S-Adenosyl-<small>L</small>-homocysteine (SAH) is the biosynthetic precursor to homocysteine. SAH is formed by the demethylation of S-adenosyl-<small>L</small>-methionine. Adenosylhomocysteinase converts SAH into homocysteine and adenosine.
DNA methyltransferases are inhibited by SAH. Two S-adenosyl-<small>L</small>-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.