Urea Cycle (Ornithine Cycle) Process

Urea Cycle Overview

• The urea cycle, also known as the ornithine cycle, is a biochemical reaction that produces urea from ammonia in many animals.
• The liver converts nitrogen from amino acids into urea, and the carbon skeletons are converted into glucose (in fasting state) or fatty acid (in fed state).

Ammonia Toxicity

• Ammonia is toxic, with normal levels ranging from 15-40 μM and a maximum of 70 μM.
• Hyperammonemia (> 500 μM) is related to inborn errors of metabolism and liver failure.

Urea Cycle Function

• The urea cycle's primary function is to rid the body of toxic ammonia, allowing amino acids to be used as an energy source.
• The liver is the major site of urea synthesis, with the small intestine also involved.

Urea Cycle Enzymes

• Carbamoyl phosphate synthetase is the rate-limiting enzyme of the urea cycle.
• Synthesis of one mole of carbamoyl phosphate requires 2 moles of ATP.

Urea and Ammonia Measurement

• Blood urea nitrogen (BUN) is measured in the blood, and elevated levels may indicate kidney failure or urinary tract obstruction.
• Normal blood urea levels range from 3.3-7.5 mmol/L.

Nitrogen Balance

• The normal nitrogen balance is a state where the amount of nitrogen ingested is balanced by the amount excreted, resulting in no net change in body nitrogen.
• Negative nitrogen balance occurs when more nitrogen is excreted than ingested, often due to starvation, disease, or injury.

Uric Acid

• Uric acid is synthesized from adenine and guanine in nucleotides, not from ammonia.
• Elevated levels of uric acid may occur in kidney diseases and leukemia, and can cause gout.