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Published in final edited form as:
Biochim Biophys Acta. 2009 July ; 1787(7): 835–840. doi:10.1016/j.bbabio.2009.04.015.
The Emergence of Nitroxyl (HNO) as a Pharmacological Agent
Christopher H. Switzer1, Wilmarie Flores-Santana1, Danellie Mancardi2, Sonia Donzelli3,
Debashree Basudhar4, Lisa A. Ridnour1, Katrina M. Miranda4, Jon M. Fukuto5, Nazerano
Paolocci6,7, and David A. Wink1,*
1Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD
20892.
2Institute
of Experimental and Clinical Pharmacology, University of Turino, Italy.
3Department
of Neurology, University Hospital Hamburg-Eppendorf, Hamburg, Germany. 27109.
4Department
of Chemistry, University of Arizona, Tucson, AZ 85721.
5Department
of Chemistry, California State University, Sonoma, CA 94928.
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6The
Johns Hopkins Medical Institutions, Baltimore, MD 21205.
7Department
of Clinical and Experimental Medicine, General Pathology and Immunology Section,
Perugia University, Italy.
Abstract
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Once a virtually unknown nitrogen oxide, nitroxyl (HNO) has emerged as a potential
pharmacological agent. Recent advances in the understanding of the chemistry of HNO has led to
the an understanding of HNO biochemistry which is vastly different from the known chemistry and
biochemistry of nitric oxide (NO), the one electron-oxidation product of HNO. The cardiovascular
roles of NO have been extensively studied, as NO is a key modulator of vascular tone and is involved
in a number of vascular related pathologies. HNO displays unique cardiovascular properties and has
been shown to have positive lusitropic and ionotropic effects in failing hearts without a chronotropic
effect. Additionally, HNO causes a release of CGRP and modulates calcium channels such as
ryanodine receptors. HNO has shown beneficial effects in ischemia reperfusion injury, as HNO
treatment during reperfusion reduces infarct size. In addition to the cardiovascular effects observed,
HNO has shown initial promise in the realm of cancer therapy. HNO has been demonstrated to inhibit
GAPDH, a key glycolytic enzyme. Due to the Warburg effect, inhibiting glycolysis is an attractive
target for inhibiting tumor proliferation. Indeed, HNO has recently been shown to inhibit tumor
proliferation in mouse xenografts. Additionally, HNO inhibits tumor angiogenesis and induces
cancer cell apoptosis. The effects seen with HNO donors are quite different from NO donors and in
some cases are opposite. The chemical nature of HNO explains how HNO and NO, although closely
chemically related, act so differently in biochemical systems. This also gives insight into the potential
molecular motifs that may be reactive towards HNO and opens up a novel field of pharmacological
development.
Correspondence should be addressed to: David A. Wink, Radiation Biology Branch, National Cancer Institute/National Institutes of
Health, Building 10, Room B3-B35, Bethesda, MD 20892, Phone: (301) 496-7511, Fax: (301) 480-2238, wink@mail.nih.gov.
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Keywords
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Nitroxyl; nitric oxide; heart failure; ischemia reperfusion injury
Introduction
Nitrogen oxides are important components of many physiological processes and with respect
to cardiovascular diseases, have the potential to be useful pharmacological agents. The advent
of nitrovasodilators such as nitroglycerin and the discovery that nitric oxide (NO) is endothelial
derived relaxation factor (EDRF) has lead to an increase of research in this field. In the
cardiovascular system, nitric oxide has been shown to regulate vascular tone, platelet function,
leukocyte adhesion and extravasations of leukocytes [1,2]. Dysfunction in the NO/cGMP
pathway leads to a number of cardiovascular disorders [3,4]. In addition to the critical functions
of NO in the circulatory system, nitric oxide plays a role in the control of heart function [5].
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One of the complexities of studying NO is the diverse array of other nitrogen oxides that must
be considered since NO metabolism can result in species such as nitrate, nitrite, peroxynitrite,
nitrogen dioxide, hydroxylamine and ammonia. One of the most overlooked, yet chemically
simple nitrogen oxide is HNO (colloquially termed nitroxyl; nitrosyl hydride in the IUPAC
nomenclature), which is the one-electron reduction product of NO. In comparison, HNO is to
NO as superoxide (O2−) is to molecular oxygen (O2) (Figure 1). Both NO and O2 are stable
paramagnetic gases with neutral charge and one-electron reduction to HNO/NO− or O2−,
respectively, results in the formation of an anion. In aqueous conditions, the formed anion has
a pKa associated with the equilibrium of protonation of the anion. For superoxide, a pKa of 6.8
has been measured, while the pKa of NO− has been calculated to be around 11.4 (the HNO/
NO− acid-base equilibrium is more complicated than common Brönsted-Lowry acid-base
conjugates and will be discussed below in more detail). The electrochemical reduction of O2
to O2− is thermodynamically unfavorable (E°= −0.33 V) and similarly, the reduction of NO to
NO− is thermodynamically very unfavorable (E°= −0.8 V) while reduction to HNO is still
unfavorable (E°= −0.6 V) [6]. Just as superoxide is easily reduced to hydrogen peroxide (E=
+0.89 V vs. NHE, pH 7), HNO can also be reduced to hydroxylamine as the two-electron
reduction potential has been calculated to be approximately E°= +0.8 V [7]. Additionally, the
one-electron reduction of HNO to aminoxyl radical (NH2O) has been calculated and is also
thermodynamically favorable (E°= +0.6 V) [7]. Another comparison lies with the selfdestruction of both superoxide and HNO; superoxide is known to undergo spontaneous
disproportionation (i.e. two molecules of superoxide react to form molecular oxygen and
hydrogen peroxide) while two molecules of HNO react to form essentially an HNO dimer
(hyponitrous acid), which decomposes to nitrous oxide and water. The disproportionation
reaction of superoxide proceeds with a relatively slow rate constant of ≥2 M−1·s−1 (and is
presumably why superoxide dismutase enzymes exist) while the dimerization reaction of HNO
proceeds with a rate constant of ~ 8×106 M−1s−1 [8]). For this reason, HNO is an inherently
unstable molecule with respect to decomposition and must be generated in situ from donor
compounds.
HNO Donor Compounds
Due to the fleeting nature of nitroxyl, vis-à-vis dimerization and formation of N2O, HNO must
be generated in situ from donor compounds (Table 1). The most common donor currently used
is Angeli’s salt (AS), which releases HNO with a half-life of approximately 2–3 minutes at
physiological pH and temperature. HNO release from this compound is observed between pH
4–8. The mechanism of HNO release from Angeli’s salt has received a theoretical
computational treatment [10]. The decomposition of Angeli’s salt to yield HNO also produces
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one equivalent of nitrite. Nitrite is not an innocuous bystander in physiological processes and
reports on the biological effects of nitrite are rapidly accumulating [9]. Thus the formation of
nitrite from Angeli’s salt can be a confounding component in studying HNO biology and
pharmacology. Another drawback to Angeli’s salt use in a physiological context is the rapid
kinetics of decomposition and lends itself to almost a “bolus dose” of HNO and nitrite. This
can often be a technical problem when studying biochemical and physiological processes that
can be several orders of magnitude slower. An often-overlooked chemical feature of Angeli’s
salt decomposition is that theoretical calculations predict that a very small amount of NO is
produced [10].
Another long known HNO donor is Piloty’s acid (or N-hydroxy-benzene sulfonamide). Piloty’s
acid releases HNO upon deprotonation and requires rather high pH to yield HNO. This is the
major issue with using Piloty’s acid in biological studies as such high pH is rarely, if ever,
encountered [11]. However a potential area of research involves the development of structural
analogs of Piloty’s acid that alter the HNO release pH profile by changing the electronic
constituents on the phenyl ring. It is hypothesized that electron withdrawing groups may allow
HNO release at more physiological conditions.
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A new class of HNO donors, acyloxy nitroso compounds, has been developed by King et al.
that release HNO upon cleavage of the ester bond [12]. Modifying the electronic and/or steric
properties of the acetyl group position changes the rate of ester cleavage and thus HNO release.
One of the major benefits of these HNO donors, besides controlling the HNO release rate, is
that nitrite is not a product as is the case with Angeli’s salt. Another feature of these compounds
that lends itself to biochemical studies is the blue color that is a result of the π→n electronic
transition of the N-O bond. Thus biochemical reaction kinetics can be easily monitored. As
novel and exciting as these compounds are, there are other reactions beyond HNO release. A
major route of decomposition in biological systems is the reactivity of the donor compound
with thiols. The characteristic blue color of these compounds due to the N-O bond implies that
the relatively low energy LUMO is associated with this bond and is susceptible to nucleophilic
attack. Thus HNO release from these compounds must compete with thiol reactivity at the
electrophilic nitroso functional group. Since the majority of HNO biochemistry and
pharmacology is associated with thiol reactions, the reactivity of “blue” HNO donors with
thiols may complicate the interpretation of cellular or in vivo experiments. However the elegant
chemistry of these compounds make it a very attractive choice for chemical and biochemical
HNO donation and future generations of “blue” compounds may limit the thiol decomposition
route.
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Another potential HNO donor compound is IPA/NO, a primary amine NONOate. NONOates
of secondary amines have traditionally been used as NO donors and have become the standard
for NO donating compounds in chemistry and biology. However primary amine NONOates
have the potential to release HNO under certain conditions [11]. IPA/NO decomposition has
received theoretical consideration, and in concert with empirical observations, it is currently
agreed that HNO is released above pH 7.8 while IPA/NO is believed to be exclusively a NO
donor at pH <7 [13].
Temporal control of HNO release from a donor compound is a feature that is lacking from the
types of donors described above. Compounds that release HNO via a retro-Diels-Alder reaction
are activated by UV-A light [14]. This gives researchers control over when HNO is released
and the only other product to consider from decomposition is a primary amine. Despite the
control of these compounds with respect to HNO release, there have been no biological reports
using these compounds, perhaps due to the use of UV irradiation.
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While there are currently only a few reasonable HNO donors available to researchers, it must
be noted that the chemical, biochemical and physiological study of HNO is achievable, if not
readily straightforward. The use of multiple HNO donors in a study is strongly encouraged due
to the confounding factors of each HNO donor (i.e. nitrite from Angeli’s salt, thiol reactions
of “blue” compounds, NO release from IPA/NO). If multiple HNO donors are used and elicit
the same biological response, then it is safe to assume that the effect is due to HNO as HNO
release is the only common characteristic of these diverse chemical compounds (Table 1). Thus
when it comes to HNO donors, fear not but be thorough.
HNO Chemistry and Biochemistry
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The orthogonal relationship between NO and HNO was not predicted from the literature prior
to 2000. Rampant misconception on the chemical nature of nitroxyl came from the reported
pKa of 4.7, which suggests that NO−, not HNO, was the most relevant species at physiological
concentration [15]. This also suggested that NO− and NO would readily interconvert with
reduction potential of NO to 3NO− of +0.39 V and 1NO− of −0.2 V [16]. If these potentials
were accurate, NO should be rapidly be converted to 3NO− which then would be expected to
react with O2 with diffusion controlled rate constants to form peroxynitrite. This implies that
NO would not be detectable in biological systems and that peroxyntrite would be the primary
effector molecule of NOS metabolism. However, NO is directly detected in vivo and from the
enzymatic turnover of NOS [17–19]. Additional experiments showed that NO donors in vivo
give different effects than HNO donors indicating that these species do not interconvert but
rather have orthogonal and distinct processes [20].
These observations taken together required a re-examination of the chemistry of HNO/NO−.
Two studies using different methods re-evaluated the pKa finding that it was 11.5 instead of
4.7 [8,21]. This reverses several previous conceptions about nitroxyl. The first is that HNO,
and not NO−, is the predominant species in vivo. The second is that the reduction potential of
NO is high < −0.8 V which is higher than reducing system occur in mammalian systems. This
explains why NO is not converted in vivo to NO− (and then to peroxynitrite via reaction with
O2) and this, in part, explains the orthogonal behavior of NO and HNO donors observed.
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Though the revised pKa of 11.5 suggests that HNO and not NO− is the dominant species at
physiological pH, this does not discount the possibility of the reactions involving the
intermediacy of 3NO−. Normally, reactions that are involved in acid base equilibrium, i.e.
protonation/deprotonation reactions, are so rapid that it does not affect the overall kinetics of
a reaction. However, it has been shown that the deprotonation of HNO involves intersystem
crossing (vibrational relaxation from one energy state to a lower energy state of differing
quantum spin states) between 1HNO and 3NO−. This causes the deprotonation reaction (HNO
↔ H+ + NO−) to be very slow compared to a spin allowed process (e.g. 2 H2O ↔ H3O+ +
OH−). This slow deprotonation of HNO has effects on its overall reactivity. For
example 3NO− can react with O2 with rate constants of >109 M−1 s−1. However at neutral pH,
HNO is the predominant species and deprotonation to 3NO− proceeds with a rate constant of
~105 M−1 s−1 and HNO has a half-life of 5000 seconds [22]. It has been argued that
the 3NO−/O2 reaction is not kinetically viable at neutral pH, even though the rate constant for
this reaction is near diffusion controlled, due to this spin-forbiden deprotonation [20,22]. This
kinetic barrier suggests that HNO cannot be converted to 3NO− that could be converted to
peroxynitrite via oxygen reaction. Therefore, NO− and HNO have sufficient kinetic and
thermodynamic barriers that they cannot be interconverted. These vastly different chemical
entities (HNO and NO−) also display very different chemical reactivities. While NO−
predominately undergoes outer sphere electron transfer (much like static electricity where the
electron jump from one atom to another) and is therefore a nucleophile, HNO is a good
electrophile preferring addition reaction to nucleophiles, especially thiols, as opposed to
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hydrogen atom donation [20,23], although HNO can potentially serve as a hydrogen atom
donor to high energy radicals such as a lipid radical.
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With this new chemical view of HNO, many of the biochemical targets can be illuminated.
The kinetics of HNO reactivity with a number of potential biomolecules revealed that HNO
reacts with ferric heme proteins, CuZnSOD, MnSOD and thiols with bimolecular rate constant
of ~106 M−1 s−1 [20,23]. HNO reacts with ferrous heme proteins approximately 100 times
slower (k = ~104 M−1 s−1) than ferric heme proteins. Interestingly the rate constant for the
reaction of HNO with O2 is <103 M−1 s−1, although the precise mechanism or the intermediates
of this reaction are not currently known [20,23]. In contrast, NO reacts preferentially with
ferrous heme protein and high-energy radicals (e.g. lipid radicals, O2−) [24]. One feature of
HNO is the relatively weak H-N bond strength of ~ 50 kcal/mol, making HNO a decent H atom
donor and thus able to quench high-energy radicals. Indeed, HNO has been shown to inhibit
lipid peroxidation in a yeast model system, demonstrating that HNO also acts as an anti-oxidant
[25]. The differences in reactivity with biochemical targets suggest that the biological
orthogonality of NO and HNO can be rationalized based on the chemical reactivity of these
related but vastly different chemical species, summarized in Table 2.
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Given the preference of HNO to react with ferric heme proteins and acidic thiols, this should
guide investigators when studying the biological effects of HNO. The facile and irreversible
reaction between HNO and the active site thiol of GAPDH represents a unique biochemical
molecular motif for further investigation [25]. Similarly, HNO inhibits cathepsin B activity,
which is a cysteine protease [26,27]. HNO also has been shown to react with metal bound
thiolates, as Angeli’s salt was used to disrupt a yeast Cu-thiolate transcription factor protein
[28]. These examples demonstrate the reactive nature of HNO in a biological setting by
targeting critical reactive thiolates (RS−). Other enzymes such as caspases, fatty acid acyl
transferase and ubiquitin ligases use acidic thiols in their respective catalytic processes and are
potentially targets of HNO. Furthermore, calcium channels, which are altered by HNO, have
critical thiols that are presumably the chemical motif that is responsible for the observed effects
of HNO. Additionally, metalloproteins are subject to reactivity with HNO. These potential
biochemical targets have not yet been investigated, but represent a large body of work that
needs to be addressed in the development of HNO biochemistry and pharmacology.
HNO in Biological Systems
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Similar to nitric oxide, HNO has been shown to induce vasorelaxation. This effect has been
known for some time as HNO, from either Angeli’s salt or cyanamide, is a potent vasodilator
similar to NO and EDRF [29,30]. HNO was also a candidate for EDRF, as thiol compounds
inhibit the vasorelaxation of EDRF, which is consistent with HNO chemistry [31]. More recent
work has shown that HNO is selectively a venodilator in dogs [32]. In contrast, NO donors
equally dilate both the arterial and venal side of the circulatory system. Administration of
baroreceptor blockers resulted in balanced dilation suggesting that the preference for AS
mediated venodilation was through constriction of the arterial side through a sympathetic
response [32]. The molecular mechanisms of HNO mediated vasodilation have been reported.
Recent studies have shown that HNO mediates relaxation in rodent models through a cGMP
mechanism as well as modulating voltage-gated potassium channels [33,34]. In rat coronary
vasculature, Angeli’s salt is a potent vasodilatory agent by sGC dependent CGRP release and
by activating KATP channels [35]. Of clinical relevance, HNO from Angeli’s salt does not
develop tolerance in isolated rat aorta, which is a concern with traditional therapies for angina
pecoralis such as nitroglycerin. [36].
An important property of HNO in the cardiovascular system is that HNO donors cause an
increase in contractility in a canine model. HNO has been shown to have positive inotropic
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(force of muscle contraction) as well as lusitropic (relaxation of cardiac muscle) properties;
both properties contribute to increased cardiac output [32]. Furthermore, in failing cainine
hearts, HNO also has positive lusitropic and inotropic effects that are independent of betaadernergic signaling [32]. Additionally, it was found that HNO donors increased circulating
CGRP levels, which increases the contractility of the heart (positive inotropy) without an
increase in the chronotropic effect (i.e. without an increase in the rate of cardiac contraction)
[32]. It was presumed that CGRP was released from NANC neurons. However, under heart
failure conditions, CGRP did not play a major role in the HNO induced contractility suggesting
another molecular target [37]. Additional mechanisms for HNO induced contractility have been
proposed. Sarcoplasmic reticulum calcium cycling has been implicated as HNO donors were
shown to modulate the RyD receptors and SERCA that result in increasing the contractility of
isolated cardiomyocytes [38]. Mechanistically, HNO has been shown to target a critical
cysteine residue of SERCA (cysteine 674) that causes an increase in SERCA activity [39].
Furthermore, SERCA activation is achieved by dephosphorylation by phospholamban (PLN),
as HNO causes the formation of a disulfide on PLN, which removes the inhibition of the
Ca2+ pump [40]. Additionally, HNO alters myofilament- Ca2+ interaction and results in
increased cardiac contraction force [41]. Collectively, these finding indicate that nitroxyl may
have a positive effect in heart failure with a number of potential targets and mechanisms.
Furthermore this represents opportunity to develop HNO donors for the pharmacological
treatment of heart failure.
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HNO has been shown to have effects in the realm of ischemia reperfusion (IR) injury. IR injury
occurs when tissue is deprived of adequate blood flow for a period of time causing among other
effects hypoxia, which is the ischemic event. The reperfusion of oxygenated blood causes the
injury and results in necrosis of the tissue. IR injury can be alleviated by preconditioning the
tissue, which involves brief occlusions of the vasculature prior to the actual cessation of blood
flow. NO donors have been shown to be protective in cardiac ischemia reperfusion injury when
given during the reperfusion phase [42]. However, HNO administration during reperfusion
was found to dramatically increase the infarct size. These results again indicate that in vivo,
NO and HNO have very different effects [43]. However, additional studies have shown that
HNO is a powerful preconditioning agent. Pre-treating hearts with Angeli’s salt followed by
an ischemic event results in a dramatic decrease in the infarct size [44]. In contrast, NO donors
are modest preconditioning agents. These in vivo results clearly suggest that HNO and NO
donors have very different pharmacological effects.
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In addition to cardiovascular effects, HNO has shown promising anti-cancer effects. Most solid
tumors thrive in hypoxic conditions and rely upon glycolysis as a major energy source. One
of the critical enzymes in the glycolytic pathway is GAPDH. HNO has been shown to inhibit
GAPDH activity in an irreversible manner [45]. Furthermore, HNO inhibits breast and
neurobalstoma cancer proliferation in mouse xenografts as well as in vitro cultures [46,47].
HNO also caused an increase of apoptosis in the breast cancer xenografts and inhibited tumor
angiogenesis [46]. In addition to the direct effects of HNO on cancer cells, HNO-donors may
be useful adjuvant agents to cancer chemotherapy. Angeli’s salt has been shown to inhibit poly
(ADP-Ribose) polymerase (PARP) in a breast cancer cell line [48]. PARP is an important
component of the DNA repair machinery and is thus a potentially important molecular target
in the treatment of cancer, since a number of chemotherapies and radiation therapy are based
on inducing DNA damage of the cancer cell. Inhibition of PARP by HNO donors would then
increase the efficacy of these therapies. These observations indicate that HNO donors may be
effective agents in cancer chemotherapy.
Due to the unique physiological properties of HNO, the endogenous formation of HNO in
vivo is an intriguing possibility and one that has been suggested for several decades. A number
of potential mechanisms have been proposed for the endogenous production of HNO (Figure
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2); however no mechanism has been clearly established to date, mainly due to a lack of detection
(vide infra). Several studies implicated that HNO could be an intermediate in catalytic process
of nitric oxide synthase (NOS) [49–51]. Detection of N2O and NH2OH, products of HNO
metabolism, was an indication that HNO was part of the enzymatic turnover of NOS [50,52].
Furthermore, the presence of CuZnSOD caused an increase in NO from NOS [53]. Also, Fe
(II)-NO was observed to be formed during enzymatic turnover in NOS which is normally in a
ferric resting state. Taken together, these findings imply that nitroxyl could be an intermediate
in the metabolism of NOS, however this has not been conclusively established.
Another potential mechanism of HNO generation in biological systems is through the
decomposition of RSNO. Thioredoxin has been shown to generate HNO and NH2OH though
the reduction of these adducts [54]. HNO is also formed from the nitrosation of dithiol
compounds (i.e. DTT and lipoic acid) [55]. Thus one of the secondary products of nitrosative
stress involves the intermediacy of HNO and eventually NH2OH. These processes have not
yet been established in vivo, but the chemistry implies that RSNO is not an evolutionary
signaling motif like phosphorylation, as has been suggested, due to the reactivity of excess
thiols to form disulfide and HNO [56].
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The oxidation of NH2OH is another potential source of HNO. The two-electron oxidation of
NH2OH whose oxidation potential is +0.7 V (vs. Ag/AgCl) results in HNO. The generation of
high-valent iron-oxo complexes from the reaction of hydrogen peroxide with ferric hemes can
oxidize NH2OH to HNO [55]. The formation of glutathione sulfinamide was used as a marker
of HNO migration from the heme pocket [55]. Since the resulting ferric heme that is formed
after the oxidation of hydroxylamine can react with HNO to form a ferrous nitrosyl complex,
there is some question as to whether HNO would be capable of escaping the heme pocket to
react with another substrate. A survey of heme proteins showed that Mb and MPO having
proximal hisitidine residues to the heme site could generate HNO that is able to migrate from
the heme pocket. Interestingly, HNO activity has been primarily shown to be associated with
the heart and under conditions of inflammation where hydrogen peroxide generation is known.
One of the current challenges with expanding HNO research is in developing reliable detection
methods of HNO generation and/or reactivity in vivo. Currently, the formation of N2O and
NH2OH are used to indicate that HNO is involved in a particular process; however this is only
indirect evidence of nitroxyl. The formation of sulfinamide with proteins such as GAPDH may
provide a unique footprint for the formation and activity of this compound [45,57]. Recently
a novel proteomic approach offers a potentially very useful tool in analyzing the reactive targets
of HNO from ex vivo samples [58]. The development of analytical methods is critically needed
for this promising pharmacological agent to advance.
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Figure 1. Comparison of HNO and Superoxide Chemistry
(A) Molecular oxygen (O2) reduction to superoxide (O2−) is thermodynamically unfavorable,
as is the reduction of NO to HNO/NO−. Further reduction of O2− to hydrogen peroxide is
thermodynamically favorable, as is the reduction of HNO. (B) Reduction of O2 or NO results
in an anionic species that has an associated pKa in aqueous conditions. Superoxide has a pKa
of 4.88 and in neutral conditions the anion is the predominant species. Nitroxyl has a pKa of
11.4 and the dominant species in biology will be HNO. (C) Superoxide and HNO both autoreact resulting in the destruction of these species.
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Figure 2. Potential Mechanisms of Endogenous HNO Formation
HNO has been postulated to be formed from the enzymatic activity of nitric oxide synthase
(NOS) in which the substrate arginine is reduced by six electron to yield HNO instead of five
electrons to yield NO. Another route is from the decomposition of nitrosothiol by thiols such
as glutathione. HNO can also be generated by the oxidation of hydroxylamine from the
peroxidase activity of heme proteins.
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Table 1
Common HNO Donors Used in Biology
Donor
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Chemical
Structure
Angeli's salt
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Piloty's acid
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Acyloxo nitroso "Blue
compounds"
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Donor
Chemical
Structure
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IPA/NO
hetero-Diels-Alder cycloadduct
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Table 2
Comparison of NO and HNO Reactivity with Biological Reactants
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Biological
Reactant
NO
HNO
NO
no reaction
Forms N2O2−/HN2O2 unknown chemistry
HNO
Forms N2O2−/HN2O2 unknown chemistry
Dimerization and decomposition to N2O
O2
Autoxidation leading to nitrosative species (i e. NO2 and N2O3)
Forms a potent 2 e− oxidant that is not ONOO−
RSH /RS−
no reaction
Forms sulfinamide or disulfide + H2NOH
2+
heme
Very stable Fe -NO
Forms coordination complexes
3+
heme
Forms unstable electrophic nitrosyl; first step in reductive nitrosylation
Very stable Fe2+-NO (except sGC)
Cu2+
no reaction
Reduces Cu2+ to Cu+ yielding NO
Lipid Radical
Yields Lipid -NO
Yields Lipid -H + NO
Fe
Fe
2+
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