ODC activity induction and modulation in mammalian cells
Stofko, Renata E.
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Ornithine decarboxylase (ODC; EC 126.96.36.199) is the rate-limiting enzyme in the polyamine biosynthetic pathway. The enzyme and its products putrescine, spermidine and spermine are found in virtually all cells. Rapid fluctuations occur in the activity of ODC and in the concentration of the polyamines in response to a wide range of stimuli. The enzyme has an extremely short half- life and it is found in very low cellular concentrations. These unusual characteristics have evoked intense interest in further investigation of ODC. ODC exists in two charged states separable by ion- exchange chromatography in a variety of mammalian tissues. The function of these alternate charged states in the regulation of ODC activity is not well understood. In this study, the ODC forms were investigated for their role in enzyme induction and rapid turnover in rat hepatoma cells (HTC), an ODC-overproducing HTC cell line, HMOA, and intact mouse kidney tissue. Form II enzyme was observed to preferentially accumulate upon ODC induction with polyamine biosynthesis inhibitors in cell culture, and with testosterone induction in mouse kidney. Conversely, exogenously added polyamines, spermidine and spermine, not putrescine, induced a very rapid loss of both forms of the enzyme. Form II was found to convert to form I in cell homogenates. By blocking energy production, both loss and conversion of ODC forms were blocked even in the presence of spermidine. It appears that form II is a post- translational modification of form I, and spermidine and spermine may be an essential part of ODC degradation which may involve conversion between these alternate enzyme- charged states. Further, the inactivation and/or modification of these forms may be an energy-requiring process.