Biochemistry: Nitrogen Metabolism and Enzymes

Urine and Ammonia

• Urine is acidic and traps ammonia in the form of ammonium ions, which are less toxic than ammonia.
• Glutaminase and Asparaginase play a role in nitrogen metabolism.
• Glutaminase removes nitrogen, generating glutamate and glutamine, which act as intracellular nitrogen carriers.
• Asparaginase is typically less critical, but it is an enzyme involved in nitrogen metabolism.
• L and D amino acid oxidases are involved in breaking down amino acids.

L and D Amino Acid Oxidases

• L amino acid oxidase uses FMN (flavine mononucleotide) and produces peroxide as a byproduct.
• D amino acid oxidase utilizes FAD and also produces peroxide.
• The D amino acid oxidase is important for breaking down D amino acids, which are often found in bacterial infections.

The Glucose-Alanine Cycle (Cahill Cycle)

• This cycle helps conserve glucose backbones.
• It involves converting alanine to pyruvate and then pyruvate back to glucose in the liver.
• Glucose is primarily used by the brain and red blood cells.
• It prevents muscle cells from using glucose, as this would lead to fatty acid and ketone body production.

The Importance of Catalase

• Catalase is an enzyme present in peroxisomes.
• It detoxifies peroxide generated by L and D amino acid oxidases.

Ammonia Toxicity

• Ammonia levels exceeding 10 micrograms per liter are toxic to cells.
• Ammonium ions are less toxic, with a toxic level of around half a milligram.
• However, physiological conditions maintain a 100-fold higher concentration of ammonium ions compared to ammonia.

Urea Cycle

• A complete blockage in any step of the urea cycle is fatal.
• The urea cycle detoxifies ammonia in the liver, converting it into urea.
• Urea is then excreted from the body.

Location of Urea Cycle Enzymes

• The first two enzymes, Carbamylphosphate synthetase 1 (CPS1) and Ornithine transcarbamylase, are located in the mitochondrial matrix.
• The remaining three enzymes are located in the cytosol.

Carbamyl Phosphate Synthetase 1 (CPS 1)

• CPS 1 is the initial enzyme in the urea cycle.
• It catalyzes the reaction between bicarbonate and ammonium ions, forming carbamyl phosphate.
• Carbamyl phosphate is an activated form of urea, containing a phosphate group on one end and a nitrogen group on the other.

Ornithine Transcarbamylase

• This enzyme catalyzes the reaction between ornithine and carbamyl phosphate, leading to the formation of citrulline.

Urea Cycle Reactions:

• CPS1: Bicarbonate + Ammonium ion → Carbamyl phosphate
• Ornithine Transcarbamylase: Ornithine + Carbamyl phosphate → Citrulline
• Arginosuccinate Synthetase: Citrulline + Aspartate → Arginosuccinate
• Arginosuccinate Lyase: Arginosuccinate → Arginine
• Arginase: Arginine → Urea

Hyperammonemia

• Hyperammonemia refers to elevated ammonia levels in the blood.
• Deficiencies in urea cycle enzymes can cause hyperammonemia.

Activation of the Urea Cycle

• The urea cycle is activated by a flow of nitrogen.
• N-Acetylglutamate Synthetase regulates this, producing N-acetylglutamate, which activates CPS1.

Urea Precursors

• Aspartate, free ammonium ions, and carbamyl phosphate are the precursors providing nitrogen for the urea cycle.

Hyperammonemia: Urea Cycle

• The urea cycle involves two key enzymes: CPS 1 and OTC.
• CPS 1 is the rate-limiting enzyme, regulated by N-acetylglutamate.

Hyperammonemia: Process

• 1. Ammonium ion + Bicarbonate → Carbamyl phosphate (via CPS 1)
• 2. Carbamyl phosphate → Citrulline (via OTC)
• 3. Citrulline → Argininosuccinate (via Arginosuccinate Synthetase)
• 4. Arginosuccinate → Arginine (via Arginosuccinate Lyase)
• 5. Arginine → Urea (via Arginase)
• 6. Urea → Ornithine (via Arginase)

Hyperammonemia: Diagnosis

• High ammonia levels are a key indicator.
• Decreased BUN (blood urea nitrogen) levels.
• Blood glutamine levels are elevated.

Enzyme Deficiencies

Ornithine Transcarbamylase (OTC) Deficiency

• Hyperammonemia, elevated carbamylphosphate levels, and elevated orotic acid levels (due to stimulated pyrimidine biosynthesis) are characteristic.
• Diagnosis is established through Orotic Acid Urea testing.

Carbamylphosphate Synthetase 1 (CPS 1) Deficiency

• Characterized by hyperammonemia, reduced carbamylphosphate levels, and reduced orotic acid levels.
• Diagnosis is confirmed by the absence of Orotic Acid Urea.

Clinical Signs and Symptoms

• Central edema

• Lethargy

• Convulsions

• Coma

• Death

• Note: Ammonia can pass the blood-brain barrier, leading to central edema.

Hyperammonemia Summary

• Enzyme Deficiency: | Symptoms: | Diagnosis:
• OTC Deficiency: | Hyperammonemia, Elevated Carbamylphosphate, Elevated Orotic Acid | Orotic Acid Urea
• CPS1 Deficiency: | Hyperammonemia, Decreased Carbamylphosphate, Decreased Orotic Acid | No Orotic Acid Urea

This quiz covers key concepts of nitrogen metabolism, including the roles of urine, ammonia, and critical enzymes like glutaminase and asparaginase. Additionally, it delves into L and D amino acid oxidases and the Glucose-Alanine Cycle, highlighting their biological significance and interconnections. Test your understanding of these biochemical processes!