S-Adenosylmethionine decarboxylase is one of a small group of enzymes that use a pyruvoyl residue as a cofactor. Histidine decarboxylase from Lactobacillus 30a, the best studied pyruvoyl-containing enzyme, has an (alpha beta)6 subunit structure with the pyruvoyl moiety linked through an amide bond to the NH2-terminal of the larger alpha subunit (Recsei, P. A., Huynh, Q. K., and Snell, E. E. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 973-977). To examine potential structural analogies between the two enzymes, we have isolated and partially characterized S-adenosylmethionine decarboxylase. The purified enzyme comprises equimolar amounts of two subunits of Mr = 14,000 and 19,000 (by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and has a native molecular weight of 136,000 (by gel filtration). Approximately 4 mol of [methyl-3H] adenosylmethionine are incorporated per mol of enzyme (Mr = 136,000) when the enzyme is inactivated with this substrate and NaCNBH3. These data suggest an (alpha beta)4 structure with 1 pyruvoyl residue for each alpha beta pair. The two subunits have been separated by reversed-phase high performance liquid chromatography after reduction and carboxymethylation. The smaller subunit (beta) has a free amino terminus. The amino terminus of the larger subunit (alpha) appears to be blocked by a pyruvoyl group; this subunit can be sequenced only after this group is converted to an alanyl residue by reduction with sodium cyanoborohydride in the presence of ammonium acetate. This work suggests that S-adenosylmethionine decarboxylase is structurally much more similar to histidine decarboxylase than previously thought.