Urea Cycle, Nitrogen Balance, and Urea Cycle Disorders

Mastering the Urea Cycle | An In-Depth Look at Its Importance, Mechanisms, and Disorders for Medical Professionals and Students 

    Welcome healthcare professionals, students, and self-learners! Today, we're going to dive into the complex world of the urea cycle. Whether you're studying for an upcoming exam or simply looking to expand your knowledge, this article will cover everything you need to know about the urea cycle.

    The urea cycle is a vital process that occurs in the liver, and it plays a crucial role in the removal of ammonia from the body. In this article, we'll discuss the definition and overview of the urea cycle, its importance, and the role of the liver in the process. We'll also go over the steps involved in the urea cycle, as well as the mechanisms that regulate it.

    Understanding the nitrogen balance mechanism is also essential for healthcare professionals, students, and self-learners as it plays a crucial role in maintaining the body's overall health. Lastly, we'll touch on urea cycle disorders (UCD), which can impact the body's ability to eliminate ammonia and can lead to severe health complications.

    Whether you're studying for medical professional exams such as MBBS-I, MBDS-I, BS Nursing, Paramedics, Pharmacy Exams, BS Biochemistry Exams, NEET, BS Nutrition, or any other entrance test, this article will provide you with the necessary knowledge to succeed. So, let's get started and explore the fascinating world of the urea cycle!

Biochemistry Clinical Note on the Urea Cycle
From the Biochemistry Library of H.E.S (Health, Education, and Skills) 

Definition and Overview of Urea Cycle | Ornithine Cycle

The Urea cycle converts toxic ammonia, which is produced during the breakdown of proteins and nucleic acids, into urea, a less toxic compound that can be excreted by the kidneys in the urine.

• The Urea cycle is a synthetic process, which converts ammonia into Urea.
• Ammonia is a toxic and water-insoluble substance produced by amino acid catabolism.
• Urea is less toxic, water-soluble, and a major disposal form of ammonia.
• Urea is readily excreted by the kidneys and it accounts for about 90% of nitrogen-containing components of urine.
• The urea cycle occurs in the liver.
• Enzymes for the first two reactions of the urea cycle are found in mitochondria, therefore the first reaction occurs in mitochondria and the enzymes for the remaining reactions are found in the cytosol, hence the remaining reactions occur in the cytosol.

Substrate for Urea Cycle | Ammonia

• The first Nitrogen of the Urea molecule is provided by free ammonia (NH3).
• The second Nitrogen is provided by aspartate.
• Glutamate is the intermediate precursor of both i.e. the NH3 (through oxidative deamination by glutamate dehydrogenase) and aspartate (through transamination of oxaloacetate by aspartate transaminase).
• The carbon and oxygen of urea molecule are derived from CO2.
• The energy required to start the cycle is provided by two molecules of ATP.  

Importance of the Urea Cycle 

i. Removal of toxic ammonia: Ammonia is highly toxic to the body, especially to the brain, and can cause serious health problems if it is not removed efficiently. The urea cycle converts ammonia into urea, a less toxic compound that can be excreted by the kidneys. You need to understand the urea cycle in order to diagnose and treat conditions where ammonia levels are elevated, such as urea cycle disorders and liver disease.

ii. Diagnosis and management of urea cycle disorders: Urea cycle disorders (UCDs) are a group of rare genetic disorders that affect the urea cycle, resulting in the accumulation of toxic ammonia in the body. It is important to be familiar with the symptoms of UCDs in order to make an accurate diagnosis and to understand the treatment options available, such as dietary modifications, medications, and liver transplantation.

iii. Role in metabolic disorders: The urea cycle is closely connected to other metabolic pathways in the body, such as the citric acid cycle, the glycogen cycle, and the fatty acid cycle. You also need to understand these connections in order to diagnose and manage metabolic disorders, such as diabetes, obesity, and hyperlipidemia.

iv. Use in diagnostic tests: The urea cycle is used in several diagnostic tests to assess liver and kidney function. For example, the blood urea nitrogen (BUN) test measures the amount of urea nitrogen in the blood, which can be used to evaluate kidney function. Similarly, the ammonia test measures the amount of ammonia in the blood, which can be used to diagnose urea cycle disorders and liver disease.

Role of the Liver in the Urea Cycle

• The urea cycle takes place primarily in the liver, although some components of the cycle also occur in other tissues such as the kidneys and intestines.
• The liver is responsible for synthesizing the enzymes (Carbamoyl phosphate synthetase I (CPS1), Ornithine transcarbamylase (OTC), Argininosuccinate synthetase (ASS), Argininosuccinate lyase (ASL), Arginase) that are involved in the urea cycle, as well as producing the amino acids that are required for the cycle to function. 
• The urea cycle begins with the breakdown of proteins into their constituent amino acids. These amino acids are transported to the liver, where they are deaminated (a process that removes the nitrogen-containing amino group and produces ammonia). 
• The liver then uses a series of enzymes to convert the ammonia into urea, which is then transported to the kidneys for excretion in the urine.
• The liver is also responsible for regulating the rate of urea production (the primary end product of the urea cycle). Which is influenced by factors such as dietary protein intake, hormonal signals, and the body's metabolic needs. If there is excess ammonia in the body, the liver can increase the rate of urea production to help remove the excess ammonia and prevent toxicity. Similarly, if there is a shortage of amino acids or other components required for the urea cycle, the liver can decrease the rate of urea production to conserve these resources.
• Dysfunction of the liver can lead to disruptions in the urea cycle, leading to conditions such as urea cycle disorders or hepatic encephalopathy, a condition characterized by cognitive and neurological symptoms due to excess ammonia in the blood.

Steps of the Urea Cycle

Step 1: Formation of Carbamoyl Phosphate

• The conversion of ammonia and bicarbonate into carbamoyl phosphate, which is catalyzed by the enzyme carbamoyl phosphate synthetase I (CPSI). 
• This reaction requires the input of energy in the form of ATP and occurs in the mitochondria of liver cells.

        NH3 + HCO3- + 2ATP → carbamoyl phosphate + 2ADP + Pi

Step 2: Formation of Citrulline

• This step involves the transfer of the carbamoyl group from carbamoyl phosphate to ornithine.
• Ornithine is regenerated with each turn of the urea cycle much in the same way, that oxaloacetate is regenerated by the citric acid cycle.  
• This reaction is catalyzed by the enzyme ornithine transcarbamylase (OTC), and it results in the formation of citrulline.
• Ornithine and Citrulline are the basic amino acid that participates in the urea cycle.

        carbamoyl phosphate + ornithine → citrulline + Pi

Step 3: Formation of Argininosuccinate

• Citrulline is transported out of the mitochondria and into the cytosol of the liver cell. 
• Here, citrulline reacts with aspartate, another amino acid, to form argininosuccinate. 
• This reaction is catalyzed by the enzyme argininosuccinate synthetase.

        citrulline + aspartate + ATP → argininosuccinate + AMP + PPi

Step 4: Formation of Arginine

• The fourth step involves the cleavage of argininosuccinate into arginine and fumarate. 
• This reaction is catalyzed by the enzyme argininosuccinate lyase.
• Arginine in the Urea Cycle acts as an immediate precursor of the Urea.
• Fumarate formed, enters the TCA Cycle

        argininosuccinate → arginine + fumarate

Step 5: Formation of Urea

• In the final step of the urea cycle, arginine is hydrolyzed by the enzyme arginase to form urea and ornithine. 
• Urea is then excreted from the body in the urine, while ornithine is transported back into the mitochondria to begin the cycle again.
• The enzyme arginase is found only in the liver therefore urea is only formed in the liver whereas other tissues can synthesize only arginine, not urea.

        arginine + H2O → urea + ornithine

Fate of Urea

• Urea diffuses out from the liver into the blood and is then transported to the kidneys, from where it is excreted out as urine.
• A portion of urea diffused out from the blood into the intestine and is cleaved to CO2 and NH3 by bacterial urease.
• This NH3 is partly lost in feces and partly reabsorbed back into the blood.

Regulation of the Urea Cycle

    The regulation of the urea cycle occurs at multiple levels, including gene expression, enzyme activity, and substrate availability. Some of the key regulatory mechanisms are:

i. Gene expression 

• The genes encoding the enzymes of the urea cycle are regulated at the transcriptional level. 
• The expression of these genes is primarily controlled by two transcription factors, CCAAT/enhancer-binding protein alpha (C/EBPα) and hepatocyte nuclear factor 4 alpha (HNF4α). 
• These transcription factors activate the expression of genes encoding the urea cycle enzymes in response to increased protein intake or increased ammonia levels.

ii. Enzyme activity 

• The activity of the enzymes in the urea cycle is regulated by allosteric modulation, post-translational modification, and protein-protein interactions. For example, the first enzyme in the cycle, carbamoyl phosphate synthetase I (CPSI), is activated by N-acetyl glutamate (NAG), which is synthesized from glutamate and acetyl-CoA. The production of NAG is stimulated by the presence of excess ammonia.

iii. Substrate availability

• The availability of substrates for the urea cycle, such as ornithine, citrulline, and arginine, can also regulate the activity of the cycle. These substrates are obtained from the diet or synthesized from other amino acids, and their levels can be influenced by dietary intake and other metabolic processes.

Nitrogen Balance 

• The regulation of the urea cycle is critical for maintaining proper nitrogen balance in the body, as well as preventing the accumulation of toxic ammonia.
• The primary source of nitrogen for the body is dietary proteins.
• As amino acids are oxidized, nitrogen is converted to urea and excreted out in the urine.
• Other nitrogen-containing compounds, formed from amino acids, are also excreted in urine e.g. uric acid, creatinine, and ammonium (NH4).
• Nitrogen balance (the normal state of nitrogen in adults) occurs when the synthesis of the body proteins equals degradation. 

Nitrogen Balance Equilibrium 

    To determine the nitrogen balance, the total amount of nitrogen ingested and eliminated from the body is measured over a certain period of time. Under conditions of equilibrium, as in normal adults, these values are equal i.e. the amount of nitrogen ingested each day equals the amount of nitrogen excreted in urine each day.

• Positive Nitrogen Balance: It can be defined as "Nitrogen ingestion is more than elimination" e.g. during growth, convalescence from wasting illnesses, and pregnancy.
• Negative Nitrogen Balance: It can be defined as "Nitrogen elimination is more than ingestion" e.g in starvation, wasting diseases, fever, after burns, surgical operations, and when food is deficient in proteins, and essential amino acids.

Urea Cycle Disorders | Clinical Note

    Urea cycle disorders (UCDs) are a group of rare genetic disorders that affect the urea cycle, the process that converts toxic ammonia into urea for excretion by the kidneys. UCDs are caused by mutations in the genes that encode enzymes involved in the urea cycle, leading to a buildup of ammonia in the blood and other tissues.

    There are several different types of UCDs, depending on which enzyme is affected. The most common types of UCDs include:

i. Ornithine transcarbamylase (OTC) deficiency

• OTC deficiency is the most common type of UCD, accounting for approximately 50% of cases. 
• It is caused by mutations in the OTC gene, which encodes for the enzyme that catalyzes the conversion of ornithine and carbamoyl phosphate to citrulline. 
• OTC deficiency can result in a buildup of ammonia in the blood, leading to symptoms such as vomiting, lethargy, and seizures.

ii. Citrullinemia 

• Citrullinemia is caused by mutations in the genes that encode for the enzymes involved in the conversion of citrulline to arginine. 
• Citrullinemia can result in a buildup of ammonia and citrulline in the blood, leading to symptoms such as hyperammonemia, lethargy, and coma.

iii. Argininosuccinic aciduria (ASA)

• ASA is caused by mutations in the gene that encodes for the enzyme argininosuccinate synthase (ASS), which converts citrulline and aspartate into argininosuccinate. 
• ASA can result in a buildup of ammonia in the blood, leading to symptoms such as vomiting, seizures, and developmental delay.

iv. Arginase deficiency

• Arginase deficiency is caused by mutations in the gene that encodes for the enzyme arginase, which catalyzes the conversion of arginine to urea and ornithine. 
• Arginase deficiency can result in a buildup of arginine in the blood, leading to symptoms such as intellectual disability, spasticity, and seizures.

Symptoms of UCDs can range from mild to severe and may include poor feeding, vomiting, lethargy, seizures, intellectual disability, and coma. UCDs are diagnosed through blood tests that measure ammonia levels and levels of other metabolites in the blood. Genetic testing can also be used to identify mutations in the genes associated with UCDs.

Treatment for UCDs typically involves a combination of dietary modifications, medications, and sometimes liver transplantation. Dietary modifications may include restricting protein intake and supplementing with specific amino acids to provide the body with the nutrients it needs without producing excess ammonia. Medications may be used to help remove excess ammonia from the body or to replace the missing enzymes in the urea cycle. In severe cases, liver transplantation may be necessary to replace the defective enzymes and restore normal urea cycle function.