Table 7: Urea cycle defects and biochemical characteristics.

Defective enzyme
or component
Citrulline ArginineAmmoniaAdditional 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].