Formation of nitric oxide by cytochrome P450-catalyzed oxidation of aromatic amidoximes

Evaluation of Nitric Oxide-Donating Properties of 11H-indeno[1,2-b]quinoxalin-11-one Oxime (IQ-1) by Electron Paramagnetic Resonance Spectroscopy

IQ-1 (11H-indeno[1,2-b]quinoxalin-11-one oxime) is a specific c-Jun N-terminal kinase (JNK) inhibitor with anticancer and neuro- and cardioprotective properties. Because aryloxime derivatives undergo cytochrome P450-catalyzed oxidation to nitric oxide (NO) and ketones in liver microsomes, NO formation may be an additional mechanism of IQ-1 pharmacological action. In the present study, electron paramagnetic resonance (EPR) of the Fe2+ complex with diethyldithiocarbamate (DETC) as a spin trap and hemoglobin (Hb) was used to detect NO formation from IQ-1 in the liver and blood of rats, respectively, after IQ-1 intraperitoneal administration (50 mg/kg). Introducing the spin trap and IQ-1 led to signal characteristics of the complex (DETC)2-Fe2+-NO in rat liver. Similarly, the introduction of the spin trap components and IQ-1 resulted in an increase in the Hb-NO signal for both the R- and the T-conformers in blood samples. The density functional theory (DFT) calculations were in accordance with the experimental data and indicated that the NO formation of IQ-1 through the action of superoxide anion radical is thermodynamically favorable. We conclude that the administration of IQ-1 releases NO during its oxidoreductive bioconversion in vivo.

A review on oxime functionality: an ordinary functional group with significant impacts in supramolecular chemistry

The oxime functional group is pivotal in chemistry, finding extensive applications in medical science, catalysis, organic functional group transformations, and the recognition of essential and toxic analytes. While the coordination chemistry of oxime derivatives has been thoroughly explored and several reviews have been published on this topic in reputable journals, a comprehensive review encompassing various aspects such as crystal engineering, cation and anion recognition, as well as coordination chemistry activities, is still in demand. This feature article highlights the diverse applications of oxime derivatives across multiple domains of chemistry, including medicine, agriculture, crystal engineering, coordination chemistry, and molecular recognition studies. Each of the oxime derivatives in this feature article are meticulously described in terms of their medicinal applications, crop protection, crystal engineering attributes, analyte recognition capabilities, and coordination chemistry aspects. By providing a comprehensive overview of their versatile applications, this article aims to inspire researchers to explore and develop novel oxime-based derivatives for future applications.

Opportunities for Nitric Oxide in Potentiating Cancer Immunotherapy

Despite nearly 30 years of development and recent highlights of nitric oxide (NO) donors and NO delivery systems in anticancer therapy, the limited understanding of exogenous NO's effects on the immune system has prevented their advancement into clinical use. In particular, the effects of exogenously delivered NO differing from that of endogenous NO has obscured how the potential and functions of NO in anticancer therapy may be estimated and exploited despite the accumulating evidence of NO's cancer therapy-potentiating effects on the immune system. After introducing their fundamentals and characteristics, this review discusses the current mechanistic understanding of NO donors and delivery systems in modulating the immunogenicity of cancer cells as well as the differentiation and functions of innate and adaptive immune cells. Lastly, the potential for the complex modulatory effects of NO with the immune system to be leveraged for therapeutic applications is discussed in the context of recent advancements in the implementation of NO delivery systems for anticancer immunotherapy applications. SIGNIFICANCE STATEMENT: Despite a 30-year history and recent highlights of nitric oxide (NO) donors and delivery systems as anticancer therapeutics, their clinical translation has been limited. Increasing evidence of the complex interactions between NO and the immune system has revealed both the potential and hurdles in their clinical translation. This review summarizes the effects of exogenous NO on cancer and immune cells in vitro and elaborates these effects in the context of recent reports exploiting NO delivery systems in vivo in cancer therapy applications.

Oximes: Novel Therapeutics with Anticancer and Anti-Inflammatory Potential

Oximes have been studied for decades because of their significant roles as acetylcholinesterase reactivators. Over the last twenty years, a large number of oximes have been reported with useful pharmaceutical properties, including compounds with antibacterial, anticancer, anti-arthritis, and anti-stroke activities. Many oximes are kinase inhibitors and have been shown to inhibit over 40 different kinases, including AMP-activated protein kinase (AMPK), phosphatidylinositol 3-kinase (PI3K), cyclin-dependent kinase (CDK), serine/threonine kinases glycogen synthase kinase 3 α/β (GSK-3α/β), Aurora A, B-Raf, Chk1, death-associated protein-kinase-related 2 (DRAK2), phosphorylase kinase (PhK), serum and glucocorticoid-regulated kinase (SGK), Janus tyrosine kinase (JAK), and multiple receptor and non-receptor tyrosine kinases. Some oximes are inhibitors of lipoxygenase 5, human neutrophil elastase, and proteinase 3. The oxime group contains two H-bond acceptors (nitrogen and oxygen atoms) and one H-bond donor (OH group), versus only one H-bond acceptor present in carbonyl groups. This feature, together with the high polarity of oxime groups, may lead to a significantly different mode of interaction with receptor binding sites compared to corresponding carbonyl compounds, despite small changes in the total size and shape of the compound. In addition, oximes can generate nitric oxide. This review is focused on oximes as kinase inhibitors with anticancer and anti-inflammatory activities. Oximes with non-kinase targets or mechanisms of anti-inflammatory activity are also discussed.

Therapeutic Effects of Tryptanthrin and Tryptanthrin-6-Oxime in Models of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease involving joint and bone damage that is mediated in part by proteases and cytokines produced by synovial macrophages and fibroblast-like synoviocytes (FLS). Although current biological therapeutic strategies for RA have been effective in many cases, new classes of therapeutics are needed. We investigated anti-inflammatory properties of the natural alkaloid tryptanthrin (TRYP) and its synthetic derivative tryptanthrin-6-oxime (TRYP-Ox). Both TRYP and TRYP-Ox inhibited matrix metalloproteinase (MMP)-3 gene expression in interleukin (IL)-1β-stimulated primary human FLS, as well as IL-1β–induced secretion of MMP-1/3 by FLS and synovial SW982 cells and IL-6 by FLS, SW982 cells, human umbilical vein endothelial cells (HUVECs), and monocytic THP-1 cells, although TRYP-Ox was generally more effective and had no cytotoxicity in vitro. Evaluation of the therapeutic potential of TRYP and TRYP-Ox in vivo in murine arthritis models showed that both compounds significantly attenuated the development of collagen-induced arthritis (CIA) and collagen-antibody–induced arthritis (CAIA), with comparable efficacy. Collagen II (CII)-specific antibody levels were similarly reduced in TRYP- and TRYP-Ox-treated CIA mice. TRYP and TRYP-Ox also suppressed proinflammatory cytokine production by lymph node cells from CIA mice, with TRYP-Ox being more effective in inhibiting IL-17A, granulocyte-macrophage colony-stimulating factor (GM-CSF), and receptor activator of nuclear factor-κB ligand (RANKL). Thus, even though TRYP-Ox generally had a better in vitro profile, possibly due to its ability to inhibit c-Jun N-terminal kinase (JNK), both TRYP and TRYP-Ox were equally effective in inhibiting the clinical symptoms and damage associated with RA. Overall, TRYP and/or TRYP-Ox may represent potential new directions for the pursuit of novel treatments for RA.

Amidoximes and Oximes: Synthesis, Structure, and Their Key Role as NO Donors

Nitric oxide (NO) is naturally synthesized in the human body and presents many beneficial biological effects; in particular on the cardiovascular system. Recently; many researchers tried to develop external sources to increase the NO level in the body; for example by using amidoximes and oximes which can be oxidized in vivo and release NO. In this review; the classical methods and most recent advances for the synthesis of both amidoximes and oximes are presented first. The isomers of amidoximes and oximes and their stabilities will also be described; (Z)-amidoximes and (Z)-oximes being usually the most energetically favorable isomers. This manuscript details also the biomimetic and biological pathways involved in the oxidation of amidoximes and oximes. The key role played by cytochrome P450 or other dihydronicotinamide-adenine dinucleotide phosphate (NADPH)-dependent reductase pathways is demonstrated. Finally, amidoximes and oximes exhibit important effects on the relaxation of both aortic and tracheal rings alongside with other effects as the decrease of the arterial pressure and of the thrombi formation

Synthesis, biological evaluation, and molecular modeling of 11H-indeno[1,2-b]quinoxalin-11-one derivatives and tryptanthrin-6-oxime as c-Jun N-terminal kinase inhibitors

c-Jun N-terminal kinases (JNKs) play a central role in many physiologic and pathologic processes. We synthesized novel 11H-indeno[1,2-b]quinoxalin-11-one oxime analogs and tryptanthrin-6-oxime (indolo[2,1-b]quinazoline-6,12-dion-6-oxime) and evaluated their effects on JNK activity. Several compounds exhibited sub-micromolar JNK binding affinity and were selective for JNK1/JNK3 versus JNK2. The most potent compounds were 10c (11H-indeno[1,2-b]quinoxalin-11-one O-(O-ethylcarboxymethyl) oxime) and tryptanthrin-6-oxime, which had dissociation constants (K_d) for JNK1 and JNK3 of 22 and 76 nM and 150 and 275 nM, respectively. Molecular modeling suggested a mode of binding interaction at the JNK catalytic site and that the selected oxime derivatives were potentially competitive JNK inhibitors. JNK binding activity of the compounds correlated with their ability to inhibit lipopolysaccharide (LPS)-induced nuclear factor-κB/activating protein 1 (NF-κB/AP-1) activation in human monocytic THP-1Blue cells and interleukin-6 (IL-6) production by human MonoMac-6 cells. Thus, oximes with indenoquinoxaline and tryptanthrin nuclei can serve as specific small-molecule modulators for mechanistic studies of JNK, as well as potential leads for the development of anti-inflammatory drugs.

A novel dual NO-donating oxime and c-Jun N-terminal kinase inhibitor protects against cerebral ischemia-reperfusion injury in mice

The c-Jun N-terminal kinase (JNK) has been shown to be an important regulator of neuronal cell death. Previously, we synthesized the sodium salt of 11H-indeno[1,2-b]quinoxalin-11-one (IQ-1S) and demonstrated that it was a high-affinity inhibitor of the JNK family. In the present work, we found that IQ-1S could release nitric oxide (NO) during its enzymatic metabolism by liver microsomes. Moreover, serum nitrite/nitrate concentration in mice increased after intraperitoneal injection of IQ-1S. Because of these dual actions as JNK inhibitor and NO-donor, the therapeutic potential of IQ-1S was evaluated in an animal stroke model. We subjected wild-type C57BL6 mice to focal ischemia (30min) with subsequent reperfusion (48h). Mice were treated with IQ-1S (25mg/kg) suspended in 10% solutol or with vehicle alone 30min before and 24h after middle cerebral artery (MCA) occlusion (MCAO). Using laser-Doppler flowmetry, we monitored cerebral blood flow (CBF) above the MCA during 30min of MCAO provoked by a filament and during the first 30min of subsequent reperfusion. In mice treated with IQ-1S, ischemic and reperfusion values of CBF were not different from vehicle-treated mice. However, IQ-1S treated mice demonstrated markedly reduced neurological deficit and infarct volumes as compared with vehicle-treated mice after 48h of reperfusion. Our results indicate that the novel JNK inhibitor releases NO during its oxidoreductive bioconversion and improves stroke outcome in a mouse model of cerebral reperfusion. We conclude that IQ-1S is a promising dual functional agent for the treatment of cerebral ischemia and reperfusion injury.

NO-donating oximes relax corpora cavernosa through mechanisms other than those involved in arterial relaxation

INTRODUCTION

Erectile dysfunction (ED), as well as many cardiovascular diseases, is associated with impaired nitric oxide (NO) bioavailability. Recently, oxime derivatives have emerged as vasodilators due to their NO-donating capacities. However, whether these oximes offer therapeutic perspectives as an alternative NO delivery strategy for the treatment of ED is unexplored.

AIMS

This study aims to analyze the influence of formaldoxime (FAL), formamidoxime (FAM), and cinnamaldoxime (CAOx) on corporal tension and to elucidate the underlying molecular mechanisms.

METHODS

Organ bath studies were carried out measuring isometric tension on isolated mice corpora cavernosa (CC), thoracic aorta, and femoral artery. After contraction with norepinephrine (NOR), cumulative concentration-response curves of FAL, FAM, and CAOx (100 nmol/L-1 mmol/L) were performed.

MAIN OUTCOME MEASURES

FAL-/FAM-induced relaxations were evaluated in the absence/presence of various inhibitors of different molecular pathways.

RESULTS

FAL, FAM, and CAOx relax isolated CC as well as aorta and femoral artery from mice. ODQ (soluble guanylyl cyclase-inhibitor), diphenyliodonium chloride (nonselective flavoprotein inhibitor), and 7-ethoxyresorufin (inhibitor of CYP450 1A1 and NADPH-dependent reductases) substantially blocked the FAL-/FAM-induced relaxation in the arteries but not in CC. Only a small inhibition of the FAM response in CC was observed with ODQ.

CONCLUSIONS

This study shows for the first time that NO-donating oximes relax mice CC. Therefore, oximes are a new group of molecules with potential for the treatment of ED. However, the underlying mechanism(s) of the FAL-/FAM-induced corporal relaxation clearly differ(s) from the one(s) involved in arterial vasorelaxation.

Participation of nitric oxide pathway in the relaxation response induced by E-cinnamaldehyde oxime in superior mesenteric artery isolated from rats

For many years, nitric oxide (NO) has been studied as an important mediator in the control of vascular tone. Endothelial deficiencies that diminish NO production can result in the development of several future cardiovascular diseases, such as hypertension and arteriosclerosis. In this context, new drugs with potential ability to donate NO have been studied. In this study, 3 aromatic oximes [benzophenone oxime, 4-Cl-benzophenone oxime, and E-cinnamaldehyde oxime (E-CAOx)] induced vasorelaxation in endothelium-denuded and intact superior mesenteric rings precontracted with phenylephrine. E-CAOx demonstrated the most potent effect, and its mechanism of action was evaluated. Vascular reactivity experiments demonstrated that the effect of E-CAOx was reduced by the presence of 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one, and (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, suggesting the participation of NO/sGC/PKG pathway. NO donation seems to be mediated through nicatinamide adenine dinucleotide phosphate-dependent reductases because 7-ethoxyresorufin decreased the effect of E-CAOx on vascular reactivity and reduced NO formation as detected by flow cytometry using the NO indicator diaminofluorescein 4,5-diacetate. Further downstream of NO donation, K+ subtype channels were also shown to be involved in the E-CAOx vasorelaxant effect. The present study showed that E-CAOx acts like an NO donor, activating NO/sGC/PKG pathway and thus K+ channels.

Nitric oxide signaling as a common target of organohalogens and other neuroendocrine disruptors

Organohalogen compounds such as polychlorinated biphenyls (PCB) and polybrominated diphenyl ethers (PBDE) are global environmental pollutants and highly persistent, bioaccumulative chemicals that produce adverse effects in humans and wildlife. Because of the widespread use of these organohalogens in household items and consumer products, indoor contamination is a significant source of human exposure, especially for children. One significant concern with regard to health effects associated with exposure to organohalogens is endocrine disruption. Toxicological studies on organohalogen pollutants primarily focused on sex steroid and thyroid hormone actions, and findings have largely shaped the way one envisions their disruptive effects occurring. Organohalogens exert additional effects on other systems including other complex endocrine systems that may be disregulated at various levels of organization. Over the last 20 years evidence has mounted in favor of a critical role of nitric oxide (NO) in numerous functions ranging from neuroendocrine functions to learning and memory. With its participation in multiple systems and action at several levels of integration, NO signaling has a pervasive influence on nervous and endocrine functions. Like blockers of NO synthesis, PCBs and PBDEs produce multifaceted effects on physiological systems. Based on this unique set of converging information it is proposed that organohalogen actions occur, in part, by hijacking processes associated with this ubiquitous bioactive molecule. The current review examines the emerging evidence for NO involvement in selected organohalogen actions and includes recent progress from our laboratory that adds to our current understanding of the actions of organohalogens within hypothalamic neuroendocrine circuits. The thyroid, vasopressin, and reproductive systems as well as processes associated with long-term potentiation were selected as sample targets of organohalogens that rely on regulation by NO. Information is provided about other toxicants with demonstrated interference of NO signaling. Our focus on the convergence between NO system and organohalogen toxicity offers a novel approach to understanding endocrine and neuroendocrine disruption that is particularly problematic for developing organisms. This new working model is proposed as a way to encourage future study in elucidating common mechanisms of action that are selected with a better operational understanding of the systems affected.

Vasorelaxant activity of some oxime derivatives

Several non-aromatic substituted oxime derivatives (formamidoxime, acetaldoxime, acetone oxime, acetohydroxamic acid, formaldoxime) function as vasorelaxant NO donors when added to precontracted aortic rings in vitro. This study was aimed to evaluate whether these substances posses vasodilator properties under in vivo conditions. We studied blood pressure changes elicited by administration of these compounds to conscious chronically catheterized Wistar rats in which endogenous NO synthesis was acutely inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME) pretreatment (30 mg/kg i.v.). Three of the tested substances (formaldoxime, acetohydroxamic acid and formamidoxime) induced pronounced dose-dependent blood pressure reduction which was further augmented when baroreflex operation was interrupted by ganglionic blockade (5 mg/kg pentolinium). Pretreatment of rats with methylene blue (soluble guanylate cyclase inhibitor) was used to estimate the contribution of NO to observed blood pressure lowering effects of the above compounds. Nitric oxide seems to be responsible for the entire formaldoxime-induced blood pressure decrease and for a considerable part of blood pressure changes elicited by formamidoxime. On the contrary, we did not find a significant NO contribution to blood pressure reduction caused by acetohydroxamic acid. In conclusion, our study confirmed in vivo vasodilator effects of three above mentioned compounds which were earlier demonstrated to induce in vitro vasorelaxation. It indicated a variable contribution of nitric oxide to blood pressure changes elicited by particular compounds. Substances with hydrophilic character (formamidoxime, acetohydroxamic acid, formaldoxime) were effective, whereas less hydrophilic substance (acetaldoxime) or slightly hydrophobic one (acetone oxime) were ineffective.

The effect of seasonality on oxidative metabolism in Nacella (Patinigera) magellanica

We studied the seasonal variation on aerobic metabolism and the response of oxidative stress parameters in the digestive glands of the subpolar limpet Nacella (P.) magellanica. Sampling was carried out from July (winter) 2002 to July 2003 in Beagle Channel, Tierra del Fuego, Argentina. Whole animal respiration rates increased in early spring as the animals spawned and remained elevated throughout summer and fall (winter: 0.09+/-0.02 micromol O2 h-1 g-1; summer: 0.31+/-0.06 micromol O2 h-1 g-1). Oxidative stress was assessed at the hydrophilic level as the ascorbyl radical content/ascorbate content ratio (A./AH-). The A./AH- ratio showed minimum values in winter (3.7+/-0.2 10(-5)AU) and increased in summer (18+/-5 10(-5) AU). A similar pattern was observed for lipid radical content (122+/-29 pmol mg-1 fresh mass [FW] in winter and 314+/-45 pmol mg-1 FW in summer), iron content (0.99+/-0.07 and 2.7+/-0.6 nmol mg-1 FW in winter and summer, respectively) and catalase activity (2.9+/-0.2 and 7+/-1 U mg-1 FW in winter and summer, respectively). Since nitrogen derived radicals are thought to be critically involved in oxidative metabolism in cells, nitric oxide content was measured and a significant difference in the content of the Fe-MGD-NO adduct in digestive glands from winter and summer animals was observed. Together, the data indicate that both oxygen and nitrogen radical generation rates in N. (P.) magellanica are strongly dependent on season.

Oxidative stress in limpets exposed to different environmental conditions in the Beagle Channel

The aim of this work was to study the oxidative profile of digestive glands of two limpets species (Nacella (Patinigera) magellanica and Nacella (Patinigera) deaurata) exposed to different environmental conditions. The intertidal population of N. (P.) magellanica is subjected to a wide variety of stresses not experienced by N. (P.) deaurata. Although a typical electron paramagnetic resonance (EPR) spectrum of ascorbyl radical in digestive gland from both limpets was observed, neither ascorbyl radical content nor the ascorbyl radical content/ascorbate content ratio was significantly different, suggesting that the difference in the environmental conditions did not appear to be responsible for developing alterations in the oxidative status of both organisms at the hydrophilic level (e.g. cytosol). Lipid peroxidation in the digestive glands was estimated, both as the content of thiobarbituric acid reactive substances (TBARS) and as the content of lipid radicals assessed by EPR, in both organisms. TBARS and lipid radical content were 34.8 and 36.5%, respectively, lower in N. (P.) magellanica as compared to N. (P.) deaurata. On the other hand, total iron content and the rate of generation of superoxide anion were 47.9 and 51.4%, respectively, lower in N. (P.) magellanica as compared to N. (P.) deaurata. The activity of catalase and superoxide dismutase (SOD) was 35.3 and 128.6% higher in N. (P.) magellanica as compared to N. (P.) deaurata, respectively. No significant differences were determined between the digestive glands of both molluscs regarding the content of total thiols. alpha-Tocopherol and beta-carotene content were significantly lower in N. (P.) magellanica as compared to N. (P.) deaurata. A distinctive EPR signal for the adduct Fe--MGD--NO (g = 2.03 and a(N) = 12.5 G) was detected in the homogenates of digestive glands of both limpets. A significant difference in the content of the Fe-MGD-NO adduct in digestive glands from N. (P.) magellanica and N. (P.) deaurata (491 +/- 137 and 839 +/- 63 pmol/g FW, respectively) was observed. Taken as a whole, the data presented here indicated that coping with environmental stressing conditions requires a complex adjustment of the physiological metabolic pathways to ensure survival by minimizing intracellular damage. It is likely that N. (P.) magellanica has a particular evolutionary adaptation to extreme environmental conditions by keeping iron content low and antioxidant activities high.

Relaxant effect of oxime derivatives in isolated rat aorta: role of nitric oxide (NO) formation in smooth muscle

Various oxime derivatives were evaluated as nitric oxide (NO) donors in arteries. Relaxation of rat aortic rings was used for bioassay of NO production, and electron paramagnetic resonance spectroscopy for demonstration of NO elevation. In rings with or without endothelium or adventitia, hydroxyguanidine and hydroxyurea were almost inactive, whereas formamidoxime, acetaldoxime, acetone oxime, acetohydroxamic acid and formaldoxime elicited relaxation. Active compounds increased NO levels in endothelium-denuded rings. Formaldoxime was the most potent agent for both relaxation and NO elevation in aortic rings, and it also increased NO in human aortic smooth muscle cells. In endothelium-denuded rings, relaxation was inhibited by a NO scavenger (2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and by inhibitors of soluble guanylyl-cyclase (1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one) or cyclic GMP-dependent protein kinases (Rp-8-bromo cyclic GMP monophosphorothioate). Neither N(omega)-nitro-l-arginine methylester (a NO synthases inhibitor) nor proadifen (a cytochrome P450 inhibitor) decreased the effect of oxime derivatives. However, 7-ethoxyresorufin (7-ER, an inhibitor of P4501A(1) which can also inhibit various NADPH-dependent reductases) abolished the relaxant effect of these compounds, without affecting the one of glyceryl trinitrate (GTN) or 2-(N,N-diethylamino)-diazenolate-2-oxide. 7-ER also abolished formaldoxime-induced NO increase in aortic rings. In rings tolerant to GTN, formaldoxime-induced relaxation and NO elevation were not different from those obtained in control rings. In conclusion, some oxime derivatives release NO by 7-ER-sensitive pathways in aortic smooth muscle, thus eliciting vasorelaxation. Pathways of NO formation are likely distinct from NO synthases and from those responsible for GTN biotransformation. Oxime derivatives could be useful for NO delivery in arteries in which endothelial NO synthase activity is impaired.

Nitric oxide, inducible nitric oxide synthase and inflammation in veterinary medicine

Inflammation is a process consisting of a complex of cytological and chemical reactions which occur in and around affected blood vessels and adjacent tissues in response to an injury caused by a physical, chemical or biological insult. Much work has been performed in the past several years investigating inducible nitric oxide synthase (NOS, EC 1.14.13.39) and nitric oxide in inflammation. This has resulted in a rapid increase in knowledge about iNOS and nitric oxide. Nitric oxide formation from inducible NOS is regulated by numerous inflammatory mediators, often with contradictory effects, depending upon the type and duration of the inflammatory insult. Equine medicine appears to have benefited the most from the increased interest in this small, inflammatory mediator. Most of the information on nitric oxide in traditional veterinary species has been produced using models or naturally occurring inflammatory diseases of this species.

Oxidation of N-hydroxyguanidines by cytochromes P450 and NO-synthases and formation of nitric oxide

Microsomal cytochromes P450 and tetrahydrobiopterin (BH4) free-NOS II catalyze the oxidation of N-hydroxyguanidines by NADPH and O2 with formation of nitrogen oxides including NO. These reactions are not selective in terms of substrates, as they occur on most N-hydroxyguanidines, and of products, as they not only lead to corresponding ureas but also to cyanamides. These non selective reactions are mainly due to O2- derived from the oxidase function of those hemeproteins. By contrast, NO synthase (NOS) containing BH4 catalyze the selective monooxygenation of some N-hydroxyguanidines by NADPH and O2 with formation of NO and the corresponding ureas in a 1:1 molar ratio. Those reactions are not inhibited by superoxide dismutase (SOD) and are performed by the NOS Fe(II)-O2 complex. The endogenous NOS substrate N(omega)-hydroxy-L-arginine (NOHA), and its close analogue homo-NOHA, are selectively oxidized in this manner by NOS whereas nor-NOHA and dinor-NOHA are not. Moreover, some non alpha-amino acid N-hydroxyguanidines act as NOS substrates in a manner similar to NOHA. This includes a small number of simple N-alkyl N'-hydroxyguanidines with R(alkyl) propyl, butyl, and pentyl, and some N-aryl N'-hydroxyguanidines that involve a relatively small and preferably electron-rich aryl substituent. The best exogenous substrate of NOS reported so far is N-butyl N'-hydroxyguanidine; this compound is oxidized by NOS II with formation of NO with a catalytic efficiency (kcat/Km) only two times lower than NOHA itself. N-butyl N'-hydroxyguanidine is also a good substrate for NOS I and NOS III. However, some N-aryl N'-hydroxyguanidines, with Ar = p-chlorophenyl and p-methylphenyl, are selective substrates of NOS II. These results show that exogenous N-hydroxyguanidines not bearing an alpha-amino acid function are efficiently and selectively oxidized by NOS with forrmation of NO. They open the way toward the research of new NO donors based on selective substrates of each class of NOS.

Current trends in QSAR on NO donors and inhibitors of nitric oxide synthase (NOS)*

This article evaluates the quantitative structure-activity relationships (QSAR) of nitric oxide (NO) radical donors and nitric oxide synthases (NOS) inhibitors, using the C-QSAR program of Biobyte. Furoxans, triazines, amidoximes, tetrazoles, imidazoles and N(omega)-2-nitroarylamino acid analogues were included in this survey. In nine out of seventeen cases, the clog P plays a significant part in the QSAR of the NO radical donors and of the NOS inhibition. Many of the compounds must be interacting with a hydrophobic space in a non-specific way. In some cases molecular refractivity CMR/MR as well as sterimol parameters (B(1) and L) are important. Electronic effects, with the exception of the Hammett's constant sigma and the Swain-Lupton parameter F, are not found to govern the biological activity. Stereochemical and electronic features are also found to be important. Indicator variables were used after the best model was found to account for the usual structural features.

Oxidation of the ketoxime acetoxime to nitric oxide by oxygen radical-generating systems

Ketoximes undergo a cytochrome P450-catalyzed oxidation to nitric oxide and ketones in liver microsomes. In addition, nitric oxide synthase (NOS) can catalyze the oxidative denitration of the >C=N-OH group of amidoximes. The objective of this work was to characterize the oxidation of a ketoxime (acetoxime) and to assess the ability of NOS to catalyze the generation of nitric oxide/nitrogen monoxide (*NO) from acetoxime. Acetoxime was oxidized to NO2- (and NO3-) by microsomes enriched with several P450 isoforms, including CYP2E1, CYP1A1, and CYP2B1. Nitric oxide was identified as an intermediate in the overall reaction. Superoxide dismutase and catalase significantly inhibited the reaction. Exogenous iron increased the microsomal generation of NO2- from acetoxime, while metal chelators (desferrioxamine, EDTA, DTPA) inhibited it. A Fenton-like system (Fe2+ plus H2O2, pH 7.4) consumed acetoxime with production of NO2- and NO3-, whereas oxidation by superoxide or by H2O2 was inefficient. The results presented suggest a role for hydroxyl radical-like oxidants in the oxidation of acetoxime to nitric oxide. O-Acetylacetoxime and O-tert-butylacetoxime were not oxidized by a Fenton system or by liver microsomes to any significant extent. Formation of the 5,5'-dimethyl-1-pyrroline-N-oxide/. OH adduct by a Fenton system was significantly inhibited by acetoxime, while O-acetylacetoxime and O-tert-butylacetoxime were inactive. These results suggest that the. OH-dependent oxidation of acetoxime initially proceeds via abstraction of a hydrogen atom from its hydroxyl group, as opposed to the oxidation of its >C=N- function. HepG2 cells with low levels of expression of P450 did not significantly produce NO2- from acetoxime, while HepG2 cells expressing CYP2E1 did, and this generation was blocked by a CYP2E1 inhibitor. Acetoxime was inactive either as a substrate or as an inhibitor of iNOS activity. These results indicate that reactive oxygen species play a key role in the oxidation of acetoxime to. NO by liver microsomes by a mechanism involving H abstraction from the OH moiety by hydroxyl radical-like oxidants and suggest the possibility that acetoxime may be an effective producer of. NO primarily in the liver by a pathway independent of NOS.

Metalloporphyrin catalyzed oxidation of N-hydroxyguanidines: a biomimetic model for the H2O2-dependent activity of nitric oxide synthase

A chemical model for the H2O2 promoted oxidation by nitric oxide synthase (NOS) has been developed. Biomimetic oxidations were carried out using H2O2 and tetrakis(perfluorophenyl)porphyrinato-iron(III) chloride (FeTPPF20) as a catalyst. Similarly to NOS our model system produces Ndelta-cyanoornithine, citrulline and NO from NOHA and did not oxidize arginine itself. Based on these results we propose a peroxide shunt to be involved in the catalytic cycle of NOS. To the best of our knowledge this is the first chemical system that semiquantitatively mimics NOS activity.

Cytochrome P450 catalyzed nitric oxide synthesis: a theoretical study

Similar to nitric oxide synthase (NOS) cytochrome P450 isoforms (e.g. 3A and 4E) can produce nitric oxide from arginine. Although the active site of both proteins contains a protoporphyrin IX unit having an axial cystein ligand, their effectiveness in the synthesis of NO differs significantly. Now the molecular basis of this functional difference was investigated. A homology model for cytochrome P450 3A4 was refined and compared to the X-ray structure of iNOS. We found the active site of iNOS to be more readily accessible for the substrate than that of P450. Docking calculations were performed using the Monte Carlo conformational analysis technique on all internal and external degrees of freedom of arginine and active site residues as well. The lowest energy conformation of the cytochrome P450 3A4-substrate complex was compared to the high resolution X-ray structure of the iNOS-arginine complex. Comparison of substrate orientations revealed that arginine binds in a similar conformation in both enzymes. In contrast to iNOS we found, however, that in P450 partially negative propionate side chains of protoporphyrin IX are located on the opposite side of the heme plane. As a result of this and the absence of other negatively charged residues the distal (substrate binding) side of P450 should be less negative than that of NOS and therefore its affinity toward the partially positive arginine is reduced. Comparison of molecular electrostatic potentials calculated within the active site of the proteins supports this proposal. Reduced affinity in combination with limited substrate access might be responsible for the less effective NO synthesis of cytochrome P450 observed experimentally.

Nitric oxide synthesis by tracheal smooth muscle cells by a nitric oxide synthase-independent pathway

Nitric oxide (NO) is known to be synthesized from L-arginine in a reaction catalyzed by NO synthase. Liver cytochrome P-450 enzymes also catalyze the oxidative cleavage of C==N bonds of compounds containing a -C(NH2)==NOH function, producing NO in vitro. The present study was designed to investigate whether there was evidence of a similar pathway for the production of NO in tracheal smooth muscle cells. Formamidoxime (10(-2) to 10(-4) M), a compound containing -C(NH2)==NOH, relaxed carbachol-contracted tracheal rings and increased intracellular cGMP in cultured tracheal smooth muscle cells, whereas L-arginine had no such effect. NO was detectable in the medium containing cultured tracheal smooth muscle cells when incubated with formamidoxime. Ethoxyresorufin (10(-7) to 10(-4) M), an alternate cytochrome P-450 substrate, inhibited formamidoxime-induced cGMP accumulation as well as tracheal ring relaxation in cultured tracheal smooth muscle cells. The NO synthase inhibitors Nomega-nitro-L-arginine (10(-3) M) and NG-monomethyl-L-arginine (10(-3) M) had no effect on formamidoxime-induced cGMP accumulation. These results suggest that NO can be synthesized from formamidoxime in tracheal smooth muscle cells, presumably by a reaction catalyzed by cytochrome P-450.

Oxidative denitrification of the antitumour drug hydroxyguanidine

The oxidative denitrification of the antitumour agent hydroxyguanidine (HOG) has been investigated by radiolysis methods and EPR spectroscopy. The azide radical (N3.), a model one-electron oxidant, reacts with HOG with the rate constant 5.1 x 10(9) dm3 mol(-1) s(-1) to yield the guanidino carbon-centred radical (HOG.) which rapidly eliminates nitric oxide (k = 3.1 x 10[3] s[-1]) with the concomitant formation of urea. The HOG. undergoes conjugation with molecular oxygen to form a peroxyl radical (HOGOO.) with a rate constant 8.8 x 10(8) dm3 mol(-1) s(-1). The HOGOO. radical also eliminates nitric oxide but may act as a precursor to the peroxynitrite (ONOO-) ion. The oxidation of HOG by the dibromide radical (Br2.-) was found to release nitric oxide with a yield of 95% relative to Br2.- as determined from the combined yields of inorganic nitrite, nitrate and a HOG/nitric oxide-adduct. This study provides a possible mechanistic basis for the oxidative denitrification of HOG which may contribute to the observed toxicity of the drug both in vitro and in vivo and for the oxidation of nonphysiological hydroxyguanidines to NO. via nitric oxide synthase-independent pathways.

New NO-donors with antithrombotic and vasodilating activities, Part 17. Arylazoamidoximes and 3-arylazo-1,2,4-oxadiazol-5-ones

Seven arylazoamidoximes (3), six phenoxycarbonyl derivatives (4), and six 1,2,4-oxadiazol-5-ones (5) have been prepared and their structure and purity established by spectroscopy and elemental analysis. In the EI mass spectra ready elimination of NO from the title amidoximes was observed. A new addition reaction of 3a with hydrochloric acid to 4-chlorophenylhydro-azoamidoxime 7 is described. The compounds were tested for nitric oxide dependent biological properties, i.e. platelet aggregation, antithrombotic effects, and decrease in blood pressure. In arterioles of rats 5/19 compounds inhibited the formation of thrombi with a laser beam by > or = 20% 2 h after oral administration of 60 mg/kg. Among these are three amidoximes (3a, 3e, 3f), one phenoxycarbonyl derivative (4a), and one oxadiazolone (5a). With the 4-chlorophenylazoamidoxime 3c a long lasting (24 h) decrease of blood pressure in spontaneously hypertensive rats was observed. Microsomal fractions of rat liver oxidize arylazoamidoximes and generate nitric oxide (e.g. 3a and 3b). NO was measured by the oxyhemoglobin assay. The influence of SOD, pretreatment of the rats with dexamethasone, as well as kinetic parameters were determined. Type 3 compounds, therefore, are a new class of NO donors. Type 4 and 5 compounds function as their prodrugs.

New NO-donors with antithrombotic and vasodilating activities, Part 16. 3-Amino-1,2,4-oxadiazol-5-ones as prodrugs for hydroxyguanidines

Nineteen 4-substituted 1,2,4-oxadiazol-5-ones (6a-s) were prepared as prodrugs for lipophilic hydroxyguanidines which should be metabolized in vivo to nitric oxide. This hypothesis was tested indirectly by measuring the antithrombotic properties of these compounds 2 h after oral administration to rats (60 mg/kg). In mesenteric arterioles seven compounds moderately (> or = 10%) inhibited the formation of thrombi by a laser beam. Maximum effects were observed in 6c (4-pentyl) and 6f (4-benzyl). The lack of activity in the corresponding 2-pentyloxadiazolone 10c, where no formation of nitric oxide seems possible, indirectly suggests that the antithrombotic properties of the title compounds could be mediated by the in vivo formation of nitric oxide.

N-hydroxylation and N-dealkylation by P4502C3 of N-methylbenzamidine: N-oxygenation and N-oxidative dealkylation of one functional group

1. The first detection of a microsomal N-hydroxylation of an N-alkylated benzamidine possessing alpha-H atoms by P450 is now reported in the present in vitro biotransformation studies. 2. The newly found metabolites, N-hydroxy-N-methylbenzamidine and N-methylbenzamidoxime, were identified after hplc separation by comparison of their retention times with those of synthetic reference compounds and by comixing methods. N-hydroxy-N-methylbenzamidine exists predominantly in the aminonitrone form and constitutes a novel type of metabolite. 3. By means of reconstitution experiments with purified P4502C3 from rabbit liver and with purified variants of 2C3 expressed in Escherichia coli, it has been shown that the N-hydroxylation and the N-dealkylation of N-methylbenzamidine are catalysed by the same P450 isoenzyme. 4. A reaction mechanism is proposed in which the P450-dependent N-oxygenations and N-dealkylation of N-methylbenzamidine are derived from a common intermediate. It is obvious that if alpha-H atoms are present N-dealkylation is observed; however, in contrast with previous concepts, N-oxygenation is also possible.

Cytokine-mediated production of nitric oxide in isolated rat hepatocytes is dependent on cytochrome P-450III activity

To investigate the role of the cytochrome P-450 system in NO synthesis, cytochrome P-450IIIA, IIE and IA activities were specifically inhibited by cimetidine (IIIA), clotrimazole (IIIA), benzoflavone (IA) and disulfiram (IIE) in a model of cultured rat hepatocytes. Cytokine-induced NO synthesis was significantly decreased in the presence of cimetidine and clotrimazole. Kinetic analysis revealed a non-competitive mode of inhibition (Ki = 21 mM, cimetidine; Ki = 13 microM, clotrimazole). Reverse transcriptase-PCR and immunoblot analysis revealed no significant change in steady state levels of iNOS mRNA and protein expression with P-450IIIA inhibition. Purified iNOS enzyme activity was not altered. These data suggest that cytokine-mediated hepatocyte synthesis of NO is dependent upon P-450IIIA activity, which functions in a post-translational capacity.

On the mechanism of nitric oxide formation upon oxidative cleavage of C = N(OH) bonds by NO-synthases and cytochromes P450

Microsomal liver cytochromes P450 catalyze the oxidative cleavage of the C = NOH bond of many ketoximes, amidoximes and guanidoximes, and NO synthases catalyze the oxidation of N omega-hydroxy-L-arginine to citrulline and NO. All these oxidations appear to be performed either by the FE(II) O2 complex of these hemoproteins or by O2.- which is formed by its decomposition. This leads to a unifying view of the mechanisms of P450- and NOS-dependent oxidative cleavage of C = NOH bonds, the relative contribution of Fe(II) O2.- being very different in NO-synthase and cytochromes P450.

Cytochrome P450 dependent N-hydroxylation of a guanidine (debrisoquine), microsomal catalysed reduction and further oxidation of the N-hydroxy-guanidine metabolite to the urea derivative. Similarity with the oxidation of arginine to citrulline and nitric oxide

The microsomal N-hydroxylation of the strongly basic guanidinium group (debrisoquine) to N-hydroxyguanidine (N-hydroxydebrisoquine) and the retroreduction of the N-hydroxyguanidine are demonstrated for the first time. The reduction of the N-hydroxyguanidine by liver homogenates and hepatocytes is catalysed by a microsomal NADH-dependent system that is strongly inhibited by hydroxylamine or N-methylhydroxylamine. In the presence of these alternate substrates for the reductase the microsomal catalysed N-hydroxylation of debrisoquine is readily characterized. The oxidation was inhibited by antibodies against NADPH cytochrome P450 reductase and the role of the P450 monooxygenase was further verified by studies with partially purified and purified P450 2C3 reconstituted systems. The transformation of N-hydroxydebrisoquine to the corresponding urea derivative was also detected in in vitro experiments with microsomal fractions and enriched P450 fractions as well as with flavin-containing monooxygenase (FMO). Experiments with catalase, superoxide dismutase and H2O2 have shown that the H2O2 or O2-, respectively, formed from the respective enzyme and the substrate, apparently participated in the reaction. Whereas the N-hydroxylation of the guanidine involves the usual monooxygenase activity of cytochrome P450 the resultant N-hydroxyguanidine decouples monooxygenases (cytochrome P450, FMO) and the H2O2 and, above all, O2- thus formed transform the N-hydroxyguanidine further to the corresponding urea derivative. The possibility for the N-hydroxylation of non-physiological guanidines to N-hydroxyguanidines and subsequent oxidative conversion to the respective urea is comparable to the physiological transformation of arginine to citrulline via N-hydroxyarginine with the liberation of nitric oxide (endothelial derived relaxing factor) and could, therefore, contribute to the efficacy of drugs containing guanidine and similar functional groups.

Biotransformation of benzamidine and benzamidoxime in vivo

After administration of benzamidine (1) or benzamidoxime (2), respectively, to rats and rabbits, plasma from rats and rabbits as well as urine from rats were examined for the presence of benzamidoxime (2) or benzamidine (1). Some of the samples were worked-up directly and the others after enzymatic pretreatment with beta-glucuronidase or arylsulfatase, respectively. HPLC analysis was employed for the detection of the metabolites. After administration of 1, an in vivo N-hydroxylation of an amidine to an amidoxime was demonstrated for the first time. The metabolite 2 could only be detected after enzymatic cleavage of the glucuronide or sulfate, respectively, and only in plasma at a low concentration. After administration of benzamidoxime (2), on the other hand, benzamidine (1) was detected in very high concentrations in all biological samples. Benzamidine was present in the free state but indications for glucuronidization and sulfatation were also detectable. These investigations suggest that the benzamidoxime (2) formed by an in vivo N-hydroxylation undergoes ready retro-reduction but that further transformations of the metabolite 2, such as conjugation to a glucuronide or a sulfate, respectively, prevent complete back reaction. Furthermore, benzamide (3) could be detected as a transformation product in urine after administration of either 1 or 2.

Structural aspects of N-hydroxy-N'-aminoguanidine derivatives as inhibitors of L1210 cell growth and ribonucleotide reductase activity

Previous studies have shown that N-hydroxy-N'-aminoguanidine (HAG) derivatives [RCH = NNHC(= NH)NHOH-tosylate] inhibit ribonucleotide reductase activity and block the growth of leukemia L1210 cells and human colon carcinoma, HT-29, cells in culture. In the current studies, the role of the side chains and the location of the bond of the side chain moiety to HAG were investigated using a new series of HAG derivatives which contained as the R-group--cyclohexyl, phenyl-, pyridyl- or napthyl moieties. The effects of these compounds as inhibitors of L1210 cell growth and ribonucleotide reductase activity were compared with the parent compound. N-hydroxy-N'-aminoguanidine was less inhibitory to ribonucleotide reductase activity and L1210 cell growth than hydroxyurea. The phenyl-HAG compounds which included 1-benzyloxybenzylidene- and 4-cyclohexylmethoxybenzylidene-HAG inhibited CDP reductase with IC50s which ranged from 50-110 microM. 1-Naphthylmethylene-HAG was more inhibitory than 2-naphthylmethylene-HAG and more inhibitory than the phenyl-HAG compounds. 2-Pyridylmethylene-HAG was more inhibitory than 3-pyridylmethylene- or 4-pyridylmethylene-HAG. While HAG inhibited CDP and ADP reductase activities essentially to the same extent, the HAG-derivatives inhibited ADP reductase activity to a greater extent than CDP reductase activity. Cyclohexylmethylene-HAG did not inhibit either L1210 cell growth or ribonucleotide reductase activity. There was good correlation between the inhibition of ribonucleotide reductase activity and L1210 cell growth by these HAG-derivatives. These data indicate that not only is the nature of the side chain substitution important, but also the location of the HAG-moiety on the ring position.

Brain nitric oxide synthase is a haemoprotein

Brain nitric oxide (NO) synthase showed pyridine haemochrome spectra typical of ferroprotoporphyrin IX-containing enzymes. The haem content of purified NO synthase was in the range 0.7-0.9 mol/mol of 160 kDa subunit. In the presence of CO, NO, KCN and miconazole, the L-citrulline-forming activity of NO synthase was markedly diminished, demonstrating that enzyme-bound haem is involved in enzymic NO synthesis.