Evidence for nuclear ornithine decarboxylase activity in different brain regions of the male rat

The subcellular distribution of ornithine decarboxylating activity in nucleus caudatus putamen, hippocampus, parietal cerebral cortex, cerebellum and hypothalamus of male rat brain has been investigated. The 7000 g supernatant (cytosolic fraction), the 7000 g sediment and the 700 g sediment (nuclear fraction) were incubated with (1 − ¹⁴C)-labeled ornithine and the ¹⁴CO₂ released was measured. The results demonstrated that 70–75% of the decarboxylating activity was present in the nuclear fraction (700 g sediment), 10% in the 7000 g sediment and 10–20% was found in the cytosol. With more vigorous homogenization (30 strokes instead of 10) an increase in the 7000 g supernatant was obtained. The activity increased linearly with time and amount of tissue added for the 770 g sediment and the 7000 g sediment. A dose-dependent inhibition was found in the whole brain in nuclear and cytosolic fractions with α-difluoromethylornithine. In all brain areas the nuclear decarboxylating activity was inhibited to 90% with 2.5 mM of α-difluoromethylornithine except in the hypothalamus, where the inhibition amounted to 20%. An equimolar formation of ¹⁴CO₂ and putrescine was found in the nuclear fraction of all brain regions except the nucleus caudatus putamen and the cerebral cortex, where ¹⁴CO₂ formation exceeded that of putrescine with about 50% suggesting that part of the putrescine is rapidly converted into higher polyamines. It is concluded that with the exception of hypothalamus the major decarboxylating activity in the above mentioned brain regions is ornithine decarboxylase activity (ODC, EC 4.1.1.17) and that the most prominent subcellular localization of this enzyme is the nucleus.