Multiple species of ornithine decarboxylase in rat tissues: effects of dexamethasone

Multisite phosphorylation of ornithine decarboxylase in transformed macrophages results in increased intracellular enzyme stability and catalytic efficiency

Ornithine decarboxylase (ODC) is the initial inducible enzyme in the polyamine biosynthetic pathway. In the transformed macrophage-derived RAW264 cell line, ODC was overproduced and existed in both unphosphorylated and phosphorylated forms. To date, the only protein kinase known to phosphorylate mammalian ODC is casein kinase II (CKII). ODC was phosphorylated in vitro by CKII and subjected to exhaustive sequential proteolysis with trypsin and V8 protease. Two-dimensional peptide mapping showed only a single phosphopeptide; two-dimensional phosphoamino acid analysis of the phosphopeptide revealed only 32P-labeled serine. ODC was metabolically radiolabeled with 32Pi in RAW264 cells and also subjected to proteolysis, two-dimensional peptide mapping, and phosphoamino acid analysis. Two phosphopeptides were generated from the metabolically radiolabeled ODC, including one that migrated similarly to the peptide phosphorylated by CKII in vitro. Each of the in situ radiolabeled ODC peptides contained both 32P-labeled serine and threonine residues. Thus, in RAW264 cells, ODC is phosphorylated on at least one serine residue in addition to that phosphorylated by CKII and on at least two threonine residues. Phosphorylated ODC had an increased stability to intracellular proteolysis compared with unphosphorylated ODC, their half-lives being 49.2 +/- 3.78 and 23.9 +/- 2.6 min (p = 0.001), respectively. The phosphorylated and unphosphorylated forms of ODC were independently purified to homogeneity. Kinetic analysis revealed that the catalytic efficiency of the phosphorylated form of ODC was 50% greater than that of the unphosphorylated form; the unphosphorylated ODC had a Vmax of 20.54 +/- 1.65 micromol/min/mg, whereas the phosphorylated form had a Vmax of 30.61 +/- 2.6 micromol/min/mg (p = 0.005). Phosphorylation of ODC by CKII has no effect on enzyme activity. Taken together, these findings demonstrate that regulation of ODC activity is governed by as yet unidentified protein kinases.

Inhibition of hormone-stimulated ornithine decarboxylase activity by lithium chloride

Effects of Li+ on hormone-stimulated ornithine decarboxylase (ODC) activity were determined in kidney and liver of rats treated with dexamethasone or prolactin (PRL) and also in cultured, PRL-stimulated Nb2 lymphoma cells. In both systems, LiCl led to rapid and marked decreases in ODC activity. The inhibitory effect of Li+ in exponentially growing Nb2 lymphoma cell cultures, measured at 45 min, was dose-dependent, ranging from 10% at 0.1 mM LiCl to 95% at 10 mM LiCl. Surprisingly, on continued incubation with 10 mM LiCl, the lymphoma cells partially overcame the inhibition, showing ODC activities which reached a maximal value of ca 50% of the control at 4.5 h. The inhibition by Li+ could not be reduced by adding myo-inositol to the culture medium. LiCl did not inhibit ODC activity when added to cell-free extracts of rat tissues and Nb2 lymphoma cells indicating it did not act directly on the enzyme; however, there is evidence that, in intact cells, Li+ enhances the rate of inactivation of the enzyme.

Changes in antizyme-ornithine decarboxylase complexes in tissues of hormone-treated rats

The presence of antizyme-ornithine decarboxylase complex in thymus and kidney of rats was demonstrated using the method of Y Murakami et al. [(1985) Biochem. J. 225, 689-697]. A very small amount of complex was found in kidney of control rats, accounting for only 1-3% of total enzyme in the tissue, while in thymus, approximately one-third of the total ornithine decarboxylase in thymus occurred as an antizyme-enzyme complex. After treatment with dexamethasone, both free ornithine decarboxylase and antizyme-ornithine decarboxylase decreased in thymus, the free enzyme activity decreasing more rapidly. In kidney, the concentration of the antizyme-ornithine decarboxylase complex increased after dexamethasone treatment, but only after the induction of free enzyme activity had reached its peak and begun to decrease. The pattern of the changes in amount of antizyme-ornithine decarboxylase complex after prolactin treatment differed from those observed in the dexamethasone-treated animals. In both kidney and thymus, the concentration of antizyme-ornithine decarboxylase complex increased concurrently with the induction of free enzyme activity. Both free and complexed ornithine decarboxylase had increased at 2.5 h after prolactin treatment and continued to increase to maximum specific activities at similar rates. In thymus, the amount of ornithine decarboxylase present as a complex reached 70% of the total in the tissue. In both thymus and kidney, the concentration of antizyme-ornithine decarboxylase complex decreased more slowly than did free enzyme activity. Free antizyme was observed only in thymus of dexamethasone-treated animals. The amount of measurable inhibitor was decreased if cycloheximide was given with dexamethasone.

Chloroform-induced multiple forms of ornithine decarboxylase: differential sensitivity of forms to enhancement by diethyl maleate and inhibition by ODC-antizyme

The role of glutathione (GSH) and ornithine decarboxylase-antizyme (ODC-AZ) in the regulation of the chloroform-mediated stimulation of rat hepatic ornithine decarboxylase (ODC) was investigated. We have previously implicated roles for each while examining the chloroform effect on crude cytosolic enzyme preparations. In this study we examined the effect of pretreatment with diethyl maleate (DEM), a GSH-depleting agent, on the chloroform stimulation of the two forms of the rat hepatic ODC enzyme and the sensitivity of these two forms to inhibition by the ODC-AZ. While the pretreatment with DEM provided a greater amount of the two forms of the ODC enzyme, it also resulted in a differential stimulation of each form when compared to chloroform alone. Additionally, Peak II was 20-25% more sensitive to the same amount of ODC-AZ then Peak I ODC activity.

Synergistic induction of rat hepatic ornithine decarboxylase by multiple doses of cobalt chloride

The level of rat hepatic ornithine decarboxylase (ODC) induced by repetitive administration of Co2+ was determined by affinity labeling with [3H]difluoromethylornithine. Such a treatment with Co2+ ion induced ODC level to a 10-fold greater extent than single dose of the metal ion or well-known inducers of the enzyme, such as thioacetamide or carbon tetrachloride. The half life of ODC activity induced by repetitive treatment with Co2+ (95 min) was substantially increased to about 10-fold over the value obtained from the enzyme induced by single treatment with the metal ion (10 min). ODC activity induced by repetitive treatment with Co2+ was separated into two peaks by DEAE-Sepharose column chromatography. The two independently collected fractions of ODC peaks exhibited different affinity for pyridoxal 5'-phosphate in vitro and sensitivity to cycloheximide in vivo.

Multiple ionic forms of ornithine decarboxylase differ in degree of phosphorylation

Two major ionic forms of ornithine decarboxylase were separated by column chromatography of extracts of kidneys from androgen-treated male CD-1 mice on DEAE-Sepharose CL-6B, and purified individually to apparent homogeneity. On SDS-PAGE, a single major protein band of Mr 50000 was present in each. When incubated with casein kinase II, purified from rat liver cytosol, only one form of the enzyme, which represented 20% of the total ornithine decarboxylase in the tissue, became phosphorylated. The major form, which was eluted later from the column, could be phosphorylated only after treatment with alkaline phosphatase, indicating that the phosphatase removed enzyme-bound phosphate already attached at the casein kinase II phosphorylation site. Evidence for the occurrence of a phosphorylated form of the enzyme in kidneys of dexamethasone-treated rats is also presented.

Interrelation between the charge isoforms of mammalian ornithine decarboxylase

Ornithine decarboxylase (ODC) isolated from a variety of tissues has been separated, using DEAE ion-exchange chromatography, into multiple peaks of activity that appear to be related to control of this enzyme stability. Reports of these charge isoforms in current literature are generally unclear as to whether these represent a covalent posttranslational modification or merely an alteration in structural conformation or association. In this study we investigated the relationship of this form separation to the degree of enzyme polymerization, interaction with other proteins and buffer components, and the multiple isoelectric forms of this enzyme noted in denaturing concentrations of urea. High-performance chromatography techniques were used to demonstrate that two of the major enzyme forms, ODC I and II, are really monomers of the enzyme, while minor peaks of activity frequently observed to elute after ODC II contain various dimeric enzyme states. Pyridoxal 5'-phosphate (0.05 mM) added to isolated enzyme preparations composed of I and II monomers induced the formation of I and II dimers as well as a mixed I-II dimer. All three dimer forms were observed to be natural components of freshly isolated crude cell homogenates. The charge distinction between the monomer forms I and II was found to be maintained during ion-exchange chromatography in the presence of 8 M urea, and the enzyme isoforms demonstrated distinct bands on isoelectric focusing gels run in the presence of 9 M urea. Thus, although some of the multiple ornithine decarboxylase forms identified by ion-exchange chromatography of crude mammalian cell homogenates are related to enzyme conformation, the two major forms are distinctly charged protein states that can be visualized using two-dimensional gel electrophoresis of highly purified samples.

A rat monoclonal antibody which interacts with mammalian ornithine decarboxylase at an epitope involved in phosphorylation

Ornithine decarboxylase was purified from androgen-treated mouse kidney to homogeneity and high specific activity. The purified enzyme was utilized for production and screening of rat monoclonal and polyclonal antibodies. A rat monoclonal antibody was isolated which was capable of immunoprecipitation of native mouse kidney ornithine decarboxylase activity or the [3H]difluoromethylornithine-inactivated enzyme. Phosphorylation of mouse ornithine decarboxylase by casein kinase-II prior to immunoprecipitation led to complete loss of the epitope recognized by the monoclonal antibody but did not alter recognition by polyclonal antibody. Mammalian ornithine decarboxylase activity obtained from several species, in crude or partially purified extracts, was subjected to quantitative immunoprecipitation with monoclonal and polyclonal antibody. Polyclonal antibody immunoprecipitated all of the ornithine decarboxylase activity from every extract tested, while monoclonal antibody was capable of only limited immunoprecipitation (60-80%). Due to the inability of the monoclonal antibody to recognize ornithine decarboxylase phosphorylated in vitro by casein kinase-II and the partial immunoprecipitation of ornithine decarboxylase activity from cell extracts, a portion of the ornithine decarboxylase molecule population must exist in a phosphorylated state. This immunological evidence further confirms existing data that the enzyme exists in at least two distinct forms.

Heart ornithine decarboxylase from control and isoproterenol-treated rats: kinetic properties, multiple forms and subcellular distribution

Heart ornithine decarboxylase (ODC) from isoproterenol treated rats was compared to heart ODC from control rats. Isoproterenol administration did not significantly change Km for ornithine, but it induced a marked increase of Vmax, X Km for pyridoxal phosphate (PLP) was somewhat reduced. Both Kornm and Vmax were a function of the dithiothreitol (DTT) concentration, in a similar way for control and stimulated enzyme. Two ODC forms were detected by ion exchange chromatography in both control and isoproterenol treated hearts. In control heart, ODC specific activity was high in cytosol and nucleoli. Isoproterenol administration induced a remarkable increase of the cytosolic enzyme only.

Ornithine decarboxylase modification and polyamine-stimulated enzyme inactivation in HTC cells

Ornithine decarboxylase isolated from HTC cells was separated into two distinct charged states by salt-gradient elution from DEAE-Sepharose columns. This charge difference between the enzyme forms was maintained in partially purified preparations, but enzyme form II was observed to change to form I in a time-dependent polyamine-stimulated fashion in crude cell homogenates. The enzyme modification that produces this charge diversity between the alternative enzyme states was further investigated for its role in enzyme activity induction, protein stability and rapid turnover. Inhibition of new protein synthesis by cycloheximide resulted in a much more rapid loss of form I enzyme than of form II, suggesting that during normal enzyme turnover the latter enzyme state may be derived from the former. Culture conditions that favour the stabilization of this usually labile enzyme generally induced an increased proportion of the enzyme in the form II charge state. In particular, inhibitors of synthesis of spermidine and spermine induced the stabilization of cellular ornithine decarboxylase and promoted a marked accumulation in form II. Conversely, polyamines added to the cells in culture induced a very rapid loss in both forms of the enzyme, an effect that could not be attributed merely to an inhibition of new enzyme synthesis. It appears that the polyamines, but not putrescine, may be an essential part of the rapid ornithine decarboxylase inactivation process and that they may function in part by stimulating the conversion of the more stable enzyme form II into the less stable enzyme state, form I.

An inhibitor of ornithine decarboxylase in the thymus and spleen of dexamethasone-treated rats

A marked decrease in activity of ornithine decarboxylase in thymus and spleen occurs soon after treatment of rats with a glucocorticoid. In the present study, evidence was obtained that extracts of these tissues prepared 5 h after administration of dexamethasone, when the enzyme activity is very low, contain an inhibitor of ornithine decarboxylase. The inhibitor is also present at 12 h after treatment and, in lesser amount, at 2.5 h, but was not evident at 24 h. The inhibitory activity was destroyed by treatment with heat or with trypsin, and was not lost on dialysis of the extract. Preliminary experiments indicate that the Mr of the inhibitor is greater than 50 000, which differentiates it from antizyme, an inhibitor of ornithine decarboxylase found in several other cell types. The inhibitor seems to act by a non-catalytic and non-competitive mechanism. The inhibition is dependent on the amount of inhibitor and does not change with time. Since inhibition is not changed by dialysis of the inhibitory extract, its activity apparently does not require small-Mr substances. This differentiates it from inhibitors which inactivate ornithine decarboxylase by covalent modification, such as the polyamine-dependent protein kinase or transglutaminase. The formation of this inhibitor is an early event in lymphoid tissues in response to dexamethasone and may be important in causing the inhibition of cell division which precedes the destruction of lymphocytes.

Two forms of ornithine decarboxylase activity in mouse kidney

Renal ornithine decarboxylase (ODC) activity was evaluated in normal female, male, testosterone-treated female and androgen-insensitive Tfm/Y mice for its heat sensitivity and in vivo half-life. ODC activity in normal female kidney consisted of 2 forms which differed in their heat sensitivity at 46 degrees C. Androgens, either endogenous in normal males or administered exogenously to females, induced primarily the heat-sensitive form. Results from mixing experiments indicated that the heat-sensitive form represented a change in the property of the ODC activity rather than a change in cytoplasmic factors. The in vivo half-life of ODC activity was increased slightly in males and short-term androgen-treated females over normal females and was markedly increased by prolonged androgen treatment. In vivo, the androgen-induced, heat-sensitive form decayed faster than did the heat-resistant form. We conclude that androgens have specific effects on both the amount as well as the biochemical properties of ODC activity in mouse kidney.

Induction by chloroform of two forms of ornithine decarboxylase in rat liver. Half-life of isozymes

Ornithine decarboxylase (ODC) in rat liver was separated into two species by DEAE-Sepharose CL-6B column chromatography. The activity of both species of ODC was increased at least 20-fold by chloroform treatment of the rats. The major species, Peak A, contained 65% of the ODC activity and possessed a half-life of 11 min. The second species, Peak B, accounted for 35% of the activity and possessed a half-life of 50 min. The long-lived species of ODC activity, induced in rat liver by chloroform, has not been reported previously and might be related to the prolonged induction of ODC activity by chloroform and to tumor promotion and growth.

Induction by 3, 5, 3' L-triiodothyronine of L-ornithine decarboxylase in rat heart muscle

Injection of 3,5,3' L-triiodothyronine (15 micrograms/100 g) induces a biphasic enhancement of rat heart ornithine decarboxylase (EC. 4.1.17) activity after 4 and 21 hours. This induction is observed after each daily injection, but to a lesser extent. The properties of partially purified basal enzyme and induced enzyme, at 21h, after single injections have been compared. 1) Affinity for ornithine is the same for both enzymes, but affinity for pyridoxal-phosphate is 40-fold higher for the induced one. 2) Thermostability studies suggest that basal and induced enzymes have different conformations. 3) The two enzymes have similar immuno-reactivity. 4) The comparisons of the time-dependent activity curve after injection and of the antigen/activity ratio suggests that triiodothyronine induces the synthesis of new molecules of enzymes and that an inhibition of the enzyme activity also occurs which explains the biphasic induction.