Essential for biosynthesis of the polyamines spermidine and spermine. Promotes maintenance and self-renewal of embryonic stem cells, by maintaining spermine levels. H(+) + S-adenosyl-L-methionine = CO2 + S-adenosyl 3-(methylsulfanyl)propylamine Binds 1 pyruvoyl group covalently per subunit. Amine and polyamine biosynthesis; S-adenosylmethioninamine biosynthesis; S-adenosylmethioninamine from S-adenosyl-L-methionine: step 1/1. First expressed at stage I of oocyte development, and is maximally expressed at stage II. Levels decline during oocyte maturation and after fertilization, and also in the early neurula. Levels increase dramatically during the late neurula stage reaching a maximum at the tail bud stage. Is synthesized initially as an inactive proenzyme. Formation of the active enzyme involves a self-maturation process in which the active site pyruvoyl group is generated from an internal serine residue via an autocatalytic post-translational modification. Two non-identical subunits are generated from the proenzyme in this reaction, and the pyruvate is formed at the N-terminus of the alpha chain, which is derived from the carboxyl end of the proenzyme. The post-translation cleavage follows an unusual pathway, termed non-hydrolytic serinolysis, in which the side chain hydroxyl group of the serine supplies its oxygen atom to form the C-terminus of the beta chain, while the remainder of the serine residue undergoes an oxidative deamination to produce ammonia and the pyruvoyl group blocking the N-terminus of the alpha chain (By similarity). Belongs to the eukaryotic AdoMetDC family. It is uncertain whether Met-1 or Met-3 is the initiator.