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

# Acid sphingomyelinase

Acid sphingomyelinase splits sphingomyelin into ceramide and phosphocholine inside the lysosome. Its absence causes Niemann–Pick types A and B.

Updated 2026-11-01

Loading the 3D model…

## Key facts

- Sphingomyelin + H₂O → ceramide + phosphocholine; a zinc-dependent phosphodiesterase.
- Its saposin domain lets it work on the inner lysosomal membrane without a separate activator.
- Part of it is secreted and makes ceramide on the cell surface during stress.
- Its deficiency causes Niemann–Pick A (infantile neurovisceral) and B (chronic visceral); both are now grouped as acid sphingomyelinase deficiency.

**Also called**
ASM, Acid sphingomyelin phosphodiesterase

**EC number**
[3.1.4.12](https://www.enzyme-database.org/query.php?ec=3.1.4.12)

**Genes**
SMPD1

**Structure (PDB)**
[5I85](https://www.rcsb.org/structure/5I85)

## Regulation

- Active at acid pH; lysosomal anionic lipids stimulate it.
- Reaches the lysosome through the mannose 6-phosphate route.

## Inhibitors and toxins

- Phosphocholine (product; the ligand in the 3D model)
- Cationic amphiphilic antidepressants (amitriptyline, desipramine): they detach it from the membrane and speed up its breakdown

## Clinical notes

- Niemann–Pick A: hepatosplenomegaly, regression, cherry-red spot and death in early childhood.
- Niemann–Pick B: hepatosplenomegaly, interstitial lung disease, dyslipidaemia and short stature, with little or no neurological disease.
- Niemann–Pick C is a different disease: cholesterol transport fails (NPC1, NPC2), not this enzyme.

## Related conditions

- [Niemann–Pick disease](https://durgical.com/en/conditions/niemann-pick-disease/)

## Related

- [Sphingomyelin](https://durgical.com/en/biochemistry/molecules/sphingomyelin/)
- [Ceramide](https://durgical.com/en/biochemistry/molecules/ceramide/)
- [Sphingolipids and sphingolipidoses](https://durgical.com/en/biochemistry/topics/sphingolipids-and-sphingolipidoses/)
- [Lysosomal storage diseases](https://durgical.com/en/biochemistry/topics/lysosomal-storage-diseases/)

## Test yourself

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

## Which products does acid sphingomyelinase release?

Ceramide and phosphocholine.

## Which metal does acid sphingomyelinase need?

Zinc: two Zn²⁺ ions in its active site.

## Is Niemann–Pick type C a sphingomyelinase deficiency?

No: it is a defect of cholesterol export from the lysosome (NPC1 or NPC2); sphingomyelinase is normal.

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

Sources: [SMPD1: Sphingomyelin phosphodiesterase (human)](https://www.uniprot.org/uniprotkb/P17405/entry) (UniProt, 2026); [SMPD1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/smpd1/) (MedlinePlus, US National Library of Medicine, 2026); [5I85: aSMase with zinc and phosphocholine](https://www.rcsb.org/structure/5I85) (RCSB Protein Data Bank, 2026); [Niemann-Pick disease: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/niemann-pick-disease/) (MedlinePlus, US National Library of Medicine, 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.

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