Answer- Case study- Urea cycle disorder

Case details

A 2-year-old girl was seen in the emergency room for vomiting and tremors. The elevated plasma ammonium ion concentration was 200 μM/L (normal 11-40 μM/L). Metabolic screens of blood and serum were ordered and were remarkable for an elevation in the amino acid arginine in serum. Which of the following enzymes would most likely cause an elevation in the plasma Arginine levels?

A. Carbamoyl phosphate synthetase I

B. Carbamoyl phosphate synthetase II

C. Ornithine transcarbamoylase

D. Arginase

E. Argino succinate lyase

Answer- The correct answer is D- Arginase

Arginine is an intermediate of urea cycle. Its high concentration indicates defect in the urea cycle at a level distal to its after formation and that can be due to Arginase deficiency. Arginase cleaves arginine to urea and ornithine (Figure)

Steps of urea formation

Figure- CPS- Carbamoyl phosphate synthetase-I; OTC-Ornithine transcarbamoylase; ASS- Argino succinic acid Synthetase ; ASL- Argino succinic acid lyase; ARG- Arginase. N-Acetyl glutamate is required as a positive allosteric modifier for CPS-I. The first two reactions are mitochondrial whereas the rest of the reactions are cytoplasmic.

Carbamoyl phosphate synthetase I is the first enzyme of urea cycle, required for the condensation of CO2 and ammonia. It is a mitochondrial enzyme.

Carbamoyl phosphate synthetase II is a cytoplasmic enzyme required for catalyzing the first step of pyrimidine biosynthesis.

Ornithine transcarbamoylase catalyzes the conversion of ornithine to citrulline (figure). Its deficiency is associated with high ornithine concentration.

Argino succinic acid lyase catalyzes the lytic cleavage of Argino succinic acid to form Arginine and Fumarate.

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