AHCY is an enzyme that catalyzes the reversible hydrolysis of S-adenosylhomocysteine(AdoHcy) to adenosine(Ado) and L-homocysteine(Hcy). AHCY controls the intracellular S-adenosylhomocysteine(SAH) concentration that is crucial for transmethylation reactions. AHCY deficiency causes hypermethioninemia.