ornithine transcarbamylase

OTC catalyzes the second step of the urea cycle in the mitochondrial matrix of hepatocytes: carbamoyl phosphate + ornithine → citrulline + phosphate. This step is essential for converting toxic ammonia into urea for excretion. OTC deficiency is inherited in an X-linked recessive pattern (unique among urea cycle disorders, which are otherwise autosomal recessive), so it predominantly affects males, though carrier females can show variable symptoms due to X-inactivation (lyonization).

Pathophysiology: Loss of OTC function causes accumulation of carbamoyl phosphate, which is shunted into the cytosol and enters the pyrimidine synthesis pathway, leading to excess orotic acid production. This results in the classic biochemical triad: hyperammonemia, elevated urine orotic acid, and LOW blood urea nitrogen (BUN) with normal/low citrulline levels. This distinguishes OTC deficiency from other urea cycle disorders like carbamoyl phosphate synthetase I (CPS1) deficiency, which also causes hyperammonemia but with LOW orotic acid (since CPS1 acts upstream of OTC).

Clinical presentation: Neonatal-onset severe deficiency presents within days of birth with poor feeding, vomiting, lethargy, tachypnea (from respiratory alkalosis due to ammonia's stimulation of the respiratory center), seizures, and progression to coma if untreated. Milder or late-onset cases may present in childhood/adulthood with episodic hyperammonemia triggered by high protein intake, illness, or metabolic stress, presenting with confusion, ataxia, and vomiting; can be mistaken for psychiatric illness or Reye syndrome.

Diagnosis: Elevated plasma ammonia, elevated urine orotic acid, low BUN, low-normal citrulline, and low arginine. Definitive diagnosis via genetic testing or liver biopsy enzyme assay.

Management: Acute hyperammonemia is treated with dialysis, IV fluids, and nitrogen-scavenging agents (sodium phenylbutyrate, sodium benzoate) that provide alternative pathways for nitrogen excretion. Long-term management includes protein restriction, arginine/citrulline supplementation, and avoidance of catabolic states. Liver transplantation can be curative in severe cases.

High-yield board associations: OTC deficiency is a classic USMLE topic for urea cycle disorders — remember 'OTC deficiency = ↑ Orotic acid, ↓ BUN, X-linked recessive' and to distinguish it from orotic aciduria due to UMP synthase deficiency (autosomal recessive, presents with megaloblastic anemia unresponsive to B12/folate, no hyperammonemia, and orotic acid crystals in urine).