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 (PubMed:18062773, PubMed:18508777, PubMed:18973822). 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 (PubMed:16916800, PubMed:24967154). Participates in the morphological integrity of the trans-Golgi network (TGN) and functions as a regulator of intracellular secretory vesicle trafficking (By similarity). Monomer. Interacts with OAZ1, OAZ2 and OAZ3; this interaction disrupts the interaction between the antizyme and ODC1. Does not form a heterodimer with ODC1. Detected as vesicle-like pattern in neurite outgrowths. Localizes to the vesicular compartments of the secretory pathway, predominantly in the trans-Golgi network (TGN). Localizes with vesicle-associated membrane protein VAMP8 in the vicinity of the plasma membrane within serotonin-containing secretory granules (By similarity). 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. Expressed in the medulla and chromaffin cells of the adrenal gland. Expressed in the Langerhans islets of the pancreas. Expressed in the inner part of the seminiferous tubules and in spermatozoa located in the lumen of the epididymis of the testis. Expressed in the cortex, hippocampus and cerebellum of the brain. Expressed in normal and neoplastic mast cells (MC) (at protein level). Expressed in testis, pancreas and brain. Expressed throughout the differentiation process from spermatids to spermatozoa in the inner part of the seminiferous tubules. Expressed in the kidney: expressed in the superficial (Cs) and the deep layer (Cd) of the cortex region and in the outer stripe (OS), inner stripe (IS) and the inner medulla papilla (IM) of the medulla region. 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. Gly-70 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. The human ortholog was initially reported to have ornithine or arginine decarboxylase activities, but it was later found to possess neither of them. Previously reported to be localized in the mitochondrion (PubMed:16916800). However, it was not confirmed by later reports (PubMed:18062773).