Transcript 32_Metabolism of ammonia. Biosynthesis of urea and its disorders

Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
amino acids
Most mammals convert
amino-acid nitrogen to
urea for excretion
The carbon chains are broken
down to molecules that feed
into the TCA cycle.
NH4+
Some animals
excrete NH4+ or
uric acid.
most terrestrial
vertebrates
fish & other aquatic
vertebrates
birds & reptiles
O
O
H2N-C-NH2
NH4+
ammonium ion
H
N
HN
urea
uric acid O
O
N
H
N
H
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Ammonia is a toxic substance to plants and animals (especially for
brain)
Normal concentration: 25-40 mol/l (0.4-0.7 mg/l)
Ammonia must be removed from the organism
Terrestrial vertebrates synthesize
urea (excreted by the kidneys) ureotelic organisms
Urea formation takes place in the
liver
Birds, reptiles synthesize uric acid
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Major fate of waste nitrogen
O
H 2N
C NH2
urea
4
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Why Urea?
Non toxic
Water soluble
Combines two waste products into one
molecule:
CO2
NH3
5
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Ammonia is highly toxic
Main reason to form urea is to reduce levels
of ammonia
“Ammonia” often refers to (NH3 + NH4+)
NH3 is really ammonia
NH4+ is the ammonium ion
pKa = 9.3
NH4+
NH3 + H+
6
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Hypotheses toxicity of ammonia
A. The binding of ammonia in the synthesis of glutamate
causes an outflow of α-ketoglutarate from the tricarboxylic
acid cycle, with decreased formation of ATP energy and
deteriorates the activity of cells.
B. Ammonium ions NH4 + caused alkalization of blood plasma.
This increases the affinity of hemoglobin for oxygen (Bohr
effect), the hemoglobin does not release oxygen to the
capillaries, resulting the cells hypoxia occurs.
C. The accumulation of free NH4 + ion in the cytosol affects
the membrane potential and intracellular enzymes work - it
competes with ion pumps, Na + and K +.
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Hypotheses toxicity of ammonia
D. The producing ammonia tramsform glutamic acid glutamine - an osmotically active substance. This leads to
water retention in the cells and the swelling that causes
swelling of tissues. In the case of nervous tissue it can cause
brain swelling, coma and death.
E. The use of α-ketoglutarate and glutamate to neutralize the
ammonia causes a decrease in the synthesis of γ-aminobutyric
acid (GABA) inhibitory neurotransmitter of the nervous
system.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Ammonia rapidly equilibrates across membranes
NH4+
NH4+
pKa = 9.3
NH3
NH3
Lipid Bilayer
Metabolic Disorders of Urea Cycle
AMMONIA METABOLISM
Metabolic & Molecular Basis of Inherited Disease
The ways of ammonia formation
1. Oxidative deamination of amino acids
2. Deamination of physiologically active amines and nitrogenous
bases.
3. Absorption of ammonia from intestine (degradation of
proteins by intestinal microorganisms results in the ammonia
formation).
4. Hydrolytic deamination of AMP in the brain (enzyme –
adenosine deaminase)
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Production of ammonia
Amino acids
-Keto acids
-Ketoglutarate
-Ketoglutarate
Glutamate
GDH
Oxaloacetate
Aspartate
Other reactions
NH+4
Urea
cycle
Urea
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Summary of sources of ammonia for urea cycle
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Peripheral Tissues Transport Nitrogen to the Liver
Two ways of nitrogen transport from peripheral
tissues (muscle) to the liver:
1. Alanine cycle. Glutamate is
formed by transamination reactions
Glutamate is not
deaminated in
peripheral tissues
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Nitrogen is then transferred to pyruvate to
form alanine, which is released into the blood.
The liver takes up the alanine and converts it back
into pyruvate by transamination.
The glutamate formed in the liver is deaminated
and ammonia is utilized in urea cycle.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Closer look at transport of waste N from peripheral tissue
to liver via alanine and glutamine
Waste N funnelled to pyruvate
via transaminations
Glucose – Alanine Cycle
Net: N (muscle)  Urea (liver)
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
2. Nitrogen can be
transported as glutamine.
Glutamine synthetase
catalyzes the synthesis of
glutamine from glutamate and
NH4+ in an ATP-dependent
reaction:
Ammonia transport in the form of
glutamine.
Excess ammonia in tissues is added
to glutamate to form glutamine, a
process catalyzed by glutamine
synthetase. After transport in the
bloodstream, the glutamine enters
the liver and NH4 is liberated in
mitochondria by the enzyme
glutaminase.
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Synthesis of Glutamine in Peripheral Tissue and
Transport to the Liver
17
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
UREA FORMATION
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Overview
Occurs primarily in liver; excreted by kidney
Principal method for removing ammonia
Hyperammonemia:
Defects in urea cycle enzymes (CPS, OTC, etc.)
Severe neurological defects in neonates
Treatment:
Stop protein intake
Dialysis
Increase ammonia excretion: Na benzoate, Na
phenylbutyrate, L-arginine, L-citrulline
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Overview
Key reaction: hydrolysis of arginine
Arginine + H2O
==>
urea + ornithine
arginase
Resynthesis of Arginine
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Blood Urea Nitrogen
Normal range: 7-18 mg/dL
Elevated in amino acid catabolism
Glutamate
N-acetylglutamate
CPS-1 activation
Elevated in renal insufficiency
Decreased in hepatic failure
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Urine
urea
NH2
C O
Mitochondrion
CO2 + NH4+
carbamoyl
2 ATP
phosphate
synthetase I
(CPSI)
2ADP + Pi
1
O
H2N C O P O-
CH2
H C NH2
Ornithine
2 ornithine
NH2
Pi
H2O
C NH
CH2 NH
CH2NH2
CH2
transcarbamoylase
Carbamoyl O
phosphate
5
Cytosol
COOH
O
NH2
NH2
C O
CH2 NH
CH2
CH2
H C NH2
COOH
Citrulline
CH2
CH2
CH2NH2
CH2
H C NH2
COOH
COOH
CH2
Arginine
H C NH2
COOH
4
argininosuccinase
Ornithine
NH2
C O
HC
CH
COOH
Fumarate
COOH
NH
CH2 NH
C
CH2
NH
CH2 NH
CH2
CH2
H C NH2
3
argininosuccinate
synthetase
COOH
Citrulline
COOH
H2N C H
CH
CH2
COOH
CH2
H C NH2
COOH
Argininosuccinate
ATP AMP + PPi
CH2
COOH
Aspartate
23
Metabolic & Molecular Basis of Inherited Disease
THE UREA CYCLE
Metabolic Disorders of Urea Cycle
Urea cycle - a cyclic pathway of urea synthesis
first postulated by H.Krebs
The sources of
nitrogen atoms in
urea molecule:
- aspartate;
- NH4+.
Carbon atom
comes from CO2.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
HCO3-
2 ATP
2 ADP + Pi
The urea cycle
O O
H2N-C-O-P-OO-
NH4+
ornithine
NH3+
Pi
H2N-CH2CH2CH2CH-CO2-
O
H2N-C-NH2
cytosol
carbamoyl
phosphate
mitochondria
citrulline
+
NH3
O
H2N-C-NH-CH2CH2CH2CH-CO2-
CO2- Asp
-O C-CH CH-NH +
2
2
3
ATP
AMP + PPi
urea
H2O
CO2- NH2+
NH3+
-O C-CH CH-NH-C-NH-CH CH CH CH-CO 2
2
2
2
2
2
argininosuccinate
arginine
NH3+
NH2+
H2N-C-NH-CH2CH2CH2CH-CO2-
-O
2C-CH=CH-CO2
-
fumarate
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Incorporation of ammonia into urea begins with
formation of carbamoyl phosphate
O O
H2N-C-O-P-Ocarbamoyl
Ophosphate
2 ADP + Pi
NH4+ + HCO32 ATP
This occurs in the mitochondrial matrix. Carbamoyl-phosphate
synthetase-1 catalyzes the reaction in three steps, using two
molecules of ATP:
HCO3ATP
(1)
ADP
O O
HO-C-O-P-OONH4+
(2)
carbamate
O
H2N-C-O-
Pi
carbonicphosphoric acid
anhydride
ATP
ADP
(3)
O O
H2N-C-O-P-OO-
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Carbamoyl phosphate reacts with ornithine to form
citrulline
O O
H2N-C-O-P-OO-
carbamoyl
phosphate
Pi
NH3+
+H N-CH CH CH CH-CO 3
2
2
2
2
ornithine
NH3+
O
H2N-C-NH-CH2CH2CH2CH-CO2+ H+
citrulline
This step also occurs in the mitochondrial matrix.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Combination of citrulline with aspartate to form
argininosuccinate is driven by breakdown of ATP to AMP
CO2-O C-CH CH-NH +
2
2
3
aspartate
NH3+
O
H2N-C-NH-CH2CH2CH2CH-CO2-
ATP
citrulline
AMP + PPi + H2O
argininosuccinate
CO2- NH2+
NH3+
-O C-CH CH-NH-C-NH-CH CH CH CH-CO 2
2
2
2
2
2
This reaction occurs only in the cytosol, so citrulline first must
leave the mitochondria. A transporter exchanges ornithine for
citrulline plus a proton across the mitochondrial inner membrane.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Argininosuccinate splits into arginine and fumarate
CO2- NH2+
NH3+
-O C-CH CH-NH-C-NH-CH CH CH CH-CO 2
2
2
2
2
2
argininosuccinate
-O
2C-CH=CH-CO2
fumarate
-
NH3+
NH2+
H2N-C-NH-CH2CH2CH2CH-CO2arginine
This reaction occurs in the cytosol.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Hydrolysis of arginine releases urea and regenerates
ornithine
NH3+
NH2+
H2N-C-NH-CH2CH2CH2CH-CO2arginine
H2O
O
H2N-C-NH2
urea
NH3+
H2N-CH2-CH2-CH2-CH-CO2H+
ornithine
This reaction occurs in the cytosol. To continue the
cycle, ornithine must return to a mitochondrion.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
HCO3-
2 ATP
2 ADP + Pi
carbamoyl
O O phosphate
H2N-C-O-P-OOPi
NH4+
NH3+
O
H2N-C-NH-CH2CH2CH2CH-CO2-
ornithine
O
H2N-C-NH2
urea
citrulline
ATP
2 Pi
PPi + AMP
CO2-O C-CH CH-NH +
2
2
3
Asp
H2O
CO2- NH2+
NH3+
-O C-CH CH-NH-C-NH-CH CH CH CH-CO 2
2
2
2
2
2
argininosuccinate
Formation of urea consumes 4 phosphate anhydride bonds
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Input-Output
1. NH4+ + CO2 + 2 ATP
carbamoyl phosphate + 2 ADP + Pi
2. carbamoyl phosphate + ornithine
citrulline + Pi
3. citrulline + aspartate + ATP
argininosuccinate + AMP + PPi
4. AMP + ATP
2 ADP
5. PPi + ATP
2 ADP
6. argininosuccinate
arginine + fumarate
7. arginine + H2O
urea + ornithine
SUM: NH4+ + CO2 + 4 ATP + aspartate
urea + fumarate + 4 ADP + 4 Pi
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
The aspartate consumed in the urea cycle can be
regenerated from the fumarate that is produced
2 ATP
HCO3+ NH4+
2 ADP + Pi
carbamoyl
phosphate
Pi
-keto acids
amino acids
aspartateoxaloacetate
aminotransferase
ornithine
citrulline
Urea
cycle
urea
ATP
AMP + PPi
arginine
oxaloacetate
aspartate
malate
dehydrogenase
argininosuccinate
NADH
malate
NAD+
fumarate
H2O
This process also uses
both cytosolic and
mitochondrial enzymes
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Oxidation of malate in mitochondria generates ATP
2 ATP
2 e- to O2 via NADH dehydrogenase
generates ~ 2.5 ATP
2 ADP + Pi
HCO3+ NH4+
carbamoyl
phosphate
oxaloacetate
mitochondrion
NAD
Pi
aspartate
ornithine
citrulline
ATP
glutamate
-ketoglutarate
-ketoglutarate
citrulline
ornithine
NADH
malate
glutamate
aspartate
urea
AMP + PPi
arginine
argininosuccinate
amino acids
-ketoacids
malate
cytosol
fumarate
H2O
NADH,
and oxaloacetate can’t cross the mitochondrial inner membrane,
but there are transporters for malate, aspartate, glutamate and -ketoglutarate.
NAD+
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Transport systems in the mitochondrial inner membrane
exchange aspartate for glutamate and a-ketoglutarate for
malate
mitochondrion
aspartate-
aspartatecytosol
glutamate- + H+
glutamate- + H+
-ketoglutarate
-ketoglutarate
malate
malate
Because the Asp/Glu transporter also moves a proton across the
membrane, it can be driven by an electrochemical potential gradient.
Mutations in this transporter have been linked to autism.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Alpha--ketoglutarate/malate and aspartate/glutamate transporters also
participate in oxidation of cytosolic NADH
mitochondrion
NADH
NAD+
2 e- to electrontransport chain
oxaloacetate
aspartate
aspartate
glutamate
glutamate
-ketoglutarate
-ketoglutarate
oxaloacetate
cytosol
NADH
NAD+
glycolysis
malate
malate
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Well Fed State
a.a.'s
Asp
a.a.'s
 KAs
 KG
NH+
Glu
4
urea
cycle
fumarate
urea
CO2 + H2O
OAA
Mal
malate
Net: 2 NH4+ + CO2 + 4 ATP  urea + 4 ADP + 4 Pi
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Fasted State
ala
Asp
ala
pyr
 KG
Gluconeogenesis
NH+
Glu
4
urea
cycle
fumarate
urea
OAA
malate
OAA
2 ala + CO2  1 urea + 1 glucose
Glucose
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Balancing the levels of ammonia and
aspartate for entry into urea cycle
(a) NH3 in excess
(b) Aspartate in excess
NH3
NH3
-Ketoglutarate
NADH
NAD
Glutamate
dehydrogenase
NADH
NAD
Glutamate
Glutamate
dehydrogenase
Glutamate
Aspartate
transaminase
Oxaloacetate
Aspartate
transaminase
Oxaloacetate
Aspartate
Citrulline
Carbamoyl
phosphate
Citrulline
Carbamoyl
phosphate
Urea
Cycle
Urea
Urea
Cycle
Urea
 
-Ketoglutarate

Aspartate
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
The urea cycle is regulated in two ways
1. Allosteric activation of carbamoylphosphate synthetase-1 by N-acetylglutamate
CO2+H N
3
C H
CO2+ acetyl-CoA
CH3CO-NH C H N-acetylglutamate
CH2
Glu
CH2
CH2
CoA-SH
CO2-
CH2
CO2-
In mammals, N-acetylGlu appears to play only a
regulatory role. Carbamoylphosphate synthetase-1
is completely inactive in its absence. A genetic
deficiency in the enzyme that forms N-acetylGlu
2 ATP
can cause a lethal defect in the urea cycle.
NH4+ + HCO3-
2 ADP + Pi
carbamoylphosphate
O O
H2N-C-O-P-OO-
2. A high-protein diet or starvation leads to increased synthesis of all five
enzymes used in the urea cycle, including carbamoylphosphate synthetase-1.
Expression of the enzyme that synthesizes N-acetylglutamate also increases.
Metabolic Disorders of Urea Cycle
The Linkage between Urea Cycle, Citric Acid Cycle
and Transamination of Oxaloacetate
Metabolic & Molecular Basis of Inherited Disease
Fumarate formed in urea cycle enters citric acid cycle
and is converted to oxaloacetate.
Fates of oxaloacetate:
(1) transamination to aspartate,
(2) conversion into glucose,
(3) condensation with acetyl CoA to form citrate,
(4) conversion into pyruvate.
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Diagnostic significance of the
determination of urea in urine.
25-30 g/day of urea is excreted in
normal conditions.
The increase of urea in urine occurs in
high fever, malignant anemia, poisoning by
phosphorus, intensive decomposition of
protein in organism.
The decrease of urea in urine occurs
in liver diseases, kidney unsufficiency,
acidosis.
11 April 2016
Total slide : 50
42
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Urea Cycle Disorders
Deficiency of any of the five enzymes in the urea cycle results
in the accumulation of ammonia and leads to encephalopathy.
Episodes of encephalopathy and associated systems are
unpredictable and, if untreated, are lethal or produce devastating
neurologic sequelae in long-term survivors.
Although these disorders do not produce liver disease, the
consequences of hyperammonemia resemble those seen in
patients with hepatic failure or in a transient interference with
the urea cycle, as seen in some forms of organic acidemias.
Investigate for hyperammonemia in any infant or child with
altered mental status
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
The urea cycle
Asterisk = N-acetyl glutamate synthetase; 1 = carbamyl phosphate
synthetase; 2 = ornithine transcarbamylase; 3 = argininosuccinate
synthetase; 4 = argininosuccinate lyase; 5 = arginase
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
UREA CYCLE DISORDERS
Disorder
Deficient Enzyme
Inheritance Pattern
Carbamyl phosphate
synthetase deficiency
Carbamyl phosphate
synthetase
Autosomal recessive
Ornithine
transcarbamylase
deficiency
Ornithine
transcarbamylase
X-linked
Citrullinemia
Argininosuccinate
synthetase
Autosomal recessive
Argininosuccinate lyase
Autosomal recessive
Arginase
Autosomal recessive
Argininosuccinic aciduria
Argininemia
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Case
The patient is a full-term newborn boy from a normal vaginal delivery.
The pregnancy was uncomplicated. At 36 hours the baby became
lethargic, irritable, and was hyperventilating. Over the next 24 hours
lethargy increased and progressed to coma requiring mechanical
ventilation. Hemodialysis was started at 5 days. Patient died
at one week of age.
Laboratory Results
At 36 hours arterial blood pH was 7.50 (7.35-7.45), carbon dioxide was
25 torr (35-45), and blood urea nitrogen was 2 mg/dl (5-20). Sepsis
workup was negative. On day 5 plasma ammonium was 1800 :mol/l
(<35). Plasma glutamine was 1500 :mol/l (550-650),arginine was
below normal, and citrulline undetectable. Orotic acid in the urine was
extremely elevated.
Family History
Two of the mother’s four brothers had died shortly after birth. Cause of
death was given as encephalitis.
Biochemical Basis of Disorder , same as..
Diagnosis: ornithine transcarbamoylase deficiency
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Biochemical explanations for ornithine
transcarbamoylase deficiency
Low BUN
Low blood arginine
Undetectable blood citrulline
Elevated blood ammonia
Elevated blood glutamine
Elevated orotic acid
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Urine
NH4+ + Glu  Gln
urea
NH2
C O
Mitochondrion
CO2 + NH4+
carbamoyl
2 ATP
phosphate
synthetase I
(CPSI)
ADP + Pi
1
O
H2N C O P O-
CH2
H C NH2
Ornithine
2 ornithine
NH2
phosphate
C NH
CH2 NH
CH2
CH2
CH2NH2
CH2
CH2 NH
CH2
CH2
H C NH2
COOH
Citrulline
H C NH2
Arginine
H C NH2
COOH
4
argininosuccinase
Ornithine
NH2
C O
HC
CH
COOH
Fumarate
COOH
NH
CH2 NH
C
CH2
NH
CH2 NH
CH2
CH2
H C NH2
3
argininosuccinate
synthetase
COOH
Citrulline
COOH
H2N C H
Carbamoyl P  orotic acid
COOH
COOH
CH2
C O
Pi
H2O
CH2NH2
CH2
transcarbamoylase
Carbamoyl O
5
Cytosol
COOH
O
NH2
NH2
CH2
COOH
Aspartate
CH2
H C NH2
COOH
Argininosuccinate
ATP AMP + PPi
CH
CH2
COOH
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Carbamoyl P synthetase deficiency
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Urine
NH4+ + Glu  Gln
urea
NH2
C O
Mitochondrion
CO2 + NH4+
carbamoyl
2 ATP
phosphate
synthetase I
(CPSI)
ADP + Pi
1
O
Cytosol
CH2
H C NH2
Ornithine
H2N C O P O-
2 ornithine
transcarbamoylase
Carbamoyl O
NH2
phosphate
C NH
CH2 NH
CH2
CH2
CH2NH2
CH2
CH2 NH
CH2
CH2
H C NH2
COOH
Citrulline
H C NH2
Arginine
H C NH2
COOH
4
argininosuccinase
Ornithine
NH2
C O
HC
CH
COOH
Fumarate
COOH
NH
CH2 NH
C
CH2
NH
CH2 NH
CH2
CH2
H C NH2
3
argininosuccinate
synthetase
COOH
Citrulline
COOH
H2N C H
CH2
Carbamoyl P  orotic acid
COOH
COOH
CH2
C O
Pi
5
H2O
CH2NH2
CH2
COOH
O
NH2
NH2
COOH
Aspartate
CH2
H C NH2
COOH
Argininosuccinate
ATP AMP + PPi
CH
CH2
COOH
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Autism is a neurodevelopmental genetic disorder
 Deficits in verbal & nonverbal communication and social interactions
 Repetitive or stereotyped behaviors
 Incidence ~1 per 1000 people (possibly higher)
 Strong evidence for heritability
 Polygenic - between 5 & 10 genes may be involved
Single-nucleotide polymorphisms (SNPs) in the gene for a mitochondrial,
Ca2+-dependent Asp/Glu exchanger increase the risk by a factor of 3 to 4.
This is the main form of the Asp/Glu exchanger that is expressed in the
brain. Mutations in the gene impair the urea cycle.
N. Ramoz et al., Am. J. Psychiatry 161: 662 (2004)
L. Palmieri et al., EMBO J. 20: 5060 (2001)
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Urea Cycle Disorders
(Diagnosis)
Cultured skin fibroblasts may be desirable if
prenatal diagnosis is considered in future
pregnancies.
Carbamyl phosphate synthetase I and ornithine
transcarbamylase (OTC) are not expressed in
cultured fibroblasts.
The enzymatic diagnosis of CPSD and OTCD
requires liver biopsy.
Biopsy should be done when establishing the
diagnosis of the first case in a family.
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Urea Cycle Disorders
(Treatment)
Once hyperammonemia is demonstrated in an infant,
protein-containing feedings should be discontinued
immediately,
appropriate supportive care, (mechanical ventilation)
Maximal calories should be provided in the form of
intravenous glucose and lipids in an effort to reduce
catabolism.
Plans should be immediately made to initiate hemodialysis
in infants who are encephalopathic and have plasma
ammonia levels over 10 times the upper limit of normal.
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Urea Cycle Disorders
(Treatment)
Maintenance therapy
dietary protein restriction+supplementation with citrulline or
arginine+ the use of drugs
The primary drug now used( provides an alternate pathway
for waste nitrogen excretion) for maintenance therapy in
patients with urea cycle disorders is sodium phenylbutyrate
(Buphenyl).
The drug is typically administered four times a day in a
dose of 0.4 to 0.6 g/kg/day. It is supplied as a powder,
which can be mixed with food or formula, or as a tablet.
Metabolic & Molecular Basis of Inherited Disease
Metabolic Disorders of Urea Cycle
Urea Cycle Disorders
(Treatment)
Liver transplantation for
Severe neonatal OTC and CPS deficiency.
Liver failure and cirrhosis in ASL deficiency.
Failed medical-pharmacologic treatment.
Pretransplant care by
aggressively managing intercurrent hyperammonemia,
vaccinations and prophylaxis are given against
infectious
appropriate caloric intake
Gene replacement
Metabolic Disorders of Urea Cycle
Metabolic & Molecular Basis of Inherited Disease
Genetic deficiencies in some of the urea-cycle enzymes can be
treated pharmacologically
benzoate
CO2-
CO2-
ATP + CoA-SH
AMP + PPi
ATP + CoA-SH
AMP + PPi
O
S-CoA
S-CoA
benzoyl-CoA
O
glycine
CoA-SH
hippurate
(benzoylglycine)
phenylacetate
phenylacetyl-CoA
glutamine
CoA-SH
O
N
H
H
N
CO2-
CO2-
phenylacetylglutamine
O
O
NH2
The amide products of these reactions (hippurate and phenylacetylglutamine) are excreted in the
urine. Replenishing the Gly or Gln removes ammonia.