Thiocyanate Cannot Inhibit the Formation of Reactive Nitrogen Species in the Human Oral Cavity in the Presence of High Concentrations of Nitrite: Detection of Reactive Nitrogen Species with 4,5-Diaminofluorescein

Pitfalls and limitations in using 4,5-diaminofluorescein for evaluating the influence of polyphenols on nitric oxide release from endothelial cells

The reagent 4,5-diaminofluorescein (DAF-2) is a widely utilized and sensitive fluorescent probe for real-time assessment of nitric oxide (NO) production. In this study we investigated the feasibility of using DAF-2 for detection of NO release from EA.hy 926 human endothelial cells stimulated with plant polyphenols. Flavonoids have recently gained much interest because of reported beneficial effects on vasodilatation, which have been ascribed to stimulation of endothelial NO production. DAF-2 shows moderate fluorescence, and because certain phenolic compounds quench fluorescence or fluoresce themselves, we utilized liquid chromatography to avoid interference. Our investigations with (+)-catechin and trans-resveratrol as test phenolic compounds revealed various previously undescribed principal methodologic pitfalls and limitations. Under assay conditions (+)-catechin displayed a highly significant increase in fluorescence intensity so that a control of test compound stability is advisable. Moreover, DAF-2 was subject to conversion to triazolofluorescein (DAF-2T) under certain assay and storage conditions; thus control of spontaneous reagent conversion is advisable. Finally, formation of DAF-2T was dose-dependently inhibited by polyphenols to a degree consistent with their free radical scavenging activity. The inhibition of DAF-2T generation seems to contradict previous reports on enhanced NO release from endothelial cells by (+)-catechin and resveratrol. Therefore, the planning of experiments involving NO measurement in biological systems and interpretation of results requires substantial scrutiny.

Interaction of 5-aminosalicylic acid with nitrous acid: formation of the diazonium derivative and nitric oxide release

The reaction of 5-aminosalicylic acid (5-ASA) with nitrous acid has been studied at low pH under conditions that simulate a gastric environment. The course of the reaction was followed by UV–visible and fluorescence spectroscopy and the products were analyzed by high performance liquid chromatography (HPLC) with UV–visible and mass spectroscopic detectors. In addition, the formation of nitric oxide (NO) was estimated electrochemically. 5-ASA was readily consumed in a process catalyzed by chloride and thiocyanate, whose rate is first order in 5-ASA and second order in nitrous acid. 2-Hydroxy-5-diazonium benzoic acid (diazonium derivative) and NO were detected as products of the reaction. From the NO formation profiles, it is concluded that NO is produced as a minor product in a process parallel to the path that leads to generation of the diazonium derivative. While the formation of NO could be beneficial for the protection of the stomach, the generation of the diazonium derivative could be considered a potentially toxic process.

Nitrogen dioxide-dependent oxidation of uric acid in the human oral cavity under acidic conditions: implications for its occurrence in acidic dental plaque

The pH in dental plaque falls to below 5 after the ingestion of foods, and it may remain low if acid-tolerant bacteria grow in the plaque. Certain nitrate-reducing bacteria in the oral cavity can proliferate in dental plaque at low pH, and nitrite is detected in such plaque. In acidic dental plaque, NO(2) can be produced by self-decomposition of nitrous acid and also by peroxidase-catalyzed oxidation of nitrite, and it may oxidize uric acid, a major antioxidant in the oral cavity. Under experimental conditions that simulate oral cavity, the oxidation of uric acid by nitrite and by nitrite/peroxidase systems was much more rapid at pH 5 than at pH 7, suggesting the more rapid production of NO(2) in dental plaque at lower pH. We propose that if the pH of plaque developed in a dental crevice decreased, NO(2) and other nitrogen oxides produced in the plaque would diffuse into the adjoining gingival tissues. The results of this study seem to contribute to the understanding of the induction of periodontal diseases in the context of nitrite-dependent production of nitrogen oxides in acidic dental plaque.

Micro gas analyzer measurement of nitric oxide in breath by direct wet scrubbing and fluorescence detection

A novel method is proposed to measure NO in breath. Breath NO is a useful diagnostic measure for asthma patients. Due to the low water solubility of NO, existing wet chemical NO measurements are conducted on NO(2) after removal of pre-existing NO(2) and conversion of NO to NO(2). In contrast, this study utilizes direct measurement of NO by wet chemistry. Gaseous NO was collected into an aqueous phase by a honeycomb-patterned microchannel scrubber and reacted with diaminofluorescein-2 (DAF-2). Fluorescence of the product was measured using a miniature detector, comprising a blue light-emitting diode (LED) and a photodiode. The response intensity was found to dramatically increase following addition of NO(2) into the absorbing solution or air sample. By optimizing the conditions, the sensitivity obtained was sufficient to measure parts per billion by volume levels of NO continuously. The system was applied to real analysis of NO in breath, and the effect of coexisting compounds was investigated. The proposed system could successfully measure breath NO.

Nitration of the salivary component 4-hydroxyphenylacetic acid in the human oral cavity: enhancement of nitration under acidic conditions

4-Hydroxyphenylacetic acid (HPA) and nitrite are present in human mixed whole saliva, and HPA can be nitrated by peroxidase/hydrogen peroxide (H(2)O(2))/nitrite systems in the oral cavity. Thus, the objectives of the present study were to estimate the concentrations of HPA, nitrated HPA [4-hydroxy-3-nitrophenylacetic acid (NO(2)HPA)], nitrite, and thiocyanate (SCN(-)) in saliva from 73 patients with periodontal diseases and to elucidate the conditions necessary to induce nitration of HPA. High concentrations of HPA, nitrite, and SCN(-) were found in the saliva of patients older than 50 yr of age. NO(2)HPA was detected in seven patients who were older than 60 yr of age. Nitrite-dependent formation of NO(2)HPA by a bacterial fraction prepared from mixed whole saliva was faster at pH 5.3 than at pH 7, and increased as the rate of H(2)O(2) formation increased. The formation of NO(2)HPA was inhibited by SCN(-) and by salivary antioxidants such as uric acid, ascorbic acid, and glutathione. These results suggest that nitration can proceed at an acidic site in the oral cavity where H(2)O(2) is produced under conditions of decreased concentrations of SCN(-) and of antioxidants.

Reactions of thiocyanate in the mixture of nitrite and hydrogen peroxide under acidic conditions: Investigation of the reactions simulating the mixture of saliva and gastric juice

Nitrite and SCN(-) in saliva can mixes with H(2)O(2) in the stomach. The mixing can result in the formation of ONOOH. It is not yet known how salivary SCN(-) reacts with ONOOH. An objective of the present study was to elucidate the reaction between ONOOH and SCN(-). In nitrite/H(2)O(2) systems at pH 2, SCN(-) inhibited the consumption of nitrite and the formation of O(3)(-). SCN(-) enhanced the decomposition of ONOOH and H(2)O(2) in HNO(2)/H(2)O(2) systems. Accompanying the reactions, sulfate was formed, suggesting that ONOOH oxidized SCN(-). SCN(-) inhibited the nitration of phenolics induced by HNO(2)/H(2)O(2). The inhibition is discussed taking SCN(-)-dependent reduction of ONOOH to HNO(2) into consideration. SCN(-) also inhibited H(2)O(2)-induced consumption of nitrite and nitration of phenolics in acidified saliva. The result obtained in this study suggests that salivary SCN(-) can reduce ONOOH to O(2)(-)/HNO(2) inhibiting nitrating reactions in the stomach.

NO/peroxynitrite dynamics of high glucose-exposed HUVECs: chemiluminescent measurement and computational model

Pathogenesis of many of diabetes-related vascular complications is associated with endothelial cell (EC) dysfunction, which is reduced bioavailability of EC-released nitric oxide (NO). Interaction dynamics of NO, superoxide (O(2)(-)) and peroxynitrite (ONOO(-)) are dependent on both their productions and consumptions through various pathways. Quantitative knowledge of these interaction dynamics in high glucose-induced EC dysfunction remains poorly understood. We developed an integrated experimental and computational approach to gain a quantitative understanding of the interactions of NO, O(2)(-) and ONOO(-) in high glucose-exposed ECs. End-products, nitrite and nitrate, were measured using a chemiluminescence analyzer. A computational biochemical reaction network model was developed to predict the effect of high glucose on ECs NO, O(2)(-) and ONOO(-). ECs NO and O(2)(-) productions increased in high glucose as evidenced by increased total NOx concentration, primarily increasing nitrate concentration. The model predicted an increase in O(2)(-) and ONOO(-) concentrations and a decrease in NO concentration in high glucose conditions. Administration of superoxide dismutase (SOD) decreased O(2)(-) concentration and increased NO concentration, thus SOD improved high glucose-induced changes in these interactions. An important finding of this study was that the NO bioavailability decreased in high glucose conditions even though NO production of EC increased. The integrated approach provides a framework to predict NO, O(2)(-) and ONOO(-) concentrations and productions that are difficult to measure in one experiment and will be useful in further EC dysfunction studies.

Reduction of nitrous Acid to nitric oxide by coffee melanoidins and enhancement of the reduction by thiocyanate: possibility of its occurrence in the stomach

Reactions of nitrous acid with freeze-dried instant coffee and its methanol-insoluble melanoidin fractions were studied at pH 2 in the presence and absence of thiocyanate (SCN (-)), simulating the mixture of coffee, saliva, and gastric juice. Coffee contained stable radicals, and the radical concentration increased by ferricyanide and decreased by ascorbic acid. This result indicates that the radical concentration was affected by the redox state of coffee and that the nature of the radical was due to quinhydrone structure that might be included in coffee melanoidins. Nitrite also increased the electron spin resonance (ESR) signal intensity at pH 2, suggesting that nitrite oxidized melanoidins producing nitric oxide (NO). The formation of NO could be detected by oxygen uptake due to the autoxidation of NO and using an NO-trapping agent. SCN (-) largely enhanced NO formation in coffee and methanol-insoluble melanoidin fractions but only slightly in a methanol-soluble fraction, and the enhancement accompanied the consumption of SCN (-) but did not accompany the formation of a stable ESR signal. The enhancement was explained by the reduction of NOSCN by melanoidins in methanol-insoluble fractions and that the consumption was due to binding of SCN (-) to melanoidins during their oxidation by nitrous acid. The result obtained in this study suggests that when coffee is ingested, in addition to chlorogenic acid and its isomers, melanoidins can also react with salivary nitrite and SCN (-) in the gastric lumen, producing NO.

Chlorogenic acid in coffee can prevent the formation of dinitrogen trioxide by scavenging nitrogen dioxide generated in the human oral cavity

Coffee contains antioxidants like chlorogenic acid and its isomers. In this report, effects of coffee on the nitrite-induced N2O3 formation were studied using whole saliva and bacterial fraction prepared from the saliva. The formation of N2O3 was measured by fluorescence increase due to the transformation of 4,5-diaminofluorescein to triazolfluorescein. Coffee inhibited the nitrite-induced fluorescence increase, and 50% inhibition was observed at several microg of coffee/mL in bacterial fraction of saliva as well as whole saliva. During the inhibition of the fluorescence increase, concentration of chlorogenic acid and its isomers decreased. It is discussed that the reduction of NO2 by chlorogenic acid and its isomers contributed to the coffee-dependent inhibition of the fluorescence increase as N2O3 is formed from NO and NO2. When coffee was added to whole saliva, chlorogenic acid and its isomers bound to cells in the saliva. The rate of the fluorescence increase in bacterial fraction, which was prepared at defined periods after the ingestion of coffee, was increased to the rate before the ingestion of coffee with a half-time of about 1 h. This result suggests that chlorogenic acid and its isomers remained in the oral cavity for a few hours after ingestion of coffee. The significance of coffee drinking and rinsing of the mouth with coffee for the health of the oral cavity is proposed.