Detection of dinitrosyl iron complexes by ozone-based chemiluminescence

Artifacts Introduced by Sample Handling in Chemiluminescence Assays of Nitric Oxide Metabolites

We recently developed a combination of four chemiluminescence-based assays for selective detection of different nitric oxide (NO) metabolites, including nitrite, S-nitrosothiols (SNOs), heme-nitrosyl (heme-NO), and dinitrosyl iron complexes (DNICs). However, these NO species (NOx) may be under dynamic equilibria during sample handling, which affects the final determination made from the readout of assays. Using fetal and maternal sheep from low and high altitudes (300 and 3801 m, respectively) as models of different NOx levels and compositions, we tested the hypothesis that sample handling introduces artifacts in chemiluminescence assays of NOx. Here, we demonstrate the following: (1) room temperature placement is associated with an increase and decrease in NOx in plasma and whole blood samples, respectively; (2) snap freezing and thawing lead to the interconversion of different NOx in plasma; (3) snap freezing and homogenization in liquid nitrogen eliminate a significant fraction of NOx in the aorta of stressed animals; (4) A "stop solution" commonly used to preserve nitrite and SNOs leads to the interconversion of different NOx in blood, while deproteinization results in a significant increase in detectable NOx; (5) some reagents widely used in sample pretreatments, such as mercury chloride, acid sulfanilamide, N-ethylmaleimide, ferricyanide, and anticoagulant ethylenediaminetetraacetic acid, have unintended effects that destabilize SNO, DNICs, and/or heme-NO; (6) blood, including the residual blood clot left in the washed purge vessel, quenches the signal of nitrite when using ascorbic acid and acetic acid as the purge vessel reagent; and (7) new limitations to the four chemiluminescence-based assays. This study points out the need for re-evaluation of previous chemiluminescence measurements of NOx, and calls for special attention to be paid to sample handling, as it can introduce significant artifacts into NOx assays.

Nitric Oxide Affects Heme Oxygenase-1, Hepcidin, and Transferrin Receptor Expression in the Placenta

Nitric oxide (NO) is a gasotransmitter that avidly binds both free and heme-bound iron, forming relatively stable iron nitrosyl compounds (FeNOs). We have previously demonstrated that FeNOs are present in the human placenta and are elevated in preeclampsia and intrauterine growth restriction. The ability of NO to sequester iron raises the possibility of the NO-mediated disruption of iron homeostasis in the placenta. In this work, we tested whether exposure of placental syncytiotrophoblasts or villous tissue explants to sub-cytotoxic concentrations of NO would elicit the formation of FeNOs. Furthermore, we measured changes in the mRNA and protein expression levels of key iron regulatory genes in response to NO exposure. Ozone-based chemiluminescence was used to measure concentrations of NO and its metabolites. Our results showed a significant increase in FeNO levels in placental cells and explants treated with NO (p < 0.0001). The mRNA and protein levels of HO-1 were significantly increased in both cultured syncytiotrophoblasts and villous tissue explants (p < 0.01), and the mRNA levels of hepcidin and transferrin receptor were significantly increased in culture syncytiotrophoblasts and villous tissue explants, respectively, (p < 0.01), while no changes were seen in the expression levels of divalent metal transporter-1 or ferroportin. These results suggest a potential role for NO in iron homeostasis in the human placenta and could be relevant for disorders of pregnancy such as fetal growth restriction and preeclampsia.

On-Site Detection of Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs) are of interest in many different fields. Among them are food and fragrance analysis, environmental and atmospheric research, industrial applications, security or medical and life science. In the past, the characterization of these compounds was mostly performed via sample collection and off-site analysis with gas chromatography coupled to mass spectrometry (GC-MS) as the gold standard. While powerful, this method also has several drawbacks such as being slow, expensive, and demanding on the user. For decades, intense research has been dedicated to find methods for fast VOC analysis on-site with time and spatial resolution. We present the working principles of the most important, utilized, and researched technologies for this purpose and highlight important publications from the last five years. In this overview, non-selective gas sensors, electronic noses, spectroscopic methods, miniaturized gas chromatography, ion mobility spectrometry and direct injection mass spectrometry are covered. The advantages and limitations of the different methods are compared. Finally, we give our outlook into the future progression of this field of research.

Chronic High-Altitude Hypoxia Alters Iron and Nitric Oxide Homeostasis in Fetal and Maternal Sheep Blood and Aorta

The mammalian fetus thrives at oxygen tensions much lower than those of adults. Gestation at high altitude superimposes hypoxic stresses on the fetus resulting in increased erythropoiesis. We hypothesized that chronic hypoxia at high altitude alters the homeostasis of iron and bioactive nitric oxide metabolites (NOx) in gestation. To test for this, electron paramagnetic resonance was used to provide unique measurements of iron, metalloproteins, and free radicals in the blood and aorta of fetal and maternal sheep from either high or low altitudes (3801 or 300 m). Using ozone-based chemiluminescence with selectivity for various NOx species, we determined the NOx levels in these samples immediately after collection. These experiments demonstrated a systemic redistribution of iron in high altitude fetuses as manifested by a decrease in both chelatable and total iron in the aorta and an increase in non-transferrin bound iron and total iron in plasma. Likewise, high altitude altered the redox status diversely in fetal blood and aorta. This study also found significant increases in blood and aortic tissue NOx in fetuses and mothers at high altitude. In addition, gradients in NOx concentrations observed between fetus and mother, umbilical artery and vein, and plasma and RBCs demonstrated complex dynamic homeostasis of NOx among these circulatory compartments, such as placental generation and efflux as well as fetal consumption of iron-nitrosyls in RBCs, probably HbNO. In conclusion, these results may suggest the utilization of iron from non-hematopoietic tissues iron for erythropoiesis in the fetus and increased NO bioavailability in response to chronic hypoxic stress at high altitude during gestation.

Iron nitrosyl complexes are formed from nitrite in the human placenta

Placental nitric oxide (NO) is critical for maintaining perfusion in the maternal-fetal-placental circulation during normal pregnancy. NO and its many metabolites are also increased in pregnancies complicated by maternal inflammation such as preeclampsia, fetal growth restriction, gestational diabetes, and bacterial infection. However, it is unclear how increased levels of NO or its metabolites affect placental function, or how the placenta deals with excessive levels of NO or its metabolites. Since there is uncertainty over the direction of change in plasma levels of NO metabolites in preeclampsia, we measured the levels of these metabolites at the placental tissue level. We found that NO metabolites are increased in placentas from patients with preeclampsia compared to healthy controls. We also discovered by ozone-based chemiluminescence and electron paramagnetic resonance that nitrite is efficiently converted into iron nitrosyl complexes (FeNOs) within the human placenta, and also observed the existence of endogenous FeNOs within placentas from sheep and rats. We show these nitrite-derived FeNOs are relatively short-lived, predominantly protein-bound, heme-iron nitrosyl complexes. The efficient formation of FeNOs from nitrite in the human placenta hints towards the importance of both nitrite and iron nitrosyl complexes in placental physiology or pathology. As iron nitrosylation is an important post-translational modification that affects the activity of multiple iron-containing proteins such as those in the electron transport chain, or those involved in epigenetic regulation, we conclude that FeNOs merit increased study in pregnancy complications.

Neuroprotective role of nitric oxide inhalation and nitrite in a Neonatal Rat Model of Hypoxic-Ischemic Injury

Background

There is evidence from various models of hypoxic-ischemic injury (HII) that nitric oxide (NO) is protective. We hypothesized that either inhaled NO (iNO) or nitrite would alleviate brain injury in neonatal HII via modulation of mitochondrial function.

Methods

We tested the effects of iNO and nitrite on the Rice-Vannucci model of HII in 7-day-old rats. Brain mitochondria were isolated for flow cytometry, aconitase activity, electron paramagnetic resonance, and Seahorse assays.

Results

Pretreatment of pups with iNO decreased survival in the Rice-Vannucci model of HII, while iNO administered post-insult did not. MRI analysis demonstrated that pre-HII iNO at 40 ppm and post-HII iNO at 20 ppm decreased the brain lesion sizes from 6.3±1.3% to 1.0±0.4% and 1.8±0.8%, respectively. Intraperitoneal nitrite at 0.165 μg/g improved neurobehavioral performance but was harmful at higher doses and had no effect on brain infarct size. NO reacted with complex IV at the heme a₃ site, decreased the oxidative stress of mitochondria challenged with anoxia and reoxygenation, and suppressed mitochondrial oxygen respiration.

Conclusions

This study suggests that iNO administered following neonatal HII may be neuroprotective, possibly via its modulation of mitochondrial function.

Tri-iodide and vanadium chloride based chemiluminescent methods for quantification of nitrogen oxides

Nitric Oxide (NO) is an important signaling molecule that plays roles in controlling vascular tone, hemostasis, host defense, and many other physiological functions. Low NO bioavailability contributes to pathology and NO administration has therapeutic potential in a variety of diseases. Thus, accurate measurements of NO bioavailability and reactivity are critical. Due to its short lifetime in vivo and many in vitro conditions, NO bioavailability and reactivity are often best determined by measuring NO congeners and metabolites that are more stable. Chemiluminescence-based detection of NO following chemical reduction of these compounds using the tri-iodide and vanadium chloride methods have been widely used in a variety of clinical and laboratory studies. In this review, we describe these methods used to detect nitrite, nitrate, nitrosothiols and other species and discuss limitations and proper controls.

Chemiluminescence Measurement of Reactive Sulfur and Nitrogen Species

Significance: Reactive sulfur and nitrogen species such as hydrogen sulfide (H₂S) and nitric oxide (NO^•) are ubiquitous cellular signaling molecules that play central roles in physiology and pathophysiology. A deeper understanding of these signaling pathways will offer new opportunities for therapeutic treatments and disease management. Recent Advances: Chemiluminescence methods have been fundamental in detecting and measuring biological reactive sulfur and nitrogen species, and new approaches are emerging for imaging these analytes in living intact specimens. Ozone-based and luminol-based chemiluminescence methods have been optimized for quantitative analysis of hydrogen sulfide and nitric oxide in biological samples and tissue homogenates, and caged luciferin and 1,2-dioxetanes are emerging as a versatile approach for monitoring and imaging reactive sulfur and nitrogen species in living cells and animal models. Critical Issues: This review article will cover the major chemiluminescence approaches for detecting, measuring, and imaging reactive sulfur and nitrogen species in biological systems, including a brief history of the development of the most established approaches and highlights of the opportunities provided by emerging approaches. Future Directions: Emerging chemiluminescence approaches offer new opportunities for monitoring and imaging reactive sulfur and nitrogen species in living cells, animals, and human clinical samples. Widespread adoption and translation of these approaches, however, requires an emphasis on rigorous quantitative methods, reproducibility, and effective technology transfer. Antioxid. Redox Signal. 36, 337-353.

S-nitrosothiols, and other products of nitrate metabolism, are increased in multiple human blood compartments following ingestion of beetroot juice

Ingested inorganic nitrate (NO₃⁻) has multiple effects in the human body including vasodilation, inhibition of platelet aggregation, and improved skeletal muscle function. The functional effects of oral NO₃⁻ involve the in vivo reduction of NO₃⁻ to nitrite (NO₂⁻) and thence to nitric oxide (NO). However, the potential involvement of S-nitrosothiol (RSNO) formation is unclear. We hypothesised that the RSNO concentration ([RSNO]) in red blood cells (RBCs) and plasma is increased by NO₃⁻-rich beetroot juice ingestion. In healthy human volunteers, we tested the effect of dietary supplementation with NO₃⁻-rich beetroot juice (BR) or NO₃⁻-depleted beetroot juice (placebo; PL) on [RSNO], [NO₃⁻] and [NO₂⁻] in RBCs, whole blood and plasma, as measured by ozone-based chemiluminescence. The median basal [RSNO] in plasma samples (n = 22) was 10 (5–13) nM (interquartile range in brackets). In comparison, the median values for basal [RSNO] in the corresponding RBC preparations (n = 19) and whole blood samples (n = 19) were higher (p < 0.001) than in plasma, being 40 (30–60) nM and 35 (25–80) nM, respectively. The median RBC [RSNO] in a separate cohort of healthy subjects (n = 5) was increased to 110 (93–125) nM after ingesting BR (12.8 mmol NO₃⁻) compared to a corresponding baseline value of 25 (21–31) nM (Mann-Whitney test, p < 0.01). The median plasma [RSNO] in another cohort of healthy subjects (n = 14) was increased almost ten-fold to 104 (58–151) nM after BR supplementation (7 × 6.4 mmol of NO₃⁻ over two days, p < 0.01) compared to PL. In conclusion, RBC and plasma [RSNO] are increased by BR ingestion. In addition to NO₂⁻, RSNO may be involved in dietary NO₃⁻ metabolism/actions.

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Human ingestion of NO₃⁻-rich beetroot juice caused increased plasma S-nitrosothiol levels compared with baseline.

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Beetroot juice ingestion also caused increased S-nitrosothiol and NO₂⁻ levels in red blood cells compared with baseline.

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RSNO formation may contribute to the physiological effects of dietary NO₃⁻.

Reference method for off-line analysis of nitrogen oxides in cell culture media by an ozone-based chemiluminescence detector

Nitric oxide (NO) and its by-products are important biological signals in human physiology and pathology particularly in the vascular and immune systems. Thus, in situ determination of the NO-related molecule (NO_x) levels using embedded sensors is of high importance particularly in the context of cellular biocompatibility testing. However, NO_x analytical reference method dedicated to the evaluation of biomaterial biocompatibility testing is lacking. Herein, we demonstrate a PAPA-NONOate-based reference method for the calibration of NO_x sensors. After, the validation of this reference method and its potentialities were demonstrated for the detection of the oxidative stress-related NO secretion of vascular endothelial cells in a 3D tissue issued from 3D printing. Such NO_x detection method can be an integral part of cell response to biomaterials. Graphical abstract.

Deferoxamine produces nitric oxide under ferricyanide oxidation, blood incubation, and UV-irradiation

Deferoxamine (DFO), an iron chelator, is used therapeutically for the removal of excess iron in multiple clinical conditions such as beta thalassemia and intracerebral hemorrhage. DFO is also used as an iron chelator and hypoxia-mimetic agent in in vivo and in vitro basic research. Here we unexpectedly discover DFO to be a nitric oxide (NO) precursor in experiments where it was intended to act as an iron chelator. Production of NO from aqueous solutions of DFO was directly observed by ozone-based chemiluminescence using a ferricyanide-based assay and was confirmed by electron paramagnetic resonance (EPR). DFO also produced NO following exposure to ultraviolet light, and its incubation with sheep adult and fetal blood resulted in considerable formation of iron nitrosyl hemoglobin, as confirmed by both visible spectroscopy and EPR. These results suggest that experiments using DFO can be confounded by concomitant production of NO, and offer new insight into some of DFO's unexplained clinical side effects such as hypotension.

Evidence for placental-derived iron-nitrosyls in the circulation of the fetal lamb and against a role for nitrite in mediating the cardiovascular transition at birth

Circulating metabolites of nitric oxide, such as nitrite, iron nitrosyls (FeNO), and nitrosothiols, have vasodilatory bioactivity. In both human and sheep neonates, plasma concentrations of these NO metabolite (NOx) concentrations fall >50% within minutes after birth, raising the possibility that circulating NOx plays a role in maintaining low fetal vascular resistance and in the cardiovascular transition at birth. To test whether the fall in plasma NOx concentrations at birth is due to either ligation of the umbilical cord or oxygenation of the fetus to newborn levels, plasma NOx concentrations were measured during stepwise delivery of near-term fetal lambs. When fetal lambs were intubated and mechanically ventilated with 100% O₂ to oxygenate the arterial blood while still in utero with the umbilical circulation still intact, there was no change in plasma NOx levels. In contrast, when the umbilical cord was ligated while fetal lambs were mechanically ventilated with O₂ levels that maintained fetal arterial blood gases, plasma NOx levels decreased by nearly 50%. Characterization of the individual NOx species in plasma revealed that the overall fall in NOx at birth was attributable mainly to FeNO compounds. Finally, when the typical fall in NOx after birth was prevented by intravenous nitrite infusion, birth-related changes in blood pressure, heart rate, and carotid flow changes were little affected, suggesting the cardiovascular transition at birth is not dependent on a fall in plasma NOx. In conclusion, this study shows FeNO is released from the placenta and that its decline accounts for most of the measured fall in plasma NOx at birth.

Nitric oxide metabolism in the human placenta during aberrant maternal inflammation

KEY POINTS

Nitric oxide (NO) is a gasotransmitter with important physiological and pathophysiological roles in pregnancy. There is limited information available about the sources and metabolism of NO and its bioactive metabolites (NOx) in both normal and complicated pregnancies. The present study characterized and quantified endogenous NOx in human and mouse placenta following determination of the stability of exogenous NOx in placental homogenates. NOx have differential stability in placental homogenates. NO and iron nitrosyl species (FeNOs), are relatively unstable in placental homogenates from normal placentas. Exogenous NO, nitrite and nitrosothiols react with placental homogenates to form iron nitrosyl complexes. FeNOs were also detected endogenously in mouse and human placenta. NOx levels in placental villous tissue are increased in fetal growth restriction vs. placentas from women with normal pregnancies, particularly in fetal growth restriction associated with pre-eclampsia. Villitis was not associated, however, with an increase in NOx levels in either normotensive or pre-eclamptic placentas. The results call for further investigation of FeNOs in normal and complicated pregnancies.

ABSTRACT

Nitric oxide (NO) is a gasotransmitter with important roles in pregnancy under both physiological and pathophysiological conditions. Although products of NO metabolism (NOx) also have significant bioactivity, little is known about the role of NO and NOx under conditions of aberrant placental inflammation during pregnancy. An ozone-based chemiluminescence approach was used to investigate the stability and metabolic fate of NOx in human placental homogenates from uncomplicated pregnancies in healthy mothers compared to that in placental tissue from normotensive and pre-eclamptic pregnancies complicated with fetal growth restriction (FGR) with and without villitis of unknown aetiology. We hypothesized that placental NOx would be increased in FGR vs. normal tissue, and be further increased in villitis vs. non-villitis placentas. Findings indicate that nitrate, nitrite and nitrosothiols, but not NO or iron nitrosyl species (FeNOs), are relatively stable in placental homogenates from normal placentas, and that NO, nitrite and nitrosothiols react with placental homogenates to form iron nitrosyl complexes. Furthermore, NOx levels in placental villous tissue are increased in FGR vs. placentas from women with normal pregnancies, particularly in FGR associated with pre-eclampsia. However, in contrast to our hypothesis, villitis was not associated with an increase in NOx levels in either normotensive or pre-eclamptic placentas. Our results also strongly support the involvement of FeNOs in both mouse and human placenta, and call for their further study as a critical mechanistic link between pre-eclampsia and fetal growth restriction.

L-NAME releases nitric oxide and potentiates subsequent nitroglycerin-mediated vasodilation

L-N^G-Nitro arginine methyl ester (L-NAME) has been widely applied for several decades in both basic and clinical research as an antagonist of nitric oxide synthase (NOS). Herein, we show that L-NAME slowly releases NO from its guanidino nitro group. Daily pretreatment of rats with L-NAME potentiated mesenteric vasodilation induced by nitrodilators such as nitroglycerin, but not by NO. Release of NO also occurred with the NOS-inactive enantiomer D-NAME, but not with L-arginine or another NOS inhibitor L-NMMA, consistent with the presence or absence of a nitro group in their structure and their nitrodilator-potentiating effects. Metabolic conversion of the nitro group to NO-related breakdown products was confirmed using isotopically-labeled L-NAME. Consistent with Fenton chemistry, transition metals and reactive oxygen species accelerated the release of NO from L-NAME. Both NO production from L-NAME and its nitrodilator-potentiating effects were augmented under inflammation. NO release by L-NAME can confound its intended NOS-inhibiting effects, possibly by contributing to a putative intracellular NO store in the vasculature.

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NOS-inhibitor L-NAME is also a precursor of NO.

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ROS releases NO from the nitro group of L-NAME via Fenton Chemistry.

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L-NAME potentates nitrodilator-mediated vasodilation.

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Nitroglycerin may cause vasodilation via activation of an intracellular NO store.