Binding of antizyme to mammalian ornithine decarboxylase and the influence of changes in the enzyme protein structure
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Ornithine decarboxylase (ODC; E.C.18.104.22.168), is the rate-limiting enzyme of the polyamine biosynthesis pathway in eukaryotic cells. Its activity responds to a variety of hormones and other stimuli that control cell growth and differentiation. The enzyme is distinguished by having the shortest half-life of any known mammalian enzyme. The mechanism of this rapid in vivo inactivation is still unknown. In this project an attempt has been made to increase our understanding of this complex cellular event. To correlate the structural changes of ODC with the rapid in vivo inactivation of this enzyme, several important physiological factors that influence the conformation of ODC in vitro were examined. It was found that pyridoxal 5'-phosphate is the key factor in ODC dimer formation and to a lesser extent L-ornithine also induced a limited amount of monomer ODC to form dimer. High concentration of salt (NaCl), spermidine, and ³H-DFMO induced ODC monomerization effectively. Triton X-100 had no effect on the ODC monomer/dimer transition. The two charge isoforms of ODC, ODC-I and ODC-II, had the same effciency in dimer formation. It has been suggested by several research groups in this field that antizyme plays an important role in ODC degradation; therefore, its interaction with different forms of ODC was examined. The results showed that 3 antizyme only binds to monomer ODC but not dimer ODC. ³H- DFMO labeled ODC which is mostly in the monomeric state was being recognized by antizyme more readily than native ODC. Antizyme does not appear to contain phosphatase activity which would be responsible for ODC-II to ODC-I conversion. Between the two major charge isoforms of ODC, antizyme was found to bind to ODC-II preferentially.