Mechanism-based triarylphosphine-ester probes for capture of endogenous RSNOs

Article abstract of DOI:10.1021/ja401565w

Nitrosothiols (RSNOs) have been proposed as important intermediates in nitric oxide (NO^?) metabolism, storage, and transport as well as mediators in numerous NO-signaling pathways. RSNO levels are finely regulated, and dysregulation is associated with the etiology of several pathologies. Current methods for RSNO quantification depend on indirect assays that limit their overall specificity and reliability. Recent developments of phosphine-based chemical probes constitute a promising approach for the direct detection of RSNOs. We report here results from a detailed mechanistic and kinetic study for trapping RSNOs by three distinct phosphine probes, including structural identification of novel intermediates and stability studies under physiological conditions. We further show that a triarylphosphine-thiophenyl ester can be used in the absolute quantification of endogenous GSNO in several cancer cell lines, while retaining the elements of the SNO functional group, using an LC-MS-based assay. Finally, we demonstrate that a common product ion (m/z = 309.0), derived from phosphine-RSNO adducts, can be used for the detection of other low-molecular weight nitrosothiols (LMW-RSNOs) in biological samples. Collectively, these findings establish a platform for the phosphine ligation-based, specific and direct detection of RSNOs in biological samples, a powerful tool for expanding the knowledge of the biology and chemistry of NO^?-mediated phenomena.

Full text of DOI:10.1021/ja401565w

Article
pubs.acs.org/JACS
Mechanism-Based Triarylphosphine-Ester Probes for Capture of
Endogenous RSNOs
Uthpala Seneviratne,† Luiz C. Godoy,† John S. Wishnok,† Gerald N. Wogan,†,‡
and Steven R. Tannenbaum*,†,‡
‡
†Departments of Biological Engineering and Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
ABSTRACT: Nitrosothiols (RSNOs) have been proposed as
important intermediates in nitric oxide (NO•) metabolism,
storage, and transport as well as mediators in numerous NO-
signaling pathways. RSNO levels are finely regulated, and
dysregulation is associated with the etiology of several
pathologies. Current methods for RSNO quantification
depend on indirect assays that limit their overall specificity
and reliability. Recent developments of phosphine-based
chemical probes constitute a promising approach for the
direct detection of RSNOs. We report here results from a
detailed mechanistic and kinetic study for trapping RSNOs by
three distinct phosphine probes, including structural identi-
fication of novel intermediates and stability studies under
physiological conditions. We further show that a triarylphosphine-thiophenyl ester can be used in the absolute quantification of
endogenous GSNO in several cancer cell lines, while retaining the elements of the SNO functional group, using an LC−MS-
based assay. Finally, we demonstrate that a common product ion (m/z = 309.0), derived from phosphine−RSNO adducts, can be
used for the detection of other low-molecular weight nitrosothiols (LMW-RSNOs) in biological samples. Collectively, these
findings establish a platform for the phosphine ligation-based, specific and direct detection of RSNOs in biological samples, a
powerful tool for expanding the knowledge of the biology and chemistry of NO•-mediated phenomena.
mation can also take place through diffusion-controlled limits in
a radical recombination between NO• and a thiyl radical (RS•)
and transition metal-catalyzed pathways.2 Once formed,
nitrosothiols can transnitrosate other thiols on peptides and
proteins.2,8,9
INTRODUCTION
■
Nitric oxide (NO•) is produced from L-arginine by three nitric
oxide synthase isoforms (NOS1, NOS2, NOS3) at low levels as
a signaling molecule and at higher concentrations in
pathophysiological conditions.1,2 Under aerobic solution
conditions NO• generates N2O3, an efficient nitrosating agent
which reacts rapidly with nucleophiles such as water to form
nitrite, and with glutathione (GSH, γ-Glu-Cys-Gly) to form S-
nitrosoglutathione (GSNO) (Figure 1).3−7 Nitrosothiol for-
GSNO is the main low-molecular weight nitrosothiol in
mammalian cells, and can promote S-nitrosation of proteins
that modulate numerous physiological functions.2,10,11 For
example, GSNO induces S-nitrosation of proteins regulating
responses to tissue hypoxia, thereby promoting angiogenesis
and vascular remodeling.12 Importantly, it has been shown to
be the key intermediate in the endogenous transnitrosation of
both reduced and oxidized thioredoxin, which in turn regulates
the activity of caspase 3 through secondary transnitrosation,
inhibiting apoptosis.13−15 Other physiological systems are
regulated through S-nitrosation by micromolar levels of
GSNO (and possibly other low-molecular weight RSNOs)
including the nucleus tractus solitarious region of the
brainstem.16 GSNO-induced transnitrosation has also been
implicated in the etiology of diverse disease states, including
Figure 1. Structures of S-nitrosoglutathione (GSNO) and triarylphos-
phine probes used in this study.
Received: February 12, 2013
© XXXX American Chemical Society
A
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asthma, sickle cell disease, pulmonary hypertension, cancer,
muscle disorders, and neurodegeneration.17−24
for discovery, detection, and elucidation of the role of RSNOs
in physiological systems.
Quantification and elucidation of RSNO behavior in vivo, as
well as the development of its full therapeutic potential,25
remain important goals. Presently, no universal direct method
exists specifically to identify or quantify RSNOs in cells. Most
widely used indirect methods rely on complete removal of the
NO• group from RSNO followed by labeling of the nascent
thiol via biotin switch, d-switch, derivatizations, AuNP- or
phenylmercury-based methods,26−30 or by detecting the
liberated NO• (chemiluminescence and Saville assays).31−35
In such methods, indirect labeling of RSNOs is generally
accomplished by blocking free thiols, reducing the RSNO with
ascorbate, and then derivatizing nascent thiols with a tagged
alkylating reagent such as fluorescent- or biotin-tagged-
iodoacetamide or similar label.36−38
The specificity and sensitivity of these procedures are
therefore dependent on reduction of RSNO by ascorbate and
detection of the liberated free thiols.39,40 Difficulty in complete
blocking of free thiols and distinguishing unblocked from S-
nitrosated thiols represent major limitations of these
approaches. The accuracy of such assays has been challenged
on the basis of variability in data they generate and has driven a
search for alternative detection systems.41−43
Xian and co-workers recently reported that treatment of
various organic S-nitrosothiols with derivatized triphenylphos-
phines yielded a variety of products depending on reaction
conditions and the structures of both RSNO and phos-
phine.42,44−47 In the presence of a properly positioned
electrophile on the triphenylphosphine, the azaylide inter-
mediate reacts to form sulfenamides or disulfide-iminophos-
phoranes, through a Staudinger ligation-type mechanism.48,49
These processes are rich in reaction chemistry, but their
application is generally limited to RSNOs derived from N- and
C-terminal protected amino acids, and they must be carried out
in organic or organic-buffer systems to circumvent limitations
in solubility. Additionally, these reactions have generally been
carried out under stoichiometric conditions (RSNO/phosphine
= 1:2) to minimize undesirable side reactions.
EXPERIMENTAL METHODS
■
General. Chemical reagents were obtained from commercial
sources (Sigma-Aldrich for chemicals, Cambridge Isotope Laboratories
for deuterated solvents, Isotec for 13C2,15N-labeled G*SH, Nanocs for
mPEG−maleimide) and were used without additional purification.
Extraction and silica chromatography solvents were reagent grade.
LC−MS and HPLC solvents were HPLC grade. Acetonitrile was
distilled for HPLC and LC−MS. Distilled water was obtained in-house
and redistilled for HPLC and LC−MS experiments. Vivaspin 3000
MWCO membrane filters were from Sartorius Stedim NA. Unless
otherwise stated, all the sample preparations, nitrosothiol preparations,
and probe−SNO reactions were carried out in the dark at ambient
temperature.
1
NMR analysis: H NMR spectra were recorded on Bruker Avance-
600 and Varian Inova-500 instruments at 600.13 and 500.13 MHz,
respectively. 13C NMR spectra were recorded on a Varian Inova-500
instrument operating at 125.76 MHz. 31P NMR spectra were recorded
on a Varian Inova-500 instrument operating at 202.46 MHz, and 31P
chemical shifts are relative to 3% H3PO4 (δ = 0 ppm) contained in a
concentric internal capillary (Wilmad). NMR spectra were obtained
using Bruker 5 mm TXI cryo-probes and Varian 5 mm PFG-probes
held at 22 °C unless otherwise stated.
HPLC Purification of Phosphine−GSNO Reaction Mixture.
HPLC was carried out with an Agilent Technologies model 1100
HPLC system equipped with a photodiode array UV detector
(Wilmington, DE). Unless specified otherwise, UV absorbance was
monitored at 254 nm. HPLC columns and solvent elution systems
were as follows: A semipreparative Phenomenex Luna C18 (25 cm ×
9.4 mm, 10 μm) column was eluted with a linear gradient of 0.1%
formic acid in water (A) and methanol (B) at a flow rate of 2.5 mL/
min. Solvent composition was initially at 25% for 5 min, changed from
25% to 60% B over 22 min, and then further to 95% B over 3 min,
held for 10 min, followed by returning to 25% B over 2 min for a total
run time of 42 min. The column was equilibrated for 10 min before
injections. This system was used for isolation of 4a, 4b, 6a, 6b, 7, and
8.
S-Alkylthiophosphonium Adducts. Glutathionyl-diphenyl-2-
(((3-sulfopropylthio)carbonyl)phenyl)phosphonium Salt (6a). To a
stirred solution of 1 (200 mg) in Tris-HCl buffer pH 7.4 was added
GSNO (50 mg). The reaction mixture was stirred for 1 h at room
temperature in the dark and then completely dried and purified by
1
We designed an approach to address these limitations by
developing reactions of derivatized triphenylphosphines with
GSNO that are capable of detecting SNO at physiological levels
and that utilize phosphine probes at concentrations adequate
for complete labeling. To accomplish these objectives, we
synthesized a panel of phosphine probes containing sulfonate
esters and tertiary amine functional groups to facilitate water
solubility and aid in mass spectrometric detection (Figure 1)
and demonstrated trapping of GSNO under physiologically
relevant conditions. We characterized the kinetics of trapping
and the applicability of the probes for GSNO detection in
aqueous buffer and also determined optimal pH, reagent
concentrations, and time required for maximal ligation. Using
GSNO labeled with probe 3, triarylphosphine thiophenylester,
we developed an integrated high-performance liquid chroma-
tography−mass spectrometric (HPLC−MS/MS)-based meth-
od capable in cell extracts of absolute quantification of GSNO
while retaining the nitrogen atom and thiol (RS) moiety in a
single, stable product. Importantly, discovery of a common ion,
m/z = 309.0, during fragmentation of phosphine-mediated
ligated products provided a basis for identification of other
endogenous low-molecular weight RSNOs in cell lysates. Our
results highlight the potential of phosphine-ligation chemistry
HPLC to isolate 6a (yield 44%). Retention time, tR = 18.8 min. H
NMR (600 MHz, D2O, δ) 7.98 (m, 2H), 7.65 (m, 6H), 7.57 (m, 4H),
7.45 (m, 1H), 7.37 (m, 1H), 4.18 (t, J = 6.2 Hz, 1H), 3.64 (s, 2H),
3.56 (t, J = 6.5 Hz, 1H), 2.91 (m, 2H), 2.65 (m, 2H), 2.50 (m, 2H),
2.25 (t, J = 6.5, 2H), 2.05−1.95 (m, 2H), 1.45 (m, 2H); 31P NMR
(202.46 MHz, D2O, δ) 51.6; HRMS-ESI+ (m/z): [M]+ calcd for
C32H37N3O10PS3, 750.1373; found, 750.1379.
Glutathionyl-diphenyl-2-(((3-sulfopropoxy)carbonyl)phenyl)-
phosphonium Salt (6b). To a stirred solution of 2 (50 mg) in Tris-
HCl buffer pH 7.4 was added GSNO (10 mg). The reaction mixture
was stirred for 30 min at room temperature in the dark and then
purified by HPLC to isolate 6b (yield =38%). Retention time, tR
=
1
18.4 min. H NMR (600 MHz, D2O, δ) 8.08 (m, 2H), 7.68 (m, 6H),
7.54 (m, 4H), 7.50−7.40 (m, 2H), 4.18 (t, J = 6.2 Hz, 1H), 3.84 (m,
2H), 3.71 (s, 2H), 3.62 (t, 7.0, Hz, 1H), 2.95 (m, 2H), 2.40 (m, 2H),
2.31 (t, J = 6.5, 2H), 2.07−1.98 (m, 2H), 1.65 (m, 2H); 31P NMR
(202.46 MHz, D2O, δ) 51.9; HRMS-ESI+ (m/z): [M]+ calcd for
C32H37N3O11PS2, 734.1607; found, 734.1602.
Formation of 8 via GSNO−Phosphine Probe 2. 3-((2-
(Diphenylphosphoryl)phenyl)(imino)methoxy)propane-1-sulfonic
Acid (8). To a stirred solution of 2 (100 mg) in Tris-HCl buffer pH 7.4
was added GSNO (20 mg). The reaction mixture was stirred for 30
min at room temperature in the dark and then purified by HPLC to
1
isolate 8 (yield =36%). Retention time, tR = 24.9 min. H NMR (600
MHz, CDCl3, δ) 8.32 (m, 1H), 7.82 (m, 1H) 7.71−7.55 (m, 11H),
7.15 (m, 1H) (m, 1H), 5.80 (bs, 1H), 4.30 (t, J = 6.5 Hz, 2H), 2.86 (t,
B
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J = 6.5 Hz, 2H), 2.10 (m, J = 6.5, 2H); 13C NMR (125.76, CDCl3, δ)
167.1, 136.8, 135.3, 134.9, 133.9, 133.1, 130.4, 127.3, 126.5, 125.0,
124.1, 66.9, 48.2, 25.2; 31P NMR (202.46 MHz, CDCl3, δ) 39.6;
HRMS-ESI+ (m/z): [M + H]+ calcd for C22H23NO5PS, 444.1029;
found, 444.1029.
min. The mass spectrometer was operated in the positive ion mode
with nitrogen as sheath gas (8 L/min). Electrospray ionization was
achieved at a spray voltage of 3.0 kV and a capillary temperature of 250
°C. The mass spectrometer parameters were optimized for maximum
response during infusion of standard solutions of 9 (fragmentor; 60 V,
collision energy; 7 V).
Preparation of Disulfide-Iminophosphorane 9. To a stirred
solution of 3 (70 mg, 0.18 mmol) in dry DMSO (4 mL) was added
freshly prepared GSNO (30 mg, 0.09 mmol, predissolved in DMSO/
H2O, 1:1, 1 mL). The reaction mixture was stirred for 30 min at room
temperature in the dark and then completely dried. Disulfide-
iminophosphorane 9 was obtained by flash column chromatography
with a 2-propanol/H2O gradient and further purified by HPLC (yield
ESI+-QqQ-MS Analysis (Precursor-Ion Analysis). Precursor-ion
analyses were carried out under conditions the same as those above,
selecting m/z = 309.0 as the product ion with a precursor-ion scan
window of 100−1000 m/z. For the analysis of 9g, the Agilent Eclipse
XDB-C18 reverse phase column (1.0 mm × 50 mm, 3.5 μm) was
eluted with 15% acetonitrile in 2 mM ammonium acetate in water with
a scan window of 200−1500 m/z.
1
= 50 mg, 77%). Retention time, tR = 9.0 min. H NMR (500 MHz,
D2O, δ) 8.01−7.97 (m, 1H), 7.61−7.50 (m, 5H) 7.45−7.41 (m, 5H),
7.25−7.13 (m, 2H), 6.96−6.92 (m, 1H), 3.93 (m, 2H), 3.86 (s, 1H),
3.70 (s, 1H), 3.65 (d, J = 3.5 Hz, 1H), 3.56 (t, J = 6.5 Hz, 1H), 2.88
(m, 1H), 2.81 (s, 3H), 2.78 (s, 3H), 2.64−2.58 (m, 1H), 2.29−2.23
(m, 2H), 1.93−1.85 (m, 2H); 13C NMR (125.76, D2O, δ) 172.6,
165.9, 132.9, 131.6, 131.5, 130.9, 130.2, 129.0, 64.8, 55.7, 48.9, 47.6,
41.2, 31.0, 25.4, 23.2; 31P NMR (202.46 MHz, D2O, δ) 25.1; HRMS-
ESI+ (m/z): [M + H]+ calcd for C32H39N5O7PS2, 700.2023; found,
700.2026; [M + 2H]+2 calcd for C32H40N5O7PS2, 350.6048; found,
350.6044.
Liquid Chromatography−Mass Spectrometry. Liquid chro-
matography separations were achieved using an Agilent Eclipse XDB-
C18 reverse phase column (2.1 mm × 150 mm, 3.5 μm) eluted with a
gradient of 0.1% formic acid in water (A) and acetonitrile (B) at 0.3
mL/min at 24 °C. Gradient for GSNO + arylphosphine reaction
analyses: The solvent composition was held at 2% B for 2 min, ramped
to 98% B over 5 min, and held for 3 min, for a total run time of 10
min.
Kinetics. Calibration plots relating LC−MS peak areas to GSNO
concentrations were determined for GSNO standards. For LC−MS
analysis, five-point calibration standards were prepared ranging from
0.01 to 5 μM in buffer solution (for reactions with probes 1 and 2:
Tris-HCl at pH 7.4; for probe 3: potassium phosphate at pH 7.4 and
pH 5.7) and 20 μL injections.
The rate constants for the reaction of phosphine and GSNO at 24
°C were determined under pseudo-first-order kinetics. Phosphine
stock solutions (50 mM) were prepared in Tris or phosphate buffers
(probes 1 and 2: Tris-HCl at pH 7.4; for probe 3: potassium
phosphate at pH 7.4 and pH 5.7 in 20% acetonitrile), with phosphine
concentrations ranging from 0.1 to 1 mM. To initiate the reactions,
phosphine was dissolved in the appropriate volume of buffer, followed
by addition of a corresponding volume of GSNO. Aliquots (20 μL)
were removed periodically and analyzed using LC−MS.
Preparation of 15N-Labeled 9 and 9a−9i Standards for LC−
MS/MS (precursor ion mode). 15N-labeled GSNO (GS15NO) (5
μM, final concentration) was added to a solution of phosphine 3 in
phosphate buffer (0.5 mM) held at ambient temperature for 15 min in
the dark and then analyzed by LC−MS/MS. The target mass was set
to m/z 351.1 for 15N-labeled 9.
Freshly prepared nitrosothiols (1 μM) were added to stirred
solutions of phosphine 3 (0.5 mM) in phosphate buffer at pH 5.7. The
resulting reaction mixtures were analyzed by LC−MS/MS in precursor
ion mode: 9a, 9.0 min, m/z 257.6 (M + 2H)2+; 9b, 11.4 min, m/z
278.6 (M + 2H)2+; 9c, 11.6 min, m/z 285.6 (M + 2H)2+; 9d, 9.7 min,
m/z 264.6 (M + 2H)2+; 9e, 8.0 min, m/z 286.1 (M + 2H)2+; 9f, 9.5
min, m/z 322.1 (M + 2H)2+; 9g, 6.8 min, m/z 580.6 (M + 2H)2+; 9h,
12.8 min, m/z 292.6 (M + 2H)2+; 9i, 10.1 min, m/z 271.6 (M + 2H)2+.
Preparation of Cells. Human cell lines were A375 (malignant
melanoma), HCT116 (colorectal carcinoma), MCF7 (breast adeno-
carcinoma), and TK6 (lymphoblasts). Macrophages were from the
mouse cell line RAW264.7. HCT116 cells were grown in McCoy’s
Medium, MCF7 cells in IMEM; all other cell lines were grown in
DMEM. Culture media were supplemented with 10% fetal bovine
serum (Atlanta Biochemical), 0.2 mM L-glutamine, 10 U/mL
penicillin, and 10 μg/mL streptomycin. All culture reagents were
from Lonza unless otherwise stated. Cells were kept at 37 °C in 5%
CO2 incubators. Macrophages were activated by treatment with 20 U/
mL IFN-γ and 20 ng/mL LPS for 24 h. For analysis of SNOs, cells
were washed with ice-cold PBS containing 0.1 mM ethylenediaminete-
traacetic acid (EDTA) under protection from light to minimize
denitrosation of SNOs. Cells were counted and the wet weights of
pellets recorded and frozen at −80 °C until analysis (addition of N-
ethyl maleimide (NEM) at the initial washing steps was avoided to
keep cells intact until lysis).
Sample Preparation for LC−MS/MS. Samples were first spiked
with 500 fmol of isotopically labeled internal standard (13C2,15N-
labeled G*SNO) and added with mPEG−maleimide (10 mM) in PBS
buffer containing 1 mM EDTA. This was then followed by three
freeze/thaw cycles (dry ice to RT) while frequently vortexing to
facilitate complete cell lysis. The lysed sample was then kept 15 min at
RT to allow complete blocking by mPEG−maleimide. Centrifugation
was performed (14k rpm, 10 min at 4 °C) to remove cell debris,
followed by filtration using membrane filters 3000 MWCO, (15k rcf,
15 min at 4 °C). The filtrate (low-molecular weight fraction) was then
treated with phosphine 3 (3 mM final concentration, prepared by
dissolving 4 mg of phosphine in 400 μL of 2:1 acetonitrile/methanol
mixture). The resulting clear reaction mixture was then freeze-dried to
a final volume of 30 μL; 8 μL of this mixture was injected into the
LC−MS/MS. To generate negative controls, the LMW-filtrate was
treated with 3 mM DTT (15 min, 24 °C, see the Supporting
Information for more experimental details).
ESI-TOF or QTOF Analyses (High-Resolution ESI-MS). ESI-
TOF MS or MS/MS data were collected on an Agilent Technologies
(1290 infinity) LC/MSD TOF system (model 1969A) or on an
Agilent Technologies (1200) LC-ESI-QTOF system (models 6510
and 6530). Typical operating conditions for ESI-TOF experiments
were the following: positive ion mode: gas temperature, 350 °C; gas
flow, 10 L/min; nebulizer, 30 psi; capillary voltage (VCap), 3500 V;
fragmentor, 90 V. Those for ESI-QTOF were the following: positive
ion mode: gas temperature, 335 °C; gas flow, 8L/min; nebulizer, 30
psi; capillary voltage (VCap), 3500 V; fragmentor, 150 V, Skimmer 65
V. Averaged MS spectra were obtained using MassHunter Software
(Agilent Technologies) in the 100−1000 MW range.
ESI+-QqQ-MS Analysis (MRM). An Agilent 1200 capillary HPLC
system interfaced to an Agilent Triple Quad LC/MS (model 6430)
was used in MRM and precursor-ion analyses. Chromatography was
based on an Agilent Eclipse XDB-C18 reverse phase column (1.0 mm
× 50 mm, 3.5 μm) eluted with a gradient of 0.1% formic acid in water
(A) and acetonitrile (B) at 20 μL/min. The solvent composition was
held at 15% B at 0 min and then to 20% over 2 min, followed by a
linear increase to 40% B over 8 min, further to 98% B over 2 min, held
for 6 min, followed by returning to 15% B over 2 min for a total run
time of 20 min. The column was equilibrated for 10 min before
injections. Injection volumes were typically 8 μL. With this solvent
system, the retention time of disulfide-iminophosphorane, 9, was 9.6
Endogenous GSNO Quantification. LC−MS quantitation was
performed by MRM, using the internal standard (G*SNO). A method
calibration curve for LC−MS/MS was obtained by spiking 500 fmol of
13C2,15N-labeled internal standard (G*SNO) into the reaction
between phosphine 3 (5 mM) and GSNO standards (0−10 pmol,
in PBS buffer containing 1 mM EDTA) (Supporting Information).
Regression analysis of the relative response ratio, calculated from LC−
MS/MS peak area ratios corresponding to analytes and internal
standards, was then used to calculate the amount of GSNO (in
C
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Figure 2. Reaction between GSNO and phosphine probe 1 (A, B) and 2 (C, D) (in excess, in Tris-HCl, pH 7.4), analyzed by LC−TOF-MS. Upper
panel (A and C): time-dependent LC of the reaction intermediates and products (at 254 nm); bottom panel (B and D): time-dependent high-
resolution ESI+-MS (EIC; extracted ion chromatogram). Colors: 23 min = black, 45 min = blue, 67 min = green, 89 min = purple, 111 min = yellow,
133 min = pink. Peaks corresponding to 1, 2, 4a, and 4b were excluded on LC−MS trace for clarity. GSNO, 5 μM and 50 μM with phosphine probe
(20-fold excess) were used for the LC−MS and LC−UV studies, respectively, (*) indicates the hydrolyzed 2 under our experimental conditions.
Monitored/expected masses, 5; m/z = 627.1678/627.1673 [M + H]+, 6a; m/z = 750.1376/750.1373 [M + H]+, 6b; m/z = 734.1602/734.1607 [M +
H]+, 7; m/z = 322.0995/322.0991 [M + H]+, 8; m/z = 444.1029/444.1029 [M + H]+. Y-axis represents the relative intensity.
pmoles). This was then multiplied by 3.8 (total volume 30 μL/8 μL
injection to MS = 3.8) to obtain the total amount of analyte in the cell
lysate. The GSNO concentration was determined by dividing the total
amount of analyte (pmol) per million cells by the wet weight per
million cells (mg), assuming that the wet weight of cells (10 million)
was equal to the weight of water (i.e., mg = μL).
D
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Scheme 1. Reaction Pathways Supported by 31P NMR and High-Resolution LC−MS Analysis of Intermediates and Products
When GSNO Reacts with (A) Probe 1, (B) Probe 2, and (C) Probe 3 in Buffer Conditions at Physiological pH
TOF) analysis of the reaction mixture after 15 min showed a
complex mixture comprising phosphoryl amide (7), sulfena-
mide (5), phosphine oxide (4a), and remaining starting
material (probe 1) (Figure 2, A and B). Early in the reaction,
we found higher amounts of sulfenamide 5 and amide 7, with
decreasing amounts of 5 during the course of the reaction
(Figure 2, A and B).
Detailed LC−MS study of the reaction mixture revealed a
novel compound, with a mass-to-charge ratio (m/z) of
750.1380, that was absent in previously proposed mechanisms.
RESULTS AND DISCUSSION
■
Reaction of GSNO with Water-Soluble Probe 1.
Toward the development of a biologically useful RSNO
probe, ligation in a fully aqueous system was demonstrated
using a phosphine sulfonate thioester (probe 1) in Tris-HCl
buffer at pH 7.4 (Figure 1). Although two phosphine molecules
are stoichiometrically sufficient for the consumption of one
molecule of GSNO, we added excess phosphine relative to
RSNO to compete with autooxidation in aqueous solutions and
for complete labeling in buffers. High-resolution LC−MS (ESI-
E
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Figure 3. Capturing GSNO as disulfide-iminophosphorane (9) by probe 3. Time- and concentration-dependent TOF-MS analysis of the reaction
mixture comprising GSNO (5 μM) with probe 3 (50 μM to 1 mM) in phosphate buffer (pH 5.7) at 24 °C. Colors: 2 min = black, 12 min = red, 22
min = blue, 32 min = green. When the concentration of probe 3 (1 mM) is in 200-fold molar excess, the formation of 9 occurs within 15 min with
<10% of the phosphoryl-disulfide 10. Peaks corresponding to 3 and 4c were excluded on EIC for clarity. Monitored/expected masses, GSNO: m/z =
337.0810/337.0812/[M + H]+; 9: m/z = 350.6044/350.6048 [M + 2H]2+; 10: m/z = 616.1334/616.1336 [M + H]+. Phosphate buffer at pH 5.7 was
used to increase ionization and to aid in detection of GSNO. Y-axis represents the relative intensity.
On the basis of 1H and 31P NMR results and structural
similarity to the literature compound50 we assigned the
structure to the S-alkylthiophosphonium ion, 6a (Scheme
1A). Mechanistically, the azaylide intermediate corresponds to
probe 1 and GSNO, yielding sulfenamide (5) through an
intramolecular acyl transfer followed by hydrolysis of the
phosphorane intermediate (Scheme 1A). Thus we envisioned
two pathways for the fate of the sulfenamide (5) in aqueous
conditions. At high concentrations of probe 1, sulfenamide 5
reacts with 1 to yield 6a (path A) or via the thiolate anion to a
disulfide (GS-SR1) and phosphorylamide 7 (path B) (Scheme
1A, Figure 2A, and Supporting Information S1). Moreover, 31P
NMR kinetic analysis of the reaction between GSNO and
probe 1 (1:3) revealed that 6a is the major product in buffered
conditions (Supporting Information S2). Stability studies on
isolated 6a demonstrated that a quantitative conversion of 6a to
GSH and phosphine oxide, 4a, was achieved within 12 h in
aqueous conditions (Supporting Information S3). This was
consistent with the proposed structure for 6a, i.e. composed of
phosphine 1 and GSH. Further, this S-alkylthiophosphonium
adduct represents the known intermediate in disulfide
reduction by triscarboxyethylphosphine (TCEP), where the
S-alkylthiophosphonium intermediate rapidly hydrolyzes to free
thiol and phosphine oxide.51−53 Taken together, these
experiments revealed (1) the formation of S-alkylthiophospho-
nium ion 6a, phosphoryl amide 7, GSH, and GS-SR1 in the
presence of excess phosphine; (2) the complexity and the
instability of the resulting chemical species (sulfenamide and
disulfide; GS-SR1); and (3) their cross-reactivity during GSNO
trapping in buffer conditions at physiological pH.
Phosphoryl amide (7), which retains the nitrogen atom of
the RSNO, can be detected at levels as low as 100 amol (100 ×
10−18 mol) due to its superior ionization in LC−ESI+-MS/MS.
Recent reports,54,55 however, indicate that the 7 formed by the
reaction of nitroxyl (HNO) with phosphines make it unsuitable
as a biomarker for RSNOs.
Moreover, the S-alkylthiophosphonium adduct 6a, due to the
possibility of its formation from the reaction between sulfenic
acids and phosphines, is also not specific for RSNOs.50
According to our results, probe 1 can be used to detect and
quantify RSNOs under physiological conditions by simulta-
neously considering both 6a, which carries the thiol moiety of
RSNO, and 7, which carries the nitrogen atom, with necessary
control experiments (disulfides, nitroxyl and sulfenic acids)
(Scheme 1A). Further, probe 1 may also be used as a specific
reducing agent to convert RSNOs to RSH completely (Scheme
1A; path A).56
Reaction of GSNO with Water-Soluble Probe 2. A
similar analysis was performed to detect ligated products arising
from the phosphine sulfonate ester (probe 2) and GSNO. At
physiological pH, in the presence of excess 2, we observed the
expected S-alkylthiophosphonium ion (6b), phosphine oxide
(4b), and the remaining starting material (probe 2) (Figure 2,
C and D and Supporting Information S4). In a situation similar
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Scheme 2. 15N Fragments Derived from the Reaction of Probe 3 and GS15NO upon Collision Induced Dissociation (CID) in
MS and Possible Resonance Stabilization of m/z = 309.0
to the reaction of GSNO with probe 1, an unexpected product
appeared. The potential sulfenamide product with expected
mass ([M + H]+, m/z = 627.1673) was not detected. Instead,
we observed a new LC−MS peak at m/z = 444.1029. On the
formation of 6b from 2 (Supporting Information S6).
Collectively, these studies revealed that the mild reducing
ability of probe 2 could hamper RSNO trapping to a single
stable conjugate in biological systems and at physiological
conditions. However, the stable phosphoryl-benzimidate 8,
which carries the nitrogen atom, and 6b, which carries the thiol
moiety of RSNO, can be used simultaneously to detect and
quantify RSNOs in physiological conditions.
1
basis of H, 13C, and 31P NMR and MS/MS fragmentation, we
propose a stable phosphoryl-benzimidate structure (compound
8) for the new peak (Scheme 1B). We envision that in aqueous
systems, the azaylide intermediate arising from phosphine 2 and
GSNO undergoes an intramolecular nucleophilic attack from
the N atom to the carbonyl carbon to generate an
amidophosphonium salt. This is then followed by hydrolysis
of the P−N bond to produce a stable phosphoryl-benzimidate 8
(Scheme 1B) and the corresponding S-alkylthiophosphonium
adduct (6b) (Supporting Information S5). However, because
these reactions were carried out in aqueous buffers, we ruled
out the formation of a strained bicyclic oxazaphosphetane
intermediate which could undergo an intramolecular aza-
Wittig-type reaction to produce 8 (Scheme 1B).57 The HPLC
isolated phosphoryl-benzimidate 8 appeared to be stable in
aqueous systems. This apparent discrepancy of the intermediate
formation (amidophosphonium salt vs phosphorane) in
phosphine-thioester (1) and phosphine-ester (2), could be
due to the leaving group effect, such that the thiolate is a better
leaving group than alkoxide (or amidophosphonium salt)
leading to a phosphorane in the case of 1 and GSNO.
To test the selectivity of these phosphine probes (1 and 2)
over other biologically relevant glutathione-related species, we
carried out reactions with GSH and glutathione disulfide
(GSSG). These phosphines appeared to be quite stable toward
GSH as indicated by previous reports.47,50 In contrast, under
aqueous conditions the reaction of phosphine probes 1 and 2
with GSSG generated the corresponding S-alkylthiophospho-
nium ions 6a and 6b, which could complicate the RSNO
quantification.50
Reaction of GSNO with Probe 3 in Buffer Conditions.
Reaction of probe 3 with GSNO in phosphate buffer (pH 5.7)
led to the desired disulfide-iminophosphorane 9 (Scheme 1).
Mechanistically, when the sulfur atom is directly bonded to the
phenyl ring (probe 3), the resulting azaylide leads to a pseudo-
sulfenamide intermediate which is then attacked intramolecu-
larly by the phenylthiolate to yield a disulfide-iminophosphor-
ane (9) (Scheme 1). MS analysis of 9 revealed an abundant
doubly charged ion [(M + 2H)2+ m/z = 350.6048] and a low
abundance pseudomolecular ion [(M + H)+ m/z = 700.2023]
in phosphate buffer. Ionization of the tertiary-amine-containing
disulfide-iminophosphorane 9 in aqueous buffers, relative to
that of GSNO, was surprisingly efficient (note the extracted ion
chromatogram in Figure 3). When the concentration of probe 3
(1 mM) was 200 times higher than that of GSNO (5 μM), the
reaction was typically complete within 15 min at 24 °C. In
addition, this chromatogram (Figure 3) illustrates the stability
of 9 in the presence of excess phosphine without reduction of
the disulfide bond. The only byproduct observed during the
reaction was phosphoryl-disulfide 10 (yield <10%), and the
corresponding S-alkylthiophosphonium ion was not observed
under our experimental conditions (Figure 3 and Supporting
Information S6). Furthermore, the HPLC-isolated compound 9
appeared to be quite stable in neutral or alkaline buffer
conditions (phosphate or HEPES) for a prolonged period of
time; however, the presence of excess phosphine 3 (100-fold)
in reaction medium led to the deterioration of the signal
(Supporting Information S7).
In our LC−MS (ESI-TOF) experiments, the formation of 6a
from 1 with GSSG was comparatively low compared to the
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Figure 4. Disulfide-iminophosphorane 9 is formed in activated macrophage lysate (106 cells) upon treatment with probe 3. (A) LC−MS (TIC; total
ion chromatogram) of cell lysate. (B) LC−MS/MS (MRM) of DTT-treated cell lysate (negative control). (C) MRM of cell lysate without treatment
with probe 3, analyte corresponds to GSNO. (D) m/z = 350.6 → 309.0, (G) m/z = 350.6 → 487.1, (H) m/z = 350.6 → 571.1, and (E) internal
standard (m/z = 352.1 → 309.0) in cell extracts. (F) Product ion spectra of 9, m/z = 350.6 [M + 2H]2+, derived from cell lysate (top), authentic
standard (middle), and 15N fragment, m/z = 351.1 [M + 2H]2+, derived from the reaction of probe 3 and GS15NO (bottom). Y-axis represents the
relative intensity.
To confirm further that both the nitrogen atom and the thiol
(RS) moiety were maintained in the label of the original
RSNO, probe 3 was incubated with 15N-labeled GSNO
(GS15NO) and analyzed by LC−MS/MS (Scheme 2). These
analyses revealed that both m/z = 572.1 and m/z = 396.1
contained the 15N label and hence the corresponding precursor,
disulfide-iminophosphorane 9 (m/z = 351.1), compared to that
of unlabeled fragments (Figure 4F).
and N as a single entity; and (3) favorable ionization of the
tertiary-amine-containing product for quantitative analysis.
Kinetic Analysis of GSNO Trapping by Phosphine
Probes. Rate constants for the aqueous trapping of GSNO by
water-soluble phosphine probes 1, 2, and 3 were determined by
LC−MS under pseudo-first-order conditions, i.e. 100-fold
excess of phosphine in buffer at room temperature (24 °C)
(Table 1).
These unique features support the implementation of an
LC−MS-based assay for detection and quantitation of GSNO
(or RSNOs) using probe 3 as an RSNO trapping agent. In
biological systems, however, care must be used to prevent
crossover product formation between cellular thiols and
disulfide-iminophosphorane, 9, by blocking in advance all free
GSH and accessible sulfhydryl groups in cysteine-containing
peptides in the sample. Collectively, these results illustrate: (1)
the ability of GSNO-derived azaylides to undergo aqueous
Staudinger ligations to generate disulfide-iminoposphorane; (2)
the ability of probe 3 to trap biological RSNOs under
physiological conditions, while retaining the elements of RS
Table 1. Pseudo-First-Order Rate Constants of Phosphine
Probes (1 mM) 1, 2, and 3, and GSNO (5 μM) in Buffer
Conditions
a
a
b,c
phosphine probe
(kobs SD) × 10−3 s−1
1
2
3
3.3 0.3
7.3 0.3
5.0 0.4
a
b
Reactions in Tris-HCl buffer, pH 7.4, 24 °C. Reactions in phosphate
c
buffer, pH 7.4, 24 °C. kobs = (2.7 0.2) × 10−3 s−1 in phosphate
buffer, pH 5.7.
H
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Observation of pseudo-first-order kinetics for bis-ligation
with phosphine probe 3 supports the proposed kinetic model
(Scheme 3). Kinetic studies of the Staudinger ligation have
including proteins, DNA, maleimide-reacted-GSH, and excess
alkylating agents, from the lysate.
The filtered lysates, comprising low-molecular weight
components, were then treated with probe 3 in the dark at
room temperature and were vacuum-concentrated and analyzed
by LC−MS in positive ion mode. Compound 9 (m/z = 350.6)
showed a clear fragmentation pattern corresponding to
disulfide-iminophosphorane (Figure 4F), with the loss of
glutamyl (m/z = 130.0), then to the fragment (m/z = 571.1)
comprising the cysteinyl-glycine moiety in disulfide-iminopos-
phorane (m/z = 487.1) and an oxa-thia-phospholanium-like
(m/z = 309.0) fragment, which is the most abundant daughter
ion, and to the final cysteinyl-glycyl moiety (m/z = 179.1). It
should be noted that both the m/z = 571.1 and m/z = 487.1
fragments (Figure 4F) contain the characteristic amino acids of
GSH which ensures unbiased MS detection and identification
of 9 in biological samples. By using 15N-labeled GS15NO, we
further confirmed the identity of the fragments generated in
CID (Figure 4F).
Scheme 3. Proposed Kinetic Model for Capturing GSNO by
Probe 3
Quantification of GSNO was done in multiple reaction
monitoring mode (MRM), with the isotopically labeled internal
standard (13C2,15N-labeled GSNO; G*SNO), using the
following transitions: m/z = 350.6 → 309.0, m/z = 350.6 →
487.1, m/z = 350.6 → 571.1 for analyte 9 derived from GSNO,
and m/z = 352.1 → 309, m/z = 352.1 → 490.1, m/z = 352.1 →
574.1 for analyte 9 derived from G*SNO (Figure 4D,E and
Supporting Information S10). Calculations were based on the
peak areas in the MRM chromatograms to yield the ratio of
analyte to internal standard (relative response ratio). Standard
curves were constructed from solutions containing 0−10 pmol
GSNO and 500 fmol of internal standard (G*SNO), followed
by regression analysis of the relative response ratios
(Supporting Information S11). In addition, integrated areas
of GSNO peaks were corrected for recovery against internal
standard peak areas, giving a relative response ratio for each
sample, to account for the loss of GSNO during sample
preparation.
Figure 4A shows a typical LC−MS/MS chromatogram
obtained from the phosphine-treated cell lysate and indicates
the quality and sensitivity of the method for the detection of
GSNO. Cell lysates treated with DTT (dithiothreitol, which
reduces all RSNOs to their free thiols) or phosphine-untreated
cells did not lead to m/z = 350.6 → 309.0 formation (Figures
4B,C and Supporting Information S10).
Endogenous GSNO Quantification in Cancer Cells and
Macrophages. Numerous studies have documented the
production of NO by tumor cells in various types of cancer
in vitro and in vivo. This feature has been strongly associated
with sustained tumor growth and resistance to drugs and
radiotherapy, both of which seem to be mediated, at least in
part, by the signaling properties of NO via S-nitrosation and
other mechanisms.63−68 Typically, the concentrations of NO
(and those of NO metabolites such as GSNO) produced by
these cells is in the nanomolar range and therefore difficult to
quantify with precision, if at all.
shown that the process is second-order with a likely irreversible
and rate-limiting first step between arylphosphines and azides.58
The major difference between the Staudinger ligation and
phosphine−RSNO ligation is that the three-membered ring
intermediate (Scheme 3), corresponding to phosphazide
complex in Staudinger ligation, is reacting with another
phosphine molecule to produce the azaylide.44 We presume
that the three-membered ring intermediate reacts via a four-
membered ring transition state to yield the azaylide with loss of
phosphine oxide 4c and finally to disulfide-iminophosphorane 9
via an intramolecular rearrangement. Probes 2 and 3 both trap
GSNO faster than probe 1 at physiological pH (Table 1 and
Supporting Information S8), and this may be due to the
differences in electronic properties of the phenyl ring in
phosphine substrates. Moreover, the observed rate constants
(kobs) are in good agreement with those of reported kinetic
studies on the triarylphosphine and a benzyl azide,58 supporting
an overall second-order reaction kinetics.
A Quantitative Assay for GSNO Detection by Mass
Spectrometry. As in other analytical determinations, sample
acquisition and preparation represent critical issues, due to
possible artifactual generation of RSNO or its degradation.59 In
the present studies, at the time of sample preparation cells were
washed at 4 °C with PBS containing EDTA while minimizing
exposure to light (Supporting Information S9). Metal chelators
EDTA or DTPA were included to scavenge adventitious metals,
which might catalyze RSNO decomposition.60 Cells were lysed
using repeated freeze−thaw cycles (−80 to 24 °C) instead of
sonication, which may result in a loss of RSNO due to sulfur−
nitrogen bond breaking.61 A PEG-modified maleimide
(mPEG−maleimide, 20 kDa) was used for alkylating the free
sulfhydryl groups on GSH and proteins. More importantly, due
to the recent report that concentration of protein thiols is much
higher than that of GSH in mammalian cells,62 this blocking
step is required to make thiols unavailable for possible
transnitrosation reactions, or for reductions, e.g., Cu(II) to
Cu(I), thus minimizing RSNO decomposition. After blocking,
ultrafiltration through 3000-molecular-weight cutoff (MWCO)
filters was used to remove the higher molecular weight fraction,
With the above-described approach, involving probe 3, we
measured endogenously produced GSNO in several human
cancer cell lines (Table 2). Higher levels of GSNO were found
in malignant cells of epithelial origin (A375, MCF7, and
HCT116) when compared to lymphoblastoid (TK6) cells. In
addition to corroborating previous reports suggesting the use of
NOS expression and activity as markers of poor prognosis in
various cancers,69−73 our procedure provides a specific and
I
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binding ability of inflammation-related transcription factors),74
these results suggest that increased S-nitrosation may also
contribute to macrophage activation. Furthermore, we also
observed a linear relationship between cell numbers and
endogenous GSNO levels, which was detectable in as few as
105 cells (Supporting Information S12), thus confirming the
sensitivity of the assay.
Capture of Other Endogenous Low-Molecular Weight
S-Nitroso Thiols (LMW-RSNOs). Based on the fragmentation
pattern of disulfide-iminophosphorane 9, with an abundant oxa-
thia-phospholanium-like daughter ion at m/z = 309.0 (Figure
4F), we conjectured that any S-nitroso compound could react
with probe 3 to give the corresponding disulfide-iminophos-
phoranes and, upon CID, produce the common product ion m/
z = 309.0.
Table 2. Endogenous GSNO Quantification in Different
Cancer Cells and Macrophages (n = 6)
a
wet weight (mg/
106 cells)
GSNO amount
(pmol/106 cells)
[GSNO]
SD (μM)
cell line
A375
6.0
4.1
9.7
2.2
2.9
23.7
5.6
3.9 0.1
1.4 0.1
1.7 0.2
0.9 0.1
2.2 0.1
HCT116
MCF7
16.8
2.0
TK6
macrophages
(activated)
6.1
b
macrophages
(nonactivated)
2.2
1.2
0.7 0.1
a
The GSNO concentration was calculated, assuming wet weight =
b
volume for 106 cells. Macrophages were activated by treatment with
20 U/mL IFN-γ and 20 ng/mL LPS for 24 h.
Subsequent precursor-ion analysis of various nitrosothiols
confirmed this conjecture and led to the discovery of a set of
low-molecular weight S-nitroso thiols (LMW-SNOs) that
produced m/z = 309.0, including S-nitrosocysteine (9a), S-
nitroso-N-acetyl-cysteine (9b), S-nitrosohomocysteine (9c), S-
nitroso-N-acetyl-homocysteine (9d), S-nitrosocysteinylglycine
(a breakdown product of GSNO) (9e), S-nitrosocysteinylglut-
amine (9f), and S-nitrosated-CoA (9g) (Figure 5, Supporting
sensitive means to detect and quantify GSNO, a molecule of
fundamental importance in modulation of cell signaling by NO
via S-nitrosation. We found that activation of RAW 264.7
mouse macrophages with LPS and γ-IFN resulted in 3-fold
increases in GSNO content compared to that in nonstimulated
macrophages (Table 2). Since S-nitrosation has been reported
to affect key steps in the activation of macrophages, (e.g., DNA-
Figure 5. (Left) Low-molecular weight nitrosothiols (LMW-RSNOs) that produce common product ion, m/z = 309.0, upon CID. (Right)
Capturing LMW-RSNOs, by LC−MS/MS using the common product ion m/z 309.0 in cell lysates: (A) S-nitrosocysteine (9a) m/z = 309.0 →
257.6. (B) S-nitrosated N-acetyl-penicillamine (9h) m/z = 309.0 → 292.6. Y-axis represents the relative intensity.
J
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corresponding disulfide-iminophosphoranes but also phosphor-
yl-thioimidates produced the typical daughter ion at m/z =
309.0, leading to the detection of nitrosated N-acetyl-
penicillamine (9h) (a metabolite of the penicillin regularly
added to cell culture media) (Figure 5 and Supporting
Information). The m/z = 309.0 ion was present in the product
ion spectra of all of these species (Supporting Information). In
cancer cells, however, we detected GSNO, S-nitrosocysteine
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CONCLUSIONS
■
In summary, we report findings from investigations of the use
of phosphines as selective and efficient reagents for the
detection and quantification of GSNO and other low-molecular
weight nitrosothiols. Reaction kinetics of disulfide-iminophos-
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ASSOCIATED CONTENT
* Supporting Information
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product ion spectra for 9−9i; and experimental details for the
synthesis of compounds 1−3, 13C2,15N-labeled G*SNO, and
other S-nitrosothiols. This material is available free of charge via
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AUTHOR INFORMATION
Corresponding Author
srt@mit.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NIH (CA26731) and by the
MIT Center for Environmental Health Sciences (ES002109).
We acknowledge Agilent Technologies for access to the
UHPLC 1290 binary pump, autosampler, degasser, and the
triple quadrupole mass spectrometer. U.S. kindly thanks Drs.
Charles Knutson, Ujjal Sarkar, Erika Bechtold (MIT), and
Brian Smith (The Scripps Research Institute, CA) for helpful
discussions.
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