Distinction between nitrosating mechanisms within human cells and aqueous solution

HIF-1 alpha protein as a target for S-nitrosation

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a master regulator to sense decreased oxygen partial pressure. HIF-1 alpha stability regulation initiates a complex biological response that allows cells to act appropriately to meet patho-physiological situations of decreased oxygen availability. Recently, nitric oxide emerged as a messenger with the ability to stabilize HIF-1 alpha and to transactivate HIF-1 under normoxia. Considering that reactive nitrogen species are recognized for post-translation protein modifications, among others S-nitrosation, we asked whether HIF-1 alpha is a target for S-nitrosation. In vitro NO+ donating NO donors such as GSNO and SNAP provoked massive S-nitrosation of purified HIF-1 alpha. All 15 free thiol groups found in human HIF-1 alpha are subjected to S-nitrosation. Thiol modification is not shared by spermine-NONOate, a NO radical donating compound. However, spermine-NONOate in the presence of O(2)(-), generated by xanthine/xanthine oxidase, regained S-nitrosation, most likely via formation of a N(2)O(3)-like species. In vitro, S-nitrosation of HIF-1 alpha was attenuated by the addition of GSH or ascorbate. In RCC4 and HEK293 cells GSNO or SNAP reproduced S-nitrosation of HIF-1 alpha, however with a significantly reduced potency that amounted to modification of three to four thiols, only. Importantly, endogenous formation of NO in RCC4 cells via inducible NO synthase elicited S-nitrosation of HIF-1 alpha that was sensitive to inhibition of inducible NO synthase activity with N-monomethyl-L-arginine. NO-stabilized HIF-1 alpha was susceptible to the addition of N-acetyl-cysteine that destabilized HIF-1 alpha in close correlation to the disappearance of S-nitrosated HIF-1 alpha. In conclusion, HIF-1 alpha is a target for S-nitrosation by exogenously and endogenously produced NO.

Mitochondrial reactive oxygen species regulate spatial profile of proinflammatory responses in lung venular capillaries

Cytokine-induced lung expression of the endothelial cell (EC) leukocyte receptor P-selectin initiates leukocyte rolling. To understand the early EC signaling that induces the expression, we conducted real-time digital imaging studies in lung venular capillaries. To compare receptor- vs nonreceptor-mediated effects, we infused capillaries with respectively, TNF-alpha and arachidonate. At concentrations adjusted to give equipotent increases in the cytosolic Ca(2+), both agents increased reactive oxygen species (ROS) production and EC P-selectin expression. Blocking the cytosolic Ca(2+) increases abolished ROS production; blocking ROS production abrogated P-selectin expression. TNF-alpha, but not arachidonate, released Ca(2+) from endoplasmic stores and increased mitochondrial Ca(2+). Furthermore, Ca(2+) depletion abrogated TNF-alpha responses partially, but arachidonate responses completely. These differences in Ca(2+) mobilization by TNF-alpha and arachidonate were reflected in spatial patterning in the capillary in that the TNF-alpha effects were localized at branch points, while the arachidonate effects were nonlocalized and extensive. Furthermore, mitochondrial blockers inhibited the TNF-alpha- but not the arachidonate-induced responses. These findings indicate that the different modes of Ca(2+) mobilization determined the spatial patterning of the proinflammatory response in lung capillaries. Responses to TNF-alpha revealed that EC mitochondria regulate the proinflammatory process by generating ROS that activate P-selectin expression.

Is N2O3 the main nitrosating intermediate in aerated nitric oxide (NO) solutions in vivo? If so, where, when, and which one?

The widespread opinion that N(2)O(3) as a product of NO oxidation is the only nitros(yl)ating agent under aerobic conditions is based on experiments in homogeneous buffered water solutions. In vivo NO is oxidized in heterogeneous media and this opinion is not correct. The equilibrium in the system being dependent on temperature and DeltaG((sol)) for NO, NO(2), isomers of both N(2)O(3), and N(2)O(4). For polar solvents including water, DeltaG((sol)) for N(2)O(3) is high enough, and a stationary concentration of N(2)O(3) in the mixture with other oxides is sufficient to guarantee the hydrolysis of N(2)O(3) to nitrite. In heterogeneous media, the mixture contains solvates NO(2(sol)), N(2)O(3(sol)), and N(2)O(4(sol)) at stationary nonequilibrium concentrations. As far as DeltaG((sol)) is decreased in heterogeneous mixtures with low polar solvents and/or at increased temperatures, the equilibrium in such a system shifts to NO(2). Although NO(2) is a reactive free radical, it almost does not react with water. In contrast, the reaction with most functional protein groups efficiently proceeds by a radical type with the formation of nitrite and new radicals (X) further stabilized in various forms. Therefore, the ratio of the nitrosylated and nitrated products yields depends on actual concentrations of all NO(x).

Novel role for low molecular weight plasma thiols in nitric oxide-mediated control of platelet function

Nitric oxide (NO) is a powerful antiplatelet agent, but its notoriously short biological half-life limits its potential to prevent the activation of circulating platelets. Here we used diethylamine diazeniumdiolate (DEA/NO) as an NO generator to determine whether the antiplatelet effects of NO are prolonged by the formation of a durable, plasma-borne S-nitrosothiol reservoir. Preincubation of both platelet rich plasma (PRP) and washed platelets (WP) with DEA/NO (2 microm) for 1 min inhibited collagen-induced platelet aggregation by 82 +/- 5 and 91 +/- 2%, respectively. After 30 min preincubation with DEA/NO, NO was no longer detectable in either preparation, but aggregation remained markedly inhibited (72 +/- 7%) in PRP. In contrast, the inhibitory effect in WP was almost completely lost at this time (5 +/- 3%) but was partially restored (39 +/- 10%) in WP containing human serum albumin (1%) and fully restored by co-incubation with albumin and the low molecular weight (LMW) thiols, glutathione, (5 microm), cysteinyl-glycine (10 microm), or cysteine (10 microm). This NO-mediated effect was not seen with LMW thiols in the absence of albumin and was associated with S-nitrosothiol formation. Our results demonstrate that LMW thiols play an important role in both the formation and activation of an S-nitrosoalbumin reservoir that significantly prolongs the duration of action of NO.

Ingress and reactive chemistry of nitroxyl-derived species within human cells

The mechanisms that control the biological signaling and toxicological properties of the nitrogen oxide species nitroxyl (HNO) are largely unknown. The ingress and intracellular reactivity of nitroxyl-derived species were examined using Angeli's salt (AS), which decomposes initially to HNO and nitrite at physiologic pH. Exposure of 4,5-diaminofluorescein (DAF) to AS resulted in fluorescent product formation only in the presence of molecular oxygen. Kinetic analysis and the lack of signal from a nitric oxide (NO)-sensitive electrode suggested that these processes did not involve conversion of HNO to NO. On an equimolar basis, bolus peroxynitrite (ONOO(-)) exposure generated only 15% of fluorescent product formation observed from AS decomposition. Moreover, infusion of synthetic ONOO(-) at a rate comparable to AS decomposition resulted in only 4% of the signal. Quenching of AS-mediated product formation within intact human MCF-7 breast carcinoma cells containing DAF by addition of urate to buffer suggested involvement of an oxidized intermediate formed from reaction between HNO and oxygen. Conversely, intact cells competitively sequestered the HNO-derived species from reaction with DAF in solution. These data show this intermediate to be a long-lived diffusible species. Relative product yield from intracellular DAF was decreased 5- to 8-fold when cells were lysed immediately prior to AS addition, consistent with the partitioning of HNO and/or derived species into the cellular membrane, thereby shielding these reactive intermediates from either hydrolysis or cytoplasmic scavenger pools. These findings establish that oxygen-derived species of nitroxyl can readily penetrate and engage the intracellular milieu of cells and suggest this process to be independent of NO and ONOO(-) intermediacy. The substantial facilitation of oxygen-dependent nitroxyl chemistry by intact lipid bilayers supports a focusing role for the membrane in modulation of cellular constituents proteins by this unique species.

Increased nitric oxide-dependent nitrosylation of 4,5-diaminofluorescein by oxidants: implications for the measurement of intracellular nitric oxide

4,5 diaminofluorescein (DAF-2) is increasingly utilized as a fluorescent detector for nitric oxide (*NO) in cells and tissues. In oxygenated solutions, reactive nitrogen species derived from (*) NO autoxidation nitrosate DAF-2 to yield the highly fluorescent DAF-2 triazole. In the present study, we investigated the nitrosation of DAF-2 at a neutral pH by absorption and fluorescence spectroscopy using NONOates as chemical sources of (*) NO. We found that both chemically synthesized peroxynitrite and horseradish peroxidase in the presence of hydrogen peroxide (H(2)O(2)) oxidized DAF-2 to a relatively stable nonfluorescent intermediate (t(1/2) approximately 90 s). Oxidation of DAF-2 prior to the addition of the z.rad;NO donor DEA/NO resulted in an increase in fluorescence that was approximately 7-fold higher than treatment with DEA/NO alone. The increase in DAF-2 triazole formation upon oxidation of DAF-2 was confirmed by high performance liquid chromatography. Peroxynitrite generated in situ from the equimolar production of (*) NO and superoxide (O(2)(*-)) also increased the yields of DAF-2 triazole formation, which was completely inhibited when O(2)(*-) was in excess of (*) NO. We propose that DAF-2 is oxidized to a free radical intermediate that directly reacts with (*) NO, thereby bypassing the requirement for (*)NO autoxidation for the formation of DAF-2 triazole. Our findings indicate that DAF-2 fluorometric assays are quantitatively difficult to interpret in cells and in solution when oxidants and (*) NO are co-generated.

Focusing of nitric oxide mediated nitrosation and oxidative nitrosylation as a consequence of reaction with superoxide

The impact of nitric oxide (NO) synthesis on different biological cascades can rapidly change dependent on the rate of NO formation and composition of the surrounding milieu. With this perspective, we used diaminonaphthalene (DAN) and diaminofluorescein (DAF) to examine the nitrosative chemistry derived from NO and superoxide (O2-) simultaneously generated at nanomolar to low micromolar per minute rates by spermine/NO decomposition and xanthine oxidase-catalyzed oxidation of hypoxanthine, respectively. Fluorescent triazole product formation from DAN and DAF increased as the ratio of O2- to NO approached equimolar, then decreased precipitously as O2- exceeded NO. This pattern was also evident in DAF-loaded MCF-7 carcinoma cells and when stimulated macrophages were used as the NO source. Cyclic voltammetry analysis and inhibition studies by using the N2O3 scavenger azide indicated that DAN- and DAF-triazole could be derived from both oxidative nitrosylation (e.g., DAF radical + NO) and nitrosation (NO+ addition). The latter mechanism predominated with higher rates of NO formation relative to O2-. The effects of oxymyoglobin, superoxide dismutase, and carbon dioxide were examined as potential modulators of reactant availability for the O2- + NO pathway in vivo. The findings suggest that the outcome of NO biosynthesis in a scavenger milieu can be focused by O2- toward formation of NO adducts on nucleophilic residues (e.g., amines, thiols, hydroxyl) through convergent mechanisms involving the intermediacy of nitrogen dioxide. These modifications may be favored in microenvironments where the rate of O2- production is temporally and spatially contemporaneous with nitric oxide synthase activity, but not in excess of NO generation.

A chemical perspective on the interplay between NO, reactive oxygen species, and reactive nitrogen oxide species

Nitric oxide (nitrogen monoxide, NO) plays a veritable cornucopia of regulatory roles in normal physiology. In contrast, NO has also been implicated in the etiology and sequela of numerous neurodegenerative diseases that involve reactive oxygen species (ROS) and nitrogen oxide species (RNOS). In this setting, NO is often viewed solely as pathogenic; however, the chemistry of NO can also be a significant factor in lessening injury mediated by both ROS and RNOS. The relationship between NO and oxidation, nitrosation, and nitration reactions is summarized. The salient factors that determine whether NO promotes, abates, or interconnects these chemistries are emphasized. From this perspective of NO chemistry, the type, magnitude, location, and duration of either ROS or RNOS reactions may be predicted.

Direct real-time evaluation of nitration with green fluorescent protein in solution and within human cells reveals the impact of nitrogen dioxide vs. peroxynitrite mechanisms

3-Nitrotyrosyl adducts in proteins have been detected in a wide range of diseases. The mechanisms by which reactive nitrogen oxide species may impede protein function through nitration were examined by using a unique model system, which exploits a critical tyrosyl residue in the fluorophoric pocket of recombinant green fluorescent protein (GFP). Exposure of purified GFP suspended in phosphate buffer to synthetic peroxynitrite in either 0.5 or 5 microM steps resulted in progressively increased 3-nitrotyrosyl immunoreactivity concomitant with disappearance of intrinsic fluorescence (IC(50) approximately 20 microM). Fluorescence from an equivalent amount of GFP expressed within intact MCF-7 tumor cells was largely resistant to this bolus treatment (IC(50) > 250 microM). The more physiologically relevant conditions of either peroxynitrite infusion (1 microM/min) or de novo formation by simultaneous, equimolar generation of nitric oxide (NO) and superoxide (e.g., 3-morpholinosydnonimine; NONOates plus xanthine oxidase/hypoxanthine, menadione, or mitomycin C) were examined. Despite robust oxidation of dihydrorhodamine under each of these conditions, fluorescence decrease of both purified and intracellular GFP was not evident regardless of carbon dioxide presence, suggesting that oxidation and nitration are not necessarily coupled. Alternatively, both extra- and intracellular GFP fluorescence was exquisitely sensitive to nitration produced by heme-peroxidase/hydrogen peroxide-catalyzed oxidation of nitrite. Formation of nitrogen dioxide (NO(2)) during the reaction between NO and the nitroxide 2-phenyl-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide indicated that NO(2) can enter cells and alter peptide function through tyrosyl nitration. Taken together, these findings exemplified that heme-peroxidase-catalyzed formation of NO(2) may play a pivotal role in inflammatory and chronic disease settings while calling into question the significance of nitration by peroxynitrite.

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.