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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# 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).

**EC number**
[3.5.4.4](https://www.enzyme-database.org/query.php?ec=3.5.4.4)

**Genes**
ADA

**Structure (PDB)**
[3IAR](https://www.rcsb.org/structure/3IAR)

## Regulation

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

## Inhibitors and toxins

- Pentostatin (2'-deoxycoformycin)
- EHNA (experimental)

## Clinical notes

- 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).

## Pathways

- [Purine degradation and salvage](https://durgical.com/en/biochemistry/pathways/purine-degradation-and-salvage/)

## Related conditions

- [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/)
- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)
- [Pleural effusion](https://durgical.com/en/conditions/pleural-effusion/)

## Related

- [Ribonucleotide reductase](https://durgical.com/en/biochemistry/enzymes/ribonucleotide-reductase/)
- [Hypoxanthine](https://durgical.com/en/biochemistry/molecules/hypoxanthine/)
- [Drugs acting on nucleotide metabolism](https://durgical.com/en/biochemistry/topics/drugs-acting-on-nucleotide-metabolism/)
- [Nucleotide structure](https://durgical.com/en/biochemistry/topics/nucleotide-structure/)

## Test yourself

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

## 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.

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

Sources: [ADA: Adenosine deaminase (human)](https://www.uniprot.org/uniprotkb/P00813/entry) (UniProt, 2026); [ADA gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/ada/) (MedlinePlus, US National Library of Medicine, 2026); [Adenosine deaminase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/adenosine-deaminase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [3IAR: The crystal structure of human adenosine deaminase](https://www.rcsb.org/structure/3IAR) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/adenosine-deaminase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/adenosine-deaminase/