Ornithine

L-Ornithin2.svg

Ornithine is an amino acid that plays a pivotal role in the urea cycle. It is not incorporated into proteins but is crucial for the disposal of excess nitrogen in the body. Ornithine is synthesized from the amino acid arginine and is a key intermediate in the conversion of ammonia to urea.

Biochemical Role

Ornithine is primarily involved in the urea cycle, also known as the ornithine cycle. This cycle is a series of biochemical reactions that produce urea ((NH2)2CO) from ammonia (NH3). The urea cycle occurs in the liver and is essential for the detoxification of ammonia, which is a byproduct of protein metabolism.

Urea Cycle

The urea cycle consists of the following steps: 1. **Formation of Carbamoyl Phosphate**: Ammonia combines with carbon dioxide to form carbamoyl phosphate. 2. **Formation of Citrulline**: Carbamoyl phosphate reacts with ornithine to form citrulline. 3. **Formation of Argininosuccinate**: Citrulline combines with aspartate to form argininosuccinate. 4. **Formation of Arginine**: Argininosuccinate is cleaved to form arginine and fumarate. 5. **Formation of Urea**: Arginine is hydrolyzed to form urea and regenerate ornithine.

Metabolism

Ornithine is produced from the amino acid arginine through the action of the enzyme arginase. It can also be converted back into arginine in a process that involves the intermediate citrulline. This interconversion is important for maintaining the balance of nitrogen in the body.

Clinical Significance

Ornithine levels can be indicative of certain metabolic disorders. For example, elevated levels of ornithine may be seen in ornithine transcarbamylase deficiency, a genetic disorder that affects the urea cycle. Symptoms of this disorder can include hyperammonemia, lethargy, and vomiting.

Dietary Sources

Ornithine is not an essential amino acid, meaning the body can synthesize it. However, it can also be obtained from dietary sources such as meat, fish, dairy products, and eggs.

See Also

• Amino acid
• Urea cycle
• Arginine
• Citrulline
• Ornithine transcarbamylase deficiency