AHCY (EC 3.3.1.1) 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. The recombinant human AHCY produced in E.Coli is a single, non-glycosylated polypeptide chain and is purified by proprietary chromatographic techniques.