Inactivation of ribonucleotide reductase by nitric oxide

Nitric oxide and intracellular heme

Figure 2 depicts a working hypothesis for these results. Activation of .NO synthesis results in nitrogen oxide-induced loss of protein-bound heme from CYP proteins, which remain relatively intact. This heme liberation results in a decrease in heme synthesis (decreased ALAS) and an increase in heme degradation (increased HO). In addition, .NO synthesis results in direct inhibition of ferrochelatase, which further contributes to inhibition of heme synthesis. There also appears to be a mechanism to repair or resynthesize CYP after .NO synthesis is inhibited. Finally, a result of this effect may be protection against cellular injury, since increased HO is an important response against cellular injury from a variety of insults.

Factors influencing macrophage activation by muramyl peptides: inhibition of NO synthase activity by high levels of NO

Treatment with muramyldipeptide (MDP) or a lipophilic derivative (MTP-Chol) included in nanocapsules renders macrophages cytostatic towards tumor cells. At the same time, nitric oxide (NO) synthase (EC 1.14.23) activity is induced, as determined by measurement of the two end products of the reaction (nitrite and L-citrulline). The objective of this study was to investigate some factors which might influence this activation and explain the decreased response observed at high nanocapsule concentrations. The glucose content of the medium did not seem to be limiting. Addition of indomethacin decreased nitrite production in the effector phase, suggesting a role for prostaglandins in the maintenance of the activated state. We also tested the hypothesis that NO itself might regulate inducible nitric oxide synthase activity. The addition of NO donors (SIN-1 and nitrosoglutathione) or superoxide dismutase to cultures of activated macrophages inhibited the NO synthase activity. Since these NO donors were non toxic towards macrophages, these observations indicate clearly that the addition of exogenous NO to that formed by the enzymatic reaction can cause inhibition of the inducible NO synthase.

Nitric oxide donor-mediated killing of bioluminescent Escherichia coli

The antimicrobial activities of two nitric oxide-releasing compounds against Escherichia coli were investigated by using recombinant E. coli cloned with a luciferase gene from Pyrophorus plagiophthalamus. Since luciferase uses intracellular ATP to generate visible light which can be measured from living cells in real time, we wanted to compare the extent to which cell viability parallels light emission. Results from luminescence measurements and CFU counts were in good agreement, and the decrease in light emission was shown to provide a rapid and more sensitive indication of cytotoxicity.

Duration of expression of inducible nitric oxide synthase in glial cells

Lipopolysaccharide (LPS) or a combination of interleukin (IL)-1 beta and interferon (IFN)-gamma cause transcriptional induction of a calcium-independent nitric oxide synthase (NOS) in astrocytes and C6 glioma cells. LPS induction of NOS in C6 cells was evidenced by a small amount of nitrite accumulation as compared with cells exposed to IL-1 beta/IFN-gamma, but the maximal NOS activity achieved (as revealed by cGMP formation) was the same. The NOS activity induced by LPS in C6 cells was maximal at 4 to 8 hr and then rapidly decreased, while NOS activity induced by IL-1 beta/IFN-gamma slowly decreased after 4 hr. In addition, the effects of re-presenting IL-1 beta/IFN-gamma to both astrocytes and C6 cells after maximal induction of activity of the inducible form of NOS were studied. The re-addition of cytokines prolonged both NOS mRNA expression and also enzyme activity, suggesting effects at either the transcriptional (further induction) or translational level (mRNA stability). These results imply that the time course of NO production by induced astrocytes depends both upon the nature of the inducing stimulus and the frequency of the cells' exposure to it.

Effect of deoxyribonucleosides on the hypersensitivity of human peripheral blood lymphocytes to UV-B and UV-C irradiation

We have previously shown that non-cycling (unstimulated) human lymphocytes from normal donors show extreme hypersensitivity to UV-B irradiation, and are killed by an excisable lesion which is not a pyrimidine dimer or 6-4 photoproduct. In this paper we show that addition of the 4 deoxyribonucleosides to the medium, each at 10(-5) M, substantially increased the survival of non-cycling normal human T-lymphocytes following UV-B irradiation and substantially reduced the frequency of excision-related strand breaks in human mononuclear cells. Addition of ribonucleosides to the medium did not enhance excision-break rejoining. The survival of fibroblasts, of cycling T-lymphocytes and of unstimulated xeroderma pigmentosum T-lymphocytes was not enhanced by deoxyribonucleosides. This suggests that the hypersensitivity is due to reduced rejoining of excision breaks as a consequence of low intracellular deoxyribonucleotide pools and that it can be redressed by supplementation of the medium with deoxyribonucleosides or upregulation of ribonucleotide reductase following mitogen stimulation. We suggest that UV-B forms an additional DNA lesion which is not a pyrimidine dimer or 6-4 photoproduct, which is relatively common, and at which incision is particularly efficient. In fibroblasts, repair of this lesion is completed with high efficiency, whereas in normal unstimulated T-lymphocytes, rapid incision exacerbates the effects of the reduced rate of strand rejoining and leads to cell death.

Inhibitory mechanisms of antibody production by nitrogen oxides released from activated macrophages during the immune response: relationship to energy consumption

We investigated the relationship between the sensitivity of mouse splenocytes in immune response to nitrogen oxides and energy consumption rate of the cells. Macrophage-like cells (Mm1) pretreated with IL-6 served as the source of the nitrogen oxides. The antibody production of both 2,4,6-trinitrophenyl-keyhole limpet haemocyanin-primed splenocytes and B cell hybridomas was markedly reduced; about 20-40% of splenocytes and B cell hybridomas were killed by co-culture with IL-6-treated Mm1. Cell viability and antibody production were completely restored by the addition of NG-monomethyl L-arginine to the culture medium. The cytotoxicity of the nitrogen oxides was correlated with the distance between effector and target cells. Under conditions of low cytotoxicity, antibody production by B cell hybridomas was suppressed by the nitrogen oxides, this suppression not being correlated with the reduction in cell growth. The sensitivity of the target cells differed in co-cultures of antigen-primed splenocytes and B cell hybridomas with IL-6-treated Mm1. The nitric oxide-sensitivity of the cells corresponded to their 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reducing activity and ATP consumption rate. These findings suggest that nitrogen oxides act as regulatory molecules in immune response in three ways: cytostasis, reduction of cell growth and suppression of antibody synthesis.

Possible role of nitric oxide in malarial immunosuppression

We have tested the hypothesis that nitric oxide may be responsible for the immunosuppression reported during malaria infections. We first showed that reactive nitrogen intermediates, which indicate nitric oxide generation, were increased in the plasma of Plasmodium vinckei-infected mice. We next found that Concanavalin A-induced proliferation of spleen cells from these mice was reduced compared with that observed in uninfected animals. The addition of NG-methyl-L-arginine (L-NMMA) for the duration of the cultures restored the malarial proliferative response to normal. We then tested the effect of oral L-NMMA on the proliferative response of P. chabaudi-infected mice to a human red blood cell lysate. The secondary response to this antigen, measured as spleen cell proliferation in vitro ten days after immunization and when there was no discernible parasitaemia, remained normal in L-NMMA-treated P. chabaudi mice, but was decreased in the untreated infected mice. These results suggest a role for nitric oxide in malarial immunosuppression.

Visible light photochemical release of nitric oxide from S-nitrosoglutathione: potential photochemotherapeutic applications

Some aspects of the physiological role of NO may be mediated by stable NO-carriers such as S-nitrosoglutathione and related S-nitrosothiols. In this report we show that irradiation of S-nitrosoglutathione at either absorption band (lambda max = 340 nm or 545 nm) results in the release of nitric oxide. Photolysis of S-nitrosoglutathione at 545 nm exhibited a quantum yield of 0.056 +/- 0.002 and was best approximated by a first-order process with kobs = 4.9 x 10(-7) +/- 0.3 x 10(-7) s-1. The photolytic release of NO from S-nitrosoglutathione resulted in an enhanced cytotoxic effect of S-nitrosoglutathione on HL-60 leukemia cells. That the cytotoxic effect of S-nitrosoglutathione was diminished by the addition of oxyhemoglobin strongly suggests that NO is the cytotoxic species. The finding that NO can be readily liberated from S-nitrosoglutathione by visible radiation indicates that the photochemical properties of this compound in the visible spectrum must be considered in order to obtain meaningful data as to its physiological role and the S-nitrosoglutathione and related compounds may find use as photochemotherapeutic agents.

Nitric oxide blocks the cell cycle of mouse macrophage-like cells in the early G2+M phase

The effects of nitric oxide produced by macrophage-like cells (Mm1) on the cell cycle were investigated. Mm1 cells lost proliferative activity in the presence of interleukin-6 (IL-6) and a subpopulation accumulated in the G2+M phase. This level increased in proportion to the incubation time. The DNA content of the cells was slightly lower than that of Mm1 cells treated with vinblastine or demecolcine, drugs which block the cell cycle in the M phase. The peak of the early G2+M phase was reduced by treatment with NG-mono-methyl-L-arginine. However, after treatment with exogenous nitric oxide or sodium nitroprusside, the G0/G1 phase increased, but the early-G2+M and the S phase decreased. The flow cytometry pattern in IL-6-treated Mm1 was the same as that of cytochalasin B-treated Mm1. These data suggest that endogenous nitric oxide affects the microfilament system of IL-6-treated Mm1 cells and blocks the cell cycle in the early G2+M phase.

[Nitric oxide: a biological effector. Detection using electron paramagnetic resonance]

Nitric oxide is synthesized in mammalian cells from L-arginine or from pharmaceutical drugs. It forms paramagnetic complexes with some metalloproteins, including hemoglobin. Induction of NOSi following LPS or cytokine activation of murine macrophages has various effects, such as inhibition of mitochondrial respiration and that of DNA biosynthesis through interaction of NO with specific metalloenzymes. Induction of NOSi in a generator cell such as macrophage gives the same metabolic effects in target cells. NO is also detected in pathological states such as septic shock, diabetes mellitus and allograft, where the inducible L-arginine-NO pathway plays an important role. Electron Paramagnetic Resonance spectroscopy enables to detect unambiguously such specific molecular targets for NO in mammalian whole cells and organelles.

Pancreatic islet cells are highly susceptible towards the cytotoxic effects of chemically generated nitric oxide

To compare the sensitivity of different mammalian cell types towards the cytotoxic action of nitric oxide, freshly isolated rat pancreatic islet cells, hepatocytes, resident and activated macrophages, cultured aortic endothelial cells and two murine tumor cell lines were tested for susceptibility towards exogenous nitric oxide. As sources for nitric oxide nitroprusside, S-nitroso-N-acetyl-penicillamine and the sydnonimine-derivative SIN-1 were used. These generate nitric oxide by different mechanisms and kinetics. Among the cell types tested we found large differences in their susceptibility towards the three nitric oxide donors. Islet cells were by far the most sensitive of the investigated cells and were completely lysed by all three nitric oxide donors. Hepatocytes and endothelial cells were sensitive towards nitroprusside but relatively resistant towards toxicity of SIN-1 and S-nitroso-N-acetyl-penicillamine. Activated and resident macrophages were lysed by SIN-1, whereas high concentrations of nitroprusside and S-nitroso-N-acetyl-penicillamine led to partial cell lysis only. The tumor cell lines were both lysed by SIN-1 but showed differences in their sensitivity towards S-nitroso-N-acetyl-penicillamine. Nitric oxide, which is produced in large amounts during infection and inflammation, may play an important role in the destruction of islet cells during insulitis leading to insulin-dependent diabetes mellitus.

Evidence for antiviral effect of nitric oxide. Inhibition of herpes simplex virus type 1 replication

Nitric oxide (NO) has antimicrobial activity against a wide spectrum of infectious pathogens, but an antiviral effect has not been reported. The impact of NO, from endogenous and exogenous sources, on herpes simplex virus type 1 (HSV 1) replication was studied in vitro. HSV 1 replication in RAW 264.7 macrophages was reduced 1,806-fold in monolayers induced to make NO by activation with gamma IFN and LPS. A competitive and a noncompetitive inhibitor of nitric oxide synthetase substantially reduced the antiviral effect of activated RAW macrophages. S-nitroso-L-acetyl penicillamine (SNAP) is a donor of NO and was added to the media of infected monolayers to assess the antiviral properties of NO in the absence of gamma IFN and LPS. A single dose of S-nitroso-L-acetyl penicillamine 3 h after infection inhibited HSV 1 replication in Vero, HEp2, and RAW 264.7 cells in a dose-dependent manner. Neither virucidal nor cytocidal effects of NO were observed under conditions that inhibited HSV 1 replication. Nitric oxide had inhibitory effects, comparable to that of gamma IFN/LPS, on protein and DNA synthesis as well as on cell replication. This report demonstrates that, among its diverse properties, NO has an antiviral effect.

Nitric oxide (NO) and nociceptive processing in the spinal cord

There is considerable evidence to implicate N-methyl-D-aspartate (NMDA) receptor activation in the mechanisms that underly thermal hyperalgesia in the spinal cord. As many of the effects of NMDA receptor activation appear to be ultimately mediated through production of nitric oxide (NO), recent reports have begun to define the role of NO in spinal nociceptive processing. From this evidence, it is likely that NO, produced in neurons in the spinal cord that contain NO synthase, like NMDA, plays a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. Collectively, these reports suggest that the reflex withdrawal response to noxious heat is not mediated through activation of NMDA receptors and subsequent production of NO and cGMP, but that the acute NMDA-produced facilitation of thermal reflexes is NMDA-, NO- and cGMP-mediated and that a sustained production of NO and subsequent activation of soluble guanylate cyclase (GC-S) in the lumbar spinal cord appears to be required for maintenance of the thermal hyperalgesia produced in persistent pain models. As our knowledge and understanding of the new and intriguing class of neurotransmitters typified by NO emerges, it is likely that the next few years of pain and analgesia research will focus on the cellular events underlying mechanisms of chronic pain.

Neutralization of gamma interferon and tumor necrosis factor alpha blocks in vivo synthesis of nitrogen oxides from L-arginine and protection against Francisella tularensis infection in Mycobacterium bovis BCG-treated mice

Peritoneal cells from Mycobacterium bovis BCG-infected C3H/HeN mice produced nitrite (NO2-, an oxidative end product of nitric oxide [NO] synthesis) and inhibited the growth of Francisella tularensis, a facultative intracellular bacterium. Both NO2- production and inhibition of bacterial growth were suppressed by NG-monomethyl-L-arginine, a substrate inhibitor of nitrogen oxidation of L-arginine, and monoclonal antibodies (MAbs) to gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). Intraperitoneal injection of mice with BCG increased urinary nitrate (NO3-) excretion coincident with development of activated macrophages capable of secreting nitrogen oxides and inhibiting F. tularensis growth in vitro. Eight days after BCG inoculation, mice survived a normally lethal intraperitoneal challenge with F. tularensis. Treatment of these BCG-infected mice with MAbs to IFN-gamma or TNF-alpha at the time of BCG inoculation reduced urinary NO3- levels to those found in normal uninfected mice for up to 14 days. The same anticytokine antibody treatment abolished BCG-mediated protection against F. tularensis: mice died within 4 to 6 days. Intraperitoneal administration of anti-IFN-gamma or anti-TNF-alpha antibody 8 days after BCG infection also reduced urinary NO3- and abolished protection against F. tularensis. Isotype control (immunoglobulin G) or anti-interleukin 4 MAbs had little effect on these parameters at any time of treatment. IFN-gamma and TNF-alpha were clearly involved in the regulation of macrophage activation by BCG in vivo. Protection against F. tularensis challenge by BCG depended upon the physiological generation of reactive nitrogen oxides induced by these cytokines.

Cytotoxicity of activated rat macrophages against syngeneic islet cells is arginine-dependent, correlates with citrulline and nitrite concentrations and is identical to lysis by the nitric oxide donor nitroprusside

Lysis of rat islet cells by syngeneic activated macrophages in vitro can be completely inhibited by the nitric oxide-synthase-inhibitor NG-methyl-L-arginine. This inhibition can be reversed by an excess of L-arginine. Time-dependent lysis of islet cells by activated macrophages is accompanied by increasing concentrations of nitrite and citrulline in the culture medium both of which are measures of nitric oxide formation derived from L-arginine. Lysis of isolated islet cells and disintegration of isolated whole islets is also obtained within 15 h by culture in the presence of the nitric oxide generating vasodilator sodium nitroprusside. We thus conclude that nitric oxide is extremely toxic for islet cells and that nitric oxide alone and in the absence of other macrophage-generated potentially toxic products can rapidly and completely kill islet cells.

Structural aspects of N-hydroxy-N'-aminoguanidine derivatives as inhibitors of L1210 cell growth and ribonucleotide reductase activity

Previous studies have shown that N-hydroxy-N'-aminoguanidine (HAG) derivatives [RCH = NNHC(= NH)NHOH-tosylate] inhibit ribonucleotide reductase activity and block the growth of leukemia L1210 cells and human colon carcinoma, HT-29, cells in culture. In the current studies, the role of the side chains and the location of the bond of the side chain moiety to HAG were investigated using a new series of HAG derivatives which contained as the R-group--cyclohexyl, phenyl-, pyridyl- or napthyl moieties. The effects of these compounds as inhibitors of L1210 cell growth and ribonucleotide reductase activity were compared with the parent compound. N-hydroxy-N'-aminoguanidine was less inhibitory to ribonucleotide reductase activity and L1210 cell growth than hydroxyurea. The phenyl-HAG compounds which included 1-benzyloxybenzylidene- and 4-cyclohexylmethoxybenzylidene-HAG inhibited CDP reductase with IC50s which ranged from 50-110 microM. 1-Naphthylmethylene-HAG was more inhibitory than 2-naphthylmethylene-HAG and more inhibitory than the phenyl-HAG compounds. 2-Pyridylmethylene-HAG was more inhibitory than 3-pyridylmethylene- or 4-pyridylmethylene-HAG. While HAG inhibited CDP and ADP reductase activities essentially to the same extent, the HAG-derivatives inhibited ADP reductase activity to a greater extent than CDP reductase activity. Cyclohexylmethylene-HAG did not inhibit either L1210 cell growth or ribonucleotide reductase activity. There was good correlation between the inhibition of ribonucleotide reductase activity and L1210 cell growth by these HAG-derivatives. These data indicate that not only is the nature of the side chain substitution important, but also the location of the HAG-moiety on the ring position.

Biochemistry of nitric oxide and its redox-activated forms

Nitric oxide (NO.), a potentially toxic molecule, has been implicated in a wide range of biological functions. Details of its biochemistry, however, remain poorly understood. The broader chemistry of nitrogen monoxide (NO) involves a redox array of species with distinctive properties and reactivities: NO+ (nitrosonium), NO., and NO- (nitroxyl anion). The integration of this chemistry with current perspectives of NO biology illuminates many aspects of NO biochemistry, including the enzymatic mechanism of synthesis, the mode of transport and targeting in biological systems, the means by which its toxicity is mitigated, and the function-regulating interaction with target proteins.

Neutrophils, nitrogen oxides, and inflammatory bowel disease

The etiologic factors responsible for IBD remain only speculative. It does appear that the inappropriate activation of the immune system, whether by immune complex deposition, infectious agents or vascular impairment, is important in the pathogenesis of these diseases. Interaction of certain cytokines known to be produced in human IBD with specific immune cells such as neutrophils and macrophages results in the induction of the enzyme NO. synthase with the concomitant release of large amounts of NO.. Nitric oxide is known to mediate many of the pathophysiological alterations associated with IBD including cell injury, intestinal hyperemia and intestinal smooth muscle dysfunction. In addition, NO. is known to decompose in solution to yield potent N-nitrosating agents which will N-nitrosate certain amines to yield potent carcinogenic nitrosamines. Because antioxidants (including 5-ASA) are potent inhibitors of nitrosamine formation, they may prove useful in attenuating the formation of potentially carcinogenic agents in vivo.

A possible new class of ribonucleotide reductase from Methanobacterium thermoautotrophicum

The ribonucleotide reductase from the strictly anaerobic methanogen Methanobacterium thermoautotrophicum has been partially purified by ion-exchange and gel-filtration chromatography. Its molecular weight is estimated to be 100,000 by the latter step. Unlike all previously studied ribonucleotide reductases, the enzyme does not employ dithiol compounds such as dithiothreitol as artificial electron donors in in vitro assays. Inhibition of the enzyme by S-adenosylmethionine, oxygen, and azide further distinguishes it from the Escherichia coli anaerobic enzyme, the iron- and manganese-containing, and the adenosylcobalamin-dependent enzymes. Our preliminary results suggest that this enzyme has an activation mechanism different from the known classes of ribonucleotide reductases.

Nitric oxide can modify amino acid residues in proteins

Nitric oxide derived from sodium nitroprusside binds to the heme moiety of hemoglobin and also modifies some functional groups in the protein. As hemoglobin concentration is increased, globin modification is decreased presumably due to formation of the NO complex with heme. The SH groups of hemoglobin are probably not involved in the formation of the stable product formed by NO. In the presence of inositol hexaphosphate, which binds preferentially in the cleft between the two beta-chains of hemoglobin, formation of one modified derivative was selectively reduced. With hemoglobin specifically blocked on its N-terminal residues, globin modification was also significantly reduced. Carbonic anhydrase, which is blocked at its N-terminus, was also refractory to modification. The results suggest that the N-terminal groups of some proteins can be modified by nitric oxide, perhaps by deamination.