Most common enzyme decient in urea cycle:
Urea cycle defects result from  deficiency or total absence of the activity of any of the enzymes along the pathway, particularly carbamoyl phosphate synthase I (CPSI), ornithine transcarbamylase (OTC), argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL), arginase (ARG) or a deficiency of the co-factor N-acetylglutamate (NAG).  
Urea cycle disorders
Disorder
Inheritance
Enzyme defect Clinical Manifestations Hyperammonemia I Inborn defect CPS I
● Ammonia toxicity
● Ataxa
● Protein intolerance Hyperammonemia II X-linked deciency OTC
● Ammonia toxicity
● Increases levels of glutamine, ornithine and ammonia in the blood
Citrullinemia
Double recessive
Argininosuccinate synthetase
● Increased levels of citrutlhe in the blood
● Hyperammonemia
● Mental retardation
Argininosuccinic
aciduria
Double
recessive
Arginino succinate lyase
● High levels of arginino-succinate in blood, urine andCSF
● Mental retardation
● Friable tufted hair
Arginemia
Arginase
● Increased Arg levels in the blood, CSF and brain
● Increased levels of amino acids transported by the COAL transporter
Arginase
Ornithine transcarbamoylase
Carbamoyl phosphate synthase I
Argininosuccinate synthetase