Photoinstability of S-nitrosothiols during sampling of whole blood: a likely source of error and variability in S-nitrosothiol measurements

S-Nitrosoglutathione exhibits greater stability than S-nitroso-N-acetylpenicillamine under common laboratory conditions: A comparative stability study

S-Nitrosothiols (RSNOs) such as S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) are susceptible to decomposition by stimuli including heat, light, and trace metal ions. Using stepwise isothermal thermogravimetric analysis (TGA), we observed that NO-forming homolytic cleavage of the S-N bond occurs at 134.7 ± 0.8 °C in GSNO and 132.8 ± 0.9 °C in SNAP, contrasting with the value of 150 °C that has been previously reported for both RSNOs. Using mass spectrometry (MS), nuclear magnetic resonance (NMR), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), we analyzed the decomposition products from TGA experiments. The organic product of GSNO decomposition was glutathione disulfide, while SNAP decomposed to form N-acetylpenicillamine disulfide as well as other products, including tri- and tetrasulfides. In addition, we assessed the relative solution stabilities of GSNO and SNAP under common laboratory conditions, which include variable temperature, pH, and light exposure with rigorous exclusion of trace metal ions by chelation. GSNO exhibited greater stability than SNAP over a 7-day period except in one instance. Both RSNOs demonstrated an inverse relationship between solution stability and temperature, with refrigeration considerably extending shelf life. A decrease in pH from 7.4 to 5.0 also enhanced the stability of both RSNOs. A further decrease in pH from 5.0 to 3.0 resulted in decreased stability for both RSNOs, and is notably the only occasion in which SNAP proved more stable than GSNO. After 1 h of exposure to overhead fluorescent lighting, both RSNOs displayed high susceptibility to light-induced decomposition. After 7 h, GSNO and SNAP decomposed 19.3 ± 0.5% and 30 ± 2%, respectively.

Attenuation of Thrombin-Mediated Fibrin Formation via Changes in Fibrinogen Conformation Induced by Reaction with S-nitroso-N-acetylpenicillamine, but not S-nitrosoglutathione

Previous work in a 4 h rabbit thrombogenicity model has shown that a nitric oxide- (NO) generating polymer extracorporeal circuits (ECC) with infusion of S-nitroso-N-acetyl-penicillamine (SNAP) preserved platelets eventhough platelets were activated as shown by an increase in the glycoprotein, p-selectin. The platelet preservation mechanism was shown to be due to a changing fibrinogen structure leading to attenuation of platelet aggregation. Understanding the effects that SNAP, another RSNO, S-nitroso-glutathione (GSNO) as well as the non-RSNO, sodium nitroprusside (SNP), may have on human fibrinogen polymerization, this in vitro study evaluated the released NO effects on the thrombin-mediated fibrin formation and fibrinogen structure. Thrombin-induced fibrin formation at 300 μM SNAP (50 + 11% of baseline) was significantly reduced compared to SNAP's parent, N-acetyl-penicillamine (NAP) (95 + 13%) after 1 h of RSNO exposure. GSNO, its parent, glutathione (GSH) and 1000 ppm NO gas did not attenuate the thrombin-mediated fibrin formation. SNAP, NAP and SNP exposure for 1 h, however, did not decrease thrombin activity by directly inhibiting thrombin itself. Changes in fibrinogen conformation as measured by intrinsic tryptophan fluorescence significantly decreased in the 300 μM SNAP (38057 + 1196 mean fluorescence intensity (MFI) and SNP (368617 + 541 MFI) groups versus the NAP control (47937 + 1196 MFI). However, infused 1000 ppm NO gas had no direct effect on the ITF after 1 h incubation at 37°C. High performance liquid chromatography (HPLC) showed that fibrinogen degradation by 0.03 U/ml thrombin was concentration-dependently reduced after 1 h with SNAP but not with NAP or SNP. Western blotting showed RSNOs, SNAP, NAP and the non-RSNO, SNP-incubated fibrinogen solutions showed that the percent level of the Aγ dimer to total Aγ dimer + γ monomer was significantly reduced in the case of the SNAP group when compared to SNP group. These results suggest that NO donors such as SNAP and SNP induce fibrinogen conformational changes by potentially nitrosating fibrinogen tyrosine residues. These NO-mediated fibrinogen changes induced via NO donors may provide another mechanism of NO for improving thromboresistance in ECC.

Volatile S-nitrosothiols and the typical smell of cancer

An unconventional approach to investigations into the identification of typical volatile emissions during illnesses gives rise to the proposal of a new class of cancer markers. Until now, cancer markers seem not to have been conclusively identified, though the obvious behavior of dogs points to their existence. The focus has been directed towards molecules containing sulfurous functionalities. Among such compounds, S-nitrosothiols (SNOs) are known to be involved in important physiological processes in living organisms and they are described as being typically elevated in cancer. Volatile SNOs (vSNOs) are proposed to be the source of the significant smell of cancer. Synthetic vSNOs are known to have lifetimes of between some minutes and several hours, which may be the main reason as to why they have been ignored until now, and also for the inability of analytics to detect them in vivo. Based on typical structures occurring in the volatile sulfur organics being emitted from human breath, four vSNOs have been synthesized and characterized by tandem mass spectrometry and gas chromatography/mass spectrometry. Simulating the relatively fatty consistency of cancer tissue by diluting the samples in n-decane, surprisingly reduces their tendency to decompose to lifetimes of weeks even at room temperature. A sniffer dog was trained with the synthetic vSNOs, and the results of the tests indicate that synthetic and cancer smells are very similar or even the same. The findings can be a clue for further target-oriented systematic optimization of existing sensitive measurement methods to prove vSNOs as cancer emissions and finally establish future methods for cancer diagnosis based on screening for this new class of volatile illness markers.

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.

Visible photolysis and amperometric detection of S-nitrosothiols

The concentration of S-nitrosothiols (RSNOs), endogenous transporters of the signaling molecule nitric oxide (NO), fluctuate greatly in physiology often as a function of disease state. RSNOs may be measured indirectly by cleaving the S-N bond and monitoring the liberated NO. While ultraviolet photolysis and reductive-based cleavage both decompose RSNOs to NO, poor selectivity and the need for additional reagents preclude their utility clinically. Herein, we report the coupling of visible photolysis (i.e., 500-550 nm) and amperometric NO detection to quantify RSNOs with greater selectivity and sensitivity. Enhanced sensitivity (up to 1.56 nA μM(-1)) and lowered theoretical detection limits (down to 30 nM) were achieved for low molecular weight RSNOs (i.e., S-nitrosoglutathione, S-nitrosocysteine) by tuning the irradiation exposure. Detection of nitrosated proteins (i.e., S-nitrosoalbumin) was also possible, albeit at a decreased sensitivity (0.11 nA μM(-1)). This detection scheme was used to measure RSNOs in plasma and illustrate the potential of this method for future physiological studies.