Osmotically-induced modification of ornithine decarboxylase in Physarum

Equilibrium between active and inactive forms of rat liver ornithine decarboxylase mediated by L-ornithine and salts

The mechanisms controlling the activity of ornithine decarboxylase (ODC) are complex and only partly understood. This study shows that ODC can exist as two different aggregation states, that differ in catalytic activity, the dimeric form being active and the monomeric form inactive. While L-ornithine shifts the association-dissociation equilibrium to the dimeric form, salts produce an opposite effect leading to subunit dissociation. alpha-DFMO, an enzyme-activated irreversible inhibitor of ODC, does not react with the monomeric form and therefore the influence of substrate and salts on the aggregation equilibrium must be taken into account in titration experiments with alpha-DFMO of the total amount of ODC in tissue preparations.

Polyamine-stimulated alteration of the ornithine decarboxylase molecule in Physarum polycephalum

The molecular mechanism for polyamine-stimulated feedback modification of ornithine decarboxylase isolated from Physarum polycephalum was investigated by using two-dimensional polyacrylamide-gel electrophoresis. Partially purified A-form enzyme was converted into the B-form enzyme by isolated fractions of the Physarum A-B-converting protein, and the substrates and products were subsequently labelled by covalent addition of alpha-difluoro[14C]methylornithine, an enzyme-activated irreversible inhibitor. The active (A-form) and inactive (B-form) states of this enzyme were found to have the same Mr value, 52 000, yet they differed noticeably in their pI values, 5.45 and 5.65 respectively. In further experiments, the use of high-specific-radioactivity [3H]spermidine to stimulate this enzyme modification was shown not to result in the covalent attachment of this polyamine to ornithine decarboxylase. These results demonstrate that the polyamine-induced modification of ornithine decarboxylase in Physarum is not due to any of the mechanisms previously suggested for ornithine decarboxylase inactivation in this and other eukaryotes, namely phosphorylation, covalent polyamine addition or the non-covalent association of a specific low-Mr protein.

Amine-specificity of the inactivating ornithine decarboxylase modification in Physarum polycephalum

The enzyme catalysing the polyamine-stimulated modification of Physarum ornithine decarboxylase in vivo was partially purified and its activity on purified ornithine decarboxylase was examined with respect to its specificity for various amines. Spermidine, spermine and several polyamine analogues strongly promoted this reaction in vitro (apparent Km in the 0.1--0.5 mM range), whereas putrescine (apparent Km 5.33 mM) and several related diamines were not nearly as effective. In agreement with this, sensitivity studies performed in vivo also suggested that cellular spermidine, and not putrescine, is critical in modulating ornithine decarboxylase activity by this post-translational control. Unlike putrescine, or other diamines, 1,3-diaminopropane demonstrated a functional similarity to the polyamines in stimulating this reaction. This study has demonstrated a method whereby non-physiological amines capable of depressing ornithine decarboxylase activity by this natural feedback mechanism can be readily identified for further evaluation of their potential use in the experimental and medical control of polyamine biosynthesis.

Osmotically induced changes in the ornithine decarboxylase activity of Dictyostelium discoideum

Myxamoebae of Dictyostelium discoideum from exponentially growing cultures showed altered ornithine decarboxylase activity upon external osmotic perturbation. On transfer to hypotonic NaCl solutions (20 mosmol/kg), cells showed high enzyme activity which was relatively independent of the concentration of the coenzyme pyridoxal phosphate (assay concentrations, 5 and 200 microM). In hypertonic solution (400 mosmol/kg) cells had a reduced level of ornithine decarboxylase activity which was dependent on the coenzyme concentration. The changes in activity were freely reversible in further external osmotic manipulation. The response to osmotic change occurred rapidly, within a few minutes. The changes still occurred at 7 degrees C but 2 mM sodium azide prevented the formation of the high activity form, although this effect was reversed when azide was removed. Cycloheximide had no effect on the osmotically induced changes. Addition of putrescine caused ornithine decarboxylase eventually to the converted to the low-activity form regardless of the osmolality of the solution. The characteristic cofactor concentration dependence of the high- and low-activity form were retained on storage of the cell extracts. No evidence was found for diffusible effectors which stabilized one or the other form of the activity. The enzymes responsible for the two forms were of the same molecular size as judged by gel filtration, and the activities had similar thermostabilities. The results are interpreted in terms of an osmotically induced interconversion of two forms of a single ornithine decarboxylase.

Protein factor which induces conversion between Physarum ornithine decarboxylase forms in vitro

The rapid activity modulation of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) in Physarum polycephalum is closely correlated with a reversible post-translational modification of this enzyme. A factor has now been isolated from homogenates of exponentially-grown microplasmodia which catalyzes the conversion of the active enzyme form, A, into its less active, B state. Partial purification of this A-B converting factor has been achieved using DEAE-Sephacel chromatography and Ultrogel AcA-34 gel filtration. It appears to be a heat labile, acidic protein with a molecular weight of about 35 000 which binds to large macromolecules in crude fractions isolated using low ionic strength buffers. The in vitro converting reaction requires the presence of spermidine or spermine (1 mM) while putrescine is much less effective, and inorganic cations are ineffective at levels up to 5 mM. Enzyme conversion is reduced in elevated ionic strengths and in the presence of polyamine chelators such as ATP, ADP or GTP (1.0 mM). Under current assay conditions the interaction between ornithine decarboxylase and this factor is stoichiometric, yet it is not reversed even by conditions which favor dissociation of protein-protein interactions. This is the first report of the isolation of a protein factor which is involved in the interconversion of ornithine decarboxylase between its alternate enzyme states.