Urea Cycle and Nitrogen Removal

Study Notes

Urea synthesis involves a process to remove nitrogen from the body
Three ways nitrogen is removed from the body: transamination, oxidative deamination, and the urea cycle

Dietary amino acids are absorbed into the amino acid pool within cells
Amino acids are either synthesized into proteins and non-protein compounds, or used for CO2, H2O, ammonia and energy production via oxidation
Amino acids are transferred back into the pool when proteins are broken down

Transamination: Transaminases transfer the amino group from an amino acid to alpha-ketoglutarate, which then becomes glutamate
Glutamine Synthetase: Glutamate is converted to glutamine by glutamine synthetase

Ammonia, present as ammonium ions in body fluids at physiological pH, is toxic to humans
Significant ammonia amounts are generated from the breakdown of dietary proteins and by intestinal bacteria
Excess ammonia is destroyed by the liver, preventing it from entering systemic circulation

The amount of ammonia is higher in the portal vein compared to systemic circulation
Since ammonia is toxic, especially to brain cells, it needs to be quickly removed from tissues

Ammonia intoxication symptoms include tremors, fluttering movements, slurred speech, and blurred vision, potentially leading to coma and death
Hepatic coma due to liver failure is linked to ammonia toxicity
Ammonia is converted into urea, a harmless form, in the liver

Glutamine breakdown releases ammonia, which was previously drawn from the environment
Ammonia levels in the blood are low (10-20 µg/ml), maintained by:
- Glutamine synthesis converting glutamate + NH3 to glutamine
- Conversion of NH3 to urea in the liver

Ammonia is passed into the urine as ammonium ions when excess ammonia in tissues is converted to glutamine via glutamate
Glutamine is then converted back to glutamate in the kidneys
Glutamine acts as a temporary carrier for ammonia

Reactions within the body release ammonia
Glutamine formation is the way to remove ammonia from brain cells, since ammonia is toxic
A small amount of urea synthesis occurs in the brain, but not sufficiently enough to remove all the ammonia
Glutamine synthetase is an important enzyme for brain cells because it removes excess ammonia by converting glutamate to glutamine

Second method for conversion of ammonia is to convert it to urea in the liver by urea synthesis.
Urea synthesis is a general metabolic pathway for amino acids
Ammonia produced from amino acids, CO2 released from metabolism are converted to harmless urea which is then excreted in urine
Urea is synthesized in the liver, transported via blood to the kidneys, and excreted in urine
After 100g of protein intake, 16.5g of nitrogen is excreted as urea
Healthy urea level in blood is 20-40 mg/dL, which equates to 20-25g in 24 hour urine

Since the enzymes for the two urea synthesis are in the mitochondria, the reaction occurs in both the mitochondria and cytoplasm
The equation of the reaction is: NH3 + Aspartate + CO2 + 3ATP → Urea + Fumarate + 2ADP + Pi + H2O
Aspartate brings nitrogen from amino acids for transamination in the urea cycle
One nitrogen molecule in urea comes from aspartate and the other results from removal of nitrogen from the oxidative deamination
The source of the carbon and oxygen in urea is CO2
CO2 produced is removed from the body via respiration and the urea cycle

Aspertate can leave the urea cycle as fumarate and return to the TCA cycle
Aspartate, in the cycle, connects the urea and TCA cycles
Of the amine molecules, it is discovered the first comes from glutamate and the second stems from apartate
For the synthesis of 1 mole of urea, 3 moles of ATP are consumed

First, Carbamoyl phosphate is formed from ammonia and CO2 by carbamoyl phosphate synthetase in the mitochondria.
Second, Carbamoyl phosphate combines with ornithine, the essential component in urea synthesis, to form citrulline.
Citrulline synthesis takes place in the mitochondria, using ornithine from the cytoplasm
Enzyme used to convert ornithine to citrulline is ornithine transcarbomylase in the mitochondria
Third, Arginosuccinate synthesis occurs, requiring ATP, citrulline, aspartic acid, and aspartate; the catalysis for this reaction is arginosuccinic acid synthetase.
Fourth, Argininosuccinase causes fumarate and arginine to form.
For these reasons or kidney failure or a disturbance, urea may be elevated, and clinical findings will point to ammonia intoxication, and if left untreated death results.

N acetylglutamate activates carbamoyl phosphate synthetase I, meaning it is the rate-limiting step of the urea cycle
N acetylglutamate is synthesized from acetyl COA and glutamate, meaning protein rich meals increase the acetylglutamate in the liver
This is because this then provides regulation of the synthesis of both glutamate and N acetylglutamate
Accelerates the synthesis of urea, and stabilizes the nitrogen balance