Ammonia Metabolism and Urea Cycle

Overview of Amino Acid Metabolism

• Amino acid metabolism includes catabolism, specialized products (heme, purines, pyrimidines, creatine), and ammonia formation with the urea cycle.
• The carbon skeletons from amino acids can be glucogenic or ketogenic.

Transport of Nitrogen from Peripheral Tissues

• Ammonia is neurotoxic, so it requires a non-toxic transport form.
• Blood ammonia is elevated in urea cycle defects and severe liver disease.
• Glutamine is a non-toxic transport form; it incorporates NH3 into glutamate and is elevated in urea cycle defects.
• Alanine is another transport form of NH3, coming from muscle.

Glutamine and Alanine

• Glutamine: formed in most tissues (brain) using glutamine synthetase to generate from glutamate
• Glutamine is elevated in urea cycle defects.
• Alanine: synthesized in muscle through transamination from pyruvate involving ALT

Aminotransferase Reaction

• Most amino acids undergo transamination with α-KG as the amino acceptor.
• Amino group transfer: Free ammonia is retained. ALT and AST enzymes are used to generate glutamate
• Amino group from amino acids is transferred to glutamate

Ammonia Formation in Liver

• Glutamate dehydrogenase forms free ammonia and enters the urea cycle.
• Glutaminase reaction generates free ammonia and glutamate.
• In the renal tubule generates NH3 converts to NH4+ (excreted in urine) which is key for acid-base balance and H+ excretion

Ammonia Formation in the Gut and Fate

• Bacterial ureases in the colon produce ammonia.
• Ammonia goes through the portal circulation into the liver to form urea.
• Intestinal ammonia formation is exaggerated when there is liver disease which results in neurotoxicity

Urea Cycle

• Urea cycle takes place in the liver, partially in mitochondria and cytosol
• First N atom from Ammonia and Aspartate donates second N
• The source of nitrogen is glutamate via the glutamate dehydrogenase reaction; aspartate forms from oxaloacetate

Urea Cycle Reactions

• (1) Carbamoyl phosphate synthetase-I (CPS-I): mitochondrial, activated by N-acetyl glutamate (NAG), incorporates free ammonia.
• (2) Ornithine transcarbamoylase (OTC): mitochondrial.
• (3) Argininosuccinate synthetase (ASS): occurs in the cytosol, uses aspartate.
• (4) Argininosuccinate lyase (ASL).
• (5) Arginase (ARG): forms urea.

Hyperammonemia and Urea Cycle Defect

• Newborns may present with vomiting, grunting respiration (hyperventilation), lethargy, and unresponsiveness.
• Metabolic screens show elevated serum ammonia and glutamine.
• Laboratory studies will likely show respiratory alkalosis.

Inherited Disorders of the Urea Cycle

• Enzyme deficiency leads to a buildup up of substrate
• Elevated blood ammonia (hyperammonemia) and glutamine are typically seen
• Decreased urea formation happens in all disorders
• CPS-1 or OTC Deficiency is most severe because it happens in the beginning

Management of Hyperammonemia (All UCD)

• Acute emergency: Dialysis is used
• A low protein/high carbohydrate diet helps lower ammonia levels
• Stress prevention is important
• Administer benzoic acid and / or phenylacetate to provide alternate routes of nitrogen excretion
• Arginine administration and liver transplantation in the long term

Management: Benzoic Acid and Phenylbutyrate

• Phenylbutyrate/phenylacetate + glutamine yields phenylacetylglutamine to be excreted out
• Benzoic acid reacts with glycine to form hippuric acid to be excreted out

Hyperammonemia: Arginine Administration

• Can be used for all UCD except for arginase deficiency
• Increases substrate concentration to boost urea cycle activity
• Increases synthesis of NAG, CPS-1 activator

CPS-1 Deficiency (Type I Hyperammonemia)

• Presents with severly high blood ammonia and glutamine; Low levels of ALL urea cycle intermediates
• Can be treated with L-Arginine supplements which stimulates formation of N-acetylglutamate
• High levels of NAG stimulate the deficient CPS-I

OTC Deficiency (X-Linked)

• This deficiency results in hyperammonemia type II and is a common UCD; Blood glutamine is elevated
• OTC combines ornithine and carbamoyl phosphate together which produces citrulline

Ornithine Transcarbamoylase Deficiency - OTC

• Presents with elevated serum ammonia and glutamine
• Elevated serum and urine orotic acid
• Elevated carbamoyl phosphate causes pyrimidine biosynthesis

Argininosuccinate Synthetase Deficiency (Citrullinemia)

• Diagnosis: Hyperammonemia and high glutamine
• Very high levels of serum and urinary citrulline

Argininosuccinate Lyase Deficiency (Argininosuccinic Aciduria)

• Differential diagnosis for hyperammonemia
• Elevated ammonia and glutamine
• Elevated argininosuccinate and elevated citrulline

Arginase Deficiency (Hyperargininemia)

• Elevated serum ammonia and elevated arginine
• Blood NH3 may not be high
• Exclude essential amino acids diet and arginine

Formation and Fate of Urea

• Dietary proteins generate amino acids which generate ammonia; that is transported to the liver and converted Urea
• The waste product of urea is transported to the kidney for excretion
• In gut degraded and ammonia reenters circulation and the liver detoxifies
• Renal failure leads to elevated BUN levels
• Severe liver disease and Inherited Urea cycle defects leads to elevated blood ammonia.

Acquired Hyperammonemia

• Liver disease is the main reason that is caused by drugs or viral hepatitis
• Blood from porto-systemic shunting sends ammonia into the bloodstream
• Ammonia passes through intestine and directly enters circulation which results in neurotoxicity

Acquired Hyperammonemia Scenario

• Liver cirrhosis from alcoholism occurs due to liver damage
• The liver is unable to process out NH3 in circulation
• Diagnosed with alcohol-induced liver cirrhosis 4 yrs and a history of excessive alcohol use for ~20 yrs
• Examination results include being apathetic, confused, disoriented, slurred speech.

Treatment of Acquired Hyperammonemia: Basis

• Low protein/high carb diet (Intuitive)
• Lactulose is a disaccharide that prevents digestion in the small intestine
• Neomycin antibiotics reduces bacterial urease in the gut that breaks down urea

Ammonia Neurotoxicity

• Hyperammonemia interferes with the balance of neurotransmitters
• Mechanism of ammonia neurotoxicity is complex; can cause impaired Brain ATP metabolism
• Shows widespread loss of cortical sulci and gray-white differentiation with cerebral edema

Mechanism: Glutamate and GABA

• Glutamate [Excitatory] goes to Glutamate Decarboxylase
• The output goes to produces y-aminobutyric acid (GABA) which is [Inhibitory]
• Hyperammonemia can alter the balance of neurotransmitter in the brain

Elevated Blood Ammonia - ATP

• Alpha-Ketoglutarate is then switched to Glutamate which depletes the TCA cycle
• Reduces APT levels
• Increased glutamine levels causes reduced Na+/K+ ATPase activity

Explore the link between elevated blood ammonia and reduced ATP synthesis in the brain. Learn about glutamine synthetase, cerebral edema, vitamin B6, and alanine's role in ammonia transport. Understand urea cycle enzymes and their role in ammonia metabolism.