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

# Arylsulfatase A (ARSA)

Arylsulfatase A removes the sulfate from sulfatide in the lysosome, helped by saposin B. Without it sulfatide destroys myelin: metachromatic leukodystrophy.

Updated 2026-11-01

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## Key facts

- Reaction: sulfatide (cerebroside 3-sulfate) + H₂O → galactosylceramide + sulfate; saposin B lifts the lipid out of the membrane so the enzyme can reach it.
- ARSA gene (chromosome 22), autosomal recessive; more than 110 variants that lower its activity cause metachromatic leukodystrophy (MLD).
- A formylglycine and a Ca²⁺ ion form the catalytic centre; the 1N2K model (human enzyme) caught the sulfated formylglycine intermediate.
- Common pseudodeficiency alleles give low measured activity in healthy people, so low activity alone does not prove MLD.
- Saposin B deficiency gives an MLD-like disease with normal arylsulfatase A activity.

**Also called**
ARSA, Cerebroside-sulfatase

**EC number**
[3.1.6.8](https://www.enzyme-database.org/query.php?ec=3.1.6.8)

**Genes**
ARSA

**Structure (PDB)**
[1N2K](https://www.rcsb.org/structure/1N2K)

## Regulation

- A dimer at neutral pH that assembles into an octamer at the acid pH of the lysosome.
- Reaches the lysosome tagged with mannose 6-phosphate.

## Inhibitors and toxins

- No inhibitor is used in medicine; the laboratory measures its activity with an artificial aryl sulfate substrate, hence the name.

## Clinical notes

- Late-infantile MLD, the most common form, starts in the second year of life with loss of walking, then speech, spasticity and peripheral neuropathy.
- Juvenile and adult forms begin with school or psychiatric problems before motor decline.
- Diagnosis combines low ARSA activity, raised urine sulfatides and ARSA gene testing, which also separates true disease from pseudodeficiency.
- Haematopoietic stem cell transplant and gene therapy with the patient's own corrected stem cells work best before symptoms or very early.

## Related conditions

- [Metachromatic leukodystrophy](https://durgical.com/en/conditions/metachromatic-leukodystrophy/)
- [Krabbe disease (globoid cell leukodystrophy)](https://durgical.com/en/conditions/krabbe-disease/)

## Related

- [Sulfatide](https://durgical.com/en/biochemistry/molecules/sulfatide/)
- [Galactosylceramide (galactocerebroside)](https://durgical.com/en/biochemistry/molecules/galactosylceramide/)
- [Sphingolipids and sphingolipidoses](https://durgical.com/en/biochemistry/topics/sphingolipids-and-sphingolipidoses/)
- [Lysosomal storage diseases](https://durgical.com/en/biochemistry/topics/lysosomal-storage-diseases/)
- [Galactocerebrosidase (GALC)](https://durgical.com/en/biochemistry/enzymes/galactocerebrosidase/)
- [Iduronate 2-sulfatase (I2S)](https://durgical.com/en/biochemistry/enzymes/iduronate-2-sulfatase/)
- [Mannose 6-phosphate tagging (sending enzymes to the lysosome)](https://durgical.com/en/biochemistry/topics/mannose-6-phosphate-tagging/)

## Test yourself

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

## Which activator protein does arylsulfatase A need to break down sulfatide?

Saposin B, which pulls sulfatide out of the membrane; its deficiency mimics metachromatic leukodystrophy with normal enzyme activity.

## Why is low arylsulfatase A activity alone not enough to diagnose MLD?

Pseudodeficiency alleles lower the measured activity in healthy people, so urine sulfatides and gene testing are needed to confirm.

## What does arylsulfatase A turn sulfatide into?

Galactosylceramide plus free sulfate; galactocerebrosidase then removes the galactose.

## Which cell types are damaged by stored sulfatide in MLD?

Oligodendrocytes and Schwann cells, the myelin-forming cells, so both the brain's white matter and peripheral nerves lose myelin.

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

Sources: [ARSA: Arylsulfatase A (human)](https://www.uniprot.org/uniprotkb/P15289/entry) (UniProt, 2026); [ARSA gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/arsa/) (MedlinePlus, US National Library of Medicine, 2026); [Metachromatic leukodystrophy: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/metachromatic-leukodystrophy/) (MedlinePlus, US National Library of Medicine, 2026); [RCSB PDB - 1N2K: Crystal structure of a covalent intermediate of endogenous human arylsulfatase A](https://www.rcsb.org/structure/1N2K) (RCSB Protein Data Bank, 2026); [Glycosphingolipid catabolism](https://reactome.org/PathwayBrowser/#/R-HSA-9840310) (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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