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.

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

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.

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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.

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) (PubChem, NCBI, 2026) · Public domain; Adenosylhomocysteinase (AHCY) - Homo sapiens (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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