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  • About UCD
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  • Low protein foods
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About UCD

Urea cycle disorders are inherited inborn errors of metabolism in which the body lacks or is deficient in the enzymes necessary to break down protein into urea and excrete it in the urine. Since our bodies lack the capacity to store protein (unlike fats and carbohydrates), the ongoing excretion of excess protein is critical for survival. Protein is broken down in the liver and tissues through the urea cycle into urea and carbon dioxide. These end products are excreted via the kidneys in our urine. When one of the necessary enzymes is missing in the urea cycle or deficient in some way, protein cannot be broken down and instead is degraded to ammonia (NH3+). High levels of ammonia in the blood (hyperammonemia) are very toxic to brain cells and may lead to cell damage or cell death. 

The following are the names of the enzymes that make up the urea cycle. When one is deficient or absent a urea cycle disorder will occur.

 

Symbol	Name	Prevalence/ Inheritance	Symptoms	Symptom Onset
N-Acetylglutamate Synthase Deficiency1	NAGS	Few Cases Worldwide Autosomal recessive	Lethargy Persistent vomiting Poor feeding              Hyperventilation        Enlarged liver Seizures	Symptoms occur shortly after birth in a total deficiency. A partial deficiency may occur later in life following a stressful event such as infection or a viral illness.
Carbamyl Phosphate Synthetase Deficiency 2	CPS	1:60,000 Autosomal recessive	Lethargy Coma Seizures Vomiting Poor Feeding Hyperventilation Hepatomegaly	Complete Deficiency: 24-72 hrs. after birth Partial Deficiency: Childhood
Ornithine Transcarbamylase Deficiency 3	OTC	1:30,000 X-linked	Lethargy Coma Seizures Vomiting Poor Feeding Hyperventilation Hepatomegaly	Hemizygote males: onset in 24-72 hrs. after birth Heterozygote females: 10% are symptomatic in childhood
Argininosuccinate Synthetase Deficiency 4 (Citrullinemia)	ASS	Rare Autosomal Recessive	Lethargy Coma Seizures Vomiting Poor Feeding Hepatomegaly	Complete Deficiency: 24-72 hrs. after birth Partial Deficiency: Childhood
Argininosuccinate Lyase Deficiency 5 (Argininosuccinc Acidura)	ASL	1:70,000 Autosomal Recessive	Lethargy Seizures Vomiting Poor Feeding Hyperventilation Hepatomegaly	Complete Deficiency: 24-72 hrs. after birth Partial Deficiency: Childhood
Arginase Deficiency	ARG	Rare Autosomal Recessive	Delayed development Protein intolerance Spasticity	Slower onset of symptoms than other urea cycle deficiencies. Often present with symptoms of loss of muscle control. Hyperammonemia is rare. Seizures, irritability, poor appetite and vomiting can occur.

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References: The following references may need to be accessed through a search engine.

1. Roth, Karl S.,MD “N-Acetylglutamate Synthetase Deficiency”, eMedicine, May 20, 2003 http://www.emedicine.com/ped/topic10.htm (May 4, 2005)

2. Roth, Karl S., MD “Carbamoyl Phosphate Deficiency”, eMedicine, March 4, 2004 http://www.emedicine.com/ped/topic314.htm (February 8, 2005)

3. Roth, Karl S., MD “Ornithine Transcarbamylase Deficiency” eMedicine, May 20, 2003 http://www.emedicine.com/ped/topic2744.htm (February 8, 2005)

4. Roth, Karl S., MD “Citrullinemia” (Argininosuccinate Synthetase Deficiency) eMedicine, July 10, 2003 http://www.emedicine.com/ped/topic406.htm ( February 8, 2005)

5. Roth, Karl S., MD “Argininosuccinate Lyase Deficiency” cMedicine, July 10, 2003 http://www.emedicine.com/ped/topic2745.htm ( February 8, 2005)

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