1. Urea Cycle & OTC Deficiency
Dolores, Jimenez, Kinomes, Legitimas, Manapat

2. Where does excess Nitrogen go?
Amino portion of amino acids undergoes transamination reactions in breakdown as well as biosynthesis
Excreted in 1 of 3 forms: Ammonia, Urea , Uric acid
Fishes Terrestrial animals Birds

3. Amino acid catabolism
If there are more than enough amino acids for protein synthesis, amino acids can be used as metabolic fuel (cell lining in small intestine uses a.a. as a main source of energy)
Glucogenic AA-yields alpha-ketoglutarate, oxaloacetate, pyruvate
Ketogenic AA-yields acetoacetyl-CoA, acetyl CoA

5. Ammonia (NH3) is a waste product from amino acid catabolism
But excreting ammonia is impractical in humans:
-High concentration can cause alkalosis or the increase of alkalinity in the blood
-Ammonia is highly toxic! (can enter the brain, combine with alpha KG-->glutamate)
Story of Urea Cycle: Toxic Ammonia must become a nontoxic substance that can be excreted

6. enzyme: Glutamate dehydrogenase (mitochondria)
Glutamate is the main supplier of nitrogen to the urea cycle
enzyme: Glutamate dehydrogenase (mitochondria)

7. Urea Cycle

8. Steps

9. Step 1 Rate defining step Investment of 2 ATP Reactants: NH4+ and CO2
Products: Carbamoyl Phosphate
Catalyzed by: Carbamoyl Phosphate Synthetase I (CPS1)
Located: Liver Cell Mitochondria

10. Step 2 Reactants: Carbamoyl phosphate and ornithine
Products: citrulline
catalyzed by: Ornithine transcarbamoylase (OTC)

11. Step 3 Citrulline leaves the mitochondria via citrulline transporter
Reactants: ATP, Citrulline, Aspartate
Products: Argininosuccinate
Catalyzed by: Argininosuccinate synthetase (ASS)

12. Step 4 Argininosuccinate gets broken down Reactants: Argininosuccinate
Products: arginine and fumarate*
Catalyzed by: argininosuccinate lyase (ASL)

13. Step 5 Reactants: Arginine and water Products: Urea and ornithine
Catalyzed by Arginase 1 (ARG1)

14. Regulation and Control
N-acetylglutamate
Arginine

15. Connection with the TCA Cycle

16. Ornithine Transcarbamylase (OTC) Deficiency
PIC: OTC enzyme. it’s a trimer. seen in the last reaction of the proximal phase of the urea cycle

17. INTRODUCTION Most common urea cycle disorder X-linked disorder
Deletion in OTC gene (Xp21.1)
Lack of the enzyme ornithine transcarbamylase
Ornithine transcarbamylase (OTC) deficiency is a rare X-linked defect that is the most common of the urea cycle disorders.
It is inherited in an X-linked recessive manner, meaning males are more commonly affected than females
The gene coding for ornithine transcarbamylase is located on the short arm of the X chromosome at position p21.1
There are no common mutations that cause disease, however % of disease causing mutations are deletions
Ornithine transcarbamylase, found primarily in liver mitochondria, is an early enzyme in the urea cycle.
When ornithine transcarbamylase is missing, carbamyl phosphate cannot combine with ornithine.
As carbamyl phosphate concentration rises in the liver, ammonia is released. When ammonia reaches the CNS via the blood, nervous system degradation leads to the symptoms of the defect

18. PREVALENCE Approximately 1 in 80,000 live births
The estimated incidence of early-onset OTC deficiency is 1 in 80,000 live births. When male late-onset OTC deficiency and variable female expression are included, estimates run as high as 1 in 20,000 live births

19. SYMPTOMS Hyperammonemia
The primary symptom of OTC deficiency is hyperammonemia.
Metabolic condition characterized by elevated levels of ammonia in the blood

20. Early-Onset OTC Deficiency
Lethargy
Anorexia
Failure to thrive
Hypotonia
Vomiting
Disorientation
Seizures
Somnolence
In early-onset OTC deficiency, as the excess ammonia reaches the central nervous system, lethargy (abnormal drowsiness/state of being lazy,sluggish), anorexia (loss of appetite), and a general failure to thrive (subsequent growth delay and cognitive deficiencies) are often the first readily apparent symptoms.
These are followed by hypotonia (decrease muscle tone), vomiting, disorientation, seizures, somnolence (sleeping more than often), coma, and finally death.
In those that survive, mental retardation is common.
Many of these same symptoms, sometimes less severe, are seen in late-onset OTC deficiency

21. What exactly happens? This is the step where OTC is so crucial.
PIC:

22. What exactly happens?
LACK OF OTC = Ornithine and carbamoyl phosphate do not get turned into citrulline
But doesn’t it just lead to carbamoyl phosphate and ornithine accumulation?
PIC:

23. What exactly happens?
How does this lead to ammonia build-up, lethargy, coma etc?
PIC:

24. Hyperammonemia Substrate build-up due to lack of OTC
Build-up goes all the way back to NH4⁺
Hyperammonemia is common in most urea cycle disorders. Basically, the deficiency in enzymes leads to increased respective substrates, continuing all the way back to NH4+. The rate of the rxn is decreased + build-up causes problems
When the capacity of the liver to metabolize ammonia is overcome, either because ammonia production exceeds the metabolic capacity of the liver or because the liver is unable to metabolize ammonia, elimination is dependent on the kidney, muscle, and brain. In the setting of hyperammonemia, the kidney decreases ammonia production and increases urinary excretion of ammonia.4,10 The muscle4–6,11–12 and brain metabolize excess ammonia to glutamine.1,11–13 The process of metabolizing ammonia to glutamine is physiologically costly, particularly in the brain where the symbiotic relationship between neurons and astrocytes is disrupted by excess glutamine production.
PIC:

25. Lethargy, Confusion, Vomiting
Elevated NH4⁺ leads to brain complications affecting movement, behavior, appetite
short-circuits potassium transport in glial cells leading to swelling
drives glutamine synthase rxn (glutamate → glutamine) hence depleting glu (precursor of GABA)
high ammonia + low glu reverses glutamate dehydrogenase rxn (glutamate → a-ketoglutarate). low a-ketoglutarate inhibits Krebs cycle thus less energy
early symptoms.
Ammonia is especially toxic to CNS. How, no one is sure. Some theories above.
addtional:
Ammonia crosses the blood–brain barrier through passive diffusion and cation channels. It affects multiple neurotransmitters, specifically the excitatory glutaminergic NMDA receptors and modulation of GABA receptors. Ultimately, neuronal apoptosis occurs.10Hyperammonemia alone appears sufficient to cause brain edema in vivo and astrocytic swelling in vitro, increasing intracranial pressure via an increase in cerebral blood volume.11

26. EARLY ONSET
LATE ONSET
Rapid decline (~24 hours) in health if OTC deficiency is not treated in time
Intellectual disabilities, Coma, Brain death, Death
Hyperammonemia induced by stress or large protein intake
Developmental delays, Protein aversion
Later symptoms include these

27. Without efficient urea cycle, ammonia disposal is left to muscle, kidney, brain. Again, especially taxing on brain.
PIC:

28. Diagnosis & Treatment
PIC:

29. Diagnosis Demonstration of hyperammonemia
Very low blood urea nitrogen (BUN) level
Normal liver and kidney function in most cases, unless hypoxia or shock supervenes
Elevated ornithine, glutamine, and alanine levels; low citrulline level
Elevated urinary orotic acid level

30. TREATMENT & MEDICATION
Hemodialysis
Dietary Management
- regulated protein, carbohydrates, and fat intake
- supplements (Cyclinex, EAA)
- calorie modules (Prophree, Polycose)
- antioxidants
hemodialysis - fastest method for lowering ammonia level
too much dietary protein - excessive ammonia production
too restricted proteins/insufficient calories - body will catabolize lean muscle mass (catabolism creates excessive ammonia)
supplementation wtih special amino acid formulas such as Cyclinex and EAA. can provide ~50% daily dietary protein allowance
UCD people often lack appetite
prescription of calorie modules such as Prophpree, Polycose to be used in combination with AA formulas
antioxidants - suggested to be helpful in minimizing free radical damage to cells in tissue and brain

31. TREATMENT & MEDICATION
Drug Treatment
Buphenyl
Sodium benzoate
L-citrulline
Antacids
Buphenyl aka sodium phenylbutyrate primary medication being used to treat urea cycle disorder
sodium benzoate is almost same

32. TREATMENT & MEDICATION
Last Resort
Liver Transplant

33. THANK YOU