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Urea Cycle

Urea Cycle

The catabolism of amino acids Catabolism of amino acids Amino acids (AAs) can be acquired through the breakdown of intracellular or ingested dietary proteins. Amino acids can enter 3 metabolic routes within the body. They can 1) be recycled to synthesize new proteins; 2) combine with cofactors and substances to create amino acid derivatives; or 3) be catabolized into their functional groups and carbon skeletons. Catabolism of Amino Acids results in the release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology of nitrogen in the form of ammonium. This excess nitrogen is transported to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy and eliminated from the body in the form of urea via the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat. The urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation (or ornithine cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation) takes place mainly in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and comprises the synthesis Synthesis Polymerase Chain Reaction (PCR) of urea from ammonium, CO2, aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids, and bicarbonate Bicarbonate Inorganic salts that contain the -HCO3 radical. They are an important factor in determining the ph of the blood and the concentration of bicarbonate ions is regulated by the kidney. Levels in the blood are an index of the alkali reserve or buffering capacity. Electrolytes. The cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation involves 1 feeder reaction to incorporate the ammonium and 4 reactions in the cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation. It prevents cytotoxic Cytotoxic Parvovirus B19 hyperammonemia Hyperammonemia Elevated level of ammonia in the blood. It is a sign of defective catabolism of amino acids or ammonia to urea. Cirrhosis levels.

Last updated: 11 Jan, 2021

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Contents

Overview of Nitrogen Metabolism

Nitrogen transport

Schematic diagram of the catabolism of amino acids Catabolism of amino acids Amino acids (AAs) can be acquired through the breakdown of intracellular or ingested dietary proteins. Amino acids can enter 3 metabolic routes within the body. They can 1) be recycled to synthesize new proteins; 2) combine with cofactors and substances to create amino acid derivatives; or 3) be catabolized into their functional groups and carbon skeletons. Catabolism of Amino Acids, resulting in the release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology of amino groups that are excreted from the body as urea.

Image by Lecturio.
  • The catabolism of amino acids Catabolism of amino acids Amino acids (AAs) can be acquired through the breakdown of intracellular or ingested dietary proteins. Amino acids can enter 3 metabolic routes within the body. They can 1) be recycled to synthesize new proteins; 2) combine with cofactors and substances to create amino acid derivatives; or 3) be catabolized into their functional groups and carbon skeletons. Catabolism of Amino Acids (AAs) involves transamination Transamination Transamination is the transfer of an amino group from an alpha-AA to an alpha-keto acid, which is an AA with an alpha-keto group (=O) instead of an alpha-amino group (NH2). Catabolism of Amino Acids and deamination Deamination The removal of an amino group (NH2) from a chemical compound. Catabolism of Amino Acids reactions, which result in the release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology of ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance (NH3). 
  • The regulation of nitrogen levels within the body is crucial due to the toxicity Toxicity Dosage Calculation of ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance. 
  • Excess nitrogen in the form of ammonium is then bonded to AAs via transamination Transamination Transamination is the transfer of an amino group from an alpha-AA to an alpha-keto acid, which is an AA with an alpha-keto group (=O) instead of an alpha-amino group (NH2). Catabolism of Amino Acids reactions and transported to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy. 
    • The most important AAs in nitrogen transport are glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids and alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids.
  • Within the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy, ammonium ions are released through glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids deamination Deamination The removal of an amino group (NH2) from a chemical compound. Catabolism of Amino Acids and glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids and directly excreted into the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat.  
  • Within the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy, the amino groups from alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids and glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids are transferred via aminotransferases, resulting in ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance and aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids, which are shunted into the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation:
    • Involves 1 feeder reaction (incorporation of ammonium and CO2) and 4 cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation reactions (creating 1 molecule of urea)
    • Overall reaction equation:
      NH3 + CO2 + aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids + 3 ATP + 2 H2O → urea + fumarate Fumarate Citric Acid Cycle + 2 ADP + 2 Pi + AMP + PPi
    • Defects in any of the cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation’s catalyzing enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes result in hyperammonemia Hyperammonemia Elevated level of ammonia in the blood. It is a sign of defective catabolism of amino acids or ammonia to urea. Cirrhosis.

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Transport of Nitrogen in the Blood

Nitrogen atoms are produced in skeletal musculature via the catabolism of proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis. They are bound to AAs in the form of amino groups → bloodstream → liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy.

  • Nitrogen can be synthesized in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy from intermediate byproducts of the citric acid cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation.
  • Nitrogen can also be produced in the intestines by bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology or via the breakdown of dietary protein, then shunted directly to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy via the venous portal system.
Essential role of glutamate

Schematic diagram of the essential role of glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids (glutamic acid) in the transport of nitrogen from the peripheral sites of breakdown of amino acids Amino acids Organic compounds that generally contain an amino (-NH2) and a carboxyl (-COOH) group. Twenty alpha-amino acids are the subunits which are polymerized to form proteins. Basics of Amino Acids to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and kidney for excretion from the body.

Image by Lecturio.

Glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids (or glutamic acid) is essential for nitrogen transport to the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy and liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy.

  • Forms in peripheral cells by transamination Transamination Transamination is the transfer of an amino group from an alpha-AA to an alpha-keto acid, which is an AA with an alpha-keto group (=O) instead of an alpha-amino group (NH2). Catabolism of Amino Acids to an α-ketoglutarate 
  • Can receive another amino group via glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids synthetase → glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids
    • In the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy, glutaminase deaminates glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids, releasing ammonium ions into the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat.
    • In the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy, glutamine Glutamine A non-essential amino acid present abundantly throughout the body and is involved in many metabolic processes. It is synthesized from glutamic acid and ammonia. It is the principal carrier of nitrogen in the body and is an important energy source for many cells. Synthesis of Nonessential Amino Acids releases ammonium ions via glutamate dehydrogenase Glutamate dehydrogenase An enzyme that catalyzes the conversion of l-glutamate and water to 2-oxoglutarate and NH3 in the presence of NAD+. Synthesis of Nonessential Amino Acids, which are shunted into the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation.
    • Can also be converted into alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids via transamination Transamination Transamination is the transfer of an amino group from an alpha-AA to an alpha-keto acid, which is an AA with an alpha-keto group (=O) instead of an alpha-amino group (NH2). Catabolism of Amino Acids
Alaline role in nitrogen transport

Schematic diagram of the essential role of alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids in the transport of nitrogen from muscular tissue to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy before its introduction into the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation.

Image by Lecturio.

Alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids is essential for nitrogen transport from the muscles to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy.

  • Forms when glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids transfers its amino group to pyruvate Pyruvate Derivatives of pyruvic acid, including its salts and esters. Glycolysis → alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids
  • This reaction reverts within the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy:
    • Resulting pyruvate Pyruvate Derivatives of pyruvic acid, including its salts and esters. Glycolysis is used in gluconeogenesis Gluconeogenesis Gluconeogenesis is the process of making glucose from noncarbohydrate precursors. This metabolic pathway is more than just a reversal of glycolysis. Gluconeogenesis provides the body with glucose not obtained from food, such as during a fasting period. The production of glucose is critical for organs and cells that cannot use fat for fuel. Gluconeogenesis → glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance, which is released into the bloodstream and used again in skeletal muscle ( glucose Glucose A primary source of energy for living organisms. It is naturally occurring and is found in fruits and other parts of plants in its free state. It is used therapeutically in fluid and nutrient replacement. Lactose Intolerance– alanine Alanine A non-essential amino acid that occurs in high levels in its free state in plasma. It is produced from pyruvate by transamination. It is involved in sugar and acid metabolism, increases immunity, and provides energy for muscle tissue, brain, and the central nervous system. Synthesis of Nonessential Amino Acids or Cahill cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation).
    • Resulting glutamate Glutamate Derivatives of glutamic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that contain the 2-aminopentanedioic acid structure. Synthesis of Nonessential Amino Acids can be used to release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance via deamination Deamination The removal of an amino group (NH2) from a chemical compound. Catabolism of Amino Acids OR convert oxaloacetate Oxaloacetate Derivatives of oxaloacetic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that include a 2-keto-1, 4-carboxy aliphatic structure. Citric Acid Cycle into aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids via transamination Transamination Transamination is the transfer of an amino group from an alpha-AA to an alpha-keto acid, which is an AA with an alpha-keto group (=O) instead of an alpha-amino group (NH2). Catabolism of Amino Acids; both products are shunted into the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation.
Ammonia and urea removal

Overview of the transport of excess nitrogen in the form of amino groups to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy and kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy. Excess nitrogen is secreted directly into the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat in the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy or shunted into the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy. The resulting urea is then transported to the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy to be excreted in the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat.

Image by Lecturio.

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Steps of the Urea Cycle

  • Urea cycle takes place exclusively in the liver, within the mitochondria and cytosol of hepatocytes. 
  • Nitrogen atoms reach the liver in the form of amino groups bound to AAs (alanine or glutamate). 
  • Involves 1 feeder reaction and 4 cycle reactions, and requires 3 ATP
Urea cycle

Schematic diagram of the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation (feeder reaction outlined in the blue square).

Image by Lecturio.

Reaction steps of the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation

  • Step 1 or feeder reaction: ammonia (NH3) + CO2 → carbamoyl phosphate 
    • In the mitochondria, catalyzed by carbamoyl phosphate synthetase, requires N-acetylglutamate as an activator and 2 ATP
    • Rate-limiting reaction
  • Step 2: carbamoyl phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes + ornithine → citrulline
    • In the mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles, catalyzed by ornithine carbamoyltransferase
  • Step 3: citrulline + aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids → argininosuccinate 
    • In the cytosol Cytosol A cell’s cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. The Cell: Cytosol and Cytoskeleton, catalyzed by argininosuccinate synthetase, requires 1 ATP
  • Step 4: argininosuccinate → arginine + fumarate Fumarate Citric Acid Cycle
    • In the cytosol Cytosol A cell’s cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. The Cell: Cytosol and Cytoskeleton, catalyzed by argininosuccinate lyase
    • Fumarate Fumarate Citric Acid Cycle either enters the CAC or transforms into oxaloacetate Oxaloacetate Derivatives of oxaloacetic acid. Included under this heading are a broad variety of acid forms, salts, esters, and amides that include a 2-keto-1, 4-carboxy aliphatic structure. Citric Acid Cycle, which can be turned into aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids and re-enter the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation ( aspartate Aspartate One of the non-essential amino acids commonly occurring in the l-form. It is found in animals and plants, especially in sugar cane and sugar beets. It may be a neurotransmitter. Synthesis of Nonessential Amino Acids cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation). 
  • Step 5: arginine + H2O → urea + ornithine
    • In cytosol Cytosol A cell’s cytoskeleton is a network of intracellular protein fibers that provides structural support, anchors organelles, and aids intra- and extracellular movement. The Cell: Cytosol and Cytoskeleton, catalyzed by arginase
    • Urea enters the bloodstream.
    • Ornithine is transported back into the mitochondrial matrix → step 2.
  • Net reaction per cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation: 2 NH3 + CO2 + 3 ATP + H2O → urea + 2 ADP + 4 Pi + AMP

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Elimination of Urea

  • Approximately 30 g of urea are produced → bloodstream every day 
    • Varies greatly according to diet
  • Urea is soluble in water → the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy → excreted in the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat
    • Biggest portion of the nitrogen-containing compounds is in the urine Urine Liquid by-product of excretion produced in the kidneys, temporarily stored in the bladder until discharge through the urethra. Bowen Disease and Erythroplasia of Queyrat
  • Urea is filtered via the glomerular capillaries Glomerular capillaries Kidneys: Anatomy and partially reabsorbed (can be measured in the laboratory).

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Clinical Relevance

Congenital Congenital Chorioretinitis deficiencies of the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation

  • Ornithine transcarbamylase (OTC) deficiency: X-linked X-linked Genetic diseases that are linked to gene mutations on the X chromosome in humans or the X chromosome in other species. Included here are animal models of human X-linked diseases. Common Variable Immunodeficiency (CVID) recessive condition considered to be the most common type of urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation disorder in humans. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship suffering from this congenital Congenital Chorioretinitis disorder lose the ability to eliminate ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance due to a lack of OTC. The high levels of ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance caused by this condition produce a coma Coma Coma is defined as a deep state of unarousable unresponsiveness, characterized by a score of 3 points on the GCS. A comatose state can be caused by a multitude of conditions, making the precise epidemiology and prognosis of coma difficult to determine. Coma in newborns within 72 hours of birth.

Acquired conditions that affect Affect The feeling-tone accompaniment of an idea or mental representation. It is the most direct psychic derivative of instinct and the psychic representative of the various bodily changes by means of which instincts manifest themselves. Psychiatric Assessment the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation

  • Acute and chronic liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy diseases: caused by hepatic parenchymal necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage leading to hepatic encephalopathy Encephalopathy Hyper-IgM Syndrome, attributed to hyperammonemia Hyperammonemia Elevated level of ammonia in the blood. It is a sign of defective catabolism of amino acids or ammonia to urea. Cirrhosis because of the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy’s inability to metabolize ammonia Ammonia A colorless alkaline gas. It is formed in the body during decomposition of organic materials during a large number of metabolically important reactions. Note that the aqueous form of ammonia is referred to as ammonium hydroxide. Acid-Base Balance by the urea cycle Cycle The type of signal that ends the inspiratory phase delivered by the ventilator Invasive Mechanical Ventilation. The liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy’s synthetic functions are defective, so there is often a concomitant coagulopathy. Jaundice Jaundice Jaundice is the abnormal yellowing of the skin and/or sclera caused by the accumulation of bilirubin. Hyperbilirubinemia is caused by either an increase in bilirubin production or a decrease in the hepatic uptake, conjugation, or excretion of bilirubin. Jaundice occurs because of the impaired conjugation Conjugation A parasexual process in bacteria; algae; fungi; and ciliate eukaryota for achieving exchange of chromosome material during fusion of two cells. In bacteria, this is a unidirectional transfer of genetic material; in protozoa it is a bi-directional exchange. In algae and fungi, it is a form of sexual reproduction, with the union of male and female gametes. Bacteriology of bilirubin Bilirubin A bile pigment that is a degradation product of heme. Heme Metabolism and excretion of the conjugated product Product A molecule created by the enzymatic reaction. Basics of Enzymes.
    • Acute liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy failure: usually caused by hepatotoxins or viral infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease. In the USA and UK, acetaminophen overdose Acetaminophen Overdose Acetaminophen (APAP) is an over-the-counter nonopioid analgesic and antipyretic medication. Acetaminophen is the most commonly used analgesic worldwide. Acetaminophen overdose is also one of the most common causes of medication poisoning and death. Acetaminophen Overdose is the most common cause, while viral hepatitis predominates in Asia ASIA Spinal Cord Injuries.
    • Advanced liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy failure in cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic parenchymal necrosis and scarring (fibrosis) most commonly due to hepatitis C infection and alcoholic liver disease. Patients may present with jaundice, ascites, and hepatosplenomegaly. Cirrhosis can also cause complications such as hepatic encephalopathy, portal hypertension, portal vein thrombosis, and hepatorenal syndrome. Cirrhosis: a condition caused by chronic damage and fibrosis Fibrosis Any pathological condition where fibrous connective tissue invades any organ, usually as a consequence of inflammation or other injury. Bronchiolitis Obliterans, with liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy insufficiency evident in the advanced stages. Common causes of cirrhosis Cirrhosis Cirrhosis is a late stage of hepatic parenchymal necrosis and scarring (fibrosis) most commonly due to hepatitis C infection and alcoholic liver disease. Patients may present with jaundice, ascites, and hepatosplenomegaly. Cirrhosis can also cause complications such as hepatic encephalopathy, portal hypertension, portal vein thrombosis, and hepatorenal syndrome. Cirrhosis include chronic viral hepatitis ( hepatitis B Hepatitis B Hepatitis B virus (HBV) is a partially double-stranded DNA virus, which belongs to the Orthohepadnavirus genus and the Hepadnaviridae family. Most individuals with acute HBV infection are asymptomatic or have mild, self-limiting symptoms. Chronic infection can be asymptomatic or create hepatic inflammation, leading to liver cirrhosis and hepatocellular carcinoma (HCC). Hepatitis B Virus, C), alcoholic Alcoholic Persons who have a history of physical or psychological dependence on ethanol. Mallory-Weiss Syndrome (Mallory-Weiss Tear) liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy disease, hemochromatosis Hemochromatosis A disorder of iron metabolism characterized by a triad of hemosiderosis; liver cirrhosis; and diabetes mellitus. It is caused by massive iron deposits in parenchymal cells that may develop after a prolonged increase of iron absorption. Hereditary Hemochromatosis, and nonalcoholic fatty liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy disease.
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