Mechanism For NO Production Research Articles

(206) Strategies of Nitric Oxide Augmentation for PDE5 Inhibitors

Abstract Introduction Erectile function and endothelial functions are closely related entities. Nitric oxide NO regulates circulation and microcirculation involved in erection.NO concentration is reduced with aging, diabetes, environment pollution smoking, obesity, inflammation, high prolactin, low testosterone oxidative stress and atherosclerosis. Many of erectile dysfunction patients are PDE5 inhibitor resistant. Strategies to enhance NO expression, can be employed to convert treatment resistant patient into responder. Objective Employ every possible mechanism to augment nitric oxide expression. Greater base line nitric oxide better would be PDE5 inhibitor respose. Methods Internet search was made on various website including pub med, science direct, google scholars using key words like erection, erectile dysfunction, nitric oxide, l arginine, oxidative stress, antioxidant, NOS, endothelial dysfunction, atherosclerosis and etc. This was followed by literature study on given topic. Results Efficacy of PDE 5 inhibitor in ED treatment is 70%. NO concentration decline to 50% by age of 40 and is only 15% at age 60. Humming can increase nasal NO level up to 15 time. Conclusions NO regulates endothelial function, vasodilatation, platelet aggregation inhibition, free radical scavenging, testosterone synthesis and neuronal protection. Study of mechanism of NO production provides, strategies for its enhancement. L arginine, L citrulline, serve as substrate and vitamin C, tetrahydrobiopterin (BH4) act as co factors in production of NO. Endothelial nitric oxide synthase e NOS, neuronal nitric oxide synthase n NOS are major enzymes act involved in synthesis. Oxygen is required for NO synthesis process. Supplementation of substrate and cofactors help increase NO levels. Oxygen availability can be increased through exercise and raising hemoglobin level. Reactive oxygen species ROS convert NO into bioinactive peroxynitrite which itself is ROS. Oxidative stress can cause endothelial nitric oxide synthase uncoupling. Consequently eNOS start producing ROS. This causes oxidation of NOS and as well as of cofactor like BH4 and vitamin C. ROS also increase level of methylarginines. Antioxidant used to neutralize ROS include Co Q10, alpha lipoic acid, flavonoids, bioflavonoids poly phenols, pycnogenol, resveterol, fulvic acid and etc. Second mechanism NO production is from dietary nitrates. Nutritional nitrates are converted into nitrite by oral anaerobic bacteria’s. Nitrate rich foods like beet root or spinach are added to enhance nitrate based NO. Nitric oxide thus produced is used for vasodilatation of penile vessels as well sinuses of corpus cavernousm. Apart from that it inhibits uncoupling of NOS. This support L arginine based mechanism of NO production. Nitroglycerin or minoxidal act as NO donors. These agents can be used locally at glans penis to augment NO availability. Bee breathing is maneuver in which humming sound vibrates nasal sinus increasing NO production up to 15 folds. Inhaled NO act as bronchodilator which increase lung capacity leading to higher blood oxygen concentration. Various NO enhancement strategies thus applied, may convert non responder PDE5 inhibitor to responders. Disclosure No.

The vascular dilatation induced by Hydroxysafflor yellow A (HSYA) on rat mesenteric artery through TRPV4-dependent calcium influx in endothelial cells

Ethnopharmacological relevanceHydroxysafflor yellow A (HSYA) is the principal constituent of the flowers of Carthamus tinctorius L., a traditional Chinese herbal medicine, which has been used for the treatment of cerebrovascular and cardiovascular diseases due to its property of promoting blood circulation and removing blood stasis. It is dominated in the water extract of Carthamus tinctorius L., which has been used in the clinical treatment for cardiovascular diseases. HSYA exerts a variety of pharmacological efficacy upon the vascular system. However, the underlying mechanisms remain unclear. Aim of the studyTo investigate the vascular dilatation effect of HSYA on rat mesenteric artery (MA) and its potential mechanism. Materials and methodsAdult male Wistar rats were applied to the study. Tension studies were conducted to determine the dilatation activity of HSYA against pre-contracted mesenteric arterial (MA) rings by U 46619 and Phenylephrine (PE). The vascular activities were measured with or without incubation with some selective inhibitors, including L-N(ω)-nitro-L-arginine methyl ester (L-NAME, a nitro oxide synthase inhibitor), HC-067047 (a selective TRPV4 antagonist), BaCl2 (a Kir channel blocker), and Indomethacin (Indo, a nonselective cyclooxygenase inhibitor), respectively. Immunocytochemistry, Calcium Imaging, NO Production detection, and Western Blot were also employed to further study the underlying mechanism. ResultsHSYA reversed the constriction of MAs induced by U 46619 in a manner of concentration dependency, and the dilatation capability was reversed by L-NAME. This effect was significantly dependent on the intactness of MA endothelium, accompanying an increment of NO production in mesenteric arterial endothelium cells. The increment of NO production was reversed by inhibiting the PKA. Also, the expression of p-eNOS was activated by HSYA shown in Western Blot assays. The cells imaging revealed a significant increase and drop of the influx of Ca2+ before and after treatment with HC-067047. ConclusionsThese findings suggest that HSYA exerts vessel dilation effect on MAs via a TRPV4-dependent influx of Ca2+ in endothelium cells, PKA-dependent eNOS phosphorylation and NO production mechanism. The present study indicates that HSYA has the potential to be a future candidate for the treatment of hypertension.

Development of combustion strategy for the internal combustion engine fueled by ammonia and its operating characteristics

Focusing on the possibility of using ammonia as a hydrogen carrier, a combustion strategy for pure ammonia as an engine fuel to convert the stored energy into a usable form has been proposed, which uses ammonia itself as a combustion promotor. Under this strategy, the conditions of sufficiently high temperature and pressure to cause the spray combustion of ammonia are obtained by the auto-ignition of a mixture of pilot-injected ammonia and air. To confirm the feasibility and the operating characteristics of an engine with the proposed ammonia combustion strategy, engine modeling has been conducted, the combustion strategy has been simulated, and a parametric study has been performed. To analyze the NO production mechanisms in an engine with the proposed combustion strategy, the NO production process has been classified into four phases, and the NO production in each phase has been analyzed under various start of injection timing and fuel amounts.

Effects of strain rate and CO2 on no formation in CH4/N2/O2 counter-flow diffusion flames

The reduction of nitrogen oxides in the high temperature flame is the key factor affecting the oxygen-enriched combustion performance. A numerical study using an OPPDIF code with detailed chemistry mechanism GRI 3.0 was carried out to focus on the effect of strain rate (25-130 s?1) and CO2 addition (0-0.59) on the oxidizer side on NO emission in CH4 / N2 / O2 counter-flow diffusion flame. The mole fraction profiles of flame structures, NO, NO2 and some selected radicals (H, O, OH) and the sensitivity of the dominant reactions contributing to NO formation in the counter-flow diffusion flames of CH4\/ N2 /O2 and CH4 / N2 / O2 / CO2 were obtained. The results indicated that the flame temperature and the amount of NO were reduced while the sensitivity of reactions to the prompt NO formation was gradually increased with the increasing strain rate. Furthermore, it is shown that with the increasing CO2 concentration in oxidizer, CO2 was directly involved in the reaction of NO consumption. The flame temperature and NO production were decreased dramatically and the mechanism of NO production was transformed from the thermal to prompt route.

ВПЛИВ БЛОКАДИ Ш-СИНТАЗИ НА ВАЗОДИЛАТАЦІЙНИЙ ЕФЕКТ ДІЕТИЛОВОГО ЕСТЕРУ 5-АЛКІЛАМІНО-2-{М-[М-БЕНЗОЇЛ-(4-МЕТИЛБЕНЗИЛІДЕН) ГЛІЦИЛ] АМІНОМЕТИЛ}-1,3-ОКСАЗОЛ-4-ІЛФОСФОНОВОЇ КИСЛОТИ НА СЕГМЕНТАХ ІЗОЛЬОВАНОЇ АОРТИ ЩУРІВ

ВПЛИВ БЛОКАДИ Ш-СИНТАЗИ НА ВАЗОДИЛАТАЦІЙНИЙ ЕФЕКТ ДІЕТИЛОВОГО ЕСТЕРУ 5-АЛКІЛАМІНО-2-{М-[М-БЕНЗОЇЛ-(4-МЕТИЛБЕНЗИЛІДЕН) ГЛІЦИЛ] АМІНОМЕТИЛ}-1,3-ОКСАЗОЛ-4-ІЛФОСФОНОВОЇ КИСЛОТИ НА СЕГМЕНТАХ ІЗОЛЬОВАНОЇ АОРТИ ЩУРІВ

Nitrosylhemoglobin in photodynamically stressed human tumors growing in nude mice

The role of nitric oxide in human tumor biology and therapy has been the subject of extensive studies. However, there is only limited knowledge about the mechanisms of NO production and its metabolism, and about the role NO can play in modern therapeutic procedures, such as photodynamic therapy. Here, for the first time, we report the presence of nitrosylhemoglobin, a stable complex of NO, in human lung adenocarcinoma A549 tumors growing in situ in nude mice. Using electron paramagnetic resonance spectroscopy we show that the level of nitrosylhemoglobin increases in the course of photodynamic therapy and that the phenomenon is local. Even the destruction of strongly vascularized normal liver tissue did not induce the paramagnetic signal, despite bringing about tissue necrosis. We conclude that photodynamic stress substantiates NO production and blood extravasation in situ, both processes on-going even in non-treated tumors, although at a lower intensity.

Increased exhaled nitric oxide and its oxidation metabolism in eosinophilic chronic rhinosinusitis

Monitoring of fractional concentrations of exhaled nitric oxide (FeNO) has become a reliable marker of inflammation in human nose and paranasal sinuses. However, it is still unknown to what extent nasal NO levels contribute to the pathology of chronic rhinosinusitis (CRS). In the present study, we aimed to examine FeNO levels and the underlying mechanism of NO production and metabolism in patients with eosinophilic chronic rhinosinusitis (ECRS) and non-ECRS. Thirty-three untreated ECRS patients, 16 non-ECRS patients, and 38 normal subjects were enrolled in this cross-sectional study of FeNO levels. Oral and nasal FeNO levels were measured before treatment using an electrochemical NO analyzer (NObreath(®)) with a nose adaptor. The mRNA expression of three nitric oxide synthase (NOS) isoforms, interleukin-5 (IL-5), and transforming growth factor-beta (TGF-β) in the ethmoid sinus mucosa and nasal polyps were analyzed by real-time PCR. Immunohistological localization of inducible NOS (iNOS) and nitrotyrosine (NT), a marker for oxidized NO metabolites, was also examined. ECRS patients showed significantly higher oral FeNO levels compared to non-ECRS patients and normal subjects (mean values, 47.6, 13.5, and 15.3ppb, respectively). Nasal FeNO levels of the non-ECRS patients (30.5ppb) were significantly lower than those of the ECRS patients (53.9ppb) and normal subjects (45.5ppb). Positive correlations existed between the blood eosinophil percentage and FeNO levels in ECRS patients. Histologically, ECRS patients showed higher eosinophil accumulation in the ethmoid mucosa than non-ECRS patients (103.1 vs. 16.3cells/HPF). Real-time PCR analysis showed significant upregulation of iNOS and IL-5 mRNA expression in the ethmoid mucosa of the ECRS patients compared to those of non-ECRS patients. Positive iNOS immunoreactivity was observed in ciliated epithelial cells, submucosal glands and associated inflammatory cells in both groups. NT immunoreactivity was detected in the epithelium and around inflammatory cells. Intense NT staining was co-localized with eosinophil accumulation and ECRS patients showed significantly higher rates of NT-positive cells than non-ECRS patients. A combination of oral and nasal FeNO measurement is a valid marker for the classification and definition of different CRS subtypes in Japan. Higher levels of oral and nasal FeNO in ECRS patients may reflect the persistence of eosinophilic inflammation in sinus mucosa with concomitant iNOS upregulation and accompanying deposition of oxidized NO metabolites.

NOx Formation in H2-CH4 Blended Flame Under MILD Conditions

In this article, NO production mechanisms for CH4-H2 combustion under MILD (moderate or intense low-oxygen dilution) conditions are studied using CDF and also zero-dimensional well stirred reactor (WSR) analysis. A H2/CH4 jet into a heated and diluted coflow is modeled in CFD analysis. The RANS equations with modified k − ϵ equations are solved in an axisymmetric 2D computational domain. The GRI2.11 full mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration, fuel hydrogen content, and fuel jet Reynolds number are studied on NO formation reactions. Results show that the measurements are predicted with an acceptable accuracy. The NNH and N2O routes are the most important pathways in NO formation under MILD conditions. An increase in oxidizer O2concentration or decrease in fuel hydrogen content addressesthe decrease in importance of the mentioned NO formation routes.

Numerical Study on Effect of Hydrogen Peroxide Additive on Ethanol HCCI Engine

In this article, based on a combined chemical mechanism with detailed ethanol oxidization and NO production mechanisms, a single cylinder ethanol HCCI engine model was established using the software CHEMKIN. Comparing with experimental data, this model can well predict cylinder pressure and NO emission. By changing mole fraction of hydrogen peroxide in initial ethanol mixture at different conditions, the effect of hydrogen peroxide additive on ethanol HCCI engine performance was investigated. The results show that hydrogen peroxide can effectively improve cylinder pressure and advance heat release progress, without notably increasing NO production.

Kinetics and mechanism of NO production in the RuIII-(edta) mediated oxidation of l-arginine with H2O2

The kinetics of the Ru(III)-(edta) (edta(4-) = ethylenediaminetetraacetate) catalyzed oxidation of l-arginine by H(2)O(2) mimicking the action of nitric oxide synthases (NOSs) has been studied spectrophotometrically. The time course of the reaction of [Ru(V)(edta)O](-) with l-arginine was followed at 390 nm under catalytic turn-over conditions. Formation of NO in the reacting system has been confirmed with an isolated nitric oxide free radical analyzer. A detailed reaction mechanism in agreement with the spectral and kinetic data is presented.

The characteristics of NO production mechanism on flue gas recirculation in oxy-firing condition

The characteristics of NO production mechanisms in counter-flow flames with gasified coal syngas have been computationally studied through detailed chemical reactions and transport properties. The combustion of gasified coal syngas with N2/O2 mixture causes a significant increase in nitric oxide emission due to the higher flame temperature and considerable amount of nitrogen in the oxidant. The nitric oxide emission can be reduced efficiently by using a CO2/O2 mixture instead of an N2/O2 mixture as the oxidant. The NO production mechanism was analyzed in detail through the overall chemical reaction pathways. The phenomenon of NO decomposition was observed for flue gas recirculation when the CO2/O2 mixture was used as the oxidizer and coal was gasified with O2/steam. The adoption of O2/steam gasification in the oxy-firing condition could considerably reduce the thermal NO and fuel NO due to the low nitrogen in the fuel and low flame temperature. However, reverse reactions of fuel NO as in NO→HNO and HNO→NH were observed when combustion flue gases were recirculated. Also, the fuel-N conversion rate of coal syngas combustion was investigated in the oxy-firing condition.

Laser-saturated fluorescence of nitric oxide and chemiluminescence measurements in premixed ethanol flames

In this study, nitric oxide laser-saturated fluorescence (LSF) measurements were acquired from premixed ethanol flames at atmospheric pressure in a burner. NO-LSF experimental profiles for fuel-rich premixed ethanol flames ( Φ = 1.34 and Φ = 1.66) were determined through the excitation/detection scheme of the Q 2(26.5) rotational line in the A 2 Σ + − X 2 Π (0, 0) vibronic band and γ(0, 1) emission band. A calibration procedure by NO doping into the flame was applied to establish the NO concentration profiles in these flames. Chemiluminescent emission measurements in the (0, 0) vibronic emission bands of the OH * ( A 2 Σ + − X 2 Π) and CH * ( A 2 Δ − X 2 Π) radicals were also obtained with high spatial and spectral resolution for fuel-rich premixed ethanol flames to correlate them with NO concentrations. Experimental chemiluminescence profiles and the ratios of the integrated areas under emission spectra (A CH*/A CH*(max.) and A CH*/A OH*) were determined. The relationships between chemiluminescence and NO concentrations were established along the premixed ethanol flames. There was a strong connection between CH * radical chemiluminescence and NO formation and the prompt-NO was identified as the governing mechanism for NO production. The results suggest the optimum ratio of the chemiluminescence of two radicals (A CH*/A OH*) for NO diagnostic purposes.

Nitric oxide density measurements in air and air/fuel nanosecond pulse discharges by laser induced fluorescence

Laser induced fluorescence is used to measure absolute nitric oxide concentrations in air, methane–air and ethylene–air non-equilibrium plasmas, as a function of time after initiation of a single pulse, 20 kV peak voltage, 25 ns pulse duration discharge. A mixture of NO and nitrogen with known composition (4.18 ppm NO) is used for calibration. Peak NO density in air at 60 Torr, after a single pulse, is ∼8 × 1012 cm−3 (∼4.14 ppm) occurring at ∼250 µs after the pulse, with decay time of ∼16.5 ms. Peak NO atom mole fraction in a methane–air mixture with equivalence ratio of φ = 0.5 is found to be approximately equal to that in air, with approximately the same rise and decay rate. In an ethylene–air mixture (also with equivalence ratio of φ = 0.5), the rise and decay times are comparable to air and methane–air, but the peak NO concentration is reduced by a factor of approximately 2.5. Spontaneous emission measurements show that excited electronic states N2(C 3Π) and NO(A 2Σ) in air at P = 60 Torr decay within ∼20 ns and ∼1 µs, respectively. Kinetic modelling calculations incorporating air plasma kinetics complemented with the GRI Mech 3.0 hydrocarbon oxidation mechanism are compared with the experimental data using three different NO production mechanisms. It is found that NO concentration rise after the discharge pulse is much faster than predicted by Zel'dovich mechanism reactions, by two orders of magnitude, but much slower compared with reactions of electronically excited nitrogen atoms and molecules, also by two orders of magnitude. It is concluded that processes involving long lifetime (∼100 µs) metastable states, such as N2(X 1Σ,v) and O2(b 1Σ), formed by quenching of the metastable N2(A 3Σ) state by ground electronic state O2, may play a dominant role in NO formation. NO decay, in all cases, is found to be dominated by the reverse Zel'dovich reaction, NO + O → N + O2, as well as by conversion into NO2 in a reaction of NO with ozone.

Computational and experimental investigation of the interaction of soot and NO in coflow diffusion flames

A combined computational and experimental investigation that examines the relationship of soot formation and NO in coflow ethylene air diffusion flames is presented. While both NO and soot formation are often studied independently, there is a need to understand their coupled relationship as a function of system parameters such as fuel type, temperature and pressure. The temperature decrease due to radiative losses in systems in which significant soot is produced can affect flame length and other temperature-dependent processes such as the formation of NO. The results of a computational model that includes a sectional representation for soot formation with a radiation model are compared against laser-induced fluorescence measurements of NO. The sooting characteristics of these flames have been studied previously. Experimentally, a laser near 225.8nm is used to excite the γ(0,0) band in NO. Spectrally resolved fluorescence emission is imaged radially, for the (0,0), (0,1), (0,2), (0,3), and (0,4) vibrational bands, at varying axial heights to create a two-dimensional image of NO fluorescence. A reverse quenching correction is applied to the computational results to determine an expected fluorescence signal for comparison with experimental results. Modeling results confirm that Fenimore NO is the dominant mechanism for NO production and suggest that for lightly sooting flames (peak soot volume fraction<0.5ppm), soot reduces only the Zeldovich NO formation (by a factor of two). For flames with increased soot levels (peak soot volume fraction∼4ppm), the model indicates not only that Zeldovich NO decreases by a factor of 2.5 through radiation loss, but that non-Zeldovich NO is reduced in the top center of the flame by about 30% through the oxidation of soot.

Novel Mechanism of Endothelial Nitric Oxide Synthase Activation Mediated by Caveolae Internalization in Endothelial Cells

Caveolin-1, the caveolae scaffolding protein, binds to and negatively regulates eNOS activity. As caveolin-1 also regulates caveolae-mediated endocytosis after activation of the 60-kDa albumin-binding glycoprotein gp60 in endothelial cells, we addressed the possibility that endothelial NO synthase (eNOS)-dependent NO production was functionally coupled to caveolae internalization. We observed that gp60-induced activation of endocytosis increased NO production within 2 minutes and up to 20 minutes. NOS inhibitor N(G)-nitro-L-arginine (L-NNA) prevented the NO production. To determine the role of caveolae internalization in the mechanism of NO production, we expressed dominant-negative dynamin-2 mutant (K44A) or treated cells with methyl-beta-cyclodextrin. Both interventions inhibited caveolae-mediated endocytosis and NO generation induced by gp60. We determined the role of signaling via Src kinase in the observed coupling of endocytosis to eNOS activation. Src activation induced the phosphorylation of caveolin-1, Akt and eNOS, and promoted dissociation of eNOS from caveolin-1. Inhibitors of Src kinase and Akt also prevented NO production. In isolated perfused mouse lungs, gp60 activation induced NO-dependent vasodilation, whereas the response was attenuated in eNOS(-/-) or caveolin-1(-/-) lungs. Together, these results demonstrate a critical role of caveolae-mediated endocytosis in regulating eNOS activation in endothelial cells and thereby the NO-dependent vasomotor tone.

Evaluation of chemical effects of added CO2 according flame location

A numerical study has been conducted to clearly grasp the impact of chemical effects caused by added CO2 and of flame location on flame structure and NO emission behaviour. Flame location affects the major source reaction of CO formation, CO2+H→CO+OH and the H-removal reaction, CH4+H→CH3+H2. It is, as a result, seen that the reduction of maximum flame temperature due to chemical effects for fuel-side dilution is mainly caused by the competition of the principal chain branching reaction with the reaction, CH4+H→CH3+H2, while that for fuel-side dilution is attributed to the competition of the principal chain branching reaction with the reaction, CO2+H→CO+OH. The importance of the NNH mechanism for NO production, where the reaction pathway is NNH→NH→HNO, is recognized. In C-related reactions most of NO is the direct outcome of (R171) and the contribution of (R171) becomes more and more important with increasing amount of added CO2 as much as the reaction step (R171) competes with the key reaction of thermal mechanism, (R237), for N atom. This indicates a possibility that NO emission in hydrogen flames diluted with CO2 shows less dependent behaviour upon flame temperature. Copyright © 2004 John Wiley & Sons, Ltd.

Differences in the mechanism of nitric oxide production between mouse vascular endothelial cells and macrophages.

The detailed mechanism of NO production in mouse vascular endothelial cells, END-D, was studied. The NO production in END-D cells was triggered by gamma interferon (IFN-gamma), but not LPS. However, LPS augmented the NO production in IFN-gamma-stimulated END-D cells. A high level of NO production was due to the expression of an inducible type of NO synthase (iNOS) in those cells. A significant amount of NO was detected 18 h after IFN-gamma stimulation, accompanied by the delayed iNOS expression. The JAK/STAT signal pathway mediated IFN-gamma-induced NO production, but did not participate in the LPS-induced augmentation. Further, no activation of nuclear factor (NF)-kappaB was involved in the NO production in END-D cells stimulated with either IFN-gamma and/or LPS. The mechanism of NO production in END-D cells was suggested to be different from that in mouse macrophages. The differential regulation of NO production in mouse vascular endothelial cells and macrophages is discussed.

Differences in the mechanism of nitric oxide production between mouse vascular endothelial cells and macrophages

The detailed mechanism of NO production in mouse vascular endothelial cells, END-D, was studied. The NO production in END-D cells was triggered by gamma interferon (IFN-γ), but not LPS. However, LPS augmented the NO production in IFN-γ-stimulated END-D cells. A high level of NO production was due to the expression of an inducible type of NO synthase (iNOS) in those cells. A significant amount of NO was detected 18 h after IFN-γ stimulation, accompanied by the delayed iNOS expression. The JAK/STAT signal pathway mediated IFN-γ-induced NO production, but did not participate in the LPS-induced augmentation. Further, no activation of nuclear factor (NF)-κB was involved in the NO production in END-D cells stimulated with either IFN-γ and/or LPS. The mechanism of NO production in END-D cells was suggested to be different from that in mouse macrophages. The differential regulation of NO production in mouse vascular endothelial cells and macrophages is discussed.

P-183 - The mechanism of NO production induced by H2P2 in cultured endothelial cells

P-183 - The mechanism of NO production induced by H2P2 in cultured endothelial cells

Evidence for a new no production mechanism in flames

Bozzelli and Dean [1] recently proposed a new mechanism for NO production via the reaction O + NNH ⇔ NH + NO that predicts formation of significant amounts of NO in flames burning in air. To test the predictions of this new NNH mechanism, we designed low-pressure flame experiments to mininize the amount of NO made through other, more established mechanisms and to yield measurable amounts of NO through NNH chemistry. We used laser-induced fluorescence (LIF) to measure NO in 38 and 78 torr, Φ =1.5, hydrogen/air flames. No is observed in both flames at∼0.3 ppm, consistent with model predictions using the Bozzelli and Dean mechanism. We also measured OH and temperature profiles and tested thoroughly for potential contamination sources. Though agreement of experiment and model is imperfect, the NNH mechanism provides a viable explanation for the presence of ppm levels of NO in these flames. The NNH mechanism should therefore be included in NO x combustion models and may be a significant route to NO in many flames.