approximately 100 times smaller than the smallest emulsified fat droplets. As part of micelles, higher fatty acids and monoglycerides are transferred from the site of fat hydrolysis to the absorption surface of the intestinal epithelium. There is no consensus regarding the mechanism of absorption of fat micelles. Some researchers believe that as a result of so-called micellar diffusion, and possibly pinocytosis, micelles penetrate into the epithelial cells of the villi as a whole particle. Here the breakdown of fat micelles occurs; in this case, bile acids immediately enter the bloodstream and enter the liver through the portal vein system, from where they are again secreted as part of bile. Other researchers admit the possibility that only the lipid component of fat micelles passes into the villi cells. And bile salts, having fulfilled their physiological role, remain in the intestinal lumen. And only then, in the overwhelming majority, they are absorbed into the blood (in the ileum), enter the liver and are then excreted in the bile. Thus, both researchers recognize that there is a constant circulation of bile acids between the liver and intestines. This process is called hepatic-intestinal (enterohepatic) circulation.

Using the labeled atom method, it was shown that bile contains only a small part of bile acids (10-15% of the total) newly synthesized by the liver, i.e. the bulk of bile acids in bile (85-90%) are bile acids , reabsorbed in the intestine and re-secreted as part of bile. It has been established that in humans the total pool of bile acids is approximately 2.8-3.5 g; at the same time, they make 5-6 revolutions per day.

Resynthesis of fats in the intestinal wall. The intestinal wall synthesizes fats that are largely specific to a given animal species and differ in nature from dietary fat. To a certain extent, this is ensured by the fact that in the synthesis of triglycerides (as well as phospholipids) in the intestinal wall, along with exogenous and endogenous fatty acids, they take part. However, the ability to carry out the synthesis of fat specific to a given animal species in the intestinal machine is still limited. A. N. Lebedev showed that when feeding an animal, especially a previously starved one, large quantities of foreign fat (for example, flaxseed oil or camel fat), part of it is found in the fatty tissues of the animal unchanged. Fat depots are most likely the only tissue where foreign fats can be deposited. Lipids that make up the protoplasm of cells of other organs and tissues are highly specific; their composition and properties depend little on dietary fats.

The mechanism of resynthesis of triglycerides in the cells of the intestinal wall in general terms boils down to the following: initially, their active form, acyl-CoA, is formed from fatty acids, after which acylation of monoglycerides occurs with the formation of first diglycerides and then triglycerides:

Thus, in the cells of the intestinal epithelium of higher animals, monoglycerides formed in the intestine during the digestion of food can be acylated directly, without intermediate stages.

However, the epithelial cells of the small intestine contain enzymes - monoglyceride lipase, which breaks down monoglyceride into glycerol and fatty acid, and glycerol kinase, which can convert glycerol (formed from monoglyceride or absorbed from the intestine) into glycerol-3-phosphate. The latter, interacting with the active form of fatty acid - acyl-CoA, produces phosphatidic acid, which is then used for the resynthesis of triglycerides and especially glycerophospholipids (see details below).

Digestion and absorption of glycerophospholipids and cholesterol. Glycerophospholipids introduced with food are exposed in the intestine to specific hydrolytic enzymes that break the ester bonds between the components that make up the phospholipids. It is generally accepted that in the digestive tract, the breakdown of glycerophospholipids occurs with the participation of phospholipases secreted with pancreatic juice. Below is a diagram of the hydrolytic cleavage of phosphatidylcholine:

There are several types of phospholipases.

Phospholipase A 1 hydrolyzes the ester bond at position 1 of the glycerophospholipid, as a result of which one molecule of fatty acid is split off and, for example, when phosphatidylcholine is broken down, 2-acylglycerylphosphorylcholine is formed.
Phospholipase A 2 , formerly simply called phospholipase A, catalyzes the hydrolytic cleavage of the fatty acid at position 2 of glycerophospholipid. The resulting products are called lysophosphatidylcholine and lysophosphatidylethanolamine. They are toxic and cause destruction of cell membranes. The high activity of phospholipase A 2 in the venom of snakes (cobra, etc.) and scorpions leads to the fact that when they bite, red blood cells are hemolyzed.
Phospholipase A 2 of the pancreas enters the cavity of the small intestine in an inactive form and only after exposure to trypsin, leading to the cleavage of the heptapeptide from it, becomes active. The accumulation of lysophospholipids in the intestine can be eliminated if both phospholipases act simultaneously on glycerophospholipids: A 1 and A 2. As a result, a product that is non-toxic to the body is formed (for example, when phosphatidylcholine is broken down - glycerylphosphorylcholine).
Phospholipase C causes hydrolysis of the bond between phosphoric acid and glycerol, and phospholipase D cleaves the ester bond between the nitrogenous base and phosphoric acid to form the free base and phosphatidic acid.

So, as a result of the action of phospholipases, glycerophospholipids are broken down to form glycerol, higher fatty acids, nitrogenous base and phosphoric acid.

It should be noted that a similar mechanism for the breakdown of glycerophospholipids also exists in body tissues; This process is catalyzed by tissue phospholipases. Note that the sequence of reactions for the cleavage of glycerophospholipids into individual components is not yet known.

We have already discussed the mechanism of absorption of higher fatty acids and glycerol. Phosphoric acid is absorbed by the intestinal wall mainly in the form of sodium or potassium salts. Nitrogenous bases (choline and ethanolamine) are absorbed in the form of their active forms.

As already noted, resynthesis of glycerophospholipids occurs in the intestinal wall. Necessary components for synthesis: higher fatty acids, glycerol, phosphoric acid, organic nitrogenous bases (choline or ethanolamine) enter the epithelial cell upon absorption from the intestinal cavity, since they are formed during the hydrolysis of dietary fats and lipids; These components are partially delivered to the intestinal epithelial cells through the bloodstream from other tissues. Resynthesis of glycerophospholipids proceeds through the stage of formation of phosphatidic acid.

As for cholesterol, it enters the human digestive organs mainly with egg yolk, meat, liver, and brain. The body of an adult daily receives 0.1-0.3 g of cholesterol contained in food products either in the form of free cholesterol or in the form of its esters (cholesterides). Cholesterol esters are broken down into cholesterol and fatty acids with the participation of a special enzyme in pancreatic and intestinal juices - cholesterol esterase. Water-insoluble cholesterol, like fatty acids, is absorbed in the intestine only in the presence of bile acids.

Chylomicron formation and lipid transport. Triglycerides and phospholipids resynthesized in intestinal epithelial cells, as well as cholesterol entering these cells from the intestinal cavity (here it can be partially esterified) combine with a small amount of protein and form relatively stable complex particles - chylomicrons (CM). The latter contain about 2% protein, 7% phospholipids, 8% cholesterol and its esters and over 80% triglycerides. The diameter of the CM ranges from 100 to 5000 nm. Due to the large particle size, CMs are not able to penetrate from the intestinal endothelial cells into the blood capillaries and diffuse into the intestinal lymphatic system, and from it into the thoracic lymphatic duct. Then, from the thoracic lymphatic duct, HMs enter the bloodstream, i.e., with their help, exogenous triglycerides, cholesterol and partially phospholipids are transported from the intestine through the lymphatic system into the blood. Already 1-2 hours after ingestion of food containing lipids, nutritional hyperlipemia is observed. This is a physiological phenomenon, characterized primarily by an increase in the concentration of triglycerides in the blood and the appearance of CM in it. The peak of nutritional hyperlipemia occurs 4-6 hours after ingestion of fatty foods. Usually, 10-12 hours after eating, the triglyceride content returns to normal values, and CM completely disappear from the bloodstream.

It is known that the liver and adipose tissue play the most significant role in the further fate of CM. The latter diffuse freely from the blood plasma into the intercellular spaces of the liver (sinusoids). It is assumed that the hydrolysis of CM triglycerides occurs both inside liver cells and on their surface. As for adipose tissue, chylomicrons are not able (due to their size) to penetrate its cells. In this regard, CM triglycerides undergo hydrolysis on the surface of the capillary endothelium of adipose tissue with the participation of the enzyme lipoprotein lipase, which is closely associated with the surface of the capillary endothelium. As a result, fatty acids and glycerol are formed. Some of the fatty acids pass into the fat cells, and some bind to serum albumin and are carried away with its current. Adipose tissue and glycerol can leave the bloodstream.

The breakdown of CM triglycerides in the liver and in the blood capillaries of adipose tissue actually leads to the cessation of the existence of CM.

Intermediate lipid metabolism. Includes the following main processes: the breakdown of triglycerides in tissues with the formation of higher fatty acids and glycerol, the mobilization of fatty acids from fat depots and their oxidation, the formation of acetone bodies (ketone bodies), the biosynthesis of higher fatty acids, triglycerides, glycerophospholipids, sphingolipids, cholesterol, etc. d.

Intracellular lipolysis

The main endogenous source of fatty acids used as “fuel” is reserve fat contained in adipose tissue. It is generally accepted that triglycerides in fat depots play the same role in lipid metabolism as liver glycogen in carbohydrate metabolism, and higher fatty acids in their role resemble glucose, which is formed during the phosphorolysis of glycogen. During physical work and other conditions of the body that require increased energy expenditure, the consumption of adipose tissue triglycerides as an energy reserve increases.

Since only free, i.e. non-esterified, fatty acids can be used as energy sources, triglycerides are first hydrolyzed using specific tissue enzymes - lipases - to glycerol and free fatty acids. The last of the fat depots can pass into the blood plasma (mobilization of higher fatty acids), after which they are used by the tissues and organs of the body as energy material.

Adipose tissue contains several lipases, of which the most important are triglyceride lipase (the so-called hormone-sensitive lipase), diglyceride lipase and monoglyceride lipase. The activity of the last two enzymes is 10-100 times higher than the activity of the first. Triglyceride lipase is activated by a number of hormones (for example, adrenaline, norepinephrine, glucagon, etc.), while diglyceride lipase and monoglyceride lipase are insensitive to their action. Triglyceride lipase is a regulatory enzyme.

It has been established that hormone-sensitive lipase (triglyceride lipase) is found in adipose tissue in an inactive form and is activated by cAMP. As a result of the influence of hormones, the primary cellular receptor modifies its structure, and in this form it is able to activate the enzyme adenylate cyclase, which in turn stimulates the formation of cAMP from ATP. The resulting cAMP activates the enzyme protein kinase, which, by phosphorylating inactive triglyceride lipase, converts it into an active form (Fig. 96). Active triglyceride lipase breaks down triglyceride (TG) into diglyceride (DG) and fatty acid (FA). Then, under the action of di- and monoglyceride lipases, the final products of lipolysis are formed - glycerol (GL) and free fatty acids, which enter the bloodstream.

Free fatty acids bound to plasma albumin in the form of a complex enter organs and tissues through the bloodstream, where the complex disintegrates, and the fatty acids undergo either β-oxidation, or part of them is used for the synthesis of triglycerides (which then go into the formation of lipoproteins), glycerophospholipids, sphingolipids and other compounds, as well as the esterification of cholesterol.

Another source of fatty acids is membrane phospholipids. In the cells of higher animals, metabolic renewal of phospholipids continuously occurs, during which free fatty acids are formed (a product of the action of tissue phospholipases).