Urea Cycle (Krebs-Henseleit Cycle)

Introduction

• Nitrogen enters as NH₄⁺ (from amino acid catabolism) and aspartate
• Carbon and oxygen are derived from CO₂ (HCO₃⁻)
• Ornithine is the primer - it is regenerated at the end of each turn (analogous to oxaloacetate in the TCA cycle)

Steps of the Urea Cycle

Step 1 - Formation of Carbamoyl Phosphate (Mitochondria)

• NH₄⁺ + HCO₃⁻ + 2 ATP → Carbamoyl phosphate + 2 ADP + Pi
• CPS-I requires N-acetylglutamate (NAG) as an obligatory allosteric activator
• NAG is synthesized from acetyl-CoA + glutamate by NAG synthase, which is activated by arginine
• Note: CPS-II (cytosolic) participates in pyrimidine synthesis; it uses glutamine as nitrogen source and does NOT need NAG

Step 2 - Formation of Citrulline (Mitochondria → Cytosol)

• Ornithine + Carbamoyl phosphate → Citrulline + Pi
• Citrulline is transported to the cytosol via an ornithine-citrulline antiporter in the inner mitochondrial membrane
• OTC deficiency is the most common urea cycle enzyme defect

Step 3 - Formation of Argininosuccinate (Cytosol)

• Citrulline + Aspartate + ATP → Argininosuccinate + AMP + PPi
• This step incorporates the second nitrogen (from aspartate) into the cycle
• 1 ATP is consumed (equivalent to 2 high-energy bonds as PPi is released)

Step 4 - Cleavage of Argininosuccinate (Cytosol)

• Argininosuccinate → Arginine + Fumarate
• Fumarate links the urea cycle to the TCA cycle (converted to malate → oxaloacetate → aspartate, completing the "urea cycle-TCA cycle link")

Step 5 - Cleavage of Arginine → Urea + Ornithine (Cytosol)

• Arginine + H₂O → Urea + Ornithine
• Ornithine is transported back into the mitochondria to begin another turn
• Arginase-I is virtually exclusive to the liver - only the liver synthesizes urea

Summary Table

Energy Cost

NH₄⁺ + HCO₃⁻ + Aspartate + 3 ATP + H₂O → Urea + Fumarate + 2 ADP + AMP + 2Pi + PPi

• 3 ATP molecules are consumed per mole of urea formed (equivalent to 4 high-energy phosphate bonds)
• The synthesis of urea is irreversible

Key Points (Exam Focus)

• Two nitrogens of urea: one from NH₄⁺ (via glutamate/GDH), one from aspartate (via transamination)
• Allosteric activator of CPS-I: N-acetylglutamate (NAG) - rate-limiting regulator
• Most common enzyme defect: OTC deficiency (X-linked)
• Only organ synthesizing urea: Liver
• Urea cycle-TCA cycle link: via fumarate → malate → oxaloacetate → aspartate
• Patients with liver failure accumulate ammonia → hepatic encephalopathy
• Biochemistry, 8th ed Lippincott Illustrated Reviews, p. 712-715
• Basic Medical Biochemistry - A Clinical Approach, 6e, p. 1321-1322

Disorders Linked to Deficiency of Urea Cycle Enzymes

Overview

Individual Enzyme Defects

1. N-Acetylglutamate Synthase (NAGS) Deficiency

• Enzyme blocked: NAGS (makes the allosteric activator of CPS-I)
• Accumulated metabolite: Ammonia (no diagnostic accumulation of cycle intermediates)
• Key lab: Low citrulline, low orotic acid, elevated glutamine
• Features: Clinically indistinguishable from CPS-I deficiency; episodic hyperammonemia triggered by high-protein meals, stress, or fasting
• Treatment: Carglumic acid (N-carbamylglutamate) - a synthetic NAG analogue; this is specific to NAGS deficiency and can restore full urea cycle function, rendering other therapies unnecessary

2. Carbamoyl Phosphate Synthetase I (CPS-I) Deficiency

• Enzyme blocked: CPS-I (Step 1, mitochondria)
• Accumulated metabolite: Ammonia
• Key lab: Low/absent citrulline, low orotic acid (unlike OTC deficiency), elevated glutamine
• Features: Severe neonatal hyperammonemia; autosomal recessive
• Distinguishing point from OTC: Urine orotic acid is normal/low (carbamoyl phosphate does not escape to cytosol, so pyrimidine synthesis is not overstimulated)

3. Ornithine Transcarbamylase (OTC) Deficiency (Most Common)

• Enzyme blocked: OTC (Step 2, mitochondria)
• Accumulated metabolite: Carbamoyl phosphate, ammonia
• Key lab: ↑ Orotic acid in blood and urine, ↓ citrulline, ↓ arginine, ↑ glutamine
• Why orotic acid rises: Excess carbamoyl phosphate "overflows" into the cytosol and enters pyrimidine biosynthesis, producing excess orotic acid - this is the hallmark diagnostic marker
• Inheritance: X-linked - severely affects males; females (carriers) may be symptomatic depending on X-inactivation pattern. Can present in females at time of childbirth due to catabolism
• Incidence: Most frequent urea cycle defect (57-62%)

4. Argininosuccinate Synthetase Deficiency - Citrullinemia Type 1

• Enzyme blocked: Argininosuccinate synthetase (Step 3, cytosol)
• Accumulated metabolite: Citrulline (markedly elevated in blood and urine), orotic acid
• Key lab: Very high plasma citrulline - detectable on newborn screening
• Forms:
  • Neonatal acute (classic) - severe hyperammonemia at birth
  • Milder late-onset form
  • Pregnancy-onset form
  • Asymptomatic form
• Note: Citrin deficiency (defect in aspartate/glutamate exchanger) causes Citrullinemia Type 2 - a different mechanism but also elevated citrulline

5. Argininosuccinate Lyase (ASL) Deficiency - Argininosuccinic Aciduria

• Enzyme blocked: Argininosuccinate lyase (Step 4, cytosol)
• Accumulated metabolite: Argininosuccinate in blood and urine; milder citrulline elevation
• Key lab: Argininosuccinic acid detectable on plasma amino acid analysis and newborn screening; 2nd most common urea cycle defect (11.5-18%)
• Features: Hyperammonemia + liver cirrhosis (hepatic involvement is characteristic); neurologic abnormalities, developmental delays, cognitive impairment
• Treatment: Arginine supplementation (citrulline → argininosuccinate → arginine pathway used as "nitrogen sink")

6. Arginase-I Deficiency - Argininemia / Hyperargininemia

• Enzyme blocked: Arginase-I (Step 5, cytosol; liver-specific)
• Accumulated metabolite: Arginine (elevated in blood and urine)
• Key lab: High plasma arginine; hyperammonemia is often less severe because arginine itself contains two waste nitrogens and is excreted
• Features: Unique clinical presentation - patients appear healthy at birth and develop normally for 1-3 years, then develop progressive spastic diplegia, loss of developmental milestones, intellectual disability, seizures
• Distinguishing feature: Milder and delayed hyperammonemia compared to other urea cycle defects

Summary Comparison Table

Diagnostic Key Points

• Orotic acid elevated: OTC deficiency (orotic acid is the key differentiator from CPS-I and NAGS)
• Orotic acid normal/low: CPS-I deficiency, NAGS deficiency
• High citrulline: Citrullinemia (ASS deficiency) or citrin deficiency
• Argininosuccinate in plasma/urine: ASL deficiency
• High arginine, mild NH₃, late onset: Arginase deficiency
• All defects → elevated plasma glutamine (ammonia not disposed by cycle is conjugated to glutamate by glutamine synthase in perivenous hepatocytes)

Treatment Principles

1. Acute hyperammonemia: IV glucose + lipids to reverse catabolism; IV arginine (essential in these patients except arginase deficiency)
2. Nitrogen scavengers:
   • Sodium benzoate → conjugates glycine → hippurate (excreted in urine)
   • Sodium phenylbutyrate/phenylacetate → conjugates glutamine → phenylacetylglutamine (excreted in urine)
3. Hemodialysis if ammonia does not fall promptly
4. Long-term: Protein-restricted diet, citrulline (for OTC/CPS-I deficiency), arginine (for ASS/ASL deficiency), phenylbutyrate/glycerol phenylbutyrate
5. NAGS deficiency: Carglumic acid (synthetic NAG analogue) - specific and highly effective
6. Liver transplantation for severe, medically refractory cases

• Biochemistry, 8th ed Lippincott Illustrated Reviews, p. 725-728
• Harrison's Principles of Internal Medicine 22E, p. 3415-3416
• Tietz Textbook of Laboratory Medicine, 7th Ed, p. 2474-2475
• Goldman-Cecil Medicine, Table 189-5