Ornithine Decarboxylase Antizyme Is a Normal Component of Uninduced H‐35 Cells and Rat Liver

Ornithine decarboxylase antizyme (a non‐competitive protein inhibitor of ornithine decarboxylase induced by the addition of putrescine, spermidine, spermine to a variety of cell lines and rat liver) has been detected in unstimulated H‐35 cells and in normal rat liver. The ornithine decarboxylase antizyme is bound to subcellular components in a nonreactive form from which it can be released as the free antizyme by treatment with high concentrations of salts or low concentrations of diamines or polyamines. Of the latter, putrescine is the most effective; 1 μM putrescine will release free ornithine decarboxylase antizyme from subcellular particles. The molecular weights and general physical properties of the bound ornithine decarboxylase antizyme are the same as those of the induced ornithine decarboxylase antizyme. In H‐35 cells, 80% of the bound form of the ornithine decarboxylase antizyme is localized in the nucleus whereas only 35% of the bound form of the ornithine decarboxylase antizyme is found in the rat liver nucleus. The remaining activity is not associated with ribosomes or polysomes but is localized in a particulate fraction that sediments through a 5–40% sucrose gradient. This bound form of ornithine decarboxylase antizyme also does not react with ornithine decarboxylase unless it is first liberated by polyamines. The presence of ornithine decarboxylase antizyme bound to particles in unstimulated cells and its liberation from the particles by low levels of polyamines as well as its induction in the intact cell by polyamines suggests that it may play a role in modulating ornithine decarboxylase activity in vivo .