A complete lack of any one of the enzymes of the urea cycle will result in death shortly after birth. However, deficiencies in each of the enzymes of the urea cycle have been identified. In addition, deficiencies in N-acetylglutamate synthetase, the enzyme necessary for the synthesis of N-acetylglutamate (NAG) which is required for activation of carbamoylphosphate synthetase I (CPSI), have been identifed. Collectively, these enzyme deficiencies lead to the urea cycle disorders or UCDs. The most dramatic presentation of UCD symptoms occurs in neonates between 24 and 48 hours after birth. Afflicted infants exhibit progressively deteriorating symptoms due to the elevated ammonium levels. Deficiencies in arginase do not lead to symptomatic hyperammonemia as severe or as commonly as in the other UCDs. Deficiencies in carbamoylphosphate synthetase I (CPS I), ornithine transcarbamoylase (OTC), argininosuccinate synthetase (AS) and argininosuccinate lyase (AL) comprise the common neonatal UCDs.
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Michael W. King, Ph.D / IU School of Medicine / miking at iupui.edu