Adenosylhomocysteinase

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Adenosylhomocysteinase (SAH hydrolase) is an enzyme that uses NAD+ to reversibly break down S-adenosylhomocysteine (SAH) into homocysteine and adenosine. The enzyme is highly conserved across organisms and normally consists of about 430–470 amino acids. It works as a tetramer, with each subunit binding one NAD+ cofactor.

Why it matters: SAH is produced whenever cells use methyl groups in chemical reactions. If SAH builds up, it inhibits methyltransferases and slows down important methylation processes. SAH hydrolase therefore helps keep methylation reactions running smoothly by removing SAH.

In humans, the enzyme is produced from the AHCY gene. It may be regulated by changes in its shape (allosteric regulation), allowing its activity to adapt as needed. The activity of SAH hydrolase has been studied in relation to neuroblastoma prognosis, and its function can be compromised by the buildup of adenosine derivatives, which can inhibit the enzyme and lead to higher SAH levels.