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 S-nitrosylation (PubMed:10462479, PubMed:11461922). Inhibited by antizymes (AZs) OAZ1, OAZ2 and OAZ3 in response to polyamine levels. AZs inhibit the assembly of the functional homodimer by binding to ODC monomers. Additionally, OAZ1 targets ODC monomers for ubiquitin-independent proteolytic destruction by the 26S proteasome (PubMed:17900240). Inhibited by 1-amino-oxy-3-aminopropane (APA, an isosteric analog of putrescine) (PubMed:17407445). Irreversibly inhibited by alpha-difluoromethylornithine (DFMO) (PubMed:17407445). kcat is 3.3 sec(-1) with L-ornithine as substrate. 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 (Probable). Does not form a heterodimer with AZIN2 (By similarity). Down-regulated in response to enterovirus 71 (EV71) infection (at protein level). S-Nitrosylation inhibits the enzyme. S-Nitrosylated in vitro on 4 cysteine residues. The disease is caused by variants affecting the gene represented in this entry. BABS is due to truncating variants that lead to a gain of function. This phenomenon apparently results from truncation proximal to or involving the C-terminal region of ODC1 protein, distal enough to allow escape from nonsense-mediated decay. A gain of function is corroborated by elevated plasma levels of N-acetylputrescine, with otherwise normal polyamine levels, in affected individuals. Belongs to the Orn/Lys/Arg decarboxylase class-II family. Ornithine decarboxylase entry