How is ammonia transported to the liver for urea production?

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Detailed Solution

Ammonia produced in tissues is carried to the liver safely in two main non-toxic forms: glutamine and alanine.

Transport via Glutamine

Formation: In peripheral tissues, the enzyme glutamine synthetase attaches ammonia to the amino acid glutamate, forming glutamine.
Circulation: Glutamine, being water-soluble and non-toxic, travels through the blood to the liver.
Processing: Once in the liver, the enzyme glutaminase breaks glutamine back into glutamate and ammonia. The released ammonia then enters the urea cycle for detoxification.

Transport via the Glucose-Alanine Cycle (Alanine)

Formation: In muscle cells, ammonia is first combined with glutamate.
Transamination: The enzyme alanine transaminase (ALT) transfers the amino group from glutamate to pyruvate, creating alanine (and alpha-ketoglutarate).
Circulation: Alanine is then released into the bloodstream and carried to the liver.
Processing: In the liver, ALT reverses the reaction, converting alanine back into pyruvate and glutamate. The ammonia is released and enters the urea cycle.

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