Pyridoxal 5'-phosphate and the regulation of ornithine decarboxylase activity and stability

There are two forms of ornithine decarboxylase with respect to pyridoxal 5'-phosphate (pyridoxal-P) affinity in exponentially-growing Swiss 3T3 mouse fibroblasts: form I (Km approximately 10 muM) accounts for 30% of the total activity, and form II (Km approximately 0.4 muM) the remainder. Each form of the enzyme is in rapid equilibrium with ornithine and pyridoxal-P; neither form recognizes the Schiff base between ornithine and pyridoxal-P as a substrate. Total pyridoxal-P concentrations indicate that both forms may normally be at least partially active in vivo. Upon stimulation of 3T3 cells by pituitary growth factors, form I becomes undetectable within 4 h. As total activity increases over 10-fold during this time and continues to increase thereafter, a possible conversion of form I to form II could account for this increase only if the Km change reflects other changes in preexisting enzyme. The rates of cofactor dissociation are apparently the same for each form and neither rate changes with the growth state. Since rapid equilibrium kinetics apply, the forms apparently differ in their rate of cofactor association. The half-lives of the two forms in vivo are the same in unstimulated cells when measured concurrently. Also, the half-life of total activity decreases markedly upon stimulation as form II becomes dominant. These and other observations are not consistent with pyridoxal-P serving a major protective function for the enzyme in vivo.     

Inhibition of ornithine decarboxylase potentiates nitric oxide production in LPS-activated J774 cells

We have examined whether modulation of the polyamine biosynthetic pathway, through inhibition by alpha-difluoromethylornithine (DFMO) of the rate limiting enzyme, ornithine decarboxylase (ODC), modulates NO synthesis in J774 macrophages. DFMO potentiated LPS-stimulated nitrite production in both a concentration- and time-dependent manner, increasing nitrite levels by 48+/-5% at 10 mM. This effect was observed in cells pre-treated with DFMO for 24 h prior to stimulation with LPS. Addition of DFMO 12 h after LPS failed to potentiate LPS-induced nitrite production. Supplementation of the culture medium with horse serum (10%) in place of foetal calf serum (10%) caused no significant change in either LPS-induced nitrite production or in the ability of DFMO (10 mM) to potentiate LPS-induced NO synthesis. Metabolism of L-[3H]arginine to L-[3H]citrulline by partially purified inducible nitric oxide synthase (iNOS) was not significantly altered by either DFMO (1-10 mM) or by putrescine (0.001-1 mM), spermidine (0.001-1 mM) or spermine (0.001-1 mM). iNOS activity was also unaffected by 1 mM EGTA but was markedly attenuated (70+/-0.07%) by L-NMMA (100 microM). Pre-incubation of cells with DFMO (10 mM; 24 h) prior to activation with LPS resulted in enhanced (approximately 2 fold) iNOS protein expression. These results show that DFMO potentiates LPS-induced nitrite production in the murine macrophage cell line J774. Since the only known mechanism of action of DFMO is inhibition of ODC, and thus polyamine biosynthesis, we conclude that expression of iNOS can be critically regulated by endogenous polyamines.     

Elevated expression of epidermal ornithine decarboxylase mRNA in scleroderma

Using in situ hybridization techniques, we examined the expression of ornithine decarboxylase (ODC) mRNA in the skin of five patients with systemic sclerosis (SSc) and five normal controls. Sections treated with an anti-sense probe showed concentrated grains exclusively in the epidermis of SSc patients, but not in that of normal controls. Because our subcloned anti-sense probe specifically hybridizes with ODC mRNA, these findings indicate that the expression of ODC mRNA is elevated in SSc epidermis. Possibly polyamines have an important part to play in the skin changes of SSc.     

Fine structure of the ocellus of the cabbage looper moth (Trichoplusia ni)

The dorsal ocelli of adult cabbage looper moths were studied by light microscopy, and scanning and transmission electron microscopy. Each ocellus has a cuticular lens located on the distal end of a cuticular cone which encapsulates the receptor cells. There are two distinct types of receptor cells in the ocellus. Seventy large receptor cells from plate-like rhabdoms with several adjacent cells to produce a rhabdom network in the ocellus. Proximally ninety small receptor cells which have a disorganized microvillar rhabdomere are located at the base of the rhabdoms formed by the large cells. Extensive areas of gap junctions which occur between the rhabdoms and the membranes of large and small cells suggest that the cells may be electrically coupled to one another. Axons from both large and small receptor cells leave the base of the ocellus and extend to the protocerebrum to synapse with second order neurons.     

Multiple nucleocapsid packaging of Autographa californica nucleopolyhedrovirus accelerates the onset of systemic infection in Trichoplusia ni

Among the nucleopolyhedroviruses (Baculoviridae), the occlusion-derived virus (ODV), which initiates infection in host insects, may contain only a single nucleocapsid per virion (the SNPVs) or one to many nucleocapsids per virion (the MNPVs), but the significance of this difference is unclear. To gain insight into the biological relevance of these different packaging strategies, we compared pathogenesis induced by ODV fractions enriched for multiple nucleocapsids (ODV-M) or single nucleocapsids (ODV-S) of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) containing a beta-galactosidase reporter gene. In time course experiments wherein newly molted fourth-instar Trichoplusia ni were challenged with doses of ODV-S or ODV-M that yielded the same final mortality ( approximately 70%), we characterized viral foci as either being restricted to the midgut or involving tracheal cells (the secondary target tissue, indicative of systemic infection). We found that while the timing of primary infection by ODV-S and ODV-M was similar, ODV-S established significantly more primary midgut cell foci than ODV-M, but ODV-M infected tracheal cells at twice the rate of ODV-S. The more efficient establishment of tracheal infections by ODV-M decreased the probability that infections were lost by midgut cell sloughing, explaining why higher numbers of primary infections established by ODV-S within larvae were needed to achieve the same final mortality. These results showed that the multiple nucleocapsid packaging strategy of AcMNPV accelerates the onset of irreversible systemic infections and may indicate why MNPVs have wider individual host ranges than SNPVs.     

The regulation of mouse liver ornithine decarboxylase by metabolites

The enzyme ornithine decarboxylase (L-Ornithine carboxy-lyase, EC 4.1.1.17), has been partially purified from the livers of mice subjected to partial hepatectomy (6-8 h previously). Mouse liver ornithine decarboxylase requires pyridoxal phosphate, and dithiothreitol for maximal activity. The enzyme has a pH optimum of 7.3, it is inhibited in the presence of 0.3 M phosphate, glycine, Tricine and Tris. It shows no dependence on metal ions and is inhibited by high salt concentrations, particularly ammonium salts. The kinetics of the enzyme have been studied with putrescine (and analogs), spermidine and spermine, in the presence of both high and low levels of pyridoxal phosphate. High concentrations of pyridoxal phosphate inhibit the enzyme. The enzyme is also inhibited by low concentrations of putrescine (1 mM). As the concentration of putrescine increased to 10 mM, non-competitive inhibition was observed, this could be reversed by addition of higher levels of pyridoxal phosphate. Spermidine and spermine inhibit (noncompetitively) only at high concentrations (10 mM). Ornithine inhibits at high concentrations (2 mM). Spectral studies have shown that the observed kinetics of competitive inhibition at low concentrations of polyamine changing to noncompetitive inhibition at high polyamine concentrations are due to competition between enzyme and substrate (or inhibitor) for free (non-enzyme bound) pyridoxal phosphate. Noncompetitive inhibition arises through the formation of transient Schiff base complexes between amines and free pyridoxal phosphate. It also appears that the binding of substrate to the active site takes place through Schiff base formation with enzyme bound pyridoxal phosphate.     

Modulation of the induction of ornithine decarboxylase by some opioid receptor agonists in immune cells and cardiomyocytes

The use of synthetic antimalarial compounds played a secondary role to the use of residual insecticides in post World War II antimalarial control and eradication campaigns. The discovery of chloroquine-resistant malaria in South East Asia and South America prompted an intensification of antimosquito measures, rather than a thorough investigation of resistance. It was the failure of the antimosquito measures which primarily called a halt to malaria eradication and a return to control. A focus on the role of synthetic antimalarials in Thailand thus aims to provide a complementary view to those histories being constructed around the antimosquito measures.     

Regulation of mevalonate 5-pyrophosphate decarboxylase in HeLa cells. Inhibition of enzymatic protein synthesis by serum lipoproteins

Mevalonate 5-pyrophosphate decarboxylase (EC 4.1.1.33) has been considered as a secondary site of regulation of cholesterogenesis. Because of this, we have studied the regulation of decarboxylase in HeLa cells by serum lipoproteins in the cell culture medium. A first group of experiments was performed with cells grown in Eagle's medium with 10% foetal calf serum. The specific activity of decarboxylase was increased when whole foetal calf serum was replaced with lipoprotein-poor serum. This increase was clearly reduced in the presence of cycloheximide. Addition of serum lipoproteins to a medium containing lipoprotein-poor serum led to a clear decrease in the decarboxylase activity. An identical decrease was observed after the addition of lipoproteins alone or in combination with cycloheximide. These results suggest for the first time that the effect of serum lipoproteins on decarboxylase activity should be a decrease in the rate of enzymatic protein synthesis, and corroborate the important role of reactions other than those catalysed by 3-hydroxy-3-methylglutaryl-CoA reductase in the regulation of cholesterogenesis.     

Differential inhibitory effects of three nitric oxide donors on ornithine decarboxylase activity in human colon carcinoma cells

Ornithine decarboxylase (ODC, EC 4.1.1.17) is the enzyme responsible for the synthesis of polyamines, which are absolutely necessary for cell proliferation. In the present work, we tested the effects of 3 nitric oxide (NO) donors, namely, sodium nitroprusside (SNP), (Z)-1-(N-methyl-N-[6-(N-methylammoniohexyl)amino] diazen-1-ium-1,2-diolate (MAHMA/NO) and 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine sodium (DEA/NO), on ODC activity in human-colon carcinoma cells (HT-29). SNP was the most effective inhibitor of ODC activity with a concentration of 8 micromol/L inducing 50% inhibition of basal activity. The effect of SNP was reversed by haemoglobin (Hb), but not by GSH or L-cysteine (CYS). Very little of the SNP in solution was degraded into nitrite, but the presence of cellular homogenate increased the production of nitrite. MAHMA/NO and DEA/NO were much less effective than SNP as ODC inhibitors, since the concentrations of these agents which induce 50% inhibition of basal activity were 20- to 60-fold higher than that of SNP. The effects of MAHMA/NO and DEA/NO were not reversed by haemoglobin. In solution, these latter 2 agents were totally degraded into nitrites. In conclusion, SNP on the one hand and MAHMA/NO and DEA/NO on the other appeared to release different NOx species with different efficiency on ODC activity.     

Aminopeptidase dependent pore formation of Bacillus thuringiensis Cry1Ac toxin on Trichoplusia ni membranes

Surveillance studies on cercarial dermatitis were carried out in paddy growing areas in Peninsular Malaysia. It was observed that dermatitis in paddy planters occurred in paddy fields which were cultivated using animals such as bafflos or fields where domestic animals were allowed to graze during the off planting season as these animals harbored the parasite. The causative agent of cercarial dermatitis was Schistosoma spindale. A total of 215 small mammals trapped from Alor Setar and 126 trapped from Labu were examined for the schistosome. In Alor Setar Bandicota indica, Rattus argentiventer and Rattus rattus diardii were the only wild mammals found to be infected with the parasite, while in the Labu areas only Rattus tiomanicus jalorensis was positive for the schistosome. The occurrence of S. spindale in R. argentiventer and R.r. diardii in Alor Setar and in R.t. jalorensis in Labu constitute new host and geographic distribution records of the schistosome.     

Screening of potential cancer-preventing chemicals for inhibition of induction of ornithine decarboxylase in epithelial cells from rat trachea

A 10 min exposure of rat hippocampal slices to hypoxic/hypoglycemic medium decreased tissue adenosine 5'-triphosphate (ATP) levels. Hypoxia/hypoglycemia also caused an anoxic depolarization and essentially no recovery of the synaptically evoked population spike from CA1 region recorded 30 min after re-introduction of normoxic/normoglycemic medium. Removal of Ca2+ or the addition of either the non-competitive N-methyl-D-aspartate antagonist dizocilpine maleate, the inorganic Ca2+ channel antagonist Co2+; or the Na+ channel blocker tetrodotoxin to hypoxic/hypoglycemic medium improved recovery of the evoked population spike upon re-oxygenation. Dizocilpine maleate, Co2+, and tetrodotoxin spared ATP during exposure to hypoxia/hypoglycemia. In contrast, Ca(2+)-free medium facilitated recovery of the population spike but did not preserve ATP during hypoxia/hypoglycemia. Dizocilpine maleate, Co2+ or dantrolene, when added to Ca(2+)-free medium, did not preserve ATP. Tetrodotoxin, when added to Ca(2+)-free medium, was effective in sparing ATP in hypoxic/hypoglycemic medium. To determine the effect of anoxic depolarization on ATP levels, hippocampal slices were collected just before and after the depolarization. There appeared to be an abrupt drop in ATP associated with the anoxic depolarization. We conclude that Na+ influx plays a relatively larger role in ATP consumption during hypoxia/hypoglycemia than Ca2+ influx. In addition, the anoxic depolarization imposes a large and rapid drop in ATP levels.     

Spermidine-induced destabilization of ornithine decarboxylase (ODC) is mediated by accumulation of antizyme in ODC-overproducing variant cells

The mechanism of spermidine-induced destabilization of ornithine decarboxylase (ODC) was examined in newly isolated ODC-overproducing variant cells by use of an in vitro ODC degrading system. The cells accumulated ODC protein in the presence of alpha-difluoromethylornithine. Addition of spermidine to the medium accelerated degradation of ODC protein concomitantly with induction of antizyme, a regulatory protein that binds to ODC, inhibiting its activity. Both the acceleration of ODC degradation and the induction of antizyme were inhibited by cycloheximide, but not by actinomycin D. ODC was degraded rapidly in extracts from spermidine-treated cells. The rate of ODC degradation correlated with the amount of antizyme in the extracts, and the degradation activity was abolished by treatment of the extracts with anti-antizyme antibody. Thus, antizyme induced by spermidine was essential for the accelerated degradation of ODC in the cells. ODC was phosphorylated in the cells, probably at serine residue 303 in the first internal PEST region. ODC phosphorylation occurred even when its new synthesis was inhibited by cycloheximide. Antizyme accelerated the degradations of both dephosphorylated ODC and native ODC.     

Hepatocyte growth factor-induced expression of ornithine decarboxylase, c-met, and c-myc is differently affected by protein kinase inhibitors in human hepatoma cells HepG2

Hiatal hernia is a common condition. Many patients are asymptomatic or experience symptoms of gastroesophageal reflux. However, with larger hernias, the hernia and its incarceration are the issues, and most patients with this condition need surgical attention.     

Cytocidal effect of alpha-methylornithine, a competitive inhibitor of ornithine decarboxylase, on ehrlich ascites tumor cells in vivo

The antiproliferative effect of alpha-methylornithine, an inhibitor of the initial and rate-limiting step in polyamine biosynthesis, was studied on Ehrlich ascites tumor cells grown in vivo. To assess drug-induced cell lethality, the loss of 125I from mice harboring 5-[125I] iodo-2'-deoxyuridine-labeled mouse tumor cells was determined during the course of alpha-methylornithine treatment. Mice that received the drug showed a significant increase in their rate of 125I excretion as compared to controls. This suggests that alpha-methylornithine kills an additional fraction of tumor cells beyond the fraction normally dying.