Activation of rat brain ornithine decarboxylase by GTP

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Ornithine decarboxylase activity during the development of Anastrepha fraterculus (Diptera, Tephritidae)

Ornithine decarboxylase (ODC) (EC 4.1.1.17) is very important for polyamine biosynthesis, which is required for main biological events. In the present study, ODC activity was measured in samples of Anastrepha fraterculus's egg, larva, pupa body and abdomen, adult body, ovaries, and fat body of young females, and in ovaries of mature flies. The kinetic parameters (Km app and Vmax) for ODC activity were determined for pupa, larva, and young ovary. ODC activity showed fluctuations during A. fraterculus's life development. In its earlier stages, prior to emergence, the egg has high ODC-specific activity probably due to embryogenesis, which is characterized by a high rate of cell division. This enzyme activity is also significantly high in the ovary and fat body of young females possibly related to the increased oogenesis and vitellogenesis. The kinetic parameters (Km app and Vmax) had great variation. Our results using GTP showed that the great variation in kinetic parameters can be accounted for by post-translational modifications.

Expression of a novel human ornithine decarboxylase-like protein in the central nervous system and testes

Ornithine decarboxylase (ODC) is the key enzyme of polyamine synthesis. The physiological activity of ODC is associated with cell proliferation, and high ODC activities are encountered in rapidly growing cancer cells. We have cloned a cDNA for a novel human protein that is 54% identical to ODC and 45% identical to antizyme inhibitor (AZI). mRNA for ODC-paralogue (ODC-p) was found only in the central nervous system and testes, suggesting a role in terminal differentiation rather than cell proliferation. ODC-p occurs at least in eight alternatively spliced forms. In vitro translated ODC-p did not decarboxylate ornithine, whereas, in vivo, one splice variant exerted modest ODC-like activity upon expression in COS-7 cells. ODC-p has a unique mutation in cysteine 360, where this ornithine decarboxylase reaction-directing residue is substituted by a valine. This substitution might lead to an enzymatic reaction that differs from typical ODC activity. ODC-p might also function as a brain- and testis-specific AZI.

Expression of ODC and its regulatory protein antizyme in the adult rat brain

Ornithine decarboxylase and its inhibitor protein, antizyme are key regulators of polyamine biosynthesis. We examined their expression in the adult rat brain using in situ hybridization and immunocytochemistry. Both genes were widely expressed and their expression patterns were mostly overlapping and relatively similar. The levels of antizyme mRNA were always higher than those of ornithine decarboxylase mRNA. The highest expression for both genes was detected in the cerebellar cortex, hippocampus, hypothalamic paraventricular and supraoptic nuclei, locus coeruleus, olfactory bulb, piriform cortex and pontine nuclei. Ornithine decarboxylase and antizyme mRNAs appeared to be localized in the nerve cells. ODC antibody displayed mainly cytoplasmic staining in all brain areas. Antizyme antibody staining was mainly cytoplasmic in the most brain areas, although predominantly nuclear staining was detected in some areas, most notably in the cerebellar cortex, anterior olfactory nucleus and frontal cortex. Our study is the first detailed and comparative analysis of ornithine decarboxylase and antizyme expression in the adult mammalian brain.

Oxidation of polyamines and brain injury

Several amine oxidases are involved in the metabolism of the natural polyamines putrescine, spermidine, and spermine, and play a role in the regulation of intracellular concentrations, and the elimination of these amines. Since the products of the amine oxidase-catalyzed reactions -- hydrogen peroxide and aminoaldehydes -- are cytotoxic, oxidative degradations of the polyamines have been considered as a cause of apoptotic cell death, among other things in brain injury. Since a generally accepted, unambiguous nomenclature for amine oxidases is missing, considerable confusion exists with regard to the polyamine oxidizing enzymes. Consequently the role of the different amine oxidases in physiological and pathological processes is frequently misunderstood. In the present overview the reactions, which are catalyzed by the different polyamine-oxidizing enzymes are summarized, and their potential role in brain damage is discussed.

Crystal structure of human ornithine decarboxylase at 2.1 A resolution: structural insights to antizyme binding

The polyamines spermidine and spermine are ubiquitous and required for cell growth and differentiation in eukaryotes. Ornithine decarboxylase (ODC, EC 4.1.1.17) performs the first step in polyamine biosynthesis, the decarboxylation of ornithine to putrescine. Elevated polyamine levels can lead to down-regulation of ODC activity by enhancing the translation of antizyme mRNA, resulting in subsequent binding of antizyme to ODC monomers which targets ODC for proteolysis by the 26S proteasome. The crystal structure of ornithine decarboxylase from human liver has been determined to 2.1 A resolution by molecular replacement using truncated mouse ODC (Delta425-461) as the search model and refined to a crystallographic R-factor of 21.2% and an R-free value of 28.8%. The human ODC model includes several regions that are disordered in the mouse ODC crystal structure, including one of two C-terminal basal degradation elements that have been demonstrated to independently collaborate with antizyme binding to target ODC for degradation by the 26S proteasome. The crystal structure of human ODC suggests that the C terminus, which contains basal degradation elements necessary for antizyme-induced proteolysis, is not buried by the structural core of homodimeric ODC as previously proposed. Analysis of the solvent-accessible surface area, surface electrostatic potential, and the conservation of primary sequence between human ODC and Trypanosoma brucei ODC provides clues to the identity of potential protein-binding-determinants in the putative antizyme binding element in human ODC.

Mutation of aspartate-233 to valine in mouse ornithine decarboxylase reduces enzyme activity

Ornithine decarboxylase is the first and key enzyme in mammalian polyamine biosynthesis. All eukaryotic ornithine decarboxylases contain several highly conserved regions and the amino acid residues 232-238 form one of the most highly conserved sequences. This region contains a glycine-rich sequence typically found in a number of pyridoxal 5'-phosphate-dependent or nucleotide-binding proteins. We mutated aspartate-233 which is the only acidic residue within this region to valine. This mutation causes striking sequence similarity with the guanine nucleotide binding domain of c-H-ras. Mutated ornithine decarboxylase cDNA with a mouse mammary tumor virus long terminal repeat promoter has been transfected for stable expression into ornithine decarboxylase-deficient C55.7 cells. Ornithine decarboxylase activity of the mutated enzyme was about 20% of wild-type ornithine decarboxylase activity and it was not activated by guanosine triphosphate like the ornithine decarboxylase isoform found in some tumors and rat brain. The mutation caused an increase in K(m) value of about 20-fold both for the substrate L-ornithine and for the cofactor pyridoxal 5'-phosphate. The Ki value for the irreversible inhibitor alpha-difluoromethylornithine was also increased, whereas the half-life of the enzyme was shortened. These results suggest that the region containing aspartate-233 is essential for binding of the cofactor and thus forms part of enzymatic active site, and the mutation of aspartate-233 to valine cannot, at least alone, cause the activation of ornithine decarboxylase by guanosine triphosphate (230).

The GTP effector site of ornithine decarboxylase from Lactobacillus 30a: kinetic and structural characterization

A nucleotide effector site of the biodegradative form of ornithine decarboxylase from Lactobacillus 30a (OrnDC L30a) has been identified. OrnDC L30a activity at pH 8.0, where the enzyme is normally inactive, is stimulated by GTP and dGTP and to a lesser extent by GDP but not by ATP, CTP, or UTP. The pH profile indicates that activation by GTP is reflected by an increase in kcat/KM,orn (above pH 6.8), while Vmax remains constant over the pH range 4.0-9. 0. Scatchard plot analysis shows that GTP binds to OrnDC L30a at both pH 5.8 (KD = 0.11 microM) and pH 8.0 (KD = 1.6 microM), but unexpectedly, half-site binding is observed at the higher pH. The OrnDC L30a dodecamer dissociates into dimers at high pH in the presence or absence of GTP. The GTP binding site was located in difference electron density maps using low-resolution X-ray data. This represents a new type of GTP binding site. A model explaining the activation of OrnDC L30a by GTP is presented.

Crystallization of a mammalian ornithine decarboxylase

Crystals of truncated (delta425-461) pyridoxal-5'-phosphate (PLP)-dependent mouse ornithine decarboxylase (mOrnDC') have been obtained that diffract to 2.2 angstroms resolution (P2(1)2(1)2, a = 119.5 angstroms, b = 74.3 angstroms, c = 46.1 angstroms). OrnDC produces putrescine, which is the precursor for the synthesis of polyamines in eukaryotes. Regulation of activity and understanding of the mechanism of action of this enzyme may aid in the development of compounds against cancer. mOrnDC is a member of group IV PLP-dependent decarboxylases, for which there are no known representative structures.

The expression of ornithine decarboxylase antizyme mRNA and protein in rat motoneurons

The distribution of ornithine decarboxylase antizyme messenger ribonucleic acid (AZ mRNA) and AZ-like immunoreactivity (LI) was studied in the brainstem and spinal cord motoneurons and in the extraocular and triceps surae muscles of rat. In situ hybridization showed AZ mRNA in the gray matter of the spinal cord at different levels of spinal cord with highest AZ mRNA levels in the ventral horn of the spinal cord. No apparent changes in AZ mRNA contents were seen after unilateral transection of the sciatic nerve in lumbar motoneurons. AZ-immunoreactive (IR) motoneurons were observed in the nucleus of the VI cranial nerve and in the ventral horn of the spinal cord. These motoneurons also showed ornithine decarboxylase (ODC)-LI. Subcellularly, AZ-LI was observed both in the nuclei and cytoplasm of labeled motoneurons. Heavily stained AZ-IR nerve fibers and myoneural junctions were observed among muscle fibers in different muscles. In addition, the nuclei of muscle fibers showed AZ-LI.

Rapid and regulated degradation of ornithine decarboxylase

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