Adenosine deaminase (ADA) !

Clears adenosine and deoxyadenosine; without it lymphocytes die and a severe combined immunodeficiency appears.

Updated 2026-11-01

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Key facts

Adenosine + H₂O → inosine + NH₃; also 2'-deoxyadenosine → 2'-deoxyinosine.
A zinc enzyme, highly active in lymphocytes.
Its deficiency builds up dATP, which shuts down ribonucleotide reductase: without deoxynucleotides lymphocytes cannot divide.
It causes about 15% of severe combined immunodeficiencies (SCID).

No important allosteric regulation; an extracellular isoform (ADA2) has a different role.

ADA-SCID: severe infections from the first months; treated with stem-cell transplant, enzyme replacement or gene therapy.
Pentostatin inhibits ADA and is used in hairy cell leukaemia; cladribine is an ADA-resistant analogue.
Raised ADA in pleural fluid points to pleural tuberculosis, common in Colombia.
Inherited ADA excess in red cells causes haemolytic anaemia (it depletes ATP).

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Why does ADA deficiency mainly affect lymphocytes?

Accumulated dATP inhibits ribonucleotide reductase; lymphocytes, which proliferate a lot and have high kinase activity, run out of deoxynucleotides.

Which drug inhibits ADA, and in which cancer is it used?

Pentostatin, in hairy cell leukaemia.

What does high ADA in a lymphocytic pleural effusion suggest?

Pleural tuberculosis; ADA is a useful supporting test where TB is common.

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

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