Inhibition of hormone-stimulated ornithine decarboxylase activity by lithium chloride

Effects of Li+ on hormone-stimulated ornithine decarboxylase (ODC) activity were determined in kidney and liver of rats treated with dexamethasone or prolactin (PRL) and also in cultured, PRL-stimulated Nb2 lymphoma cells. In both systems, LiCl led to rapid and marked decreases in ODC activity. The inhibitory effect of Li+ in exponentially growing Nb2 lymphoma cell cultures, measured at 45 min, was dose-dependent, ranging from 10% at 0.1 mM LiCl to 95% at 10 mM LiCl. Surprisingly, on continued incubation with 10 mM LiCl, the lymphoma cells partially overcame the inhibition, showing ODC activities which reached a maximal value of ca 50% of the control at 4.5 h. The inhibition by Li+ could not be reduced by adding myo-inositol to the culture medium. LiCl did not inhibit ODC activity when added to cell-free extracts of rat tissues and Nb2 lymphoma cells indicating it did not act directly on the enzyme; however, there is evidence that, in intact cells, Li+ enhances the rate of inactivation of the enzyme.