Table 7.

Urea cycle defects and biochemical characteristics.

Defective enzyme or component	Citrulline	Arginine	Ammonia	Additional biochemical characteristics
Ornithine transcarbamylase	↓	↓	↑	↑ Glutamine Normal orotic acid
Carbamoylphosphate synthetase I (CPS1)	↓	↓	↑	↑ Glutamine ↑ Orotic acid
N-acetyl glutamate synthase (NAGS)				Reduced CPS1 activity (NAGS is a vital cofactor)
Argininosuccinate synthase (ASS)	↑++ (10–100x normal)	↓	↑
Argininosuccinate lyase (ASL)	↑	↓	↑	↑ Argininosuccinic acid (unique to ASL deficiency)
Arginase (ARG1)			Normal in absence of metabolic stress
Ornithine transporter mitochondrial I (ornithine translocase deficiency)			↑	↑ Homocitrulline ↑ Ornithine
Citrin (solute carrier family 5) deficiency	↑

Adapted from Pearl [2].