Urea cycle disorders are conditions in which the body cannot properly dispose of ammonia, a waste product of protein digestion. Six enzymes are involved in the urea cycle, a process that converts the toxic substance ammonia into urea, which is excreted in the urine.
The six enzymes in the urea cycle are ornithine transcarbamylase (OTC), argininosuccinic acid synthetase (ASD), arginase (ARG), arginosuccinase acid lyase (ALD), carbamyl phosphate synthetase (CPS), and N-acetylglutamate synthetase (NAGS). These enzymes are found in the liver, where they process nitrogen-containing waste products (such as ammonia) into urea. Deficiencies in any of these enzymes, also known as inborn errors of urea synthesis, can lead to urea cycle disorders.
Urea cycle disorders can also occur from defects in transporter proteins that move or transport a specific substance involved in the urea cycle in and out of various parts of a cell. The two known defects are citrin deficiency (citrullinemia II) and ornithine translocase deficiency.
If an enzyme or transporter in the urea cycle is deficient, ammonia may build up in the blood and reach toxic levels. An elevated ammonia level (hyperammonemia) disrupts normal brain and central nervous system function. Some of the physical symptoms may include lack of appetite, irritability, heavy or rapid breathing, low energy, vomiting, disorientation, and combativeness. If left untreated, high ammonia levels may lead to coma, swelling of the brain, brain damage, and death. In a urea cycle disorder, ammonia levels may be increased after a meal high in protein, viral illness, childbirth, and certain prescription medications.
It is estimated that as a group, urea cycle disorders occur at a rate of one in 10,000 births. OTC deficiency is the most common of the disorders and is estimated to occur in one in 30,000 births. NAGS deficiency is the rarest of the urea cycle disorders, with only a handful of cases reported worldwide each year.
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