Ornithine decarboxylase (ODC) antizyme protein that negatively regulates ODC activity and intracellular polyamine biosynthesis and uptake in response to increased intracellular polyamine levels. Binds to ODC monomers, inhibiting the assembly of the functional ODC homodimers. Does not target the ODC monomers for degradation, which allows a protein synthesis-independent restoration of ODC activity. Stabilizes AZIN2 by interfering with its ubiquitination. Involved in the translocation of AZNI2 from ER-Golgi intermediate compartment (ERGIC) to the cytosol. Probably plays a key role in spermatogenesis by regulating the intracellular concentration of polyamines in haploid germ cells.