Bile acids, which are important components bile are synthesized directly in the liver from cholesterol. During meals, bile, which accumulates in the gallbladder, is released into the intestines. During the digestion process, it accelerates the breakdown and absorption of fats, and also promotes the preservation healthy microflora. Subsequently, 90% of bile acids return to the blood, from where they are again taken by the liver.
A blood test to determine the amount of bile acids is in an important way diagnostics of the development of various diseases. The data obtained allows you to correctly establish a diagnosis and prescribe the correct course of treatment. The following main ones are distinguished organic acids components of bile:
- Holevaya - 38%.
- Chenodeoxycholic acid - 34%.
- Deoxycholic acid - 28%.
- Lithocholic acid - 2%.
What kind of analysis is this
To test blood for the content of these substances, a unified enzymatic-colorimetric method is used. It is noteworthy that standard indicators at healthy people even after eating they change slightly.
Therefore, any deviation from the standard indicates liver pathologies and impaired bile outflow. The research does not require much time. Test results can be obtained within an hour after blood collection.
When is the test ordered?
The biochemical analysis can be prescribed by a doctor if there is any suspicion of a malfunction in liver function. This is due to the fact that the amount of bile acids in the blood increases even with slightly pronounced pathologies. Thus, the level of these substances always increases with cholestasis, which is observed against the background of a variety of liver diseases.
In order to evaluate the effectiveness of the prescribed therapy, the study is prescribed for the treatment of diseases in the field of gastroenterology and hepatology. In particular, in people suffering chronic hepatitis C, decline earlier high performance is a determining factor for a positive prognosis.
The amount of bile acids in the blood plasma is an important marker in obstetrics, since intrahepatic cholestasis can be diagnosed in pregnant women in this way. The study is indicated in the presence of the following obvious symptoms:
- Increased liver size.
- Dryness occurs skin and itching.
- In case of unexplained weight loss.
- Frequent bowel movements and skin rashes.
How to prepare for the test
A sample is taken for research venous blood. To obtain reliable results analysis before donating blood, a person must refuse to eat for at least 9-10 hours.
During this period it is prohibited to consume alcoholic drinks and sweet juices. It is also important not to smoke and remain calm for several hours before blood collection. Optimal time for testing - from 7.30 to 11.30.
Acceptable standards of analysis
Normal values are in the range of 1.25-3.41 mcg/dL (2.5-6.8 mmol/L). When bile acids in the blood correspond to them, this is evidence of optimal cholesterol metabolism. Upon confirmation normal indicators During the study, the following diseases can be excluded:
- Subhepatic jaundice.
- Alcohol intoxication.
- Hepatitis.
- Cystic fibrosis.
- Acute cholecystitis.
- Congenital pathologies of the bile ducts.
Deviation of results from the norm
An increase in the level of bile acids clearly indicates liver dysfunction, which is often accompanied by other symptoms, such as:
- Itching of the skin.
- Slowing heart rate.
- Decline blood pressure.
In addition, simultaneously with the increase in the amount of bile acids, other blood parameters also change, namely:
- Hemoglobin levels decrease.
- ESR decreases.
- Blood clotting is impaired.
- A failure occurs in the hemostasis system.
A significant increase in the amount of bile acids is observed with the development of such diseases:
- Mechanical jaundice.
- Cirrhosis of the liver.
- Alcohol intoxication.
- Viral hepatitis;
The amount of bile acids always increases with cholestasis. This condition is associated with a violation of the process of bile outflow due to blockage of the ducts. I can provoke cholestasis not only serious illnesses, but also different medications, which are used to treat a wide variety of diseases.
During pregnancy slight increase the amount of bile acids is considered natural due to changes hormonal levels and others physiological changes in organism. But exceeding the norm by more than 4 times indicates the development of cholestasis in the expectant mother.
The amount of bile acids decreases in cholecystitis. This is due to the fact that when the walls of the gallbladder are inflamed, these substances are synthesized in the liver in smaller quantities. Another reason for the decrease in bile acids may be long-term use medications, which were prescribed to improve cholesterol metabolism.
A blood test for the amount of bile acids is always used in combination with other diagnostic methods. For correction physiological abnormalities it is necessary to reconsider the diet. It is also important to maintain sufficient physical activity to prevent excess weight gain.
BILE ACIDS: GENERAL INFORMATION
Monocarboxylic hydroxy acids belonging to the class of steroids. Solid optical active substances, poorly soluble in water. Produced by the liver from cholesterol, contain (in mammals) 24 carbon atoms. In different animals, the structure of the dominant bile acids is species specific. In the body, bile acids usually form conjugates with glycine (glycolic acid) or taurine (taurocholic acid).
Primary bile acids - cholic acid and chenodeoxycholic acid - are synthesized in the liver from cholesterol, conjugated with glycine or taurine and secreted as part of bile.
Secondary bile acids, including deoxycholic acid and lithocholic acid, are formed from primary bile acids in the colon under the action of bacteria.
Lithocholic acid Absorbed much worse than deoxycholic acid. Other secondary bile acids are formed in negligible quantities. These include ursodeoxycholic acid (a stereoisomer of chenodeoxycholic acid) and a number of other unusual bile acids.
In chronic cholestasis, these acids are found in increased quantities. Normally, the ratio of the amounts of bile acids conjugated to glycine and taurine is 3:1; with cholestasis, the concentrations of bile acids conjugated with sulfuric and glucuronic acids are often increased.
Bile acids are surfactants. If their concentration is aqueous solution exceeds the critical value of 2 mmol/l, bile acid molecules form aggregates called micelles.
Cholesterol is poorly soluble in water; its solubility in bile depends on the concentration of lipids and the ratio of the molar concentrations of bile acids and lecithin. When the ratio of these components is normal, soluble mixed micelles containing cholesterol are formed; when the ratio is disturbed, cholesterol crystals precipitate.
In addition to promoting the excretion of cholesterol, bile acids are necessary for the absorption of fats in the intestine, which also occurs through the formation of micelles.
Active transport of bile acids is the most important factor ensuring the formation of bile.
Finally, in the small and large intestines, bile acids facilitate the transport of water and electrolytes.
Monocarboxylic hydroxy acids belonging to the class of steroids. Solid optically active substances, poorly soluble in water. Produced by the liver from cholesterol, they contain (in mammals) 24 carbon atoms. In different animals, the structure of the dominant bile acids is species specific.
In the body, bile acids usually form conjugates with glycine (glycolic acid) or taurine (taurocholic acid).
Bile acids are solid powdery substances with high temperature melting temperature (from 134 to 223 ° C), having a bitter taste, poorly soluble in water, better in alcohol and alkaline solutions. By chemical structure they belong to the group of steroids and are derivatives of cholanic acid (C24H40O2). All bile acids are formed only in hepatocytes from cholesterol.
Among human bile acids, Bergstrom distinguished primary (cholic and chenodeoxycholic, synthesized in the liver) and secondary (deoxycholic and lithocholic, formed in the small intestine from primary acids under the influence of bacterial intestinal microflora).
Human bile also contains allocholic and ursodoxycholic acids, stereoisomers of cholic and chenodeoxycholic acids, respectively. Under physiological conditions, free bile acids are practically not found in bile, since they are all bound in pairs with glycine or taurine. The physiological significance of bile acid conjugates is that their salts are more polar than salts of free bile acids, are more easily secreted and have a lower critical micelle concentration.
The liver is the only organ capable of converting cholesterol into hydroxyl-substituted cholanic acids, since the enzymes involved in the hydroxylation and conjugation of bile acids are located in the microsomes and mitochondria of hepatocytes. Conjugation of bile acids, carried out enzymatically, occurs in the presence of magnesium ions, ATP, NADP, CoA. The activity of these enzymes changes according to fluctuations in the circulation rate and composition of the bile acid pool in the liver. The synthesis of the latter is controlled by a negative feedback mechanism, i.e. the intensity of bile acid synthesis in the liver is inversely proportional to the flow of secondary bile acids into the liver.
IN normal conditions The synthesis of bile acids in the human liver is low - from 200 to 300 mg per day. The conversion of cholesterol into bile acids occurs as a result of oxidation of the side chain and carboxylation of the C24 atom. Next, the double bond between the C4 and C6 atoms is saturated. The optical configuration of the hydroxy group at the C3 atom changes: it moves from the para position to a position with the introduction of two hydroxyl groups. Apparently, all microsomal hydroxylation reactions in the biosynthesis of bile acids require the participation of an electron transport chain, including cytochrome P-450 and NADP-H2-cytochrome P~450 oxidoreductase.
The steps that lead to the formation of cholic acid are different from the steps in the formation of chenodeoxycholic acid. In fact, these acids do not convert into one another, at least in humans. The reaction of the formation of cholic and chenodeoxycholic acids is determined by the influence on the activity of three main hydroxylases.
The first reaction in the bile acid biosynthesis pathway—hydroxylation of cholesterol at the 1a position—is the rate-limiting step of the process as a whole. In 1972, the existence of cyclic daily fluctuations in the activity of the cellular key enzyme in the biosynthesis of bile acids - cholesterol-7a-hydroxylase, caused by changes in the synthesis of the enzyme itself - was shown. It turned out that the change in the rate of synthesis of bile acids and cholesterol during the day occurs simultaneously with a maximum around midnight. The time required for cholesterol reserves to balance with cholic acid reserves is 3-5 days, and for deoxycholic acid - 6-10 days. This is consistent with the fact that cholic acid is a direct derivative of cholesterol, and deoxycholic acid is a derivative of cholic acid.
Bile acids synthesized in hepatocytes are excreted into bile conjugated with glycine or taurine and enter the gallbladder through the biliary tract, where they accumulate. Absorption occurs in the walls of the gallbladder small amount bile acids - about 1.3%. On an empty stomach, the main pool of bile acids is located in the gallbladder, and after stimulation of the stomach with food, the gallbladder reflexively contracts and bile acids enter the duodenum. Bile acids accelerate lipolysis and enhance the solubilization and absorption of fatty acids and monoglycerides.
In the intestine, bile acids under the influence of anaerobes are mostly deconjugated and reabsorbed, mainly in the distal part small intestine, where secondary bile acids are formed by bacterial dehydroxylation from the primary ones. From the intestine, bile acids with the portal blood flow again enter the liver, which absorbs almost all bile acids (approximately 99%) from the portal blood; Absolutely not a large number of(about 1%) enters the peripheral blood. This is why, if there is pathology of the liver, its ability to absorb bile acids from the portal blood and excrete them into the common bile duct may be reduced. Thus, the level of bile acids in the peripheral blood will increase. The significance of determining serum bile acids lies in the fact that they, being indicators of cholestasis, can be an indicator of liver disease in some patients - an indicator of hepatodepression.
Determined that active suction bile acids occurs in the ileum of the small intestine, while passive absorption occurs due to the concentration of bile acids in the intestine, since it is always higher than in the portal blood. During active absorption, the bulk of bile acids are absorbed, and passive absorption involves the absorption of a small amount. Bile acids absorbed from the intestine bind to albumin and are transported back to the liver through the portal vein. In hepatocytes, toxic free bile acids, accounting for approximately 15% of the total amount of bile acids absorbed into the blood, are converted into conjugated ones. From the liver, bile acids return to bile in the form of conjugates.
Such enterohepatic circulation in the body of a healthy person occurs 2-6 times a day, depending on the diet; 10-15% of all bile acids entering the intestine after deconjugation undergo deeper degradation in lower sections small intestine. As a result of oxidation and reduction processes caused by enzymes of the colon microflora, the ring structure of bile acids breaks down, which leads to the formation of a number of substances excreted in feces during external environment. In a healthy person, about 90% of fecal bile acids are secondary, i.e. lithocholic and deoxycholic acids. When using labeled bile acids, it has been proven that only a small amount can be detected in the urine.
BASIC FUNCTIONS OF BALL ACIDS
Bile acids in the human body perform various functions, the main ones are participation in the absorption of fats from the intestines, regulation of cholesterol synthesis and regulation of bile formation and bile excretion.
Bile acids play important role in the digestion and absorption of lipids. In the small intestine, conjugated bile acids, being surfactants, are adsorbed in the presence of free fatty acids and monoglycerides on the surface of fat droplets, forming a thin film that prevents the smallest fat droplets from merging into larger ones. This happens a sharp decline surface tension at the boundary of two phases - water and fat, which leads to the formation of an emulsion with particle sizes of 300-1000 mmk and a micellar solution with particle sizes of 3-30 mmk. The formation of micellar solutions facilitates the action pancreatic lipase, which, when exposed to fats, breaks them down into glycerol, which is easily absorbed by the intestinal wall, and fatty acid, insoluble in water. Bile acids, combining with the latter, form choleic acids, which are highly soluble in water and therefore easily absorbed by the intestinal villi in the upper parts of the small intestine. Choleic acids in the form of micelles are absorbed from the lumen ileum inside cells, passing cell membranes relatively easily.
Electron microscopic studies have shown that in the cell the connection between bile and fatty acids disintegrates: bile acids enter the blood and liver through the portal vein, and fatty acids, accumulating inside the cytoplasm of cells in the form of clusters of tiny droplets, are the end products of lipid absorption.
The second essential role of bile acids is the regulation of cholesterol synthesis and its degradation. The rate of cholesterol synthesis in the small intestine depends on the concentration of bile acids in the intestinal lumen. The main part of cholesterol in the human body is formed by synthesis, and a small part comes from food. Thus, the effect of bile acids on cholesterol metabolism is to maintain its balance in the body. Bile acids minimize the buildup or deficiency of cholesterol in the body.
The destruction and release of part of the bile acids represent the most important path excretion of cholesterol end products. Cholic acids serve as a regulator of the metabolism of not only cholesterol, but also other steroids, in particular hormones.
The physiological function of bile acids is to participate in the regulation of excretory function of the liver. Bile salts act as physiological laxatives, increasing intestinal motility. This effect of cholates explains sudden diarrhea when large amounts of concentrated bile enter the intestines, for example with hypomotor dyskinesia biliary tract. When bile is thrown into the stomach, it can develop.
VARIETIES OF BALL ACIDS
CHOLIC ACID
Bile acids are formed from cholesterol in the liver. These 24-carbon steroid compounds are derivatives of cholanic acid, having one to three b-hydroxyl groups and a side chain of 5 carbon atoms with a carboxyl group at the end of the chain. Cholic acid is the most important acid in the human body. In bile at a slightly alkaline pH it is present in the form of cholate anion.
BILE ACIDS AND BILE ACIDS SALTS
In addition to cholic acid, bile also contains chenodeoxycholic acid. It differs from cholic acid in the absence of a hydroxyl group at C-12. Both compounds are commonly called bile acids. Quantitatively, these are the most important end products of cholesterol metabolism.
The other two acids, deoxycholic and lithocholic, are called secondary bile acids because they are formed by dehydroxylation at C-7 of primary acids in gastrointestinal tract. Conjugates of bile acids with amino acids (glycine or taurine) linked by peptide bonds are formed in the liver. These conjugates are more strong acids and are present in bile in the form of salts (cholates and deoxycholates Na+ and K+, called bile salts).
MICELLES
Due to the presence of b-hydroxyl groups in the structure, bile acids and bile salts are amphiphilic compounds and have detergent properties (see p. 34). The main functions of bile acids are the formation of micelles, emulsification of fats and solubilization of lipids in the intestine. This increases the effectiveness of pancreatic lipase and promotes lipid absorption.
The figure shows how bile acid molecules are fixed to the micelle with their non-polar parts, ensuring its solubility. Lipase aggregates with bile acids and hydrolyzes fats (triacylglycerols) contained in the fat droplet.
METABOLIC TRANSFORMATIONS OF BILE ACIDS
Primary bile acids are formed exclusively in the cytoplasm of liver cells. The biosynthesis process begins with hydroxylation of cholesterol at C-7 and C-12, and epimerization at C-3, followed by reduction of the double bond in the B ring and shortening of the side chain by three carbon atoms.
The rate-limiting step is hydroxylation at C-7 with the participation of 7b-hydroxylase. Cholic acid serves as a reaction inhibitor, so bile acids regulate the rate of cholesterol degradation.
Conjugation of bile acids occurs in two stages. First, CoA esters of bile acids are formed, and then the actual stage of conjugation with glycine or taurine follows to form, for example, glycocholic and taurocholic acids. Bile is drained into the intrahepatic bile ducts and accumulates in the gallbladder.
Intestinal microflora produces enzymes that chemically modify bile acids. Firstly, the peptide bond is hydrolyzed (deconjugation), and secondly, secondary bile acids are formed due to dehydroxylation of C-7. However, most bile acids are absorbed intestinal epithelium(6) and after entering the liver it is again secreted as part of bile (enterohepatic circulation of bile acids). Therefore, of the 15-30 g of bile salts that enter the body daily with bile, only about 0.5 g is found in excrement. This approximately corresponds to the daily de novo biosynthesis of cholesterol.
If the composition of bile is unfavorable, individual components may crystallize. This entails deposition gallstones, which most often consist of cholesterol and calcium salts bile acids (cholesterol stones), but sometimes these stones also include bile pigments.
Bile acids (BA) are formed exclusively in the liver. Every day, 250-500 mg of FA is synthesized and lost in feces. FA synthesis is regulated by a negative feedback mechanism. Primary FAs are synthesized from cholesterol: cholic acid and chenodeoxycholic acid. Synthesis is regulated by the amount of FAs that return to the liver during enterohepatic circulation. Under the influence of intestinal bacteria, primary FAs undergo 7a-dehydroxylation with the formation of secondary FAs: deoxycholic and a very small amount of lithocholic. Tertiary FAs, mainly ursodeoxycholic acid, are formed in the liver by isomerization of secondary FAs. In human bile, the amount of trihydroxy acid (cholic acid) is approximately equal to the sum of the concentrations of two dihydroxy acids - chenodeoxycholic and deoxycholic.
FAs combine in the liver with the amino acids glycine or taurine. This prevents their absorption in the biliary tract and small intestine, but does not prevent absorption in the terminal ileum. Sulfation and glucuronidation (which are detoxification mechanisms) may increase in cirrhosis or cholestasis, in which an excess of these conjugates is found in the urine and bile. Bacteria can hydrolyze FA salts into FA and glycine or taurine.
FA salts are excreted into the bile canaliculi against a large concentration gradient between hepatocytes and bile. Excretion depends in part on the magnitude of the intracellular negative potential, which is approximately 35 mV and provides voltage-dependent accelerated diffusion, as well as on the carrier (100 kDa glycoprotein) mediated diffusion process. FA salts penetrate micelles and vesicles, combining with cholesterol and phospholipids. In the upper parts of the small intestine, micelles of FA salts, quite large in size, have hydrophilic properties, which prevents their absorption. They are involved in the digestion and absorption of lipids. Absorption of fatty acids occurs in the terminal ileum and proximal colon, and in the ileum absorption occurs by active transport. Passive diffusion of non-ionized FAs occurs throughout the intestine and is most effective against unconjugated dihydroxy FAs. Oral administration Ursodeoxycholic acid interferes with the absorption of chenodeoxycholic and cholic acids in the small intestine.
Absorbed FA salts enter the system portal vein and into the liver, where they are intensively captured by hepatocytes. This process occurs due to the functioning of a friendly system of transport of molecules across the sinusoidal membrane, based on the Na + gradient. C1 – ions also participate in this process. The most hydrophobic FAs (unbound mono- and dihydroxy bile acids) probably penetrate the hepatocyte by simple diffusion (flip-flop mechanism) through the lipid membrane. The mechanism of transport of fatty acids through the hepatocyte from the sinusoids to the bile canaliculi remains unclear. This process involves cytoplasmic FA-binding proteins, for example Za-hydroxysteroid dehydrogenase. The role of microtubules is unknown. Vesicles participate in the transfer of FAs only at high concentrations of the latter. The FAs are reconjugated and released back into the bile. Lithocholic acid is not re-excreted.
The described enterohepatic circulation of GI occurs from 2 to 15 times a day. The absorption capacity of various FAs, as well as the rate of their synthesis and exchange, is not the same.
In cholestasis, FAs are excreted in the urine via active transport and passive diffusion. FAs are sulfated, and the resulting conjugates are actively secreted by the renal tubules.
Bile acids in liver diseases
FAs increase the excretion of water, lecithin, cholesterol and the associated bilirubin fraction from bile. Ursodeoxycholic acid leads to significantly greater bile secretion than chenodeoxycholic or cholic acid.
An important role in the formation of gallstones is played by impaired bile excretion and a defect in the formation of bile micelles). It also leads to steatorrhea in cholestasis.
FAs, combining with cholesterol and phospholipids, form a suspension of micelles in solution and, thus, contribute to the emulsification of dietary fats, participating in parallel in the process of absorption through the mucous membranes. Decreased FA secretion causes steatorrhea. FAs promote lipolysis by pancreatic enzymes and stimulate the formation of gastrointestinal hormones.
Disruption of intrahepatic FA metabolism may play an important role in the pathogenesis of cholestasis. Previously it was believed that they contribute to the development of itching in cholestasis, but latest research indicate that the itching is caused by other substances.
The entry of FA into the blood of patients with jaundice leads to the formation of target cells in the peripheral blood and the excretion of conjugated bilirubin in the urine. If FAs are deconjugated by small intestinal bacteria, the resulting free FAs are absorbed. The formation of micelles and absorption of fats are disrupted. This partly explains the malabsorption syndrome, which complicates the course of diseases that are accompanied by stasis of intestinal contents and increased bacterial growth in the small intestine.
Removal of the terminal ileum interrupts enterohepatic hepatic circulation and allows large amounts of primary FAs to reach the colon and be dehydroxylated by bacteria, thereby reducing the body FA pool. An increase in FA in the colon causes diarrhea with significant loss of water and electrolytes.
Lithocholic acid is excreted mainly in the feces, and only a small part is absorbed. Its administration causes cirrhosis of the liver in experimental animals and is used for modeling cholelithiasis. Taurolithocholic acid also causes intrahepatic cholestasis, probably due to disruption of bile flow independent of GI.
Serum bile acids
Gas-liquid chromatography can fractionate FAs, but this method is expensive and time-consuming.
The enzymatic method is based on the use of 3-hydroxysteroid dehydrogenase of bacterial origin. The use of bioluminescent analysis, capable of detecting picomolar amounts of FA, made the enzymatic method equal in sensitivity to the immunoradiological one. If you have the necessary equipment, the method is simple and inexpensive. The concentration of individual FA fractions can also be determined using the immunoradiological method; There are special kits for this.
The total serum FA level reflects the reabsorption from the intestine of those FAs that were not extracted during the first passage through the liver. This value serves as a criterion for assessing the interaction between two processes: absorption in the intestine and uptake in the liver. Serum FA levels are more dependent on intestinal absorption than on liver extraction.
An increase in serum FA levels indicates hepatobiliary disease. Diagnostic value of FA levels in viral hepatitis and chronic diseases liver was lower than previously expected. However, this indicator is more valuable than serum albumin concentration and prothrombin time, since it not only confirms liver damage, but also allows us to assess its excretory function and the presence of portosystemic blood shunting. Serum FA levels also have prognostic significance. In Gilbert's syndrome, the concentration of fatty acids is within normal limits)