S-adenosylmethionine (AdoMet, SAM) is an essential metabolite in all cells. AdoMet is a precursor in the synthesis of polyamines. Methionine adenosyltransferases (MAT) catalyse the only known AdoMet biosynthetic reaction from methionine (L-Met) and ATP. In mammalian tissues, three different forms of MAT (MAT I, MAT III and MAT II) have been identified that are the product of two different genes (MAT1A and MAT2A). A third gene, MAT2B has been identified and its protein product is known to associate as a regulatory subunit with catalytic MAT2A (LeGros et al. 2000, Halim et al. 1999).