ODC (ornithine decarboxylase), the first enzyme in the polyamine biosynthesis pathway in mammalian cells, is a labile protein. ODC degradation is stimulated by Az (antizyme), a polyamine-induced protein, which in turn is regulated by an ODC-related protein termed AzI (Az inhibitor). Recently, another ODCp (ODC paralogue) was suggested to function as AzI, on the basis of its ability to increase ODC activity and inhibit Az-stimulated ODC degradation in vitro. We show in the present study that ODCp is indeed capable of negating Az functions, as reflected by its ability to increase ODC activity and polyamine uptake and by its ability to provide growth advantage in stably transfected cells. However, ODCp is less potent than AzI1 in stimulating ODC activity, polyamine uptake and growth rate. The superiority of AzI1 to ODCp in inhibiting the Az-stimulated ODC degradation is also demonstrated using an in vitro degradation assay. We show that the basis for the inferiority of ODCp as an AzI is its lower affinity towards Az (Az1 and Az3). Further, we show here that ODCp, like AzI, is degraded in a ubiquitin-dependent manner, in a reaction that does not require either interaction with Az or the integrity of its C-terminus. Interaction with Az actually stabilizes ODCp by interfering with its ubiquitination. This results in sequestration of Az into a stable complex with ODCp, which is the central feature contributing to the ability of ODCp to function as AzI.