S -adenosylmethionine decarboxylase (AdoMetDC; EC 4.1 1.50) catalyzes the conversion of S -adenosylmethionine to S -5′-deoxyadenosyl-(5′)-3-methylthiopropylamine (decarboxylated S-adenosylmethionine) and CO₂ . The enzyme is unusual among decarboxylases in that it does not use pyridoxal phosphate as a cofactor, but rather uses a covalently bound pyruvate formed from an internal serine residue (1 ). The mammalian enzyme is synthesized as a 38 kDa proenzyme, which is cleaved in an apparently autocatalytic processing reaction to form the 7.7 and 30.6 kDa subunits of the mature enzyme, with the pyruvate cofactor covalently attached to the N-terminus of the larger subunit (2 ). The small subunit remains in the native form of the enzyme, which appears to exist as an α₂ β₂ heterotetramer (3 ). In addition, the small subunit has been shown to contain residues essential for catalytic activity (4 ).