Urea Cycle – Function, Disorders & Treatment

What Is the Urea Cycle?

The urea cycle (also known as the ornithine cycle) is a vital biochemical pathway that takes place primarily in the liver. Its main function is to detoxify the body by converting toxic ammonia (NH₃), a byproduct of protein and amino acid metabolism, into the largely harmless and water-soluble compound urea. Urea is subsequently transported through the bloodstream to the kidneys and excreted in the urine.

The cycle was first described in 1932 by biochemists Hans Adolf Krebs and Kurt Henseleit, making it one of the earliest metabolic cycles discovered in the human body.

Mechanism and Steps

The urea cycle consists of five enzymatic reaction steps that take place partly in the mitochondria and partly in the cytoplasm of liver cells (hepatocytes).

Step 1: Formation of Carbamoyl Phosphate

In the mitochondria of liver cells, ammonia is combined with carbon dioxide (CO₂) and adenosine triphosphate (ATP) by the enzyme carbamoyl phosphate synthetase I to form carbamoyl phosphate. This is the first and rate-limiting step of the cycle.

Step 2: Formation of Citrulline

Carbamoyl phosphate reacts with the amino acid ornithine, catalyzed by ornithine transcarbamylase, to form citrulline. Citrulline is then transported out of the mitochondria into the cytoplasm.

Step 3: Formation of Argininosuccinate

In the cytoplasm, citrulline combines with aspartate (another amino acid) via the enzyme argininosuccinate synthetase to form argininosuccinate. This step introduces the second nitrogen group that will ultimately be incorporated into urea.

Step 4: Formation of Arginine

Argininosuccinate is cleaved by argininosuccinate lyase into arginine and fumarate. Fumarate enters the citric acid cycle, linking the urea cycle to general energy metabolism.

Step 5: Cleavage of Arginine into Urea and Ornithine

The enzyme arginase cleaves arginine into urea and ornithine. The regenerated ornithine is transported back into the mitochondria to begin a new cycle, while the released urea enters the bloodstream and is excreted by the kidneys.

Clinical Relevance

The urea cycle is essential for detoxification. Disruptions in this metabolic pathway can have life-threatening consequences.

Urea Cycle Disorders

Urea cycle disorders (UCDs) are rare, mostly autosomal recessive inherited metabolic conditions in which one of the enzymes involved in the cycle is absent or functionally impaired. This leads to an accumulation of ammonia in the blood (hyperammonemia), which can cause severe brain damage.

• OTC deficiency (ornithine transcarbamylase deficiency): The most common urea cycle disorder, inherited in an X-linked manner.
• CPS-I deficiency: Deficiency of carbamoyl phosphate synthetase I.
• ASS deficiency (citrullinemia type I): Deficiency of argininosuccinate synthetase.
• ASL deficiency (argininosuccinic aciduria): Deficiency of argininosuccinate lyase.
• Arginase deficiency (argininemia): Deficiency of arginase.

Symptoms of Urea Cycle Disorders

The symptoms of hyperammonemia can vary in severity and include:

• Nausea and vomiting
• Confusion and altered consciousness
• Seizures
• Lethargy progressing to coma
• Developmental delays in children

Liver Dysfunction

Since the urea cycle occurs almost exclusively in the liver, severe liver diseases (e.g., liver cirrhosis, liver failure) can significantly impair the capacity of the cycle and likewise lead to dangerous hyperammonemia.

Diagnosis

Diagnosis of urea cycle disorders involves:

• Measurement of ammonia levels in the blood (elevated in disorders)
• Analysis of amino acid profiles in plasma and urine (e.g., elevated citrulline in citrullinemia)
• Molecular genetic testing to identify the causative gene mutation
• Newborn screening: Many countries include urea cycle disorders in extended newborn screening programs

Treatment

Treatment of urea cycle disorders aims to reduce ammonia levels in the blood and prevent further damage:

• Protein-restricted diet: Reducing protein intake to limit ammonia production.
• Nitrogen scavengers: Substances such as sodium benzoate or sodium phenylbutyrate enable nitrogen excretion via alternative metabolic pathways.
• Arginine supplementation: In certain defects, arginine becomes an essential amino acid and requires supplementation.
• Liver transplantation: In severe cases, a liver transplant can functionally correct the defect, as the donor liver carries intact enzyme activity.
• Dialysis: Used in acute, life-threatening hyperammonemia for rapid ammonia removal.

References

1. Krebs, H. A. & Henseleit, K. (1932). Untersuchungen über die Harnstoffbildung im Tierkörper. Hoppe-Seyler's Zeitschrift für physiologische Chemie, 210(1-2), 33-66.
2. Brusilow, S. W. & Horwich, A. L. (2001). Urea Cycle Enzymes. In: Scriver, C. R. et al. (Eds.), The Metabolic and Molecular Bases of Inherited Disease, 8th ed., McGraw-Hill.
3. Häberle, J. et al. (2012). Suggested guidelines for the diagnosis and management of urea cycle disorders. Orphanet Journal of Rare Diseases, 7:32. DOI: 10.1186/1750-1172-7-32.