Catalyzes the first and rate-limiting step of polyamine biosynthesis that converts ornithine into putrescine, which is the precursor for the polyamines, spermidine and spermine. Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis. H(+) + L-ornithine = CO2 + putrescine Inhibited by antizymes (AZs) in response to polyamine levels. AZs inhibit the assembly of the functional homodimer by binding to ODC monomers and targeting them for ubiquitin-independent proteolytic destruction by the 26S proteasome. Amine and polyamine biosynthesis; putrescine biosynthesis via L-ornithine pathway; putrescine from L-ornithine: step 1/1. Homodimer. Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers. Levels increase in the embryos 1.5-2 hours after fertilization and reach a maximum at 6 hours post-fertilization, then decrease to a very low level in early gastrulating embryos (12 hours post-fertilization). Belongs to the Orn/Lys/Arg decarboxylase class-II family.