NAD(P)H, a directly operating antioxidant?

Genome-wide profile of oxidoreductases in viruses, prokaryotes, and eukaryotes

Enzymes that utilize nicotinamide adenine dinucleotide (NAD) or its 2'-phosphate derivative (NADP) are found throughout the kingdoms of life. These enzymes are fundamental to many biochemical pathways, including central intermediary metabolism and mechanisms for cell survival and defense. The complete genomes of 25 organisms representing bacteria, protists, fungi, plants, and animals, and 811 viruses, were mined to identify and classify NAD(P)-dependent enzymes. An average of 3.4% of the proteins in these genomes was categorized as NAD(P)-utilizing proteins, with highest prevalence in the medium-chain oxidoreductase and short-chain oxidoreductase families. In general, the distribution of these enzymes by oxidoreductase family was correlated to the number of different catalytic mechanisms in each family. Organisms with smaller genomes encoded a larger proportion of NAD(P)-dependent enzymes in their proteome (approximately 6%) as compared to the larger genomes of eukaryotes (approximately 3%). Among viruses, those with large, double-strand DNA genomes were shown to encode oxidoreductases. Gram-positive and gram-negative bacteria showed some differences in the distribution of NAD(P)-dependent proteins. Several organisms such as M. tuberculosis, P. falciparum, and A. thaliana showed unique distributions of oxidoreductases corresponding to some phenotypic features.

Pathophysiological aspects of cellular pyridine nucleotide metabolism: focus on the vascular endothelium. Review

In recent years, pyridine nucleotides NAD(H) and NADP(H) have been established as an important molecules in physiological and pathophysiological signaling and cell injury pathways. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific aminoacid residues of the acceptor proteins, with significant changes in the function of these acceptors. Mono(ADP-ribosyl)ation reactions have been implicated to play a role both in physiological responses and in cellular responses to bacterial toxins. Cyclic ADP-ribose formation also utilizes NAD+ and primarily serves as physiological, signal transduction mechanisms regulating intracellular calcium homeostasis. In pathophysiological conditions associated with oxidative stress (such as various forms of inflammation and reperfusion injury), activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) occurs, with subsequent, substantial fall in cellular NAD+ and ATP levels, which can determine the viability and function of the affected cells. In addition, NADPH oxidases can significantly affect the balance and fate of NAD+ and NADP in oxidatively stressed cells and can facilitate the generation of various positive feedback cycles of injury. Under severe oxidant conditions, direct oxidative damage to NAD+ has also been reported. The current review focuses on PARP and on NADPH oxidases, as pathophysiologically relevant factors in creating disturbances in the cellular pyridine nucleotide balance. A separate section describes how these mechanisms apply to the pathogenesis of endothelial cell injury in selected cardiovascular pathophysiological conditions.

Bcl-xL blocks mitochondrial multiple conductance channel activation and inhibits 6-OHDA-induced death in SH-SY5Y cells

Apoptosis is an active process that is regulated by different signalling pathways. One of the more important organelles involved in apoptosis regulation is the mitochondrion. Electron chain transport disruption increases free radical production leading to multiple conductance channel opening, release of cytochrome c and caspase activation. This death pathway can be blocked by anti-apoptotic members of the Bcl-2 protein family that might shift redox potential to a more reduced state, preventing free radical-mediated damage. 6-Hydroxydopamine (6-OHDA) has been widely used to generate Parkinson's disease-like models. It is able to generate free radicals and to induce catecholaminergic cell death. In this paper we have used the human neuroblastoma cell line SH-SY5Y overexpressing Bcl-x(L) as a model to gain insights into the mechanisms through which Bcl-x(L) blocks 6-OHDA-induced cell death and to identify the molecular targets for this action. Herein, we present evidence supporting that the Bcl-x(L)-anti-apoptotic signal pathway seems to prevent mitochondrial multiple conductance channel opening, cytochrome c release and caspase-3 like activity following 6-OHDA treatment in the human neuroblastoma cell line SH-SY5Y.

Thiamine protects against paraquat-induced damage: scavenging activity of reactive oxygen species

To demonstrate the superoxide anion (O(2)(-)) scavenging activity of thiamine, we comparatively investigated the inhibition of cell growth reduction and repression of the oxidative stress-inducible gene expression (soxS, sodA, zwf and soi-19::lacZ) triggered by paraquat, intracellular O(2)(-) generator, using an Escherichia coli system. When thiamine (>1 μM) was added to the culture, a decrease of growth rate caused by paraquat was significantly recovered. Paraquat treatment (1 μM) to aerobically grown E. coli highly increased the expression of soxS and its regulons sodA and zwf, genes for manganese-containing superoxide dismutase (Mn-SOD) and glucose-6-phosphate dehydrogenase (G6PDH) to cope with the oxidative stress. However, the induction of Mn-SOD and G6PDH was suppressed by the thiamine supplement. The induction of the soi-19::lacZ gene, whose expression was dependent on paraquat, was also repressed by more than 10 μM of the thiamine addition to the culture. To characterize the role of thiamine, which challenges the paraquat toxicity, an in vitro experiment of nitroblue tetrazolium (NBT) reduction was performed. The NBT reduction by O(2)(-) generated in the xanthine/hypoxanthine system was inhibited by the thiamine supplement in a dose-dependent manner. Moreover, it competed with the 2-deoxy-d-ribose in absorbing the hydroxyl radical (OH) generated by γ-irradiation (800 Gy) and thus inhibited the formation of malondialdehyde in vitro. In conclusion, this evidence suggests that thiamine may partly act as an antioxidant to scavenge O(2)(-) (or OH) directly and thus affect the cellular response to oxidative stress induced by reactive oxygen species.

Product formation and kinetic simulations in the pH range 1–14 account for a free-radical mechanism of peroxynitrite decomposition

The yields of nitrate and nitrite from decomposition of peroxynitrite in phosphate buffer at 37 degrees C were determined in the pH range 1-14. The NO(2)(-)/NO(3)(-) yields showed a stepwise variation with pH, with inflection points at approximately pH 3.1, 5.8, 6.8, 8.0, and 11.9. Nitrite formation increased strongly above pH 7 at the expense of nitrate, but above pH 12 nitrate again became the major product (80% at pH 14). At this pH, the Arrhenius parameters were E(a)=24.1+/-0.2kcal mol(-1) and A=(4.9+/-1.3)x10(12)s(-1). The yields of NO(2)(-), NO(3)(-), and O(2) measured at pH 5.8, 7.4, and 8.5 as a function of the initial peroxynitrite concentration (50-1000 microM) were linear only at pH 5.8. In the presence of carbon dioxide, oxygen production at pH 7.5 and pH 10 was found to be linear on the CO(2) concentration. The experimental observations were satisfactorily reproduced by kinetic simulations including principal component analyses. These data strongly suggest that the chemistry of peroxynitrite is exclusively mediated by z.rad;NO(2) and HO(z.rad;) radicals in the absence, and by z.rad;NO(2) and CO(3)(z.rad;-) radicals in the presence of CO(2).

Alpha-lipoic acid decreases thiol reactivity of the insulin receptor and protein tyrosine phosphatase 1B in 3T3-L1 adipocytes

Alpha-lipoic acid is known to increase insulin sensitivity in vivo and to stimulate glucose uptake into adipose and muscle cells in vitro. In this study, alpha-lipoic acid was demonstrated to stimulate the autophosphorylation of insulin receptor and glucose uptake into 3T3-L1 adipocytes by reducing the thiol reactivity of intracellular proteins. To elucidate mechanism of this effect, role of protein thiol groups and H(2)O(2) in insulin receptor autophosphorylation and glucose uptake was investigated in 3T3-L1 adipocytes following stimulation with alpha-lipoic acid. Alpha-lipoic acid or insulin treatment of adipocytes increased intracellular level of oxidants, decreased thiol reactivity of the insulin receptor beta-subunit, increased tyrosine phosphorylation of the insulin receptor, and enhanced glucose uptake. Alpha-lipoic acid or insulin-stimulated glucose uptake was inhibited (i) by alkylation of intracellular, but not extracellular, thiol groups downstream of insulin receptor activation, and (ii) by diphenylene iodonium at the level of the insulin receptor autophosphorylation. alpha-Lipoic acid also inhibited protein tyrosine phosphatase activity and decreased thiol reactivity of protein tyrosine phosphatase 1B. These findings indicate that oxidants produced by alpha-lipoic acid or insulin are involved in activation of insulin receptor and in inactivation of protein tyrosine phosphatases, which eventually result in elevated glucose uptake into 3T3-L1 adipocytes.

Reactive oxygen species induce swelling and cytochrome c release but not transmembrane depolarization in isolated rat brain mitochondria

1 In this study, we have used isolated brain mitochondria to investigate the effects of superoxide anions (O(2)(-)) on mitochondrial parameters related to apoptosis, such as swelling, potential, enzymatic activity, NAD(P)H, cytochrome c release, and caspase activity. 2 Addition of the reactive oxygen species (ROS) generator KO(2) produced brain mitochondrial swelling, which was blocked by cyclosporin A (CSA), and which was Ca(2+) independent. 3 Calcium induced mitochondrial swelling only at high concentrations and in the presence of succinate. This correlated with the increase in O(2)(-) production detected with hydroethidine in mitochondrial preparations exposed to Ca(2+) and the fact that ROS were required for Ca(2+)-induced mitochondrial swelling. 4 Superoxide anions, but not Ca(2+), decreased citrate synthase and dehydrogenase enzymatic activities and dropped total mitochondrial NAD(P)H levels. 5 Calcium, but not O(2)(-), triggered a rapid loss of mitochondrial potential. Calcium-induced Deltapsi(m) dissipation was inhibited by Ruthenium Red, but not by CSA. 6 Calcium- and superoxide-induced mitochondrial swelling released cytochrome c and increased caspase activity from isolated mitochondria in a CS A-sensitive manner. 7 In summary, superoxide potently triggers mitochondrial swelling and the release of proteins involved in activation of postmitochondrial apoptotic pathways in the absence of mitochondrial depolarization.

Aldh3a1 protects human corneal epithelial cells from ultraviolet- and 4-hydroxy-2-nonenal-induced oxidative damage

Aldehyde dehydrogenase 3A1 (ALDH3A1) is one of the most abundant proteins found in corneal epithelial cells of mammalian species, with several postulated protective roles that include detoxification of peroxidic aldehydes, scavenging of free radicals, and direct absorption of ultraviolet (UV) radiation. In the present study, the protective role of ALDH3A1 against UV- and 4-hydroxy-2-nonenal- (4-HNE-) induced oxidative damage was studied. For this purpose, human ALDH3A1 was stably transfected in a human corneal epithelial cell line (HCE) lacking endogenous enzyme. Cells transfected with ALDH3A1 were more resistant to UV- and 4-HNE-induced cytotoxicity than mock-transfected cells. DNA fragmentation assays revealed that both treatments induced apoptosis in mock-transfected cells, but not in ALDH3A1-expressing cells. Apoptosis appeared to occur via caspase-3 activation and subsequent PARP cleavage. The Michaelis-Menten constant (K(m)) for 4-HNE was 54 microM in ALDH3A1-transfected cells; the addition of 100 microM 4-HNE increased NAD(P)H levels by 50% above that in mock-transfected cells. We also found that ALDH3A1 expression prevented 4-HNE-induced protein adduct formation. Taken together, these data suggest that ALDH3A1 is a regulatory element of the cellular defense system that protects corneal epithelium against UV- and 4-HNE-induced oxidative damage.

Mechanistic and comparative studies of melatonin and classic antioxidants in terms of their interactions with the ABTS cation radical

Melatonin and classic antioxidants possess the capacity to scavenge ABTSb+ with IC50s of 4, 11, 15.5, 15.5, 17 and 21 microm for melatonin, glutathione, vitamin C, trolox, NADH and NADPH, respectively. In terms of scavenging ABTSb+, melatonin exhibits a different profile than that of the classic antioxidants. Classic antioxidants scavenge one or less ABTSb+, while each melatonin molecule can scavenge more than one ABTSb+, probably with a maximum of four. Classic antioxidants do not synergize when combined in terms of scavenging ABTSb+. However, a synergistic action is observed when melatonin is combined with any of the classic antioxidants. Cyclic voltammetry indicates that melatonin donates an electron at the potential of 715 mV. The scavenging mechanism of melatonin on ABTSb+ may involve multiple-electron donations via intermediates through a stepwise process. Intermediates including the melatoninyl cation radical, the melatoninyl neutral radical and cyclic 3-hydroxymelatonin (cyclic 3-OHM) and N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) seem to participate in these reactions. More interestingly, the pH of the solution dramatically modifies the ABTSb+ scavenging capacity of melatonin while pH changes have no measurable influence on the scavenging activity of classic antioxidants. An acidic pH markedly reduces the ABTSb+ scavenging capacity of melatonin while an increased pH promotes the interaction of melatonin and ABTSb+. The major melatonin metabolites that develop when melatonin interacts with ABTSb+ are cyclic 3-OHM and AFMK. Cyclic 3-OHM is the intermediate between melatonin and AFMK, and cyclic 3-OHM also has the ability to scavenge ABTSb+. Melatonin and the metabolites which are generated via the interaction of melatonin with ABTSb+, i.e. the melatoninyl cation radical, melatoninyl neutral radical and cyclic 3-OHM, all scavenge ABTSb+. This unique cascade action of melatonin, in terms of scavenging, increases its efficiency to neutralized ABTSb+; this contrasts with the effects of the classic antioxidants.

"Mitochondrial" photochemical drugs do not release toxic amounts of 1O(2) within the mitochondrial matrix space

Previously, we demonstrated that mitochondrial NAD(P)H is the primary target of singlet oxygen (1O(2)) generated by photoactivation of mitochondria-selective rhodamine derivatives. Hence, local NAD(P)H oxidation/fluorescence decrease may be used to reveal the site of intracellular 1O(2) generation. Therefore, in addition to the previously used tetramethylrhodamine methylester (TMRM), 2('),4('),5('),7(')-tetrabromorhodamine 123 bromide (TBRB) and rhodamine 123 (Rho 123), we tested here whether mitochondrial NAD(P)H of cultured hepatocytes is directly oxidized upon irradiation of different "mitochondrial" photosensitizers (Photofrin; protoporphyrin IX; Al(III) phthalocyanine chloride tetrasulfonic acid; meso-tetra(4-sulfonatophenyl)porphine dihydrochloride; Visudyne). In contrast to TMRM and Rho 123, which directly oxidized NAD(P)H upon irradiation, irradiation of intracellular TBRB and the photochemical drugs only indirectly affected mitochondrial NAD(P)H due to loss of mitochondrial integrity. In line with this result only TMRM and Rho 123 exclusively localized within the mitochondrial matrix. Due to these results it is doubtful whether real mitochondrial photosensitizers actually exist among the photochemical drugs applicable/used for photodynamic therapy.

Chromaffin cell death induced by 6-hydroxydopamine is independent of mitochondrial swelling and caspase activation

Our results provide evidence that 6-hydroxydopamine induced, after auto-oxidation, toxic levels of hydrogen peroxide (H2O2) that caused bovine chromaffin cell toxicity and death. 6-Hydroxydopamine (6-OHDA) treatment markedly reduced, in a dose-response fashion, chromaffin cell viability. Cell death was accompanied by cell shrinkage, nuclear condensation and DNA degradation. Under our experimental conditions, 6-OHDA auto-oxidation formed quinones and reactive oxygen species (ROS) that mainly contributed to 6-OHDA-induced cytotoxicity in bovine chromaffin cells. Accordingly, different antioxidants, including catalase, vitamin E, Mn(IIItetrakis(4-benzoic acid)porphyrin chloride (MnTBAP) or ascorbic acid, provided protection against 6-OHDA-induced toxicity. Further evidence that 6-OHDA induces oxidative stress is provided by the fact that this compound decreased total mitochondrial reduced NAD(P)H levels. Our results also suggest that mitochondrial swelling and caspase activation do not play a direct role in 6-OHDA-induced death in bovine chromaffin cells.

NAD(P)H, a primary target of 1O2 in mitochondria of intact cells

Direct reaction of NAD(P)H with oxidants like singlet oxygen ((1)O(2)) has not yet been demonstrated in biological systems. We therefore chose different rhodamine derivatives (tetramethylrhodamine methyl ester, TMRM; 2',4',5',7'-tetrabromorhodamine 123 bromide; and rhodamine 123; Rho 123) to selectively generate singlet oxygen within the NAD(P)H-rich mitochondrial matrix of cultured hepatocytes. In a cell-free system, photoactivation of all of these dyes led to the formation of (1)O(2), which readily oxidized NAD(P)H to NAD(P)(+). In hepatocytes loaded with the various dyes only TMRM and Rho 123 proved suited to generating (1)O(2) within the mitochondrial matrix space. Photoactivation of the intracellular dyes (TMRM for 5-10 s, Rho 123 for 60 s) led to a significant (29.6 +/- 8.2 and 30.2 +/- 5.2%) and rapid decrease in mitochondrial NAD(P)H fluorescence followed by a slow increase. Prolonged photoactivation (> or =15 s) of TMRM-loaded cells resulted in even stronger NAD(P)H oxidation, the rapid onset of mitochondrial permeability transition, and apoptotic cell death. These results demonstrate that NAD(P)H is the primary target for (1)O(2) in hepatocyte mitochondria. Thus NAD(P)H may operate directly as an intracellular antioxidant, as long as it is regenerated. At cell-injurious concentrations of the oxidant, however, NAD(P)H depletion may be the event that triggers cell death.

Topical application of NADH for the treatment of rosacea and contact dermatitis

Among many important physiological functions played by NADH (the reduced form of beta-nicotinamide adenine dinucleotide) its antioxidative properties are remarkable. Acting directly as an antioxidant, NADH can effectively protect the cell and its membrane from destruction by free radicals. NADH can be stabilized as a suspension in hydrophobic ointments prepared in a way that prevents contact with atmosphere containing oxygen and water. We present the first report of NADH as a treatment for some inflammatory dermatoses. It was found that topical application of 1% NADH diluted in Vaseline ointment can be very effective in the treatment of rosacea and contact dermatitis. Since no adverse effects were observed, therapy with NADH can be viewed as a potential alternative to other established treatments.

Cell signalling and the glutathione redox system

The reduction/oxidation (redox) state of the cell is a consequence of the balance between the levels of oxidising and reducing equivalents. A reducing intracellular environment is often associated with cell survival; however, redox unbalance is necessary since it represents a regulatory sensor for several nuclear transcription factors. Activator protein 1 (AP-1), nuclear factor-kappaB (NF-kappaB) and protein tyrosine phosphatases 1-B (PTP-1B) are some of the well-known molecular factors for which a redox modulation of their activity has been demonstrated. The glutathione buffer system modulates cell response to redox changes induced by either external or intracellular stimuli. This paper summarises recent knowledge on the role played by several redox modulators in inducing signalling events that finally regulate cell cycle progression.

The NADH oxidase activity of the plasma membrane of synaptosomes is a major source of superoxide anion and is inhibited by peroxynitrite

Plasma membrane vesicles from adult rat brain synaptosomes (PMV) have an ascorbate-dependent NADH oxidase activity of 35-40 nmol/min/(mg protein) at saturation by NADH. NADPH is a much less efficient substrate of this oxidase activity, with a Vmax 10-fold lower than that measured for NADH. Ascorbate-dependent NADH oxidase activity accounts for more than 90% of the total NADH oxidase activity of PMV and, in the absence of NADH and in the presence of 1 mm ascorbate, PMV produce ascorbate free radical (AFR) at a rate of 4.0 +/- 0.5 nmol AFR/min/(mg protein). NADH-dependent *O2- production by PMV occurs with a rate of 35 +/- 3 nmol/min/(mg protein), and is a coreaction product of the NADH oxidase activity, because: (i) it is inhibited by more than 90% by addition of ascorbate oxidase, (ii) it is inhibited by 1 micro g/mL wheat germ agglutinin (a potent inhibitor of the plasma membrane AFR reductase activity), and (iii) the KM(NADH) of the plasma membrane NADH oxidase activity and of NADH-dependent *O2- production are identical. Treatment of PMV with repetitive micromolar ONOO- pulses produced almost complete inhibition of the ascorbate-dependent NADH oxidase and *O2- production, and at 50% inhibition addition of coenzyme Q10 almost completely reverts this inhibition. Cytochrome c stimulated 2.5-fold the plasma membrane NADH oxidase, and pretreatment of PMV with repetitive 10 microm ONOO- pulses lowers the K0.5 for cytochrome c stimulation from 6 +/- 1 (control) to 1.5 +/- 0.5 microm. Thus, the ascorbate-dependent plasma membrane NADH oxidase activity can act as a source of neuronal.O2-, which is up-regulated by cytosolic cytochrome c and down-regulated under chronic oxidative stress conditions producing ONOO-.

Reactive sulphur species: an in vitro investigation of the oxidation properties of disulphide S-oxides

We have recently proposed that disulphide S-monoxides (thiosulphinates) and disulphide S-dioxides (thiosulphonates) are formed from their parent disulphides and 'reactive oxygen species' during oxidative stress. These 'reactive sulphur species' are themselves strong oxidizing agents that preferably attack the thiol functionality. We now show that under conditions where disulphides show little effect, disulphide S-oxides rapidly modify metallothionein, alcohol and glyceraldehyde 3-phosphate dehydrogenases and a zinc finger-protein fragment in vitro. The known antioxidants ascorbate, NADH, trolox and melatonin are unable to inhibit this oxidation pathway and only an excess of the cellular redox-buffer glutathione quenches the disulphide S-oxide activity. These results suggest that, under conditions of oxidative stress, despite the presence of high concentrations of antioxidants, reactive sulphur species formation may occur and inhibit the function of thiol-dependent proteins. Such a characterization of the disulphide S-oxide-oxidation pathway might also account for some previously observed anomalies in protein oxidation.

Formation of peroxynitrite from reaction of nitroxyl anion with molecular oxygen

Peroxynitrite (ONOO(-)/ONOOH) is generally expected to be formed in vivo from the diffusion-controlled reaction between superoxide (O(2)) and nitric oxide ((*)NO). In the present paper we show that under aerobic conditions the nitroxyl anion (NO(-)), released from Angeli's salt (disodium diazen-1-ium-1,2,2-triolate, (-)ON=NO(2)(-)), generated peroxynitrite with a yield of about 65%. Simultaneously, hydroxyl radicals are formed from the nitroxyl anion with a yield of about 3% via a minor, peroxynitrite-independent pathway. Further experiments clearly underline that the chemistry of NO(-) in the presence of oxygen is mainly characterized by peroxynitrite and not by HO( small middle dot) radicals. Quantum-chemical calculations predict that peroxynitrite formation should proceed via intermediary formation of (*)NO and O(2), probably by an electron-transfer mechanism. This prediction is supported by the fact that H(2)O(2) is formed during the decay of NO(-) in the presence of superoxide dismutase (Cu(II),Zn-SOD). Since the nitroxyl anion may be released endogenously by a variety of biomolecules, substantial amounts of peroxynitrite might be formed in vivo via NO(-) in addition to the "classical" ( small middle dot)NO + O(2)() pathway.

Reduced nicotinamide nucleotides prevent nitration of tyrosine hydroxylase by peroxynitrite

Tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter dopamine (DA). TH activity is inhibited by peroxynitrite (ONOO(-)) by a mechanism that involves nitration of tyrosine residues and oxidation of cysteine residues in the enzyme. Reduced forms of the nicotinamide adenine dinucleotide cofactors, NADH and NADPH, protect TH from inhibition by ONOO(-) and prevent nitration of tyrosine residues. NAD, the oxidized form of the cofactors, neither protects TH from ONOO(-)-induced inhibition nor prevents the nitration of tyrosine residues in the enzyme. These results suggest that the redox status of the nicotinamide nucleotide cofactors could influence the ability of ONOO(-) to modify proteins that are important to the function of DA neurons.

Reactive sulfur species: an emerging concept in oxidative stress

The ingredients of oxidative stress include a variety of reactive species such as reactive oxygen and reactive nitrogen species (ROS, RNS). While sulfur is usually considered as part of cellular antioxidant systems there is mounting evidence that reactive sulfur species (RSS) with stressor properties similar to the ones found in ROS are formed under conditions of oxidative stress. Thiols as well as disulfides are easily oxidised to sulfur species with sulfur in higher oxidation states. Such agents include thiyl radicals, disulfides, sulfenic acids and disulfide-S-oxides. They rapidly oxidise and subsequently inhibit thiol-proteins and enzymes and can be considered as a separate class of oxidative stressors providing new antioxidant drug targets.

The pathobiochemistry of nitrogen dioxide

Nitrogen dioxide (*NO2) is an oxidizing free radical which can initiate a variety of destructive pathways in living systems, and several diseases are suspected to be connected with both exogenously and endogenously formed *NO2. Peroxynitrite (ONOO-/ONOOH) is believed to be an important endogenous source of *NO2 radicals, but other sources, among them enzymatically ones, have been identified recently. It also became clear during the last few years that in vivo formation of 3-nitrotyrosine strictly depends on the availability of *NO2 radicals. Since nitrogen dioxide is a very toxic compound an arsenal of antioxidants (e.g. vitamin C, glutathione, vitamin E, and beta-carotene) must eliminate this harmful radical in vivo. Here the recently identified superoxide (O2*-)-dependent formation of peroxynitrate (O2NOO-) and the central role of vitamin C are of special importance.

Synaptosomal plasma membrane Ca(2+) pump activity inhibition by repetitive micromolar ONOO(-) pulses

A sustained increase of intracellular free [Ca(2+)] ([Ca(2+)](i)) has been shown to be an early event of neuronal cell death induced by peroxynitrite (ONOO(-)). In this paper, chronic exposure to ONOO(-) has been simulated by treatment of rat brain synaptosomes or plasma membrane vesicles with repetitive pulses of ONOO(-) during at most 50 min, which efficiently produced nitrotyrosine formation in several membrane proteins (including the Ca(2+)-ATPase). The plasma membrane Ca(2+)-ATPase activity at near-physiological conditions (pH 7, submicromolar Ca(2+), and millimolar Mg(2+)-ATP concentrations), which plays a major role in the control of synaptic [Ca(2+)](i), can be more than 75% inhibited by a sustained exposure to micromolar ONOO(-) (e.g., to 100 pulses of 10 microM ONOO(-)). This inhibition is irreversible and mostly due to a decreased V(max), and to the 2-fold increase of the K(0.5) for Ca(2+) stimulation and about 5-fold increase of the K(M) for Mg(2+)-ATP. [Ca(2+)](i) increases to >400 nM when synaptosomes are subjected to this treatment. Reduced glutathione can afford only partial protection against the inhibition produced by micromolar ONOO(-) pulses. Therefore, inhibition of the plasma membrane Ca(2+)-pump activity during chronic exposure to ONOO(-) may account by itself for a large and sustained increase of intracellular [Ca(2+)](i) in synaptic nerve terminals.

Optimizing the energy status of skin cells during solar radiation

Ionizing- and ultraviolet-radiation cause cell damage or death by directly altering DNA and protein structures and by production of reactive oxygen species (ROS) and reactive carbonyl species (RCS). These processes disrupt cellular energy metabolism at multiple levels. The formation of DNA strand breaks activates signaling pathways that consume NAD, which can lead to the depletion of cellular ATP. Poly(ADP)-ribose polymerase (PARP-1) is the enzyme responsible for much of the NAD degradation following DNA damage, although numerous other PARPs have been discovered recently that await functional characterization. Studies on mouse epidermis in vivo and on human cells in culture have shown that UV-B radiation provokes the transient degradation of NAD and the synthesis of ADP-ribose polymers by PARP-1. This enzyme functions as a component of a DNA damage surveillance network in eukaryotic cells to determine the fate of cells following genotoxic stress. Additionally, the activation of PARP-1 results in the activation of a nuclear proteasome that degrades damaged nuclear proteins including histones. Identifying approaches to optimize these responses while maintaining the energy status of cells is likely to be very important in minimizing the deleterious effects of solar radiation on skin.