Urea cycle disorders

Urea cycle disorders

1. Urea cycle

   • fully expressed only in the liver
     • ammonia NH3 from the nitrogen group of all amino acids
       • forms urea
         • requires
           • many enzymes
             • any of them can be defective
           • mitochondrial transporters
             • any of them can be defective
               • impair urea cycle function

2. Urea cycle disorders

   • characterized by
     • hyperammonemia
     • encephalopathy
     • respiratory alkalosis
   • 4 of the 5 metabolic diseases
     • result in the accumulation of urea precursors
       • ammonia
         • Ammonia intoxication
           • most severe when reactions 1 or 2 are blocked
       • glutamine
   • clinical symptoms
     • vomiting
     • avoidance of high-protein foods
     • intermittent ataxia
     • irritability
     • lethargy
     • severe mental retardation
   • CPS1 Carbamoyl Phosphate Synthetase I
     • Initiates Urea Biosynthesis
     • Condensates CO2 & NH3
       • forms carbamoyl phosphate
         • reaction requires
           • allosteric activator, N-acetylglutamate (NAG)
           • Mg2+
           • ATP
     • gene location 2q25
       • deficiency
         • autosomal recessive disorder
           • one of the most severe UCDs
             • clinical symptoms
               • hyperammonemia
                 • seen during the neonatal period
                   • might results in
                     • death
                     • neurological damage
             • Lab findings
               • elevated plasma concentrations
                 • alanine
                 • glutamine
               • normal urinary orotic acid
     • rate limiting enzyme
   • NAGS N-acetylglutamate synthase
     • conversion of glutamate & acetyl coA
       • forms NAG
         • allosteric activator of CPS1
     • gene location 17q21.3
       • mutations
         • autosomal recessive disorder
           • NAG deficiency
             • CPS1 functional deficiency
   • OCT Ornithine transcarbamylase
     • from carbamoylphosphate & ornithine
       • citrulline synthesis
     • gene location Xp21.1
       • only X-linked UCD
         • most common UCD
           • Mutations
             • Clinical symptoms
               • episodic hyperamonemia symptoms
                 • hyperammonemia in neonates
             • Lab findings
               • elevated plasma concentrations
                 • glutamine
                 • alanine
               • elevated urinary orotic acid
             • deficiency
               • abrogate OCT activity
                 • severe
                   • neonatal onset
                   • hemizygous male patients
               • decrease OTC activity
                 • some enzyme function
                   • late-onset form
                     • heterozygous females
   • ASS1 Argininosuccinate synthetase 1
     • cytosol of hepatocytes
       • conjugates citrulline & aspartate
         • transported to cytoplasm from mitochondrial matrix
           • through
             • ORNT1 transporter
             • citrin (carrier)
           • forms argininosuccinate
     • gene location 9q34
       • ASS1 deficiency
         • citrullinemia type I
           • autosomal recessive disorder
             • broad phenotypic variability
               • severely affected patients
                 • neonatal hyperammonemia
               • asymptomatic children
               • adults with only biochemical manifestations
             • Lab findings
               • elevated plasma concentration
                 • glutamine
                 • alanine
               • mild elevation of urinary orotic acid
   • Citrin (aspartate–glutamate carrier)
     • mitochondrial membrane aspartate–glutamate transporter
       • encoded by SLC25A15
         • gene location 7q21.3
           • Mutations
             • defective citrin activity
               • reduced aspartate export to cytosol
                 • not available as ASS substrate
               • Lab findings
                 • neonatal period and infancy
                   • high blood amino acids
                     • citrulline
                     • phenylalanine
                     • threonine
                     • tyrosine
                     • methionine
                   • increased galactose
                     • blood
                     • urine
                   • intrahepatic cholestasis
                     • improves spontaneously
                   • asymptomatic period during childhood
                     • adolescence
                       • become symptomatic again
                     • adulthood
                       • become symptomatic again
                         • neuropsychiatric symptoms
                         • recurrent hyperammonemia
                         • increased plasma citrulline
                           • citrullinemia type 2
   • ASL Argininosuccinate lyase
     • cytosolic enzyme
       • catalyzes argininosuccinate
         • to arginine & fumarate
           • gene location 7cen-q11.2
             • Mutations
               • autosomal recessive disorder
                 • argininosuccinic aciduria
                   • argininosuccinate tissue accumulation
               • wide range of severity
                 • clinical & biochemical findings
                   • caused by combination of
                     • arginine tissue specific deficiency
                     • argininosuccinate elevation
               • onset
                 • neonatal onset
                   • severe hyperammonemia
                     • high mortality rate
                 • late-onset
                   • cognitive impairment
                   • stunted growth
                   • liver dysfunction
                     • hepatomegaly
                       • fibrosis/cirrhosis
               • clinical symptoms
                 • Hair abnormalities
                   • trichorrhexis nodosa
                 • common sequelae
                   • chronic hepatopathy
                     • with bleeding tendency
                       • caused by
                         • mild elevation of transaminases
                         • coagulation factor abnormalities
                 • patient in catabolic state
                   • during an infection
                     • acute episodes
                       • severe hyperammonemia
               • Lab findings
                 • hyperammonemia
                 • mild transaminases elevation
                 • plasma concentrations
                   • elevated
                     • glutamine
                     • alanine
                   • marked elevation
                     • argininosuccinate
                       • also found in
                         • urine
                         • CSF
                   • mild elevation
                     • citrulline
   • ARG1 Arginase 1
     • arginine hydrolysis
       • ARG1 gene
         • located on 6q23
           • Mutations
             • defective ARG1 activity
               • autosomal recessive disorder
                 • phenotypic variability
                   • usually without acute onset hyperammonemia
                   • often develop
                     • developmental delay
                     • progressive spasticity
                     • seizures
                     • hepatomegaly
                     • loss of intellectual attainments
                 • Lab findings
                   • plasma concentrations
                     • high arginine
                       • also in CSF
                     • moderate elevation of urinary orotic acid
                     • ammonia
                       • slightly elevated or normal
                   • urine
                     • normal or high
                       • arginine
                       • lysine
                       • cystine
                       • ornithine
                     • very elevated
                       • Urinary guanidino compounds
                         • α keto guanidino valerate
                         • alginate
   • ORNT1 mitochondrial ornithine transporter
     • encoded by SLC25A15
       • located on 13q1
         • transport
           • ornithine
             • from cytosol
               • to mitochondrial matrix
           • citrulline
             • from mitochondrion
               • into cytosol
         • Mutations
           • ornithine translocase deficiency
             • also called
               • hyperornithinemia
               • hyperammonemia
               • homocitrullinuria syndrome
                 • Homocitrulline
                   • originate from carbamylation of lysine
               • HHH syndrome
             • high plasma ornithine
             • clinical symptoms
               • most affected patients
                 • hyperammonemia
                   • accompanied by
                     • coagulation disorders
                     • signs of spasticity
                     • vomiting
                     • lethargy
                     • coma
                     • abnormal growth
                     • affected intellectual development

3. Citrulline CIT

   • amino acid
     • glutamine metabolism end product
       • Glutamine
         • ornithine
           • CIT
             • by intestine
     • metabolite of arginine ARG
       • small net CIT liver uptake
         • enters systemic circulation
           • transformed in kidney to ARG
             • ARG impacts hepatic enzymes
               • 5 fold increase in ureagenesis
                 • effects regulated by CIT
     • not incorporated into proteins
     • almost absent in food
   • pharmaconutrient
     • supplement
       • restore ARG metabolism
         • NO synthase (NOS) enzymes convert ARG into CIT
           • NO production
   • Administration
     • safe for oral & IV use
       • no identified toxicity
       • no side effects
       • Treatment
         • children
           • UCDs
           • congenital cardiac defects
           • possibly paediatric pulmonary hypertension
   • Effects
     • immunity & oxidative stress
       • reduces
         • leukocytes and of neutrophils in circulation
       • scavenger for
         • oxidative lipoproteins
         • ADMA
           • hydrolysed by DDAH
             • L-citrulline
       • induces
         • ARG-derived NO-mediated vasoprotection
           • suppression
             • leukocyte activation
             • cell adhesion
             • endothelial damage
           • CIT malate supplement
             • higher neutrophil nitrite concentrations
             • no oxidative damage
           • polyamines derived from ARG & ornithine might be involved
       • anti-oxidant properties
     • sports performance and recovery
       • Oral supplementation
         • increases plasma ARG
         • imporve exercise performance
           • in young healthy adults under some conditions
             • improvements in
               • reduced the exercise time by 1.5 %
               • immediate concentration after exercise
               • pulmonary O2 uptake (V̇ O2) kinetic
                 • higher tolerance
                 • increases whole work volume
               • grip strength
               • peak & explosive power
               • muscle soreness and fatigue
         • studies used CIT as malate salt
           • Krebs cycle intermediate
             • cellular energy production
     • Anabolic effects
       • CIT-enriched diet or supplementation
         • study
           • higher muscle mass & fat free mass
             • lower fat mass
               • nomal whole body protein synthesis
                 • effects of CIT on nitrogen handling are not
                   • ubiquitous
                   • uniform
         • study
           • CIT co-ingestion with low protein
             • ineffective
               • compared to NEAA
                 • Hyperargininaemia after ingestion
         • study
           • better nitrogen balance at 12 hours
         • study
           • improvements
             • cGMP activity
               • greater muscle protein synthesis
                 • (25% of whole body protein synthesis)
                   • fractional synthesis rate (FSR) higher on CIT than NEAA
                     • higher muscle performance
                   • therefore contribute less than 7% increment in whole body protein synthesis
                     • other study shows lower hepatic protein synthesis in rats
                       • lack of CIT effect at the whole-body level
         • CIT mechanisms of action begin to be understood
           • involvement of mTOR pathway
             • coordinates
               • protein synthesis & mitochondrial functions
                 • modulating
                   • mRNA translation
                   • nuclear encoded mitochondrial proteins synthesis
           • NOS activity
             • necessary for calcium-induced activation of Akt pathway
               • cGMP/PI3K dependent pathway
                 • translation initiation
                   • muscle protein synthesis
           • Nitrite
             • enhance mTOR activity
               • myoblasts proliferation

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