Antizyme inhibitor (AZI) protein that positively regulates ornithine decarboxylase (ODC) activity and polyamine uptake. AZI is an enzymatically inactive ODC homolog that counteracts the negative effect of ODC antizymes (AZs) OAZ1, OAZ2 and OAZ3 on ODC activity by competing with ODC for antizyme-binding. Inhibits antizyme-dependent ODC degradation and releases ODC monomers from their inactive complex with antizymes, leading to formation of the catalytically active ODC homodimer and restoring polyamine production. Participates in the morphological integrity of the trans-Golgi network (TGN) and functions as a regulator of intracellular secretory vesicle trafficking. Monomer. Interacts with OAZ1, OAZ2 and OAZ3; this interaction disrupts the interaction between the antizyme and ODC1. Does not form a heterodimer with ODC1. Colocalizes with KDEL receptors in ER-Golgi intermediate compartment (ERGIC). Translocates from the ERGIC structure to the cytoplasm in a antizyme-dependent manner. Localizes with vesicle-associated membrane protein VAMP8 in the vicinity of the plasma membrane within serotonin-containing secretory granules. Detected as vesicle-like pattern in neurite outgrowths. Localizes to the vesicular compartments of the secretory pathway, predominantly in the trans-Golgi network (TGN) (By similarity). Localizes with vesicle-associated membrane protein VAMP8 in the vicinity of the plasma membrane within serotonin-containing secretory granules. Up-regulated in activated mast cells (MC) (at protein level). The N-terminus domain is necessary for its localization to the ER-Golgi intermediate compartment (ERGIC). Ubiquitinated, leading to its proteasomal degradation; a process that is reduced in presence of antizymes. May also be degraded through the lysosomal degradative pathway in a proteasomal-independent manner (By similarity). Arg-68 is present instead of the conserved Lys which would otherwise be the covalent pyridoxal phosphate binding site. Belongs to the Orn/Lys/Arg decarboxylase class-II family. ODC antizyme inhibitor subfamily. Human ortholog was initially reported to have ornithine decarboxylase or arginine decarboxylase activities, but it was later found that the mouse ortholog does not possess neither of them.