What is the urea cycle and why does it matter?
The urea cycle (ornithine cycle) is the energy-dependent pathway by which the liver detoxifies ammonia into urea, which the kidneys excrete. It occurs only in the liver, starting in the mitochondrial matrix and finishing in the cytoplasm. Ammonia comes from protein catabolism, deamination, starvation and gut flora. In muscle and peripheral tissues ammonia is carried as glutamine (and alanine), which the liver then breaks down with glutaminase to release ammonia again.
What are the steps and enzymes of the urea cycle?
| Step | Reaction | Enzyme | Site |
|---|---|---|---|
| 1 | CO2 + NH3 + 2 ATP → carbamoyl phosphate | Carbamoyl phosphate synthetase 1 (CPS-1) — rate-limiting; needs N-acetylglutamate | Mitochondria |
| 2 | Carbamoyl phosphate + ornithine → citrulline | Ornithine transcarbamylase (OTC) | Mitochondria; citrulline exported by ornithine translocase |
| 3 | Citrulline + aspartate + ATP → argininosuccinate | Argininosuccinate synthetase | Cytoplasm |
| 4 | Argininosuccinate → arginine + fumarate | Argininosuccinate lyase | Cytoplasm |
| 5 | Arginine + H2O → urea + ornithine | Arginase | Cytoplasm |
- First two steps in mitochondria; the rest in the cytoplasm.
- N-acetylglutamate (NAG) is the obligate activator of CPS-1. It is made from glutamate and acetyl-CoA by N-acetylglutamate synthase (NAGS), which is upregulated by arginine.
- Fumarate produced at step 4 enters the TCA cycle (and links to tyrosine catabolism).
- Arginase regenerates ornithine, which re-enters step 2 — hence 'ornithine cycle'.
- Glutamate dehydrogenase releases more ammonia from glutamate, feeding the cycle.
Why is ammonia toxic to the brain?
Ammonia exists as ammonium at physiological pH and is extremely toxic to the central nervous system, especially in the developing brain. In astrocytes ammonia combines with glutamate through glutamine synthetase to form glutamine. Excess glutamine causes astrocyte swelling and cerebral oedema. Acute hyperammonaemia also activates NMDA receptors, leading to excitotoxic injury, mitochondrial dysfunction and disruption of the glutamate–glutamine cycle. Ammonia also pulls α-ketoglutarate toward glutamate and so impairs the TCA cycle. The picture is lethargy, vomiting, seizures, coma and, in adults, asterixis.
What are the urea cycle disorders and how do you tell them apart?
Urea cycle disorders (UCDs) are inborn errors of metabolism affecting the six enzymes or the two transporters of the cycle. All are autosomal recessive except OTC deficiency, which is X-linked. OTC deficiency causes about two-thirds of UCDs, with argininosuccinate synthetase deficiency about 20% and argininosuccinate lyase deficiency about 10%; the combined incidence is about 1 in 35,000 live births. NAGS deficiency is the rarest.
| Disorder | Defect | Inheritance | Key biochemical clue | Notes |
|---|---|---|---|---|
| NAGS deficiency | No N-acetylglutamate → CPS-1 inactive | AR | ↑ NH3, low citrulline, no orotic aciduria | Mimics CPS-1 deficiency; very rare. Carglumic acid |
| CPS-1 deficiency | Carbamoyl phosphate not formed | AR | ↑ NH3, low citrulline, orotic acid not raised | Often fatal in infancy; sepsis can trigger |
| OTC deficiency | Citrulline not formed | X-linked recessive | ↑ NH3, low citrulline, ↑ orotic acid (carbamoyl phosphate spills into pyrimidine synthesis) | Most common UCD; boys severe; girls variable |
| Citrullinaemia type 1 (ASS1 deficiency) | Argininosuccinate synthetase | AR | ↑ citrulline markedly | Second most common UCD |
| Argininosuccinic aciduria (ASL deficiency) | Argininosuccinate lyase | AR | ↑ argininosuccinate and citrulline | Liver dysfunction; cognitive effects |
| Argininaemia (arginase deficiency) | Arginase 1 | AR | ↑ arginine | Later childhood: developmental delay, limb spasticity; no rapid-onset hyperammonaemia |
| HHH syndrome (ORNT1 deficiency) | Mitochondrial ornithine transporter | AR | Hyperornithinaemia, hyperammonaemia, homocitrullinuria | Intermittent hyperammonaemia and coagulation abnormality |
| Citrin deficiency (citrullinaemia type 2) | Aspartate–glutamate carrier | AR | Citrullinaemia | Neonatal cholestasis; can present later |
How does a urea cycle disorder present and how is it diagnosed?
- Neonatal (severe, proximal defects: OTC, CPS-1, NAGS): a normal-looking newborn becomes catastrophically ill within 24 to 48 hours — poor feeding, vomiting, irritability, lethargy, tachypnoea, then seizures, coma and death. Evaluate for sepsis at the same time.
- Late-onset (infancy to adulthood): variable features involving the central nervous system, liver and other organs, from mild neurocognitive symptoms to encephalopathy. Triggers are anything that raises protein catabolism: fasting, infection, fever, a high-protein meal, surgery, pregnancy or the post-partum period, valproate, chemotherapy and high-dose steroids.
- Blood gas: typically significant respiratory alkalosis secondary to hyperventilation.
- Key test: plasma ammonia — send urgently; then plasma amino acids, urine organic acids, urine orotic acid, lactate and acylcarnitines. BUN is typically low.
- Confirmation: enzyme assays and molecular genetic testing. Liver biopsy is not recommended to diagnose OTC deficiency in females (X-inactivation can give false negatives).
| Condition | Clue |
|---|---|
| Neonatal sepsis | Evaluate for it in every sick newborn at the same time |
| Other inborn errors (for example fatty acid oxidation defects, organic acidaemias) | Urine organic acids and acylcarnitines separate them |
| Transient hyperammonaemia of the newborn | Listed in the differential; resolves without a lasting defect |
| Liver failure; congenital infection; toxins; drugs | Check liver function and history |
| Reye syndrome | Child recovering from viral infection, aspirin exposure |
How is acute hyperammonaemia treated?
Treatment is started before the final diagnosis is known. Goals: lower ammonia quickly, stop protein intake and reverse catabolism.
- Stop all protein (including parenteral nitrogen) for 24 to 48 hours; later add branched-chain amino acids and re-introduce protein cautiously.
- Calories to reverse catabolism: IV fluids with 10% dextrose or higher, and IV lipids.
- Nitrogen scavengers: IV sodium phenylacetate and sodium benzoate — they lower ammonia by giving nitrogen an alternative route of excretion.
- Arginine hydrochloride: replenishes cycle intermediates; used in most UCDs, except arginase deficiency. Citrulline is used for proximal defects (CPS-1, OTC).
- Haemodialysis for severe cases with very high ammonia; peritoneal dialysis is inefficient and used only if haemodialysis is not possible.
- Carglumic acid (N-carbamylglutamate) for NAGS deficiency (and considered in CPS-1 deficiency).
- Monitor ammonia every 2 to 4 hours; treat cerebral oedema; avoid valproate for seizures (use levetiracetam or benzodiazepines).
| Measure | Detail |
|---|---|
| Diet | Lifelong low-protein diet supervised by a metabolic dietitian |
| Drugs | Sodium phenylbutyrate or the better-tolerated glycerol phenylbutyrate (nitrogen excretion); arginine or citrulline supplements |
| Definitive | Liver transplantation corrects the defect; 5- and 10-year survival above 90% |
| Future | Gene therapy (AAV addition, gene editing) under investigation for OTC deficiency |
| Prevention | Prenatal genetic counselling in families with an affected child or unexplained neonatal death |
What causes hyperammonaemia in adults and how is hepatic encephalopathy treated?
In adults, about 90% of hyperammonaemia is due to advanced liver disease — cirrhosis or acute liver failure — through impaired hepatic detoxification and portosystemic shunting. Other causes: urea cycle disorders presenting late, valproate, some chemotherapeutic drugs, infection with urease-producing bacteria, and catabolic stress such as gastrointestinal bleeding, starvation, surgery or trauma.
| Grade | Features |
|---|---|
| 1 | Sleep–wake reversal, reduced attention, mild disorientation, mood change |
| 2 | Lethargy, apathy, disorientation, slurred speech, personality change, asterixis |
| 3 | Somnolent but arousable; marked confusion, gross disorientation |
| 4 | Coma, minimal or no response to pain |
- First line: lactulose (or lactitol) titrated to about 2 to 3 soft stools per day.
- Add-on: rifaximin reduces recurrence and persistence.
- Adjuncts if inadequate: neomycin or metronidazole (limited by toxicity), probiotics, L-ornithine L-aspartate (LOLA).
- Control the precipitant: treat GI bleeding, infection, dehydration, constipation, renal failure and drug toxicity.
- Acute liver failure: non-absorbable disaccharides and antibiotics have not shown outcome benefit; prioritise ammonia clearance and definitive therapy, with ICU care for cerebral oedema.
What do exams ask about the urea cycle?
- Rate-limiting enzyme — CPS-1 (mitochondrial), activated by N-acetylglutamate; not CPS-2 (pyrimidine synthesis, cytosolic).
- Site — liver only; first two steps in mitochondria.
- Most common UCD / only X-linked UCD — OTC deficiency.
- Raised orotic acid with hyperammonaemia — OTC deficiency; without ammonia — hereditary orotic aciduria.
- Raised citrulline — citrullinaemia type 1 (argininosuccinate synthetase deficiency).
- Raised argininosuccinate — argininosuccinate lyase deficiency.
- Spastic paraplegia, no early hyperammonaemia — arginase deficiency.
- Carglumic acid — NAGS deficiency.