Exchange of bilirubin

Bilirubin is the final product of heme decay. The main part (80-85%) of bilirubin is formed from hemoglobin and only a small part from other heme-containing proteins, for example cytochrome P450. The formation of bilirubin occurs in the cells of the reticuloendothelial system. About 300 mg of bilirubin is formed daily.

The conversion of heme to bilirubin occurs with the participation of the microsomal enzyme hemoxygenase, for which oxygen and NADPH are required. The cleavage of the porphyrin ring occurs selectively in the methane group at position a. The carbon atom that forms part of the a-methane bridge is oxidized to carbon monoxide, and instead of the bridge, two double bonds are formed with oxygen molecules coming from outside. The resultant linear tetrapyrrole is IX-alpha-biliverdin in structure. Further, it is converted by biliverdin reductase, a cytosolic enzyme, into IX-alpha-bilirubin. Linear tetrapyrrole of this structure should dissolve in water, while bilirubin is a fat soluble substance. The solubility in lipids is determined by the structure of IX-alpha-bilirubin - by the presence of 6 stable intramolecular hydrogen bonds. These bonds can be destroyed by alcohol in diazoreaction (Van den Berg), in which unconjugated (indirect) bilirubin is converted into a conjugated (direct) bilirubin. In vivo, stable hydrogen bonds are destroyed by esterification with glucuronic acid.

About 20% of circulating bilirubin is formed not from the heme of mature erythrocytes, but from other sources. A small amount comes from immature cells of the spleen and bone marrow. With hemolysis, this amount increases. The rest of bilirubin is formed in the liver from heme-containing proteins, for example myoglobin, cytochromes, and from other unidentified sources. This fraction increases with pernicious anemia, erythropoietic uroporphyrin and in Kriegler-Nayyar syndrome.

Transport and conjugation of bilirubin in the liver

Unconjugated bilirubin in plasma is firmly bound to albumin. Only a very small part of bilirubin can undergo dialysis, but under the influence of substances that compete with bilirubin for binding to albumin (for example, fatty acids or organic anions), it can increase. This is important in newborns, where a number of drugs (eg, sulfonamides and salicylates) can facilitate the diffusion of bilirubin into the brain and thus contribute to the development of nuclear jaundice.

Many organic anions, including fatty acids, bile acids and other components of bile that do not belong to bile acids, such as bilirubin (despite its strong association with albumin), are secreted by the liver. Studies have shown that bilirubin is separated from albumin in sinusoids, diffuses through a layer of water on the surface of the hepatocyte. The previously stated assumptions about the presence of albumin receptors have not been confirmed. Transfer of bilirubin through the plasma membrane into the hepatocyte is carried out with the help of transport proteins, for example transport protein of organic anions, and / or by the mechanism of "flip-flop". Capture of bilirubin is highly effective due to its rapid metabolism in the liver in the reaction of glucuronidization and excretion into bile, as well as the presence of binding proteins in the cytosol, such as ligandins (glutathione-8-transferase).

Unconjugated bilirubin is a non-polar (fat-soluble) substance. In the conjugation reaction, it turns into a polar (water-soluble substance) and can therefore be excreted into bile. This reaction proceeds via the microsomal enzyme uridine-diphosphate glucuronyltransferase (UDPGT), which converts unconjugated bilirubin into conjugated mono- and diglucuronide bilirubin. UDFGT is one of several enzyme isoforms that ensure the conjugation of endogenous metabolites, hormones and neurotransmitters.

The gene UDFGT bilirubin is on the 2nd pair of chromosomes. The structure of the gene is complex. For all isoforms of UDPGT, the constant components are exons 2-5 at the 3 'end of the DNA of the gene. To express the gene, one of the first few exons must be involved. Thus, for the formation of bilirubin-UDPGT isoenzymes 1 * 1 and 1 * 2, the exons 1A and ID, respectively, must be involved. Isozyme 1 * 1 participates in the conjugation of virtually all bilirubin, and the isoenzyme 1 * 2 is almost or completely not involved in this. Other exons (IF and 1G) encode the phenol-UDPGT isoforms. Thus, the choice of one of the sequences of exon 1 determines the substrate specificity and the properties of the enzymes.

Further expression of UDPGT 1 * 1 also depends on the promoter region at the 5 'end associated with each of the first exons. The promoter region contains the TATAA sequence.

Details of the structure of the gene are important for understanding the pathogenesis of unconjugated hyperbilirubinemia (Gilbert and Kriegler-Nayyar syndromes) when the content of enzymes responsible for conjugation in the liver is reduced or absent.

The activity of UDFGT in hepatic cell jaundice is maintained at a sufficient level, and even increases with cholestasis. In newborns, UDFGT activity is low.

In human bile, bilirubin is mainly represented by diglucuronide. The conversion of bilirubin to monoglycuronide as well as to diglucuronide occurs in the same microsomal glucuronyl transferase system. When bilirubin is overloaded, for example, during hemolysis, monoglycuronide is predominantly formed, and with a decrease in the intake of bilirubin or with the induction of the enzyme, the content of diglucuronide increases.

The most important is conjugation with glucuronic acid, but a small amount of bilirubin is conjugated to sulfates, xylose and glucose; with cholestasis, these processes are intensified.

In the late stages of cholestatic or hepatic-cell jaundice, despite the high content of plasma, bilirubin in the urine is not detected. Obviously, the reason for this is the formation of bilirubin type III, monoconjugated, which is covalently bound to albumin. It is not filtered in the glomeruli and, therefore, does not appear in the urine. This reduces the practical significance of the samples used to determine the content of bilirubin in the urine.

Excretion of bilirubin into the tubules occurs with the help of a family of ATP-dependent multispecific transport proteins for organic anions. The rate of transport of bilirubin from plasma to bile is determined by the stage of excretion of glucuronide bilirubin.

The bile acids are transported to the bile with the help of another transport protein. The presence of different mechanisms of transport of bilirubin and bile acids can be illustrated by the example of Dubin-Johnson syndrome, in which the excretion of conjugated bilirubin is disturbed, but normal excretion of bile acids remains. Most of the conjugated bilirubin in bile is in mixed micelles containing cholesterol, phospholipids and bile acids. The significance of the Golgi apparatus and microfilaments of the cytoskeleton of hepatocytes for intracellular transport of conjugated bilirubin has not yet been established.

Diglukuronid bilirubin, located in the bile, water soluble (polar molecule), so the small intestine is not absorbed. In the large intestine, conjugated bilirubin undergoes hydrolysis of b-glucuronidase bacteria with the formation of urobilinogens. With bacterial cholangitis, part of diglucuronide bilirubin is hydrolyzed already in the biliary tract, followed by precipitation of bilirubin. This process can be important for the formation of bilirubin gallstones.

Urolilinogen, having an nonpolar molecule, is well absorbed in the small intestine and in a minimal amount - in the thick. A small amount of urobilinogen, which is normally absorbed, is again excreted by the liver and kidneys (enterohepatic circulation). When the hepatocyte function is disturbed, hepatic reexecretion of urobilinogen is disrupted and renal excretion increases. This mechanism explains urobilinogenuria in alcoholic liver disease, with fever, heart failure, and also in the early stages of viral hepatitis.