The human body breaks down proteins to amino acids. Some amino acids are broken down, and one of the products is ammonia. Even a small amount of ammonia is poisonous to the nerves, so ammonia is changed into urea in the liver. The urea cycle is all of the steps the liver takes to change ammonia to urea. The urea is then sent to the kidneys and excreted in urine.
N-acetylglutamate Synthetase Deficiency
This is a hereditary disease where infants do not have enough of an enzyme called N-acetylglutamate synthetase. Enzymes are proteins that speed up the time that a process takes. The liver uses this enzyme in the first step of the urea cycle to make a substance called N-acetylglutamate. In the severe form, infants may not have any symptoms in the first few days, according to Allan Ropper, M.D., Professor of Neurology at Harvard Medical School in "Adams & Victor's Principles of Neurology." Then they will refuse to eat, have seizures, develop rigid muscles, a low body temperature, become less alert and go into a coma.
Carbamoyl Phosphate Synthetase Deficiency
Carbamoyl phosphate synthetase deficiency is also a hereditary disorder. Approximately one in 62,000 infants is born with this disease, writes Victor Rodwell, Ph.D., Emeritus Professor of Biochemistry at Purdue University in "Harper's Illustrated Biochemistry." Carbamoyl phosphate synthetase is an enzyme that the liver uses in the early part of the urea cycle. This enzyme works along with N-acetylglutamate to make a substance in the urea cycle called carbamoyl phosphate. Infants with this deficiency have the same symptoms as those with N-acetylglutamate synthetase deficiency.
Ornithine Transcarbamylase Deficiency
Approximately one child in every 80,000 is born with this disorder of the urea cycle, per the National Institutes of Health. It is a hereditary disease where the child does not have enough of the enzyme ornithine transcarbamylase. Similar to N-acetylglutamate synthetase deficiency and carbamoyl phosphate synthetase deficiency, not having enough of this enzyme stops the liver from making urea. The ammonia levels in the bloodstream increase, and the high levels affect the nervous systems and brains of infants. Thus, infants have the same symptoms as N-acetylglutamate synthetase deficiency.
Argininosuccinate Lyase Deficiency
Children in this hereditary disorder do not have enough of the enzyme argininosuccinate lyase. This prevents the urea cycle from changing argininosuccinate to an amino acid called arginine. Children usually have seizures, mental retardation, periods where they are uncoordinated, a large liver and brittle hair, writes Peter Itin, M.D., Head of the Department of Dermatology at the University Hospital Basel in "Fitzpatrick's Dermatology in General Medicine."
Argininosuccinate Synthetase Deficiency
This hereditary disease is also called citrullinemia. Without enough of the enzyme argininosuccinate synthetase, the amino acid citrulline cannot be changed to argininosuccinate. This leads to citrullinemia, or high levels of citrulline in the bloodstream. In "Adams & Victor's Principles of Neurology," Dr. Ropper explains that children develop symptoms months after birth, after their protein meals increase. The increased protein leads to high ammonia levels in the blood.
Arginase Deficiency
According to the National Institutes of Health, children with this disorder do not have any symptoms until they are approximately three years old. Then their legs get stiff and their muscles tense up. They develop seizures, become uncoordinated and mentally retarded.
References
- "Adams & Victor's Principles of Neurology"; Allan Ropper, M.D., Martin Samuels, M.D.; 2009
- "Fitzpatrick's Dermatology in General Medicine"; Klauss Wolff, M.D., Lowell Goldsmith, M.D. et al.; 2008
- "Harper's Illustrated Biochemistry"; Robert Murray, M.D., Ph.D., David Bender, Ph.D., Kathleen Botham, Ph.D. et al.; 2009
- National Institutes of Health: Arginase deficiency
- National Institutes of Health: Ornithine transcarbamylase deficiency
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