Localization of ornithine decarboxylase in mutant CHO cells that overproduce the enzyme. Differences between the intracellular distribution of monospecific ornithine decarboxylase antibodies and radiolabeled alpha-difluoromethylornithine.

The intracellular localization of ornithine decarboxylase (ODC), a key enzyme in polyamine synthesis and cell growth, is a matter of present debate. Using two independent methods of analysis, we have attempted to determine the actual distribution of ODC in a mammalian cell. To overcome the problem of a normally very low cellular ODC content, we have used ODC overproducing mutant CHO cells. These mutant cells exhibit a 10-fold higher ODC activity than do the wild type cells. The localization of ODC protein in exponentially growing cells, was determined by indirect immunofluorescence microscopy (permeabilized whole-cell preparations and 1 micron sections), using a monospecific ODC antibody. The intracellular localization of catalytically active ODC was determined by light and electron microscope autoradiography following pulselabeling of cells with alpha-difluoromethyl(5-3H)ornithine (3H-DFMO) at the time of peak ODC activity. alpha-Difluoromethylornithine (DFMO) is an enzyme-activated irreversible inhibitor of ODC and binds covalently to the active enzyme. The specificity of this reaction in the cell was ascertained by immunoprecipitation of 3H-DFMO-labeled ODC. ODC (as determined by both methods) was present in all the cells of a serum-stimulated monolayer culture. The highest concentration of ODC protein and of catalytically active ODC was observed in the smallest and most rapidly proliferating cells. Polyploid and multinuclear cells always exhibited the lowest concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)