The role of the liver in the metabolism of proteins and amino acids

The liver plays an important role in the metabolism of proteins. The greatest amount of protein is synthesized in the muscles. But in terms of 1 g of the mass their amount in the liver is more than in the muscles. Here, not only proteins of hepatocytes are produced, but also a large amount of secreted proteins that are required to meet the needs of the organism as a whole. Most of the blood plasma proteins are synthesized in the liver. The most important of them is albumin, which synthesis is 25% from the total formation of proteins in the liver and 50% - from the amount of secreted proteins.

In the synthesis of albumin mRNA translation occurs on the polyribosomes of rough endoplasmic reticulum. Albumin synthesis is regulated by the transcription rate of mRNA and tRNA availability. With a decrease of oncotic pressure of plasma albumin synthesis is increased. Albumin synthesis is also dependent on income of amino acids, especially tryptophan.

In patients with large carcinoid synthesis of albumin can decrease significantly, as the tumor cells use tryptophan to serotonin synthesis. 75-90% of α-globulins and 50% of β-globulins are synthesized by hepatocytes. γ-Globulins are produced by Kupffer cells.

12.2. Choleresis. Pigment metabolism. Types of jaundice

Bile is produced by the liver cells – hepatocytes. Choleresis is the synthesis and secretion of bile by hepatocytes; the secretion of water by epithelial cells of bile ducts; reabsorption of water as it moves through the ducts of bile; excretion by hepatocytes of organic and inorganic components. In the day the person is secreted from 250 to 1000 ml of bile. Choleresis occurs continuously. After the meal, secretion of bile is enhanced reflexively after 3-12 min, and one of the stimuli that affect the acceleration of the process is the bile itself.

Cleavage products, toxic compounds and drugs are excreted with the bile. Bile acids emulsify fats, allowing absorption of fatty acids, cholesterol, vitamins B, K, E, and calcium salts in the small intestine. Bile creates favorable conditions for digestion in the small intestine, improves digestion of proteins and carbohydrates, facilitates the absorption of their cleavage products, stimulates the motility of the small intestine, prevents putrefaction in the intestines, providing an antimicrobial effect, and stimulates the secretion of pancreatic juice and bile formation function of the liver.

The components of bile are bile acids, bile pigments, cholesterol, phospholipids, fatty acids, sodium, potassium, calcium, chlorine, phosphatidylcholine and water. Bile contains enzymes, vitamins, urea, uric acid, amino acids and other useless to the body compounds. Many substances, including drugs are excreted with bile. Substances secreted in the bile are divided into two groups:

- substances associated with proteins in plasma (e.g. hormones). They cannot penetrate the kidney filter and are excreted in bile;

- substances which are insoluble in water (cholesterol, steroids). In the liver they join with glucuronic acid and pass into a water-soluble state, after this they are excreted through the kidneys.

Cholesterol exists in a dissolved state in the complex with bile salts and phosphatidylcholine. With a lack of bile acids cholesterol precipitates, and the stones can be formed. Intensive formation of stones is marked in bile stasis and infection.

Bile pigments (bilirubin is the main) do not take part in digestion. Their secretion by the liver is only excretory process.

Bile removes from the body many drugs, toxins, and inorganic substances - compounds of copper, zinc and mercury.

The production of bile is controlled by nerve and humoral mechanisms. Increased level of bile components in blood stimulate choleresis. Hormones have significant impact on choleresis. Vasopressin, ACTH, and insulin stimulate it. The pineal gland hormone also stimulates secretion of bile, as a result of its direct effect on the liver parenchyma. Choleresis process is most closely associated with the cleavage of the hemoglobin in the tissues and the formation of bile pigments.

Pigment metabolism. Heme is the prosthetic group of hemoglobin and hemin enzymes; about 80% of the body’s heme is in hemoglobin. With the destruction of red blood cell hemoglobin is released. Its cleavege occurs in the liver, spleen and bone marrow. The degradation of the hemoglobin in the liver begins with α-methine linkage between rings I and II of the porphyrin ring. This process is catalyzed by NADPH- containing oxidase and leads to the formation of green pigment verdoglobin. Then verdoglobin spontaneously decomposes, iron and globin protein are released and one of the bile pigments – biliverdin - is formed. The formed biliverdin is enzymatically reduced to the main bile pigment bilirubin in the liver.