Biochemistry · Topic

Glucose–alanine cycle !

Muscle sends its spare nitrogen to the liver as alanine; the liver turns it into urea and sends back glucose.

Updated 2026-11-01

Key facts

In muscle, alanine aminotransferase (ALT) moves the amino group from glutamate to pyruvate: alanine forms.
In the liver, ALT returns the amino group to α-ketoglutarate; the nitrogen goes to the urea cycle and the pyruvate to gluconeogenesis.
It carries nitrogen and carbon together, with no free ammonia in the blood.
It dominates in fasting and prolonged exercise, when muscle breaks down protein.

In detail

1. Muscle proteins are broken down; amino acids hand their amino group to α-ketoglutarate, forming glutamate.
2. Muscle ALT transfers that amino group to pyruvate from glycolysis: alanine.
3. Alanine travels in the blood to the liver.
4. Liver ALT turns it back into pyruvate and forms glutamate; glutamate releases ammonia, which enters the urea cycle.
5. The pyruvate goes through gluconeogenesis and the glucose returns to muscle.

Also called: Cahill cycle

Clinical notes

Blood ALT (formerly SGPT) is a sensitive marker of liver damage because it is abundant in hepatocytes.
ALT needs pyridoxal phosphate (B6).
In liver failure nitrogen is not cleared as urea and ammonia rises (hepatic encephalopathy).

Pathways

Test yourself

Think of the answer, then tap the question to see it.

Which enzyme makes alanine from pyruvate in muscle?

Alanine aminotransferase (ALT), with pyridoxal phosphate (B6).

What advantage does the glucose–alanine cycle have over the Cori cycle?

Besides carbon it carries nitrogen to the liver without releasing toxic ammonia into the blood.

What happens to alanine's nitrogen in the liver?

It goes to glutamate and then into the urea cycle; it leaves in the urine as urea.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: GPT: Alanine aminotransferase 1 (human) (UniProt, 2026); Gluconeogenesis (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.