Nitric Oxide Cheletropic Traps (NOCTs) with Improved Thermal Stability and Water Solubility

Mechanism of Electrochemical Generation and Decomposition of Phthalimide-N-oxyl

Phthalimide N-oxyl (PINO) is a potent hydrogen atom transfer (HAT) catalyst that can be generated electrochemically from N-hydroxyphthalimide (NHPI). However, catalyst decomposition has limited its application. This paper details mechanistic studies of the generation and decomposition of PINO under electrochemical conditions. Voltammetric data, observations from bulk electrolysis, and computational studies suggest two primary aspects. First, base-promoted formation of PINO from NHPI occurs via multiple-site concerted proton-electron transfer (MS-CPET). Second, PINO decomposition occurs by at least two second-order paths, one of which is greatly enhanced by base. Optimal catalytic efficiency in PINO-catalyzed oxidations occurs in the presence of bases whose corresponding conjugate acids have pK_a's in the range of ∼11-15, which strikes a balance between promoting PINO formation and minimizing its decay.

Comment on “trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies” by J. Drujon et al., Phys. Chem. Chem. Phys., 2014, , 7513

The involvement of biradical intermediates/biradicaloid transition states in the benzocyclobutene ring opening step needs to be considered as well.

Enhanced electron transfer for hemoglobin entrapped in a cationic gemini surfactant films on electrode and the fabrication of nitric oxide biosensor

The direct electrical communication between hemoglobin (Hb) and GCE surface was achieved based on the immobilization of Hb in a cationic gemini surfactant film and characterized by electrochemical techniques. The cyclic voltammograms showed that direct electron transfer between Hb and electrode surface was obviously promoted and then a novel unmediated nitric oxide (NO) biosensor was constructed in view of this protein-based electrode. This modified electrode showed an enzyme-like activity towards the reduction of NO and its amperometric response to NO was well-behaved with a rapid response time and displaying Michaelis-Menten kinetics with a calculated Km(app) value of 84.37 micromol L(-1). The detection limit was estimated to be 2.00 x 10(-8)mol L(-1). This biosensor was behaving as expected that it had a good stability and reproducibility, a higher sensitivity and selectivity and should has a potential application in monitoring NO released from biologic samples.

First Direct Detection of 2,3-Dimethyl-2,3-diphenylcyclopropanone

3,5-dihydro-3,5-dialkyl-3,5-diaryl-4H-pyrazol-4-ones stimulate interest as potential precursors for 2,3-diarylcyclopropanones. Photoreactions of trans-3,5-dihydro-3,5-dimethyl-3,5-diphenyl-4H-pyrazol-4-one were studied by continuous-wave (CW) and pulsed laser UV photolysis revealing an intermediate that undergoes rearrangement to form cis- and trans-1,3-dimethyl-1-phenyl-2-indanones with the yield of ca. 60%. Steady-state photolysis (254 and 350 nm excitation) in different solvents produced an intermediate cyclohexadiene as evidenced by UV/vis, IR, and 1H NMR spectra. In contrast, the nanosecond laser pulsed photolysis at 355 nm produced 2,3-dimethyl-2,3-diphenylcyclopropanone along with two products of retro-1,3-dipolar addition phenylmethylketene and 1-phenyldiazoethane. These can be observed by time-resolved IR (TRIR) spectroscopy as characteristic absorption bands at 1814, 2101, and 2038 cm-1, respectively. Similar retro-1,3-dipolar addition showed 1-phenyldiazoethane formed following flash photolysis of 1-pyrazoline (trans-4,5-dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyrazol-4-ol). The formation of the corresponding cyclopropanone as well the products of retro-1,3-dipolar addition during photoreaction of starting pyrazol-4-one is directly confirmed by the nanosecond TRIR spectroscopy for the first time. On the basis of the CW and pulsed laser UV photolysis, a dynamic equilibrium between cyclopropanone and intermediate 2,4-diphenyl-3-pentanone-2,4-diyl (dimethyldiphenyloxyallyl) was proposed.

Cell type-dependent release of nitric oxide and/or reactive nitrogenoxide species from intracellular SIN-1: effects on cellular NAD(P)H

SIN-1 is frequently used in cell culture studies as an extracellularly operating generator of peroxynitrite. However, little is known about the nature of the reactive species produced intracellulary from SIN-1. SIN-1 can easily penetrate cells as exemplified for both L-929 mouse fibroblasts and bovine aortic endothelial cells (BAECs) by utilizing capillary zone electrophoresis. In L-929 cells, SIN-1 produced nitric oxide (*NO) as monitored by the fluorescent *NO scavenger FNOCT-1 and by means of a *NO electrode, as well as reactive nitrogenoxide species (RNOS, e.g. peroxynitrite, nitrogen dioxide, dinitrogen trioxide), as detected with the fluorescent indicator DAF-2. Laser scanning microscopy revealed that in L-929 cells SIN-1 -derived species initially oxidized the major fraction of the NAD(P)H within the cytosol and the nuclei, whereas the mitochondrial NAD(P)H level was somewhat increased. In marked contrast to this, in BAECs no evidence for *NO formation was found although the intracellular amount of SIN-1 was four-fold higher than in L-929 cells. In BAECs, the level of NAD(P)H was slightly decreased within the first 10 min after administration of SIN-1 in both the cytosol/nuclei and mitochondria. These observations reflect the capability of SIN-1 to generate intracellularly either almost exclusively RNOS as in BAECs, or RNOS and freely diffusing *NO as in L-929 cells. Nitric oxide as well as RNOS may decisively affect cellular metabolism as indicated by the alterations in the NAD(P)H level. Hence, care should be taken when applying SIN-1 as an exclusively peroxynitrite-generating compound in cell culture systems.

Spin trapping of nitric oxide by aci anions of nitroalkanes

In alkaline solutions, nitroalkanes (RCH2NO2) undergo deprotonation and rearrange to an aci anion (RHC=NO2-), which may function as a spin trap. Using electron paramagnetic resonance (EPR) spectroscopy, we have investigated suitability of aci anions of a series of nitroalkanes (CH3NO2, CH3CH2NO2, CH3(CH2)2NO2, and CH3(CH2)3NO2) to spin trap nitric oxide (*NO). Based on the observed EPR spectra, the general structure of the adducts, formed by addition of *NO to RHC=NO2-, was identified as nitronitroso dianion radicals of general formula [RC(NO)NO2]*2- in strong base (0.5 M NaOH), and as a mono-anion radical [RCH(NO)NO2]*- in alkaline buffers, pH 10-13. The hyperfine splitting on 14N in the -NO2 moiety (11.2-12.48 G) is distinctly different from the splitting on 14N in the -NO moiety of the adducts (5.23-6.5 G). The structure of the adducts was verified using 15N-labeled *NO, which produced radicals, in which triplet due to splitting on 14N (I = 1) in 14NO/aci nitro adducts was replaced by a doublet due to 15N (I = 1/2) in 15NO/aci nitro adducts. EPR spectra of aci nitromethane/NO adduct recorded in NaOH and NaOD (0.5 M) showed that the hydrogen at alpha-carbon can be exchanged for deuterium, consistent with structures of the adducts being [CH(NO)NO2]*2- and [CD(NO)NO2]*2-, respectively. These results indicate that nitroalkanes could potentially be used as prototypes for development of *NO-specific spin traps suitable for EPR analysis.

Stereospecific thermal isomerization of 2, 2-dimethylbenzocyclobutenols to 2-isopropenylphenyl alcohols

Thermolysis of the 1-alkyl-2,2-dimethylbenzocyclobutenol 3 at 160 degrees C gave the 2-isopropenylphenyl alcohol 8 through an (E)-dienol intermediate by a 1,5-sigmatropic hydrogen shift from the isopropylidene methyl group to the carbon bearing hydroxy group. In the thermolysis of each of the diastereomeric 2, 2-dimethylbenzocyclobutenols 6 and 7 which have a hydroxy group on the beta-carbon of the quaternary C(1)-alkyl substituent, the isomerization to the 2-isopropenylphenyl alcohols 10 and 11 took place stereospecifically through a twisted (E)-dienol intermediate. The configuration of the newly formed chiral center in 10 and 11 was the same as that of the ring carbon bearing hydroxy group in the starting 6 and 7.

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.

Nitric oxide-forming reaction between the iron-N-methyl-D-glucamine dithiocarbamate complex and nitrite

The objective of this study was to elucidate the origin of the nitric oxide-forming reactions from nitrite in the presence of the iron-N-methyl-D-glucamine dithiocarbamate complex ((MGD)(2)Fe(2+)). The (MGD)(2)Fe(2+) complex is commonly used in electron paramagnetic resonance (EPR) spectroscopic detection of NO both in vivo and in vitro. Although it is widely believed that only NO can react with (MGD)(2)Fe(2+) complex to form the (MGD)(2)Fe(2+).NO complex, a recent article reported that the (MGD)(2)Fe(2+) complex can react not only with NO, but also with nitrite to produce the characteristic triplet EPR signal of (MGD)(2)Fe(2+).NO (Hiramoto, K., Tomiyama, S., and Kikugawa, K. (1997) Free Radical Res. 27, 505-509). However, no detailed reaction mechanisms were given. Alternatively, nitrite is considered to be a spontaneous NO donor, especially at acidic pH values (Samouilov, A., Kuppusamy, P., and Zweier, J. L. (1998) Arch Biochem. Biophys. 357, 1-7). However, its production of nitric oxide at physiological pH is unclear. In this report, we demonstrate that the (MGD)(2)Fe(2+) complex and nitrite reacted to form NO as follows: 1) (MGD)(2)Fe(2).NO complex was produced at pH 7.4; 2) concomitantly, the (MGD)(3)Fe(3+) complex, which is the oxidized form of (MGD)(2)Fe(2+), was formed; 3) the rate of formation of the (MGD)(2)Fe(2+).NO complex was a function of the concentration of [Fe(2+)](2), [MGD], [H(+)] and [nitrite].

Detection and imaging of endogenously produced nitric oxide with electron paramagnetic resonance spectroscopy

Nitric oxide (NO) represents a new paradigm for second messengers in regulation. Despite the numerous physiological and pathophysiological functions of NO, its importance as an endogenous second messenger and a cytostatic and/or cytotoxic agent was unknown until 1987. Recent developments in detection methods for endogenous NO produced directly or indirectly from NO synthases (NOSs) have enabled major advances in our understanding of the role of NO in biological systems. The spin-trapping technique combined with electron paramagnetic resonance (EPR) spectroscopy is a method for analyzing NO production directly both in vivo and in vitro. Iron complexes with dithiocarbamate derivatives are noteworthy among the spin-trapping reagents for NO because NO has a high affinity for iron complexes. The resultant stable nitrosyl iron complexes exhibit an intense three-line signal at room temperature and an axial signal at low temperature. Besides the facility and wide applicability of this method, its outstanding feature is that noninvasive in vivo measurements are available by using a low-frequency EPR spectrometer. In this article, we review on previous and recent developments of in vitro, in vivo, and ex vivo EPR detection and imaging of endogenously produced NO.

Nitric oxide-forming reactions of the water-soluble nitric oxide spin-trapping agent, MGD

The objective of this study was to elucidate the nitric oxide-forming reactions of the iron-N-methyl-D-glucamine dithiocarbamate (Fe-MGD) complex from the nitrogen-containing compound hydroxyurea. The Fe2+(MGD)2 complex is commonly used in electron paramagnetic resonance (EPR) spectroscopic detection of NO both in vivo and in vitro. The reaction of Fe2+(MGD)2 with NO yields the resultant NO-Fe2+(DETC)2 complex, which has a characteristic triplet EPR signal. It is widely believed that only NO reacts with Fe2+(MGD)2 to form the NO-Fe2+(MGD)2 complex. In this report, the mechanism leading to the formation of NO-Fe2+(MGD)2 was investigated using oxygen-uptake studies in conjunction with the EPR spin-trapping technique. We found that the air oxidation of Fe2+(MGD)2 complex results in the formation of the Fe3+(MGD)3 complex, presumably concomitantly with superoxide (O3*-). Dismutation of superoxide forms hydrogen peroxide, which can subsequently reduce Fe3+(MGD)3 back to Fe2+(MGD)2. The addition of NO to the Fe3+(MGD)3 complex resulted in the formation of the NO-Fe2+(MGD)2 complex. Hydroxyurea is not considered to be a spontaneous NO donor, but has to be oxidized in order to form NO. We present data showing that in the presence of oxygen, Fe2+(MGD)2 can oxidize hydroxyurea to yield the stable NO-Fe2+(MGD)2 complex. These results imply that hydroxyurea can be oxidized by reactive oxygen species that are formed from the air oxidation of the Fe2+(MGD)2 complex. Formation of the NO-Fe2+(MGD)2 complex in this case could erroneously be interpreted as spontaneous formation of NO from hydroxyurea. The chemistry of the Fe2+(MGD)2 complexes in aerobic conditions must be taken into account in order to avoid erroneous conclusions. In addition, the use of these complexes may contribute to the overall oxidative stress of the system under investigation.

Changes of plasma L-arginine levels in spontaneously hypertensive rats under induced hypotension

Nicardipine, a dihydropyridine type calcium channel blocker, was infused at two flow-rates into spontaneously hypertensive (SH) and control normotensive Wistar-Kyoto (WKY) rats (young, 6-week-old and adult, 23-week-old, n = 5) under pentobarbital anesthesia, to cause hypotension. Mean arterial blood pressure and the concentrations of plasma amino acids and norepinephrine (NE) were measured before infusion and at each step of the infusion. The reduction in blood pressure caused by nicardipine induced a decrease in plasma L-arginine concentration in both young and adult SH rats, this effect being larger in adult rats. There was no significant change in plasma levels of L-arginine in age-matched WKY rats. The concentration of other amino acids did not change in both rat strains. On the contrary, there was an increase in plasma NE concentration in both SH and WKY rats after infusion with nicardipine. Plasma L-arginine concentration showed a good inverse correlation with the logarithm of plasma NE concentration in SH and WKY rats and the correlation was expressed as Y = -alpha log(X) + m (Y, plasma L-arginine concentration (nmol/mL); X, plasma NE concentration (pmol/mL); alpha, a slope; and m, an intercept). alpha, 43.0 and 4.35 for 23-week-old SH and WKY rats, respectively, and 17.0 and 4.0 for 6-week-old SH and WKY rats, respectively. The present data together with previous data suggest a direct noradrenergic stimulation of the synthesis of nitric oxide (NO) from L-arginine. The findings also indicate an impairment of the L-arginine metabolism or pools in SH rats compared with WKY rats. The deficiency of L-arginine increases with the age of SH rats and could be related to the development and maintenance of hypertension due to inefficient production of NO.

NO release from NO donors and nitrovasodilators: comparisons between oxyhemoglobin and potentiometric assays

Unraveling the biology, pharmacology, and toxicology of NO depends on accurate NO assays, two of the more common being the oxyHb (oxyhemoglobin) assay and potentiometric detection using a Clark-type NO-selective electrode. Comparison of the specificity and sensitivity of the oxyHb and potentiometric methods was carried out using a broad series of nitrovasodilators, including organic nitrates, nitrites, thionitrates, nitrosothiols, and diazenium diolates. Only with the more labile diazenium diolates was a linear relationship observed between the rates of NO release measured potentiometrically and the rate of oxyHb oxidation from the oxyHb assay. The nonlinear plots indicate that N,O-species other than NO itself are capable of oxidizing oxyHb.

Nitric oxide-induced oxidation of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid under aerobic conditions: non-enzymatic route to melanin pigments of potential relevance to skin (photo)protection

Diffusible melanin-related metabolites have recently been suggested to subserve a variety of functions that are critical for protection of skin against inflammatory stimuli and oxidative tissue injury. We report here the results of in vitro studies showing that 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) exhibit a marked reactivity toward potentially cytotoxic nitrogen oxides produced by autoxidation of nitric oxide (NO) under physiologically relevant conditions. Exposure of DHI or DHICA to NO in air-equilibrated 0.1 M phosphate buffer, pH 7.4, resulted in a fast, concentration-dependent consumption of the substrates and the concomitant deposition of dark melanin-like pigments. All NO-induced oxidations were completely inhibited in the absence of oxygen. Addition of 10 microM DHI and DHICA completely prevented the oxidation of 10 microM alpha-tocopherol in 0.1 M phosphate buffer, pH 7.4 in the presence of 300 microM NO. Overall, these results shed light on novel oxidative pathways of melanin-related metabolites of possible relevance to the mechanisms of skin hyperpigmentation under oxidative stress conditions.

Reaction of substituted anthracenes and a butadiene with nitric oxide: product formation determined by EPR spectroscopy

When nitric oxide (NO), generated from nitric acid and copper, was reacted with a series of 9,10-substituted anthracenes, only 9,10-dimethylanthracene gave EPR detectable nitroxide radicals, although the expected bicyclic nitroxide arising from cheletropic NO addition across the 9,10-positions was not observed. Purity of the NO is crucial as the presence of higher oxides of nitrogen leads to radicals by hydrogen abstraction which are trapped by NO and the resultant nitroso compounds produce stable nitroxides detectable by EPR. In contrast the acyclic system, 3,4-diphenyl-2,5-dimethyl-2,4-hexadiene gives rise to an EPR spectrum consistent with cheletropic NO addition, although higher oxides of nitrogen again mediate the formation of different nitroxides.

Determination of nitrite and nitrate reduction by capillary ion electrophoresis

Production of nitrates and nitrites is a common step in many methodologies used to measure nitric oxide (NO) and NO-derived products in biological fluids. We report conditions that allow the rapid separation and quantification of nitrite from nitrate ions in biological fluids by capillary ion electrophoresis (CIE). CIE can be used to directly quantify nitrites and nitrates near the millimolar range. To detect lower levels, we have used CIE to monitor the reduction of nitrites and nitrates to NO for chemiluminescence detection. For reduction reactions, we directly compared the ability of three commonly used agents--potassium iodide (KI), mercuric chloride (HgCl2) and vanadium chloride (VCl3)--to reduce nitrite and nitrate ions to NO. Nitrites/nitrates can be efficiently reduced to NO at 37 degrees C using vanadium chloride (100%) or HgCl2 (80%). However, these CE-derived conditions cannot simply be extrapolated to chemiluminescence measurements. Vanadium (III) yields high background in the photomultiplier that diminishes the sensitivity of chemiluminescence measurement to that outside of physiological ranges. We find that reactions carried out at 37 degrees C in 2 M HCl using HgCl2 is efficient using both techniques.

Reaction of Phenyl-Substituted o-Quinodimethanes with Nitric Oxide. Are Benzocyclobutenes Suitable Precursors for Nitric Oxide Cheletropic Traps?

In order to elucidate the potential of substituted o-quinodimethanes as reagents for the trapping of nitric oxide (NO) in biological systems, the reaction of alkoxyl- and alkyl-substituted 7,8-diphenyl- and 7,7,8-triphenyl-o-quinodimethanes with nitric oxide in solution was investigated by ESR spectroscopic and UV/vis stopped-flow techniques. Photolytic decarbonylation of 1,3-diphenyl- and 1,1,3-triphenylindan-2-ones gave the corresponding phenyl-substituted benzocyclobutenes as the major products and low photostationary concentrations of o-quinodimethanes. During 266-nm laser flash photolysis (LFP) of 1,3-dimethoxy-1,3-diphenylindan-2-one and 1-methoxy-1,3,3-triphenylindan-2-one in acetonitrile, species absorbing in the 400-600 nm range were produced, which were attributed to configurational isomers of the corresponding 7,7,8,8-substituted o-quinodimethanes. The isomeric o-quinodimethanes decayed at significantly different rates, indicating a strong influence of the relative orientation of the terminal substituents on their stability. Reaction of the raw photolysates of the 2-indanones with NO produced strong ESR spectra of the corresponding cyclic nitroxide radicals, isoindolin-2-oxyls. The nitroxide radicals were generated in a two-phase process, the first, rapid phase being attributed to the reaction of NO with the photolytically formed o-quinodimethanes and the second, slow phase reflecting the reaction with small amounts of o-quinodimethanes, generated by thermal ring opening of the phenyl-substituted benzocyclobutenes and probably a direct reaction of NO with the benzocyclobutenes. The kinetics of both steps, as evaluated by stopped-flow UV/vis and ESR spectroscopy, revealed a strong dependence of the rate constants of the o-quinodimethane + NO reaction on the substitution pattern of the o-quinodimethanes, with rate constants spanning a range of 10-4000 M(-)(1) s(-)(1). The rate constants ((0.4-7.5) x 10(-)(4) s(-)(1)) for the reaction of NO with the 7,7,8,8-tetrasubstituted benzocyclobutenes are much less influenced by the substitution pattern. The utility of phenyl-substituted benzocyclobutenes as "reservoirs" for o-quinodimethane-type nitric oxide traps is discussed.

Nitric oxide-induced oxidation of alpha-tocopherol

Exposure of alpha-tocopherol (alpha-T) to nitric oxide under aerobic conditions resulted in a complex oxidation process whose final outcome was dictated by the nature of the reaction medium. In a cyclohexane solution, a prevailing route led to a mixture of relatively unstable polar products positive to Griess reagent. On standing at room temperature these were partially converted to the novel 2,3-dimethyl-4-acetyl-4-hydroxy-5-nitroso-2-cyclopentenone derivative. Reaction of alpha-T via a secondary oxidation path led to the formation of alpha-tocopherylquinone (alpha-TQ) as well as of little amounts of the corresponding nitrite ester. A quite different product pattern was observed when the reaction was carried out on a suspension of alpha-T in 0.1 M phosphate buffer, pH 7.4. Besides a significant formation of alpha-TQ and its nitrite ester, product analysis revealed a characteristic pattern of apolar compounds consisting of a yellow dimer and a series of related oligomers. These results provide an improved chemical background to inquire into the role of alpha-T in nitric oxide-induced tissue injury.

In vivo EPR detection and imaging of endogenous nitric oxide in lipopolysaccharide-treated mice

Nitric oxide (NO), a simple diatomic free radical, is known to play a critical physiological role in diverse organisms. An iron complex, with N-(dithiocarboxy)sarcosine (Fe-DTCS), has a high affinity for endogenous NO and can trap, stabilize, and accumulate it. The stable NO adduct thus formed is detectable at room temperature with electron paramagnetic resonance (EPR) spectrometry. We report in vivo EPR imaging of endogenous NO, trapped by an Fe-DTCS complex, in the abdomen of a live mouse. To our knowledge, this is the first report on EPR imaging of endogenous free radicals produced in vivo. This EPR imaging method will be useful for the noninvasive investigation of the spatial distribution of NO in pathologic organs or tissues.

Continuous monitoring of cellular nitric oxide generation by spin trapping with an iron-dithiocarbamate complex

Nitric oxide (NO) generation in murine macrophages was determined in real time using the electron paramagnetic resonance (EPR) spin trapping method. An iron complex of N-methyl D-glucamine dithiocarbamate was utilized as the spin trap. This spin trapping compound reacts with NO in solution to form a specific room-temperature stable, mononitrosyl complex which is readily detected and identified by EPR spectroscopy. Mouse peritoneal macrophages were placed in an EPR sample-cell and activated by lipopolysaccharide and gamma-interferon at 37 degrees C, followed by an additional incubation in oxygenated medium without these activation agents. After various incubation periods, spin trap solution was infused to replace the medium in the sample-cell, and the time-evolution of the EPR signal of the spin adduct (NO-complex) was recorded. Rates of NO generation were calculated based upon the initial slopes of the increase in the EPR intensity with time. In comparison to the NO (or NO2-) generation rate obtained under similar experimental conditions using the Griess reaction assay, the spin trapping method was found to be more sensitive, with a lowest limit of the detection of 3 pmol/min. In addition, by using the spin trapping method, NO generation from the same cells could be measured consecutively during various stages of activation, because infusion of the spin trap solution did not affect the viability of macrophages.