Adenosylhomocysteinase/AHCY: ProductsHuman S-Adenosylhomocysteinase (AHCY) is a cytoplasmic tetramer with a tightly bound NAD co-factor for each subunit. It is the only known enzyme to catalyze the breakdown of S-adenosylhomocysteine (AdoHcy) to homocysteine and adenosine. AdoHcy hydrolysis is a reversible reaction with an equilibrium favoring AdoHcy formation, but hydrolysis prevails under physiological conditions due to the rapid removal of adenosine and homocysteine. Thus, AHCY's activity in mammals is directly related to homocysteine level, an independent risk factor for vascular disease. It also functions as a regulator of biological transmethylation by controlling the concentration of AdoHcy, a potent competitive inhibitor of all S-adonosyl-L-methionine methyltransferases. A mutation in the human AHCY results in AHCY deficiency with increase of plasma creatine kinase, methionine, S-adenosylmethionine and AdoHcy, delay of myelination, myopathy and psychomotor retardation.