Formation & Clinical significance of Urea

Dr. Rohini C Sane

Urea Urea : End of protein metabolism

Amino group of amino acids

↓

Ammonia

↓

Urea

↓

Excretion through urine

Disposal of Ammonia1) Ammonitelic : ammonia excreted in water eg aquatic animals

2) Uricotelic : ammonia excreted as uric acid eg reptiles & birds

3) Ureotelic : ammonia excreted as urea (non toxic ,water soluble )eg mammals

AMMONIA URIC ACID

UREA

Formation of Urea • Carbomoyl phosphate synthtase -CPS I of urea cycle &

Glutamate dehydrogenase of Transaminase

1. Mitochondrial

2. Coordinate formation of NH3 & Urea synthesis

3. Enzymes activity (4 enzymes )increase with substrate concentration

4. Urea synthesis – Liver

5. Urea transport –blood

6. Urea excretion - kidney

7. Urea synthesis in intestine

Small intestine (E coli contain urease which splits Urea into Ammonia & CO2 -Ammonia enters into blood , excreted in urine

Urea synthesis Characteristics of Urea synthesis : Urea -End of protein metabolism

1. Site of synthesis –Liver

2. 80-90 % N2 excreted through in urine

3. Kreb’s Henseleit cycle= Urea cycle

4. 2 Amino groups from Ammonia & Aspartate ( one each )

5. 5 enzymes involved ( 2 mitochondrial +2 cytosolic –Liver Cells )

6. Irreversible

7. 4 ATP utilized ( 2 ATP for synthesis of Carbamoyl phosphate & 2ATP as ATP AMP + PPi

8. NH4⁺ + CO2 + Aspartate + 3ATP Urea + Fumarate +2 ATP +2 Pi +AMP + PPi

9. Regulation of CPS I (rate limiting steps ,↑ by Acetyl Glutamate )

10. Protein consumption↑ NAG ↑ Urea ↑

Sources of Atoms of Urea molecule

Sources and fate of Blood Urea Nitrogen(BUN )

Sites of synthesis of Intermediates ofUrea cycle

Synthesis of Carbamoyl Phosphate in Urea cycle

1. NH2 +CO2 Carbomoyl Phosphate ( enzyme: Carbomoyl Phosphate Synthtase -CPS I )

2. 2 ATP required

3. Irreversible

4. Rate limiting

CPS I CPS II

Site Of Action ( Mitochondrial ) ( Cytosolic )

Allosteric Modulator (NAG ) increase activity by N Acetyl Glutamate (NAG )

not influenced by NAG

Activity ↑ Urea formation ↑ Pyrimidine synthesis

Synthesis of Carbomoylphosphate in Urea cycle by CPS I

Synthesis of Citrulline in Urea cycle

Carbamoyl ℗ +Ornithine Citrulline ( Ornithine trans carbamylase )

• Ornithine regenerated

Ornithine & Citrulline

1. Basic amino acids

2. No codons

3. Not found in protein structure

4. Transporter (mitochondria cytosol)

Synthesis of Arginosuccinate in Urea cycle • Citrulline +Aspartate Arginosuccinate

• Enzyme Arginosuccinate Synthtase

• NH2 group of Aspartate contributes ( second amino group of Urea )

• ATP AMP + Ppi ( Inorganic phosphate Ppi )

Cleavage of Arginosuccinate in Urea cycle

• 4. Arginosuccinate Arginine + Formate*

• Enzyme - Arginosuccinase

• Fumarate * TCA Gluconeogenesis

• Arginine Ornithine

• Arginine – next precursor for Urea cycle

Formation of Urea from Arginine in Urea cycle

5.Arginine Urea + Ornithine (enzyme –Arginase )

cytosol

Ornithine

mitochondrial

Arginase –mostly in liver

Other 4 enzymes of Urea cycle found in other tissues

Arginine synthesis occurs in other tissues

Urea synthesis occurs only in Liver cells (Arginase present only in Liver )

Metabolism of urea in Renal failureRenal failure Uremia

1. More urea transported to intestine

2. ↑ urea Urease (bacteria) ↑ NH3 (HYPERAMMONEMIA )

3. Treatment : oral administration of antibiotics (Neomycin ) kills intestinal bacteria

Aspartate & Fumarate link Urea cycle & TCA cycle

FOUR LINKING POINTS BETWEEN TCA & UREA CYCLE 1.CO2 FROM DECARBOXYLATION OF TCA USED FOR UREA CYCLE

2. 12 ATP FROM TCA CYCLE- 4 ATP USED IN UREA CYCLE

3. OAA ASP FROM TCA CYCLE USED FOR ARGINOSUCCINATE SYNTHESIS

4. ARGINOSUCCINATE - FUMARATE–USED IN TCA CYCLE

FOUR LINKING POINTS BETWEEN TCA & UREA CYCLE 1.CO2 FROM DECARBOXYLATION OF TCA USED FOR UREA CYCLE

2. 12 ATP FROM TCA CYCLE- 4 ATP USED IN UREA CYCLE

3. OAA ASP FROM TCA CYCLE USED FOR ARGINOSUCCINATE SYNTHESIS

4. ARGINOSUCCINATE - FUMARATE –USED IN TCA CYCLE

Metabolic disorders of Urea cycle-HyperammonemiaType of Hyperammonemmia Defective Enzyme

HYPER AMMONEMMIA I CPS I

HYPER AMMONEMMIA II Ornithine Transcarbomylase

HYPER AMMONEMMIA IIICitullinemia

Argino succinate synthtase

HYPER AMMONEMMIA IVArginosuccinic aciduria

Arginosuccinase

HYPER AMMONEMMIA VHyperargininemia

Arginase

Symptoms of Hyperammoninemia

Symptoms of Hyperammoninemia

• Increase in Ammonia

• Vomitting

• Lethargy

• Irritability

• Atexia

• Mental retardation

Clinical significance of Blood Urea estimation RENAL UREMIA

• Acute glomerulonephritis

• Chronic nephritis

• Nephrosclerosis

• Polycystic kidney

POST RENAL UREMIA(OBSTRUCTION TO URINARY TRACT )

1. Tumor

2. Stones Reabsorption of Urea

3. Prostrate enlargement

Clinical significance of Blood UreaNon protein nitrogen (NPN )

• Urea

• Uric acid

• Creatinine

• Creatine 20-40mg/dl

• Peptides

• Amino acids

• BLOOD UREA NITROGEN ( BUN )=14/60 XUREA = 2.14X BLOOD UREA

Formation & degradation of N-Acetyl GlutN-Acetyl Glutamate

Key enzymes & ATP utilization of Urea cycle

Chemical Reactionsof Urea cycle