S-nitrosothiol assays that avoid the use of iodine

A pilot study on the kinetics of metabolites and microvascular cutaneous effects of nitric oxide inhalation in healthy volunteers

RATIONALE

Inhaled nitric oxide (NO) exerts a variety of effects through metabolites and these play an important role in regulation of hemodynamics in the body. A detailed investigation into the generation of these metabolites has been overlooked.

OBJECTIVES

We investigated the kinetics of nitrite and S-nitrosothiol-hemoglobin (SNO-Hb) in plasma derived from inhaled NO subjects and how this modifies the cutaneous microvascular response.

FINDINGS

We enrolled 15 healthy volunteers. Plasma nitrite levels at baseline and during NO inhalation (15 minutes at 40 ppm) were 102 (86-118) and 114 (87-129) nM, respectively. The nitrite peak occurred at 5 minutes of discontinuing NO (131 (104-170) nM). Plasma nitrate levels were not significantly different during the study. SNO-Hb molar ratio levels at baseline and during NO inhalation were 4.7E-3 (2.5E-3-5.8E-3) and 7.8E-3 (4.1E-3-13.0E-3), respectively. Levels of SNO-Hb continued to climb up to the last study time point (30 min: 10.6E-3 (5.3E-3-15.5E-3)). The response to acetylcholine iontophoresis both before and during NO inhalation was inversely associated with the SNO-Hb level (r: -0.57, p = 0.03, and r: -0.54, p = 0.04, respectively).

CONCLUSIONS

Both nitrite and SNO-Hb increase during NO inhalation. Nitrite increases first, followed by a more sustained increase in Hb-SNO. Nitrite and Hb-SNO could be a mobile reservoir of NO with potential implications on the systemic microvasculature.

Alternative fluorimetric-based method to detect and compare total S-nitrosothiols in plants

Nitric oxide (NO) is an important signaling molecule occurring in virtually all organisms, whose mechanism of action relies mainly on its interaction with proteins or peptides by nitrosylation, forming compounds such as S-nitrosothiols (SNO). The Saville reaction and the ozone-based chemiluminescence method are the main techniques used for nitrosylated protein quantification. While the Saville assay is not very sensitive, the ozone-based chemiluminescence is expensive and time-consuming. Here we propose a method in which the protein-bound NO groups are exposed to UV light, cleaving the bond and allowing the quantification of the derived NO molecules by diamino-rhodamine (DAR) dyes. The DAR-based method was shown to be specific in plant tissues by pre-treatment of the samples with reducing agents and parallel EPR analysis. Spike-and-recovery assays revealed 72% recovery after a GSNO spike. Moreover, the method was significantly more sensitive than the Saville reaction, and this increase in sensitivity was crucial for detecting the reduced levels of nitrosylated proteins in plant species other than Arabidopsis. The method presented here is a suitable alternative to compare plant samples, allowing simple and fast detection of nitrosylated proteins.

A reductive ligation based fluorescent probe for S-nitrosothiols

A reductive ligation based fluorescent probe () for the detection of S-nitrosothiols (SNO) was developed. The probe showed good selectivity and sensitivity for SNO.

Detection of S-nitroso compounds by use of midinfrared cavity ring-down spectroscopy

S-Nitroso compounds have received much attention in biological research. In addition to their role as nitric oxide donors, there is growing evidence that these compounds are involved in signaling processes in biological systems. Determination of S-nitrosylated proteins is of great importance for fundamental biological research and medical applications. The most common method to assay biological S-nitroso compounds is to chemically or photochemically reduce SNO functional groups to release nitric oxide, which is then entrained in an inert gas stream and detected, usually through chemiluminescence. We report a method of S-nitroso compound detection using cavity ring-down measurements of gaseous NO absorbance at 5.2 μm. The proposed method, in contrast to the chemiluminescence-based approach, can be used to distinguish isotopic forms of NO. We demonstrated sensitivity down to ∼2 pmol of S(14)NO groups and ∼5 pmol of S(15)NO groups for S-nitroso compounds in aqueous solutions. The wide dynamic range of cavity ring-down detection allows the measurement of S-nitroso compound levels from pico- to nanomole amounts.

Hypoxia-induced changes in protein s-nitrosylation in female mouse brainstem

Exposure to hypoxia elicits an increase in minute ventilation that diminishes during continued exposure (roll-off). Brainstem N-methyl-D-aspartate receptors (NMDARs) and neuronal nitric oxide synthase (nNOS) contribute to the initial hypoxia-induced increases in minute ventilation. Roll-off is regulated by platelet-derived growth factor receptor-β (PDGFR-β) and S-nitrosoglutathione (GSNO) reductase (GSNOR). S-nitrosylation inhibits activities of NMDAR and nNOS, but enhances GSNOR activity. The importance of S-nitrosylation in the hypoxic ventilatory response is unknown. This study confirms that ventilatory roll-off is virtually absent in female GSNOR(+/-) and GSNO(-/-) mice, and evaluated the location of GSNOR in female mouse brainstem, and temporal changes in GSNOR activity, protein expression, and S-nitrosylation status of GSNOR, NMDAR (1, 2A, 2B), nNOS, and PDGFR-β during hypoxic challenge. GSNOR-positive neurons were present throughout the brainstem, including the nucleus tractus solitarius. Protein abundances for GSNOR, nNOS, all NMDAR subunits and PDGFR-β were not altered by hypoxia. GSNOR activity and S-nitrosylation status temporally increased with hypoxia. In addition, nNOS S-nitrosylation increased with 3 and 15 minutes of hypoxia. Changes in NMDAR S-nitrosylation were detected in NMDAR 2B at 15 minutes of hypoxia. No hypoxia-induced changes in PDGFR-β S-nitrosylation were detected. However, PDGFR-β phosphorylation increased in the brainstems of wild-type mice during hypoxic exposure (consistent with roll-off), whereas it did not rise in GSNOR(+/-) mice (consistent with lack of roll-off). These data suggest that: (1) S-nitrosylation events regulate hypoxic ventilatory response; (2) increases in S-nitrosylation of NMDAR 2B, nNOS, and GSNOR may contribute to ventilatory roll-off; and (3) GSNOR regulates PDGFR-β phosphorylation.

Detection of S-nitrosothiols

BACKGROUND

S-nitrosothiols have been recognized as biologically-relevant products of nitric oxide that are involved in many of the diverse activities of this free radical.

SCOPE OF REVIEW

This review serves to discuss current methods for the detection and analysis of protein S-nitrosothiols. The major methods of S-nitrosothiol detection include chemiluminescence-based methods and switch-based methods, each of which comes in various flavors with advantages and caveats.

MAJOR CONCLUSIONS

The detection of S-nitrosothiols is challenging and prone to many artifacts. Accurate measurements require an understanding of the underlying chemistry of the methods involved and the use of appropriate controls.

GENERAL SIGNIFICANCE

Nothing is more important to a field of research than robust methodology that is generally trusted. The field of S-nitrosation has developed such methods but, as S-nitrosothiols are easy to introduce as artifacts, it is vital that current users learn from the lessons of the past. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Direct methods for detection of protein S-nitrosylation

S-nitrosylation of protein cysteine residues is known to be an important mechanism for nitric oxide signaling. However, the detection of protein S-nitrosylation is still challenging due to technical limitations of current methods. This chapter provides a brief review on recent developments of methods, which directly target S-nitroso moieties for detection. We also describe in detail the protocol of an organophosphine-based biotin labeling of protein S-nitroso moieties.

Analysis of S-nitrosothiols via copper cysteine (2C) and copper cysteine-carbon monoxide (3C) methods

This chapter summarizes the principles of RSNO measurement in the gas phase, utilizing ozone-based chemiluminescence and the copper cysteine (2C)±carbon monoxide (3C) reagent. Although an indirect method for quantifying RSNOs, this assay represents one of the most robust methodologies available. It exploits the NO detection sensitivity of ozone based chemiluminescence, which is within the range required to detect physiological concentrations of RSNO metabolites. Additionally, the specificity of the copper cysteine (2C and 3C) reagent for RSNOs negates the need for sample pretreatment, thereby minimizing the likelihood of sample contamination (false positive results), or the loss of certain highly labile RSNO species. Herein, we outline the principles of this methodology, summarizing key issues, potential pitfalls and corresponding solutions.

Breath formate is a marker of airway S-nitrosothiol depletion in severe asthma

BACKGROUND

Children with severe asthma have poor symptom control and elevated markers of airway oxidative and nitrosative stress. Paradoxically, they have decreased airway levels of S-nitrosothiols (SNOs), a class of endogenous airway smooth muscle relaxants. This deficiency results from increased activity of an enzyme that both reduces SNOs to ammonia and oxidizes formaldehyde to formic acid, a volatile carboxylic acid that is more easily detected in exhaled breath condensate (EBC) than SNOs. We therefore hypothesize that depletion of airway SNOs is related to asthma pathology, and breath formate concentration may be a proxy measure of SNO catabolism.

METHODS AND FINDINGS

We collected EBC samples from children and adolescents, including 38 with severe asthma, 46 with mild-to-moderate asthma and 16 healthy adolescent controls, and the concentration of ionic constituents was quantified using ion chromatography. The concentrations of EBC components with volatile conjugates were log-normally distributed. Formate was the principal ion that displayed a significant difference between asthma status classifications. The mean EBC formate concentration was 40% higher in samples collected from all asthmatics than from healthy controls (mean = 5.7 microM, mean+/-standard deviation = 3.1-10.3 microM vs. 4.0, 2.8-5.8 microM, p = 0.05). EBC formate was higher in severe asthmatics than in mild-to-moderate asthmatics (6.8, 3.7-12.3 microM vs. 4.9, 2.8-8.7 microM, p = 0.012). In addition, formate concentration was negatively correlated with methacholine PC(20) (r = -0.39, p = 0.002, asthmatics only), and positively correlated with the NO-derived ion nitrite (r = 0.46, p<0.0001) as well as with total serum IgE (r = 0.28, p = 0.016, asthmatics only). Furthermore, formate was not significantly correlated with other volatile organic acids nor with inhaled corticosteroid dose.

CONCLUSIONS

We conclude that EBC formate concentration is significantly higher in the breath of children with asthma than in those without asthma. In addition, amongst asthmatics, formate is elevated in the breath of those with severe asthma compared to those with mild-to-moderate asthma. We suggest that this difference is related to asthma pathology and may be a product of increased catabolism of endogenous S-nitrosothiols.

Direct and indirect detection methods for the analysis of S-nitrosylated peptides and proteins

Covalent binding of nitric oxide to specific cysteine residues in proteins is a key event in cellular redox signal transduction. This modification influences both physiological and pathological processes, such as cardiovascular, neurological, and cancer-associated events. Even though, since its introduction, the biotin switch technique is the most used indirect method for the study of S-nitrosylation both in vivo and in vitro, during the last years modifications of this method have emerged, allowing more efficient sample enrichment and the precise identification of the modified aminoacidic sites. At the same time, to bypass the difficulties generated by the multiple chemical reaction steps required by these labeling methods, the direct identification of the SNO groups by mass spectrometry is emerging as a useful tool in this field, although, until now, it has been limited to the study of synthetic or purified recombinant proteins. Here we present two different techniques, developed in our laboratories, for detection of S-nitrosylation: the first is based on a modification of the biotin switch technique and is called His-tag switch, and the second is a direct mass spectrometry-based method used to detect in vivo generated SNO groups.

Structural analysis of cysteine S-nitrosylation: a modified acid-based motif and the emerging role of trans-nitrosylation

S-Nitrosylation, the selective and reversible addition of nitric oxide (NO) moiety to cysteine (Cys) sulfur in proteins, regulates numerous cellular processes. In recent years, proteomic approaches that are capable of identifying nitrosylated Cys residues have been developed. However, the features underlying the specificity of Cys modification with NO remain poorly defined. Previous studies suggested that S-nitrosylated Cys may be flanked by an acid-base motif or hydrophobic areas and show high reactivity, low pK(a), and high sulfur atom exposure. In the current study, we prepared an extensive, manually curated data set of proteins with S-nitrosothiols, accounting for a variety of biochemical functions, organisms of origin, and physiological responses to NO. Analysis of this generic NO-Cys data set revealed that proximal acid-base motif, Cys pK(a), sulfur atom exposure, and Cys conservation or hydrophobicity in the vicinity of the modified Cys do not define the specificity of S-nitrosylation. Instead, this analysis revealed a revised acid-base motif, which is located more distantly to the Cys and has its charged groups exposed. We hypothesize that, rather than being strictly used for direct activation of Cys, the modified acid-base motif is engaged in protein-protein interactions thereby contributing to trans-nitrosylation as an important and widespread mechanism for reversible modification of Cys with NO moiety. For proteins lacking the revised motif, we discuss alternative mechanisms including a potential role of nitrosoglutathione as a trans-acting agent.

Detection of protein S-nitrosylation with the biotin-switch technique

Protein S-nitrosylation, the posttranslational modification of cysteine thiols to form S-nitrosothiols, is a principle mechanism of nitric oxide-based signaling. Studies have demonstrated myriad roles for S-nitrosylation in organisms from bacteria to humans, and recent efforts have greatly advanced our scientific understanding of how this redox-based modification is dynamically regulated during physiological and pathophysiological conditions. The focus of this review is the biotin-switch technique (BST), which has become a mainstay assay for detecting S-nitrosylated proteins in complex biological systems. Potential pitfalls and modern adaptations of the BST are discussed, as are future directions for this assay in the burgeoning field of protein S-nitrosylation.