Nuclear ornithine decarboxylase. Electron microscope autoradiographic identification of the active enzyme using alpha-[5-3H]difluoromethylornithine

alpha-Difluoromethylornithine (DFMO) is an enzyme-activated irreversible inhibitor of ornithine decarboxylase, that forms a covalent bond with the active enzyme. The highly selective binding of tritium-labeled DFMO to ornithine decarboxylase in vivo, as identified by electron microscope autoradiography, was used to determine the intracellular distribution of the enzyme in the germ cells of a polychaete (Ophryotrocha labronica). In mid-oogenesis ornithine decarboxylase was predominantly located in the nurse cells, which are actively supporting growth of the oocytes. On the basis of biochemical analyses ornithine decarboxylase has been considered mainly cytoplasmic in its distribution. However, in metabolically active polychaete cells (oocytes, nurse cells, intestinal and body wall cells), binding sites for tritiated DMFO, indicating the presence of active ornithine decarboxylase, were as abundant in the nucleus. The nucleolus was the most densely labeled organelle in nurse cells and oocytes.