Urea cycle and its disorders

Dr. N. Sivaranjani

Asst. Prof.

U r e a c y c l e

H2N C

O

NH2

urea

Dr. N. Sivaranjani 1

Transport of Ammonia


* Ammonia is produced in most tissues – less than 1% is TOXICespecially to CNS

Readily ionizes to ammonium ion NH4+

* It is immediately removed from the circulation and detoxified to Urea in the LIVER.

* Three transport forms of NH3 from peripheral tissues to LIVER:1. Glutamate – ALL tissues 2. Glutamine – Brain 3. Alanine - Muscle

NH4+

pKa = 9.3

NH3 + H+


4

NH3+CH

COO-

COO-

CH2

CH2

Glutamate

NH3+CH

COO-

CO--

CH2

CH2

Glutamine

NH3+CH

COO-

COO-

CH2

CH2

Glutamate

ATP ADP+Pi NH3

NH3 H2O

Glutamine synthetase

Glutaminase

Liver

Mitochondria

NH2

Glutamine is a non-toxic carrier of ammonia from Brain . It is released into blood circulation and carried to liver.

Urea cycle

Brain

Dr. N. Sivaranjani

Glucose-Alanine cycle

Alanine is safe way to transport ammonia from muscle to liver via blood.

• Glucose Alanine Cycle / Cahill Cycle

Plays a dual role :

• Transports ammonia from muscle to liver in a non-toxic form (Alanine)

• Transports carbon skeleton to liver for gluconeogenesis


Amino acids

GLUTAMATE NH3

α Keto glutarate

Glutamate dehydrogenase

Transamination

Glutamine

Glutamate

Glutamate

NH3Urea


Glutaminase

Alanine

Brain

Liver

AspargineMuscle

All cells

Pyruvate

NH3

Aspartate

Aspartate

Liver

Liver


Amino acids

GLUTAMATE

NH3

α Keto glutarate

Glutamate dehydrogenase

Transamination

Glutamine

Glutamate

Glutamate

NH3Urea


Glutaminase

Alanine

Brain

Liver

AspargineMuscle

All cells

Pyruvate Aspartate

Aspartate

Liver

Liver


Sources of Ammonia

Intestine

GLUTAMINE Glutamate

ASPARGINE Aspartate

Bacterial degradation of Urea

GDH

GLUTAMATE

Serine

Pyruvate

Threonine

α KB

Histidine

Urocanate

NH4+

Purine Pyrimidine Catabolism

Oxidation of Monoamine by MAO

Amino Sugars

Non Oxidative Deamination

Oxidative Deamination


Biochemical basis of Ammonia toxicity

• In Brain cell Mitochondria – excess NH3 reacts with αKG to form GLUTAMATE by GDH – dec. αKG – dec. TCA cycle- dec.Glucose utilization & ATP generation. not universally accepted.

Glutamate depletion – NH3 Inhibits glutaminase – depletes glutamate which is a excitatory NT.

Glutamine is accumulated in neurons – osmotic shift of H2O into the cell- Edema & swelling of Astrocytes.


Neuronal dysfunction – inc.permeability of K+ & Cl- ions

Accumulation of Excito-toxins – inc. transport of Tryptophan across BBB – its Metabolites are accumulated – which are Excito-toxins.


Excretory forms of Nitrogen

Mammals including human beings

The basic features of nitrogen metabolism were elucidated initially in pigeons


Urea cycle

Krebs–Henseleit urea cycle / Ornithine cycle

Site – LIVER

Subcellular organelle – Mitochondria , cytoplasm – 2 steps occur in

mitochondria, remaining in the cytosol.

Converts NH3 into harmless Urea


• Disposable form of NH3

• Accounts for 90% of NPN in UrineUREA

• 1 N – Ammonia

• 2 N – Aspartate

• C & O – CO2


NH3 + CO2 + H2O

H3N-CO-O-PO3

2 ATP

2 ADP + Pi

Carbamoyl Phosphate

Citrulline

Argininosuccinate

Ornithine Aspartate

UREA

H2O

Carbamoyl PO4 synthetase I

Ornithine Trans Carbamoylase

Argininosuccinate synthetase

Argininosuccinate lyase

Arginase

H2N-CO-NH2

Transporter

Cytosol


Significance Of Urea Cycle

2 N of urea ( H2N-CO-NH2) – NH3 , amino N of Aspartate

Disposes 2 waste products – NH3, HCO3-

Arginase E – only in LIVER

Forms SEAA – Arginine

Ornithine is regenerated – Polyamine syn. , NEAA syn. - proline

Fumarate is the link b/w UREA & TCA cycle – Kreb’s bi cycle


Arginine

Urea

Ornithine

Citrulline

Malate

Fumarate

ArgininoSuccinate

Aspartate

Oxaloacetate

α-keto acid α-amino acid

Relationship b/w Urea cycle & TCA cycle

Carbamoyl PO4

Urea cycle

TCA cycle


Difference b/w CPS 1 and CPS 11

CPS -I CPS-II

Location Mitochondria Cytosol

Nitrogen donor NH3 Glutamine

Participates Urea CyclePyrimidine

Biosynthesis

Regulated Activated – NAG

(N-Acetyl glutamate) Inhibited - CTP


Over all reaction

NH3 + CO2 + Aspartate + 3 ATP

UREA


Energetics of Urea Cycle

2 ATP• Reaction 1

2 ATP• Reaction 3

TCA cycle

Via Fumarate

Malate – OAA 1 NADH=3ATP

4 ATP (UREA CYCLE) – 3 ATP (TCA CYCLE) = 1 ATPDr. N. Sivaranjani 19

Regulation

• Feed forward mechanism / Coarse regulation –regulated by substrate availability

High PROTEIN diet – induces CPS-I Prolonged starvation – inc. Catabolism of proteins -

induces GDH- inc. NH3 – urea cycle.

* Allosteric mechanism – CPS -I stimulated by N-Acetyl Glutamate (NAG)


Allosteric Mechanism

Glutamate Acetyl - CoA

N-Acetyl Glutamate

+

High protein diet,Glutamate ,

Arginine, Prolonged Starvation

+

Acetyl Glutamate synthase

CPS – I of Urea Cycle

+


Fate of UREA


Disorders of UREA cycle

• Genetic defect have been described in all enzymes of urea

cycle - results in ammonia intoxication

• These are extremely rare – 1 in 30,000 live births

• Autosomal Recessive , except OTC defect – X linked

• Defect in reaction 1 and 2 – accumulation of Ammonia directly

• Defect of later enzymes - accumulation of intermediates


Common features seen are – severity varies

Feeding difficulties , Lethargy , irritability ,

protein induced vomiting and poor intellectual

development – MR ,cerebral edema, seizures

leads to COMA and death


Disorders Defective Enzyme

Products Accumulated

Clinical features

Hyperammonaemiatype – I

CPS – I Ammonia Severe hyperammonemiaMental retardation , developmental delay

Variant of hyperammonemiatype I

N acetyl glutamate synthase

Ammonia Neonatal Hyperammonemia –Fatal. Rx -N-carbamoyl-L-glutamate– activates CPS-I

Hyperammonaemiatype – II

OTC , X-linked

Ammonia Orotic aciduria -channeling of CP to Pyrimidine syn.


Disorders Defective Enzyme Products Accumulated

Clinical features

Hyperornithinemia Defective ornithinetransporter protein.ORNT1 gene defect.

Ornithine & NH3 HHH syndrome -Hyperornithinemia, hyperammonemia & homocitrullinuria - carbamoylates lysine

Citrullinemia Argino - succinatesyntethase

Citrulline Citrullinuria,breast milk is to be avoided

Argininosuccinicaciduria

ArgininoauccinateLyase

Argininosuccinate Metabolic acidosis,Friable brittletufted hair - Trichorrhexis Nodosa

Argininemia Arginase Arginineinc. - CSF.

Instead of arginine, Cysteine & lysine are lost in urine.


Hepatic Coma (Acquired Hyperammonemia)

Portal systemic Encephalopathy / Hepatic encephalopathy

Hepatic failure –finally lead to hepatic coma and death

Hyperammonemia –characteristic feature of liver failure

C/F - Altered sensorium, convulsions, ascites, jaundice, hepatomegaly, cerebral edema, hemorrhage, spider naevi.

Urine



Treatment • Dietary restriction of protein – Mainstay of management

Replacement of EAA by their corresponding α-Keto acids –dec. N disposal – without causing EAA deficiencies.• Maximal calories should be provided in the form of I.V glucose

& lipids to reduce catabolism.

• Promote N excretion in forms other than Urea Block due to Argininosuccinate lyase defect – supplement Arginine diet – Argininosuccinate is excreted.

First 2 blocks – supplement diet with Benzoate & Phenylacetate


Treatment for first 2 defects Benzyl CoA + Glycine = Hippuric acid

Phenylacetyl CoA + Glutamine = Phenylaceylglutamine

Arginine

Urea

Citrulline

Argininosuccinate

Aspartate

Ornithine

Arginine supplements

Excreted in URINE


Treatment • Gene therapy is in experimental stage

• Neonatal Hyperammonaemia – Medical emergency - requires rapid lowering of NH3

- Hemodilaysis ,- Exchange Transfusion ,- Peritoneal dialysis

• Hepatic Encepahalopathy- sterilization of Gut – Lactulose laxative- liver transplantation - Treat the underlying cause.


Education

Urea cycle and its disorders