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

# S-Adenosylhomocysteine (SAH)

SAH is what is left of SAM after it gives away its methyl group. It must be cleared quickly because it blocks methylation.

Updated 2026-11-01

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## Key facts

- Every SAM-dependent methylation produces one SAH.
- SAH inhibits almost all methyltransferases; the SAM/SAH ratio is used as the cell's 'methylation index'.
- SAH hydrolase splits it into homocysteine and adenosine; the equilibrium favours SAH, so homocysteine and adenosine must be removed.

**Also called**
AdoHcy

**Formula**
C₁₄H₂₀N₆O₅S

**PubChem**
[CID 439155](https://pubchem.ncbi.nlm.nih.gov/compound/439155)

## Structure

- Adenosine joined through its 5′ carbon to the sulfur of homocysteine; the sulfur is neutral, not a sulfonium like in SAM.

## Clinical notes

- When homocysteine is high, SAH rises too and methylation of DNA and proteins slows.
- SAH hydrolase deficiency (AHCY) is a very rare cause of myopathy, liver disease and high methionine.

## Pathways

- [Methionine cycle and transsulfuration](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/)
- [Catecholamine synthesis and breakdown](https://durgical.com/en/biochemistry/pathways/catecholamine-synthesis-and-breakdown/)
- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## Related conditions

- [Homocystinuria](https://durgical.com/en/conditions/homocystinuria/)

## Related

- [S-adenosylmethionine (SAM)](https://durgical.com/en/biochemistry/molecules/s-adenosylmethionine/)
- [Homocysteine](https://durgical.com/en/biochemistry/molecules/homocysteine/)
- [Methionine](https://durgical.com/en/biochemistry/molecules/methionine/)
- [One-carbon metabolism (folate, B12 and SAM)](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/)

## Test yourself

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

## Which reaction produces SAH?

Any methyl transfer from SAM to an acceptor such as DNA, a protein or norepinephrine.

## Why must SAH be removed quickly?

It inhibits methyltransferases, so its build-up would stop methylation.

## Which enzyme splits SAH, and into what?

SAH hydrolase, into homocysteine and adenosine.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [Adenosylhomocysteine (CID 439155)](https://pubchem.ncbi.nlm.nih.gov/compound/439155) (PubChem, NCBI, 2026) · Public domain; [Adenosylhomocysteinase (AHCY) - Homo sapiens](https://www.uniprot.org/uniprotkb/P23526/entry) (UniProt, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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