S-Adenosyl-L-homocysteine

S-Adenosyl- L -homocysteine (SAH) is the biosynthetic precursor to homocysteine. SAH is formed by the demethylation of S-adenosyl- L -methionine. Adenosylhomocysteinase converts SAH into homocysteine and adenosine.

Biological role
DNA methyltransferases are inhibited by SAH. Two S-adenosyl- L -homocysteine cofactor products can bind the active site of DNA methyltransferase 3B and prevent the DNA duplex from binding to the active site, which inhibits DNA methylation.