Function and Origin of Multiple Ionic Forms of Hormone-Induced Ornithine Decarboxylase

Ornithine decarboxylase (ODC) occurs in liver and kidney of control and hormone-treated rats as two ionic forms, A and B in order of elution, separable by ion-exchange chromatography of cell-free extracts. These studies were designed to investigate the origin of the multiple forms and their role in the hormonal stimulation of ODC activity. The percentage of activity present in A and B was determined in liver (73/27) and kidney (50/ 50) of control animals. The proportion of activity present as form B had increased by approximately 15% at 5 h after treatment with dexamethasone, prolactin, or growth hormone. In tissues of control rats, the half-life of both A and B was short, in the range of 7-11 min. The half-life of form A was unchanged after hormone treatment, but the half-life of B increased 2- to 3-fold in liver and 5- to 15-fold in kidney. When hormone-treated rats received LiCl 0.5-1 h before sacrifice, ODC activity was less than that after hormone treatment alone, the proportion of activity in A and B reverted to that present in control animal tissues, and the half-life of B reverted to the range of 7-10 min. Thus the production or maintenance of the enhanced stability of B is disrupted by lithium. A post-translational modification of A could be responsible for producing the more acidic and more stable B in the hormone-treated rats. A 3- to 5-fold increase in ODC mRNA occurred in kidney of rats treated with dexamethasone or prolactin, and was largely reversed by LiCl. Stimulation of ODC in kidney by growth hormone did not involve a major change in ODC mRNA, nor was the amount affected by LiCl.