The coding region nucleotide sequences of rat, hamster, and bovine S-adenosylmethionine decarboxylase (AdoMetDC) cDNA exhibit over 90% homology with the human sequence. No N-terminal amino acid could be detected when either bovine or rat AdoMetDC was subjected to Edman degradation, suggesting that the beta-subunit must be blocked since the pyruvate residue is located at the amino terminus of the a-subunit. In this study, we present the primary structure, including post-translational modification, of rat prostate AdoMetDC. Our strategy was to compare the molecular masses of peptides produced by five specific cleavage methods with peptides expected from the known cDNA-derived amino acid sequence of rat AdoMetDC using matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). All AdoMetDC peptide fragments produced by the five cleavage methods could be assigned to theoretical peptides based on the rat cDNA sequence except for the peptides containing the N-terminus of the beta- and alpha-subunits. The N-terminus of the alpha-subunit was assigned as pyruvoyl peptide. Liberation of acetylmethionine was demonstrated when the peptide containing the beta-subunit N-terminal amino acid obtained by lysylendopeptidase digestion was reacted with acylamino acidreleasing enzyme. Furthermore, N-terminal acetylation of the beta-subunit was confirmed by MALDI-post source decay analysis. In conclusion, the results of the present study on amino acid full sequence of rat prostate AdoMetDC determined by the combination of five specific cleavage methods demonstrate that the N-terminus of the beta-subunit is acetylated, and the expected amino acid sequence based on the rat AdoMetDC cDNA sequence is correct.