Mouse ornithine decarboxylase is phosphorylated by casein kinase-II at a predominant single location (serine 303)

Structural basis of binding and inhibition of ornithine decarboxylase by 1-amino-oxy-3-aminopropane

Ornithine decarboxylase (ODC) is the rate-limiting enzyme for the synthesis of polyamines (PAs). PAs are oncometabolites that are required for proliferation, and pharmaceutical ODC inhibition is pursued for the treatment of hyperproliferative diseases, including cancer and infectious diseases. The most potent ODC inhibitor is 1-amino-oxy-3-aminopropane (APA). A previous crystal structure of an ODC-APA complex indicated that APA non-covalently binds ODC and its cofactor pyridoxal 5-phosphate (PLP) and functions by competing with the ODC substrate ornithine for binding to the catalytic site. We have revisited the mechanism of APA binding and ODC inhibition through a new crystal structure of APA-bound ODC, which we solved at 2.49 Å resolution. The structure unambiguously shows the presence of a covalent oxime between APA and PLP in the catalytic site, which we confirmed in solution by mass spectrometry. The stable oxime makes extensive interactions with ODC but cannot be catabolized, explaining APA's high potency in ODC inhibition. In addition, we solved an ODC/PLP complex structure with citrate bound at the substrate-binding pocket. These two structures provide new structural scaffolds for developing more efficient pharmaceutical ODC inhibitors.

Phosphorylation of pyridoxal 5'-phosphate enzymes: an intriguing and neglected topic

Pyridoxal 5'-phosphate (PLP)-dependent enzymes catalyze a wide range of reactions of amino acids and amines, with the exception of glycogen phosphorylase which exhibits peculiar both substrate preference and chemical mechanism. They represent about 4% of the gene products in eukaryotic cells. Although structure-function investigations regarding these enzymes are copious, their regulation by post-translational modifications is largely unknown. Protein phosphorylation is the most common post-translational modification fundamental in mediating diverse cellular functions. This review aims at summarizing the current knowledge on regulation of PLP enzymes by phosphorylation. Starting from the paradigmatic PLP-dependent glycogen phosphorylase, the first phosphoprotein discovered, we collect data in literature regarding functional phosphorylation events of eleven PLP enzymes belonging to different fold types and discuss the impact of the modification in affecting their activity and localization as well as the implications on the pathogenesis of diseases in which many of these enzymes are involved. The pivotal question is to correlate the structural consequences of phosphorylation among PLP enzymes of different folds with the functional modifications exerted in terms of activity or conformational changes or others. Although the literature shows that the phosphorylation of PLP enzymes plays important roles in mediating diverse cellular functions, our recapitulation of clue findings in the field makes clear that there is still much to be learnt. Besides mass spectrometry-based proteomic analyses, further biochemical and structural studies on purified native proteins are imperative to fully understand and predict how phosphorylation regulates PLP enzymes and to find the relationship between addition of a phosphate moiety and physiological response.

Subcellular localization and phosphorylation of antizyme 2

Antizymes (AZs) are polyamine-induced proteins that negatively regulate cellular polyamine synthesis and uptake. Three antizyme isoforms are conserved among mammals. AZ1 and AZ2 have a broad tissue distribution, while AZ3 is testis specific. Both AZ1 and AZ2 inhibit ornithine decarboxylase (ODC) activity by binding to ODC monomer and target it to the 26S proteasome at least in vivo. Both also inhibit extra-cellular polyamine uptake. Despite their being indistinguishable by these criteria, we show here using enhanced green fluorescent protein (EGFP)-AZ2 fusion protein that in mammalian cells, the subcellular location of AZ2 is mainly in the nucleus, and is different from that of AZ1. The C-terminal part of AZ2 is necessary for the nuclear distribution. Within a few hours, a shift in the distribution of EGFP-AZ2 fusion protein from cytoplasm to the nucleus or from nucleus to cytoplasm is observable in NIH3T3 cells. In addition, we found that in cells a majority of AZ2, but not AZ1, is phosphorylated at Ser-186, likely by protein kinase CK2. There may be a specific function of AZ2 in the nucleus.

Increasing ornithine decarboxylase activity is another way of prolactin preventing methotrexate-induced apoptosis: Crosstalk between ODC and BCL-2

Prolactin has more than 300 separate functions including affecting mammary growth, differentiation, secretion and anti-apoptosis. In the previous studies, prolactin induced Bcl-2 expression to prevent apoptosis and also provoked the activity of ornithine decarboxylase (ODC). Our previous data showed that ODC overexpression upregulates Bcl-2 and prevents tumor necrosis factor alpha (TNF-alpha)- and methotrexate (MTX)-induced apoptosis. Here, we further investigate whether prolactin prevents MTX-induced apoptosis through inducing ODC activity and the relationship between ODC and Bcl-2 upon prolactin stimulation. Prolactin prevented MTX-induced apoptosis in a dose-dependent manner in HL-60 cells. Following prolactin stimulation, ODC enzyme activity also shows an increase in a dose-dependent manner, expressing its maximum level at 3 h, and rapidly declining thereafter. Prolactin-induced ODC activity is completely blocked by a protein kinase C delta (PKCdelta) inhibitor, rottlerin. However, there are no changes in the expressions of ODC mRNA and protein level after prolactin stimulus. It indicates that prolactin may induce ODC activity through the PCKdelta pathway. Besides, Bcl-2 expresses within 1 h of prolactin treatment and this initiating effect of prolactin is not inhibited by alpha-difluoromethylornithine (DFMO). However, Bcl-2 is further enhanced following prolactin stimulation for 4 h and this enhancement is blocked by DFMO. Bcl-2 has no effect on ODC activity and protein levels, but ODC upregulates Bcl-2, which is inhibited by DFMO. Overall, there are two different forms of prolactin effect, it induces Bcl-2 primarily, and following this it stimulates ODC activity. Consequently induced ODC activity further enhances the expression of Bcl-2. The anti-apoptotic effect of prolactin is diminished by DFMO and recovered by putrescine. Obviously, ODC activity is one basis for the anti-apoptotic mechanisms of prolactin. A Bcl-2 inhibitor, HA14-1, together with DFMO, completely blocks the anti-apoptotic effects of prolactin. These results suggest that increasing ODC activity is another way of prolactin preventing MTX-induced apoptosis and that this induction of ODC activity enhances the expression of Bcl-2 strongly enough to bring about the anti-apoptotic function.

A Novel Protein Kinase CK2 Substrate Indicates CK2 Is Not Directly Stimulated by Polyamines in Vivo

The activity of the protein kinase (CK2) is enhanced in vitro by the binding of polyamines to the CK2beta regulatory subunit. The overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, also elevates CK2 kinase activity in primary keratinocytes and tissues of K6/ODC transgenic mice. In an effort to better characterize the mechanisms by which polyamines may affect CK2 in vivo, we constructed a transfectable CK2 substrate cDNA consisting of the enhanced green fluorescence protein appended with a canonical CK2 phosphorylation sequence (EGFP-S). In contrast to unmodified EGFP, the EGFP-S protein was extensively phosphorylated by CK2, and this phosphorylation was stimulated by the polyamine spermine in a dose-dependent manner. The in vivo phosphorylation of EGFP-S was examined in cell lines which inducibly express either wild-type CK2 holoenzyme or a CK2 holoenzyme which contains activating mutations in the polyamine-binding region of its CK2beta regulatory subunit. Neither the overexpression of ODC in either cell line nor the mutation of the CK2beta subunit conferred an increase in CK2 kinase activity as measured by the in vivo phosphorylation of EGFP-S. Rather, our data indicate that polyamines increase total CK2 kinase activity through increases in steady-state levels of both CK2alpha and CK2beta subunits. The overexpression of ODC resulted in a 3-fold increase in steady-state levels of both exogenous and endogenous CK2 transcripts but did not increase the half-life of wild-type or mutated CK2 protein. These data suggest that the regulation of intracellular CK2 by the polyamines may occur through mechanisms distinct from the direct stimulation of CK2 by polyamines in vitro as previously described.

Gene cloning and molecular characterization of lysine decarboxylase from Selenomonas ruminantium delineate its evolutionary relationship to ornithine decarboxylases from eukaryotes

Lysine decarboxylase (LDC; EC 4.1.1.18) from Selenomonas ruminantium comprises two identical monomeric subunits of 43 kDa and has decarboxylating activities toward both L-lysine and L-ornithine with similar K(m) and V(max) values (Y. Takatsuka, M. Onoda, T. Sugiyama, K. Muramoto, T. Tomita, and Y. Kamio, Biosci. Biotechnol. Biochem. 62:1063-1069, 1999). Here, the LDC-encoding gene (ldc) of this bacterium was cloned and characterized. DNA sequencing analysis revealed that the amino acid sequence of S. ruminantium LDC is 35% identical to those of eukaryotic ornithine decarboxylases (ODCs; EC 4.1.1.17), including the mouse, Saccharomyces cerevisiae, Neurospora crassa, Trypanosoma brucei, and Caenorhabditis elegans enzymes. In addition, 26 amino acid residues, K69, D88, E94, D134, R154, K169, H197, D233, G235, G236, G237, F238, E274, G276, R277, Y278, K294, Y323, Y331, D332, C360, D361, D364, G387, Y389, and F397 (mouse ODC numbering), all of which are implicated in the formation of the pyridoxal phosphate-binding domain and the substrate-binding domain and in dimer stabilization with the eukaryotic ODCs, were also conserved in S. ruminantium LDC. Computer analysis of the putative secondary structure of S. ruminantium LDC showed that it is approximately 70% identical to that of mouse ODC. We identified five amino acid residues, A44, G45, V46, P54, and S322, within the LDC catalytic domain that confer decarboxylase activities toward both L-lysine and L-ornithine with a substrate specificity ratio of 0.83 (defined as the k(cat)/K(m) ratio obtained with L-ornithine relative to that obtained with L-lysine). We have succeeded in converting S. ruminantium LDC to form with a substrate specificity ratio of 58 (70 times that of wild-type LDC) by constructing a mutant protein, A44V/G45T/V46P/P54D/S322A. In this study, we also showed that G350 is a crucial residue for stabilization of the dimer in S. ruminantium LDC.

Phosphorylated human keratinocyte ornithine decarboxylase is preferentially associated with insoluble cellular proteins

Ornithine decarboxylase (ODC), the first enzyme in polyamine biosynthesis, is highly regulated by many trophic stimuli, and changes in its levels and organization correlate with cytoskeletal changes in normal human epidermal keratinocytes (NHEK). NHEK ODC exhibits a filamentous perinuclear/nuclear localization that becomes more diffuse under conditions that alter actin architecture. We have thus asked whether ODC colocalizes with a component of the NHEK cytoskeleton. Confocal immunofluorescence showed that ODC distribution in NHEK was primarily perinuclear; upon disruption of the actin cytoskeleton with cytochalasin D, ODC distribution was diffuse. The ODC distribution in untreated NHEK overlapped with that of keratin in the perinuclear but not cytoplasmic area; after treatment with cytochalasin D, overlap between staining for ODC and for keratin was extensive. No significant overlap with actin and minimal overlap with tubulin filament systems were observed. Subcellular fractionation by sequential homogenizations and centrifugations of NHEK lysates or detergent and salt extractions of NHEK in situ revealed that ODC protein and activity were detectable in both soluble and insoluble fractions, with mechanical disruption causing additional solubilization of ODC activity (three- to sevenfold above controls). Fractionation and ODC immunoprecipitation from [(32)P]orthophosphate-labeled NHEK lysates showed that a phosphorylated form of ODC was present in the insoluble fractions. Taken together, these data suggest that two pools of ODC exist in NHEK. The first is the previously described soluble pool, and the second is enriched in phospho-ODC and associated with insoluble cellular material that by immunohistochemistry appears to be organized in conjunction with the keratin cytoskeleton.

Early alterations of polyamine metabolism induced after acute administration of clenbuterol in mouse heart

An acute treatment of mice with clenbuterol, a beta-adrenergic agonist, produced a marked increase of polyamines levels in heart, particularly during the early phase of administration of the drug. A single dose of 1.5 mg/kg caused as much as a 10 fold induction in activity of ornithine decarboxylase (ODC) and 3 to 4 fold increase in levels of putrescine, spermidine and spermine in mouse heart. Maximum changes were observed 3 to 4 hours post-administration of clenbuterol. This treatment did not produce any change in S-adenosylmethionine decarboxylase activity. The induction of cardiac ODC by clenbuterol was also dose dependent with a peak at about 5 micromol/kg. Co-administration of difluoromethylornithine, an irreversible inhibitor of ODC, or propranolol, a nonspecific beta-antagonist, with clenbuterol completely prevented the induction of ODC activity as well as the increase in polyamine levels in heart. However, pretreatment with alprenolol or metoprolol, the specific beta1 and beta2-antagonists, respectively, produced only partial prevention. The cardiac ODC from controls as well as clenbuterol treated mice exhibited similar affinity (Km) for its substrate, ornithine, while maximum enzyme activity (Vmax) was about 14 fold higher in clenbuterol treated mouse heart than in the control. Clenbuterol produced no change in the level of specific ODC mRNA or the protein, but the enzyme from the drug-treated mouse heart was considerably more stable than the control. Pretreatment of mice with either cycloheximide or actinomycin D followed by administration of clenbuterol could not prevent the induction in ODC activity suggesting that de novo biosynthesis of the enzyme protein or ODC mRNA was not responsible for induction of ODC activity. Post-translational changes in ODC may be responsible for an early increase of ODC activity due to clenbuterol treatment.

Structure of mammalian ornithine decarboxylase at 1.6 A resolution: stereochemical implications of PLP-dependent amino acid decarboxylases

BACKGROUND

Pyridoxal-5'-phosphate (PLP) dependent enzymes catalyze a broad range of reactions, resulting in bond cleavage at C alpha, C beta, or C gamma carbons of D and L amino acid substrates. Ornithine decarboxylase (ODC) is a PLP-dependent enzyme that controls a critical step in the biosynthesis of polyamines, small organic polycations whose controlled levels are essential for proper growth. ODC inhibition has applications for the treatment of certain cancers and parasitic ailments such as African sleeping sickness.

RESULTS

The structure of truncated mouse ODC (mODC') was determined by multiple isomorphous replacement methods and refined to 1.6 A resolution. This is the first structure of a Group IV decarboxylase. The monomer contains two domains: an alpha/beta barrel that binds the cofactor, and a second domain consisting mostly of beta structure. Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers. The interactions stabilizing the dimer shed light on its regulation by antizyme. The overall structure and the environment of the cofactor are compared with those of alanine racemase.

CONCLUSIONS

The analysis of the mODC' structure and its comparison with alanine racemase, together with modeling studies of the external aldimine intermediate, provide insight into the stereochemical characteristics of PLP-dependent decarboxylation. The structure comparison reveals stereochemical differences with other PLP-dependent enzymes and the bacterial ODC. These characteristics may be exploited in the design of new inhibitors specific for eukaryotic and bacterial ODCs, and provide the basis for a detailed understanding of the mechanism by which these enzymes regulate reaction specificity.

Apoptosis inhibition and ornithine decarboxylase superinduction as early epigenetic events in morphological transformation of Syrian hamster embryo cells exposed to 2-methoxyacetaldehyde, a metabolite of 2-methoxyethanol

We have conducted a study to determine the carcinogenic potential of ethylene glycol monomethyl ether (EGME), a member of the glycol ether family, as compared to its reactive metabolite 2-methoxy-acetaldehyde (MALD). Since disruption of equilibrium between cell proliferation and cell death is thought to play a key role in multistage carcinogenesis, we investigated, in Syrian hamster embryo (SHE) cells exposed to various doses of EGME and MALD, impairment in apoptosis rate and in ornithine decarboxylase (ODC) metabolism. The activity of this rate-limiting enzyme of polyamine biosynthesis is closely related to cell proliferation and cell transformation. At the end-point, comparative action of the two products on SHE cell morphological transformation frequency was evaluated. One-stage exposure of SHE cells to 2 mM EGME and 200 microM MALD for 5 h did not change basal apoptotic level, whereas 0.16 microM phorbol ester (TPA) decreased it. Using two-stage exposure protocol (1 h xenobiotic followed by 5 h TPA), MALD strongly inhibited apoptosis more than did TPA alone; the parent compound EGME did not have any effect on TPA inhibiting action. Western blotting analysis showed that sequential treatment (MALD/TPA) increased Bcl-2 oncoprotein expression, whereas Bcl-XL and Bax proteins were not changed. The same staged exposure of SHE cells to MALD/TPA strongly induced ODC activity, and the rate was higher than that obtained with TPA alone: this was accompanied by an increase of ODC protein level. This ODC superinduction was not observed with EGME/TPA treatment. In long-term SHE-cell morphological transformation assay, staged exposure to MALD (800 microM or 1 mM for 24 h) followed by TPA applications increased the number of transformed colonies at the seventh day. Such early cooperative events as apoptosis inhibition and ODC superinduction, followed by the increase of SHE-cell transformation frequency, are highly indicative of a carcinogenic potential for the metabolite, MALD.

The 90-kDa ribosomal S6 kinase (pp90rsk) phosphorylates the N-terminal regulatory domain of IkappaBalpha and stimulates its degradation in vitro

Nuclear factor kappaB (NF-kappaB) is a eukaryotic member of the Rel family of transcription factors whose biological activity is post-translationally regulated by its assembly with various ankyrin-rich cytoplasmic inhibitors, including IkappaBalpha. Expression of NF-kappaB in the nucleus occurs after signal-induced phosphorylation, ubiquitination, and proteasome-mediated degradation of IkappaBalpha. The induced proteolysis of IkappaBalpha unmasks the nuclear localization signal within NF-kappaB, allowing its rapid migration into the nucleus, where it activates the transcription of many target genes. At present, the identity of the IkappaBalpha kinase(s) that triggers the first step in IkappaBalpha degradation remains unknown. We have investigated the potential function of the 90-kDa ribosomal S6 kinase, or pp90(rsk), as a signal-inducible IkappaBalpha kinase. pp90(rsk) lies downstream of mitogen-activated protein (MAP) kinase in the well characterized Ras-Raf-MEK-MAP kinase pathway that is induced by various growth factors and phorbol ester. We now show that pp90(rsk), but not pp70(S6K) or MAP kinase, phosphorylates the regulatory N terminus of IkappaBalpha principally on serine 32 and triggers effective IkappaBalpha degradation in vitro. When co-expressed in vivo in COS cells, IkappaBalpha and pp90(rsk) readily assemble into a complex that is immunoprecipitated with antibodies specific for either partner. While phorbol 12-myristate 13-acetate produced rapid activation of pp90(rsk), in vivo, other potent NF-kappaB inducers, including tumor necrosis factor alpha and the Tax transactivator of human T-cell lymphotrophic virus, type I, failed to activate pp90(rsk). These data suggest that more than a single IkappaBalpha kinase exists within the cell and that these IkappaBalpha kinases are differentially activated by different NF-kappaB inducers.

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.

Gly387 of murine ornithine decarboxylase is essential for the formation of stable homodimers

In its active form mammalian ornithine decarboxylase (ODC) is a homodimer composed of two 53-kDa subunits while the monomer retains no enzymic activity. In the present study we demonstrate that Gly387 of mouse ODC plays an important role in enabling dimer formation. Gly387 of mouse ODC, an evolutionary conserved residue, was converted to all possible 19 amino acids using site-directed mutagenesis. With the exception of alanine, all other substitutions of Gly387 completely abolished enzymic activity. Cross-linking analysis and fractionation through a Superose-12 sizing column have demonstrated that mutant subunits are detected only in their monomeric form. These results strongly suggest that the primary lesion of substitution at position 387 of mouse ODC is the inability of mutant subunits to associate with each other to form the active homodimers. In agreement with this conclusion, G387A, the only mutant that retained partial activity, displayed reduced dimerization. The degradation rate of ODC mutants in which Gly387 was substituted by aspartic acid or alanine was enhanced compared to the wild-type enzyme, suggesting that monomers may be more susceptible to degradation.

Superinduction of ornithine decarboxylase by halogenated ribofuranosylbenzimidazoles

1. The effect of dichlororibofuranosylbenzimidazole (DiCl-RB), an inhibitor of hnRNA synthesis and casein kinase-2 activity, on ornithine decarboxylase (ODC) was investigated in a difluoromethylornithine (DFMO) resistant, ODC overproducing cell line. 2. In cells growing in the absence of DFMO, DiCl-RB provoked a marked, but transient increase in ODC activity and immunoreactive ODC content. 3. The ODC response to DiCl-RB was prevented by cycloheximide and was not due to stabilization of the enzyme. 4. The dibromo derivative analogue (DiBr-RB) exerted similar effects on ODC, but was effective at lower concentrations. 5. The halogenated ribofuranosylbenzimidazoles were ineffective in cells growing in the presence of DFMO and containing higher levels of ODC protein.

Structural elements of ornithine decarboxylase required for intracellular degradation and polyamine-dependent regulation

Mammalian ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is rapidly degraded in cells, an attribute important to the regulation of its activity. Mutant and chimeric ODCs were created to determine the structural requirements for two modes of proteolysis. Constitutive degradation requires the carboxy terminus and is independent of intracellular polyamines. Truncation of five or more carboxy-terminal amino acids prevents this mode of degradation, as do several internal deletions within the 37 carboxy-most amino acids that spare the last five residues. Polyamine-dependent degradation of ODC requires a distinct region outside the carboxy terminus. The ODC of a parasite, Trypanosoma brucei, is structurally very similar to mouse ODC but lacks the carboxy-terminal domain; it is not a substrate for either pathway. The regulatory properties of enzymatically active chimeric proteins incorporating regions of the two ODCs support the conclusion that distinct domains of mouse ODC confer constitutive degradation and polyamine-mediated regulation. Mouse ODC contains two PEST regions. The first was not required for either form of degradation; major deletions within the second ablated constitutive degradation. When mouse and T. brucei ODC RNAs were translated in vitro in a reticulocyte lysate system, the effects of polyamine concentration on ODC protein production and activity were similar for the two mRNAs, which contradicts claims that this system accurately reflects the in vivo effects of polyamines on responsive ODCs.

Structural elements of ornithine decarboxylase required for intracellular degradation and polyamine-dependent regulation

Mammalian ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is rapidly degraded in cells, an attribute important to the regulation of its activity. Mutant and chimeric ODCs were created to determine the structural requirements for two modes of proteolysis. Constitutive degradation requires the carboxy terminus and is independent of intracellular polyamines. Truncation of five or more carboxy-terminal amino acids prevents this mode of degradation, as do several internal deletions within the 37 carboxy-most amino acids that spare the last five residues. Polyamine-dependent degradation of ODC requires a distinct region outside the carboxy terminus. The ODC of a parasite, Trypanosoma brucei, is structurally very similar to mouse ODC but lacks the carboxy-terminal domain; it is not a substrate for either pathway. The regulatory properties of enzymatically active chimeric proteins incorporating regions of the two ODCs support the conclusion that distinct domains of mouse ODC confer constitutive degradation and polyamine-mediated regulation. Mouse ODC contains two PEST regions. The first was not required for either form of degradation; major deletions within the second ablated constitutive degradation. When mouse and T. brucei ODC RNAs were translated in vitro in a reticulocyte lysate system, the effects of polyamine concentration on ODC protein production and activity were similar for the two mRNAs, which contradicts claims that this system accurately reflects the in vivo effects of polyamines on responsive ODCs.

Ornithine decarboxylase as an enzyme target for therapy

Interest in ornithine decarboxylase (ODC) and the therapeutic effects of its inhibition with the consequent depletion of polyamine biosynthesis has been widespread since the late 1970s and 1980s. This review covers new information about the properties of ODC, recent findings with ODC inhibitors and a discussion of the mechanism of inactivation of ODC by eflornithine. Recent in vivo therapeutic approaches of ODC inhibition are also discussed including: cancer and cancer chemoprevention; autoimmune diseases; polyamines and the blood-brain barrier, ischemia and hyperplasia; the NMDA receptor and modulation by polyamines; hearing loss; African trypanosomiasis; Pneumocystis carinii pneumonia and Cryptosporidium in AIDS; and other infectious diseases/organisms.