Biochemistry of Liver PDF

# Biochemistry of Liver ## Liver Structure - sinusoids - portal vein - bile duct - hepatic artery - central vein - bile canaliculi ## Functions of the Liver - **Supply** - Vena cava - **Metabolism** - Biosynthesis - Storage - Conversion - Degradation - **Uptake**...

# Biochemistry of Liver ## Liver Structure - sinusoids - portal vein - bile duct - hepatic artery - central vein - bile canaliculi ## Functions of the Liver - **Supply** - Vena cava - **Metabolism** - Biosynthesis - Storage - Conversion - Degradation - **Uptake** - From the gastrointestinal tract, pancreas, spleen - **Excretion** - Bile duct - Gallbladder - Intestine - **Detoxification** - Biotransformation ## Liver Metabolism | | Carbohydrate Metabolism | Lipid Metabolism | Amino Acid Metabolism | Biotransformation | | :---- | :----------------------- | :--------------- | :------------------------------------ | :----------------- | | | Glucose | Fatty acids | Amino acids | Steroid hormones | | | Galactose | Fats | Urea | Bile pigments | | | Fructose | Ketone bodies | | Ethanol | | | Mannose | Cholesterol | | Drugs | | | Pentoses | Bile acids | | | | | Lactate | Vitamins | Plasma proteins | | | | Glycerol | | Lipoproteins | | | | Glycogen | | Albumin | | | | | | Coagulation factors | | | | | | Hormones | | | | | | Enzymes | | - **B** Biosynthesis - **C** Conversion and Degradation - **E** Excretion - **S** Storage ## Liver's Functions 1. Liver is a main organ which is responsible for dividing of nutritional substances in our organism (for example, glucose, triacylglicerides and ketone bodies). 2. Hepatocytes synthesizes as lot of blood plasma proteins and lipoproteins, low-weight bioactive substances (creatin, 25-oxicalciferol, hem), cholesterol. 3. Synthesis of urea (final product of nitrogen metabolism) also takes place in the liver. 4. Liver synthesizes bile acids and excrete a bile into intestinal tract. This process plays a very important role in lipids digestion and excretion of cholesterin and some products of metabolism into intestine. 5. Liver play a big desintoxification role, inactivates endogenic and exogenic substances (drugs, some hormones, different toxins). 6. Liver is a depo for iron, some another metals, vitamines A, D, E, B12, folic acid. ## Absorptive State The liver is in the center of this diagram with arrows showing the flow of nutrients. Nutrients are absorbed from the intestine and transported to the liver. - Glucose levels in the blood increase after a meal and are taken up by the liver and used for glycogen synthesis. - Amino acid levels in the blood increase after a meal and are taken up by the liver and used for protein synthesis. - Fatty acid levels in the blood increase after a meal and are taken up by the liver and used for fat synthesis. - Glycogen, proteins, and triacylglycerols are stored in the liver, muscle, and adipose tissue, respectively. ## Postabsorptive State - The liver releases glucose from glycogen stores to maintain blood glucose levels. - The liver synthesizes ketone bodies from fatty acids which are released to the blood and used as an energy source by other tissues. - The liver releases fatty acids from stored triacylglycerols. ## Role of the Liver in Carbohydrate Metabolism From intestine glucose pass into the liver, where most part of it undergone the phosphorillation. Glucose-6-phosphate formed in result of this reaction, which catalyzed by two enzymes - hexokinase and glucokinase. Glucose-6-phosphate is a key product of carbohydrates metabolism. In the liver this substance can metabolized into different ways depend of liver's and whole organism's necessity. ## The Fate of Glucose Molecule in the Cell - **Glycogenesis:** (synthesis of glycogen) is activated in well fed, resting state. - **Glycogenolyisis:** (degradation of glycogen) - **Gluconeogenesis:** is activated if glucose is required. - **Glycolysis:** is activated if energy is required. - **Glucose** is converted to **Glucose-6-phosphate** - Glucose-6-phosphate can either be converted to **Glycogen**, **Pyruvate**, or **Ribose and NADPH**. - **Pyruvate** is then converted to **TCA cycle**. ## Synthesis of Glycogen - Content in the liver - 70-100g - Glucose-6-phosphatase catalize dephosphorillation of glucose-6-phosphate and formation of free glucose - Excess of glucose-6-phosphate, which not used for synthesis of glycogen will follow to form free glucose - Glucose-6-phosphate decomposed to H₂O and CO2, and free energy for hepatocytes formed. - Part of glucose-6-phosphate oxidized in pentosophosphate cycle. - Hepatocytes content full set of gluconeogenesis necessary enzymes. So, in liver glucose can be formed from lactate, pyruvate, amino acids, glycerol. - Gluconegenesis from lactate takes place during intensive muscular work. Lactate formed from glucose in muscles, transported to the liver, new glucose formed and transported to the muscles ## Role of the Liver in Lipid Metabolism In the liver all processes of lipid metabolism take place. Most important of them are following: Lipogenesis (synthesis of fatty acids and lipids). Substrate for this process - acetyl-CoA, formed from glucose and amino acids, which are not used for another purposes. Liver more active than another tissues synthesizes saturated and monounsaturated fatty acids. Fatty acids then used for synthesis of lipids, phospholipids, cholesterol ethers. Liver play a central role in synthesis of cholesterin, because near 80% of its amount is synthesized there. Biosynthesis of cholesterin regulated by negative feedback. When the level of cholesterin in the meal increases, synthesis in liver decreases, and back to front. Besides synthesis regulated by insulin and glucagon. Liver is a place of ketone bodies synthesis. These substances formed from fatty acids after their oxidation, and from liver transported to another tissues, first of all to the heart, muscles, kidneys and brain ## Lipid Metabolism This image shows the various pathways for lipid metabolism. - The **intestine** absorbs **fats** and **cholesterol**. - These are then converted to **chylomicron residues**. - The liver receives **glucose**, which is converted to **acetyl CoA**. - The liver also receives **fatty acids**, which are either used for synthesizing **fats**, **phospholipids**, and **cholesterol** or converted to **ketone bodies**. - Ketone bodies are then sent to various organs. - VLDL, HDL, IDL, LDL, are all **apolipoproteins** involved in transporting lipids. ## Bile Acids and Bile Salts - **Cholesterol** is converted to **cholic acid** via multiple steps. - **Primary bile acids:** - Cholic acid: C-3, C-7, C-12 - Chenodeoxycholic C-3, C-7 - **Secondary bile acids:** - Deoxycholic acid: C-3, C-12 - Lithocholic acid: C-3. - **Bile salts** are conjugated bile acids - **Glycocholic acid** adds the amino acid **glycine** - **Taurocholic acid** adds the amino acid **taurine** ## Metabolism of Bile Salts - The liver synthesizes **primary bile acids** from **cholesterol**. - These are then conjugated to form **bile salts**. - The bile salts are stored in the **gallbladder**. - The bile salts are then excreted into the **intestine**. - The breakdown of bile salts by **intestinal bacteria** results in the production of **secondary bile acids** - The secondary bile acids are reabsorbed from the intestine and transported back to the liver. - This process is known as the **enterohepatic circulation**. - **Feces** will contain the remaining bile salts. ## Role of the Liver in Protein Metabolism - The liver has enzymes which are necessary for amino acids metabolism. Amino acids from food used in the liver for following pathways: 1. Protein synthesis. 2. Decomposition for the final products. 3. Transformation to the carbohydrates and lipids. 4. Interaction between amino acids. 5. Transformation to the different substances with amino group. 6. Release to the blood and transport to another organs and tissues. - Liver synthesizes 100% of albumins, 90 % of α₁-globulines, 75 % of α₂-globulines, 50 % of β-globulins, blood clotting factors, fibrinogen, protein part of blood lipoproteins, such enzyme as cholinesterase. - Liver can synthesize non-essential amino acids. - Liver synthesizes purine and pyrimidine nucleotides, hem, creatine, nicotinic acid, choline, carnitine, polyamines. ## Role of the Liver in Detoxification Processes - A xenobiotics is a compound that is foreign to the body. - The principal classes of xenobiotics of medical relevance are drugs, chemical cancerogens, and various compounds that have found their way into our environment by one route or another (insecticides, herbicides, pesticides, food additions, cosmetics, domestic chemical substances). - Some internal substances also have toxic properties (for example, bilirubin, free ammonia, bioactive amines, products of amino acids decay in the intestine). - Moreover, all hormones and mediatores must be inactivated. - Reactions of detoxification take place in the liver. - Big molecules like bilirubin excreted with the bile to intestine and leaded out with feces. Small molecules go to the blood and excreted via kidney with urine. ## Xenobiotics Biotransformation and Localization in Cell | | ENZYME | PHASE I | LOCALIZATION | | :-------------------------------------------- | :------------------------- | :-------------------- | :------------------------------------------------------------------------------------------------------------- | | **REACTION** | | | | | Hydrolysis | Esterase | | Microsomes, cytosol, lysosomes, blood lysosomes | | | Peptidase | | Microsomes, cytosol | | | Epoxide hydrolase | | | | Reduction | Azo- and nitro-reduction | | Microflora, microsomes, cytosol | | | Carbonyl reduction | | Cytosol, blood, microsomes | | | Disulfide reduction | | Cytosol | | | Sulfoxide reduction | | Cytosol | | Oxidation | Alcohol dehydrogenase | | Cytosol | | | Aldehyde dehydrogenase | | Mitochondria, cytosol | | | Aldehyde oxidase | | Cytosol | | | Xanthine oxidase | | Cytosol | | | Monoamine oxidase | | Mitochondria | | | Diamine oxidase | | Cytosol | | | Flavin-monooxygenases | | Microsomes | | | Cytochrome P450 | | Microsomes | | **PHASE 1** | | | | | **PHASE II** | | | | | Glucuronide conjugation | | | Microsomes | | Sulfate conjugation | | | Cytosol, microsomes | | Glutathione conjugation | | | Cytosol | | Amino acid conjugation | | | Mitochondria, cytosol | | Acetylation | | | Mitochondria, microsomes | | Methylation | | | Cytosol, microsomes, blood | ## The Metabolism of Xenobiotics has 2 Phases **In phase 1**, the major reaction involved is hydroxylation, catalyzed by members of a class of enzymes referred to as monooxygenases or cytochrome P-450 species. These enzymes can also catalyze deamination, dehalogenation, desulfuration, epoxidation, peroxidation and reduction reaction. Hydrolysis reactions and non-P-450-catalyzed reactions also occur in phase 2. **Phase 2** involves converting the hydroxylated or other compounds produced in Phase 1 to more polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids. ## Biotransformations This image shows various examples of how xenobiotics are biotransformed in the body. - **Drug Metabolism:** - **Acetylsalicylic acid (aspirin)** is hydrolyzed to **salicylate**. - Hydrolysis is just one type of reaction in Phase 1. - **Hormone Metabolism:** - **Norepinephrine** is methylated to **O-methylnorepinephrine**. - Methylation is just one type of reaction in Phase 2. ## Cytochrome P450-Dependent Monooxygenases: Reactions **(Phase 1)** - **Apolar substrate** can undergo various reactions, including **hydroxylation, epoxidation, and dealkylation**. ## Cytochrom P450 - The highest concentration - in endoplasmic reticulum of hepatocytes (microsomes). - Hem containing protein. - Catalyzes monooxigenation of oxygen atom into substrate; another oxygen atom is reduced to water - Electrons are transferred from NADPH to cytochrome P450 through flavoprotein NADPH-cytochrome P450 reductase. ## Reaction Mechanism of Cytochrome P450 - This image shows the various steps of cytochrome P450 reaction. - **Step 1:** - The resting state begins with the heme iron in the trivalent form. - The substrate will bind to the heme group. - **Step 2:** - An electron is transferred from FADH2, reducing the iron to the divalent form. - This allows the heme group to bind an **O2** molecule - **Step 3:** - Another electron transfer, and valence change, reduces the **O2** to a **peroxide**. - **Step 4:** - A **hydroxyl ion** is cleaved. - This results in the removal of **H2O** and the formation of reactive **ferryl radical**. - **Step 5:** - Oxygen atoms are inserted into a **C-H** bond in the substrate, resulting in the formation of an **OH** group - **Step 6:** - The product is then dissociated, returning the cytochrome P450 to its resting state. ## Conjugate Formation (Phase 2) - This image shows how **glucuronidation** is used to inactivate hormones. - **Tetrahydrocortisol** is conjugated with **UDP-GICUA** to form **tetrahydrocortisol glucuronide**. - The enzyme involved in this reaction is **glucuronosyltransferase**. ## In Phase 2 - The hydroxylated or other compounds produced in phase 1 are converted by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids, or by methylation. - In certain cases, phase 1 metabolic reaction convert xenobiotics from inactive to biologically active compounds. In these instances, the original xenobiotics are referred to as prodrugs or procarcinogens. In other cases, additional phase 1 reactions convert the active compounds to less active or inactive forms prior to conjugation. In yet other cases, it is the conjugation reactions themselves that convert the active product of phase 1 to less active or inactive species, which are subsequently excreted in the urine or bile. In a very few cases, conjugation may actually increase the biologic activity of a xenobiotics. ## 5 Types of Phase 2 Reactions 1. **Glucuronidation:** UDP-glucuronic acid is the glucuronyl donor. - A variety of glucuronyl transferases, present in both the ER and cytosol. are the catalysts. - Molecules like **bilirubin, thyroxin, 2-acetylaminofluorene, aniline, benzoic acid, meprobromate, phenol, crezol, indol, skatol, and many steroids** are excreted as glucuronides. The glucuronide may be attached to oxygen, nitrogen, or sulfur groups of substrates. 2. **Sulfation:** Some alcohols, arylamines, and phenols are sulfated. - The sulfate donor in these and other biologic sulfation reactions is **adenosine 3'-phosphate-5'-phosphosulfate (PAPS)**; this compound is called **active sulfate**. 3. **Conjugation with Glutathione:** Glutathione (γ-glutamylcysteinylglycine) is a tripeptide consisting of glutamic acid, cysteine, and glycine. - Glutathione is commonly abbreviated to GSH; - The SH indicates the sulfhydryl group of its cysteine and is the business part of the molecule. - A number of potentially toxic electrophilic xenobiotics (such as certain carcinogens) are conjugated to the nucleophilic GSH. - The enzymes catalyzing these reactions are called **glutathione S-transferases** and are present in high amounts in liver cytosol and in lower amounts in other tissues. 4. **Acetylation:** These reactions is represented by X + Acetyl- CoA → Acetyl-X + CoA, where X represents a xenobiotic. - These reactions are catalyzed by acetyltransferases present in the cytosol of various tissues, particularly liver. - The different aromatic amines, aromatic amino acids, such drug as isoniazid, used in the treatment of tuberculosis, and sulfanylamides are subjects to acetylation. - Polymorphic types of acetyltransferases exist, resulting in individuals who are classified as slow or fast acetylators, and influence the rate of clearance of drugs such as isoniazid from blood. 5. **Methylation:** - A few xenobiotics (amines, phenol, tio-substances, inorganic compounds of sulphur, selen, mercury, arsenic) are subject to methylation by methyltransferases, employing S-adenosylmethionine as methyl donor. - Also catecholamines and nicotinic acid amid (active form of vitamin PP) are inactivated due to methylation. ## Ureogenesis - Very important way of detoxification is ureogenes (urea synthesis). - Free ammonia, which formed due to metabolism of amino acids, amides and amines, removed from organism in shape of urea.