In humans and mammals, almost 80% of the nitrogen excreted is in the form of urea, which is produced through a series of reactions occurring in the cytosol and mitochondrial matrix of liver cells. These reactions are collectively called the urea cycle or the Krebs-Henseleit cycle.
Ammonia is a toxic product of nitrogen metabolism which should be removed from our body. The urea cycle or ornithine cycle converts excess ammonia into urea in the mitochondria of liver cells. The urea forms, then enters the blood stream, is filtered by the kidneys and is ultimately excreted in the urine.
The overall reaction for urea formation from ammonia is as follows:
2 Ammonia + CO2 + 3ATP ---> urea + water + 3 ADP
Steps in the Urea Cycle
The urea cycle is a series of five reactions catalyzed by several key enzymes. The first two steps in the cycle take place in the mitochondrial matrix and the rest of the steps take place in the cytosol. Thus the urea cycle spans two cellular compartments of the liver cell.
- In the first step of the Krebs-Henseleit cycle, ammonia produced in the mitochondria is converted to carbamoyl phosphate by an enzyme called carbamoyl phosphate synthetase I. The reaction can be given as follows:
NH3 + CO2 + 2ATP → carbamoyl phosphate + 2ADP + Pi
- The second step involves the transfer of a carbamoyl group from carbamoyl phosphate to ornithine to form citrulline. This step is catalyzed by the enzyme ornithine transcarbamoylase (OTC) . The reaction is given as follows:
Carbamoyl phosphate + ornithine → citrulline + Pi
Citrulline thus formed is released into the cytosol for use in the rest of the steps of the cycle.
- The third step is catalyzed by an enzyme called argininosuccinate synthetase, which uses citrulline and ATP to form a citrullyl-AMP intermediate, which reacts with an amino group from aspartate to produce argininosuccinate. This reaction can be given as follows:
Citrulline + ATP + aspartate → argininosuccinate + AMP + PPi
- The fourth step involves the cleavage of argininosuccinate to form fumarate and arginine. Argininosuccinate lyase is the enzyme catalyzing this reaction, which can be represented as follows:
Argininosuccinate → arginine + fumarate
- In the fifth and last step of the urea cycle, arginine is hydrolyzed to form urea and ornithine. This is catalyzed by arginase and can be given as follows:
Arginine → urea + ornithine
The overall reaction can be given as follows:
2NH3 + CO2 + 3ATP g urea + 2ADP + AMP + PPi + 2Pi
Significance of the Urea Cycle
The main purpose of the urea cycle is to eliminate toxic ammonia from the body. About 10 to 20 g of ammonia is removed from the body of a healthy adult every day. A dysfunctional urea cycle would mean excess amount of ammonia in the body, which can lead to hyperammonemia and related diseases. The deficiency of one or more of the key enzymes catalyzing various reactions in the urea cycle can cause disorders related to the cycle. Defects in the urea cycle can cause vomiting, coma and convulsions in new born babies. This is often misdiagnosed as septicemia and treated with antibiotics in vain. Even 1mm of excess ammonia can cause severe and irreversible damages.
Diagnosis of Urea Cycle Defects
A blood aminogram is routinely used in the diagnosis of urea cycle disorders. The concentration of the nitrogen-carrying amino acids, glutamine and alanine, in plasma is elevated in the case of OTC deficiency. In babies, elevated levels of orotic acid in the urine may be an indicator of OTC deficiency. Increased levels of blood citrulline and argininosuccinate are also seen in cases of citrullinemia.
In older children, these disorders may present in the form of growth failure, psychomotor retardation and behavioral abnormalities. Hence, blood ammonia and urinary orotic acid monitoring and quantitation are crucial in patients with unexplained neurological symptoms.