STEPTRACKERBIOCHEMISTRY + PEDIATRICS + PHARMACOLOGY | AMINO ACID METABOLISM | HIGH-YIELD REVIEW

Urea Cycle & Hyperammonemia

How the liver turns toxic ammonia into urea — and why a newborn with vomiting, lethargy and no acidosis may have a blocked cycle.

FOCUS ON

Order: "Ordinarily, Careless Crappers Are Also Frivolous About Urination" — ornithine, carbamoyl phosphate, citrulline, aspartate, argininosuccinate, fumarate, arginine, urea. CPS I (mitochondria) is rate-limiting, activated by N-acetylglutamate. OTC deficiency (most common, XR): ↑ ammonia + ↑ orotic acid, ↓ BUN. Hyperammonemia → ↓ α-KG → TCA stalls; ↑ glutamine in astrocytes → cerebral edema. Treat: low protein, lactulose, rifaximin, benzoate/phenylbutyrate, dialysis.

STEP · DISORDER

WHAT HAPPENS

FINDINGS

CLUE · TREATMENT

Transamination

Amino acids pass their amino group to α-KG → glutamate (ALT, AST; B6)

Glutamate collects nitrogen

Glutamate DH releases NH₄⁺ in liver

Ammonia transport

Muscle → alanine (glucose-alanine cycle); other tissues → glutamine

Both carried to liver

Kidney glutaminase excretes NH₄⁺ (acid buffer)

CPS I

rate-limiting · mitochondria

NH₄⁺ + CO₂ + 2 ATP → carbamoyl phosphate

Needs N-acetylglutamate (made from glutamate + acetyl-CoA, stimulated by arginine)

NAGS deficiency looks like CPS I deficiency; treat with carglumic acid

OTC

mitochondria

Carbamoyl phosphate + ornithine → citrulline

Deficiency: carbamoyl phosphate → cytosol → orotic acid

XR; male neonates or symptomatic females after protein load

Argininosuccinate synthetase / lyase

cytosol

Citrulline + aspartate → argininosuccinate → arginine + fumarate (links to TCA)

Deficiency: citrullinemia / argininosuccinic aciduria (brittle hair, trichorrhexis nodosa)

Supplement arginine

Arginase

cytosol

Arginine → urea + ornithine

Deficiency: ↑ arginine → spastic diplegia, growth delay, little hyperammonemia

Low-arginine diet

Hyperammonemia — toxicity

Ammonia drives α-KG → glutamate → glutamine; ↓ TCA; glutamine swells astrocytes

Vomiting, lethargy, tremor (asterixis), slurred speech, cerebral edema, blurred vision, coma

Respiratory alkalosis (hyperventilation) — no anion gap acidosis

Acquired hyperammonemia

Cirrhosis, portosystemic shunts, valproate, GI bleed (protein load), urease-positive UTI with obstruction

Hepatic encephalopathy

Lactulose acidifies colon (NH₃ → NH₄⁺ trapped); rifaximin ↓ ammonia-making bacteria

Nitrogen scavengers

Benzoate + glycine → hippurate; phenylbutyrate → phenylacetate + glutamine → excreted

Remove nitrogen without urea cycle

Hemodialysis for crises

Picture it — the urea cycle

Urea cycleCarbamoyl phosphate (CPS I, needs N-acetylglutamate) and OTC work in mitochondria; the rest runs in the cytosol. Aspartate brings the second nitrogen; fumarate links to the TCA cycle; arginase releases urea and regenerates ornithine.

Schematic diagrams, not to scale.

Mechanism — the urea cycle

NH₄⁺ + CO₂

→

CPS I(NAG) → carbamoyl-P

→

OTC+ ornithine → citrulline

→

+ Aspartate→ argininosuccinate

→

→ Arginine + fumarate

→

Arginase→ urea + ornithine

First two steps in mitochondria, the rest in cytosol. One nitrogen from NH₄⁺, the other from aspartate.

OTC vs CPS I vs orotic aciduria

DISORDER

AMMONIA

OROTIC ACID

ANEMIA

OTC deficiency

↑

CPS I deficiency

↑

Normal/↓

Orotic aciduria

Normal

↑

Megaloblastic

How it's tested

COMMON CONFUSIONHigh orotic acid + hyperammonemia = OTC; high orotic acid + normal ammonia + megaloblastic anemia = UMP synthase (orotic aciduria).

THE TRAPA urea cycle defect gives respiratory alkalosis, not metabolic acidosis — acidosis with hyperammonemia suggests an organic acidemia (9.04).

YOUR APPROACH↑ NH₃ + low BUN → urea cycle → orotic acid level → enzyme; then check citrulline/arginine for distal defects.

If you see… think…

Newborn boy, lethargy after feeds, ↑ ammonia, ↑ orotic acid → OTC deficiency

Hyperammonemia, normal orotic acid, low citrulline → CPS I or NAGS deficiency

Required activator of CPS I → N-acetylglutamate

Treatment for NAGS deficiency → carglumic acid

Spastic diplegia with ↑ arginine → arginase deficiency

Brittle, nodular hair + hyperammonemia → argininosuccinic aciduria

Cirrhotic with asterixis after GI bleed → hepatic encephalopathy (↑ NH₃)

Drug acidifying the colon to trap ammonia → lactulose

Antibiotic for hepatic encephalopathy → rifaximin

Urea cycle intermediate linking to TCA → fumarate