The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2-coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.

The ionophore thiomaltol induces rapid lysosomal accumulation of copper and apoptosis in melanoma

In this report, we investigate the toxicity of the ionophore thiomaltol (Htma) and Cu salts to melanoma. Divalent metal complexes of thiomaltol display toxicity against A375 melanoma cell culture resulting in a distinct apoptotic response at submicromolar concentrations, with toxicity of Cu(tma)2 > Zn(tma)2 >> Ni(tma)2. In metal-chelated media, Htma treatment shows little toxicity, but the combination with supplemental CuCl2, termed Cu/Htma treatment, results in toxicity that increases with suprastoichiometric concentrations of CuCl2 and correlates with the accumulation of intracellular copper. Electron microscopy and confocal laser scanning microscopy of Cu/Htma treated cells shows a rapid accumulation of copper within lysosomes over the course of hours, concurrent with the onset of apoptosis. A buildup of ubiquitinated proteins due to proteasome inhibition is seen on the same timescale and correlates with increases of copper without additional Htma.

Expanding the Reactive Sulfur Metabolome: Intracellular and Efflux Measurements of Small Oxoacids of Sulfur (SOS) and H2S in Human Primary Vascular Cell Culture

Hydrogen sulfide (H2S) is an endogenous signaling molecule which is important for cardiovascular health, but its mechanism of action remains poorly understood. Here, we report measurements of H2S as well as its oxidized metabolites, termed small oxoacids of sulfur (SOS = HSOH and HOSOH), in four human primary vascular cell lines: smooth muscle and endothelial cells derived from both human arterial and coronary tissues. We use a methodology that targets small molecular weight sulfur species; mass spectrometric analysis allows for species quantification to report cellular concentrations based on an H2S calibration curve. The production of H2S and SOS is orders of magnitude higher in smooth muscle (nanomolar) as compared to endothelial cell lines (picomolar). In all the primary lines measured, the distributions of these three species were HOSOH >H2S > HSOH, with much higher SOS than seen previously in non-vascular cell lines. H2S and SOS were effluxed from smooth muscle cells in higher concentrations than endothelial cells. Aortic smooth muscle cells were used to examine changes under hypoxic growth conditions. Hypoxia caused notable increases in HSOH and ROS, which we attribute to enhanced sulfide quinone oxidase activity that results in reverse electron transport.

Characterization of Endogenous and Extruded H2S and Small Oxoacids of Sulfur (SOS) in Cell Cultures

This report characterizes and quantifies endogenous hydrogen sulfide (H₂S) and small oxoacids of sulfur (SOS = HOSH, HOSOH) in a panel of cell lines including human cancer (A375 melanoma cells, HeLa cervical cells) and noncancer (HEK293 embryonic kidney cells), as well as E. coli DH5α and S. cerevisiae S288C. The methodology used is a translation of well-studied nucleophilic and electrophilic traps for cysteine and oxidized cysteines residues to target small molecular weight sulfur species; mass spectrometric analysis allows for species quantification. The observed intracellular concentrations of H₂S and SOS vary in different cell types, from nanomolar to femtomolar, typically with H₂S > HOSOH > HOSH. We propose the term sulfome, a subset of the metabolome, describing the nonproteinaceous metabolites of H₂S; the sulfomic index is as a measure of the S-oxide redox status, which gives a profile of endogenous sulfur at different oxidation states. An important observation is that H₂S and SOS were found to be continuously extruded into surrounding media against a concentration gradient, implying an active efflux process. Small molecule inhibition of several H₂S generating enzymes suggest that SOS are not derived solely from H₂S oxidation. Even after successful inhibition of H₂S production, cells maintain constant efflux and repopulate H₂S and SOS over time. This work proves that these small sulfur oxoacids are generated in cells of all types, and their efflux implies that they play a role in cell signaling and possibly other vascular physiology attributed to H₂S.

Oxidative Stress and Arterial Dysfunction in Peripheral Artery Disease

Peripheral artery disease (PAD) is an atherosclerotic disease characterized by a narrowing of the arteries in the lower extremities. Disease manifestations are the result of more than just reduced blood flow, and include endothelial dysfunction, arterial stiffness, and inflammation. Growing evidence suggests that these factors lead to functional impairment and decline in PAD patients. Oxidative stress also plays an important role in the disease, and a growing amount of data suggest a link between arterial dysfunction and oxidative stress. In this review, we present the current evidence for the involvement of endothelial dysfunction, arterial stiffness, and inflammation in the pathophysiology of PAD. We also discuss the links between these factors and oxidative stress, with a focus on nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2)-derived reactive oxygen species (ROS) and decreased nitric oxide (NO) bioavailability. Finally, the potential therapeutic role of NOX2 antioxidants for improving arterial function and functional status in PAD patients is explored.

Chemical trapping and characterization of small oxoacids of sulfur (SOS) generated in aqueous oxidations of H2S

Small oxoacids of sulfur (SOS) are elusive molecules like sulfenic acid, HSOH, and sulfinic acid, HS(O)OH, generated during the oxidation of hydrogen sulfide, H2S, in aqueous solution. Unlike their alkyl homologs, there is a little data on their generation and speciation during H2S oxidation. These SOS may exhibit both nucleophilic and electrophilic reactivity, which we attribute to interconversion between S(II) and S(IV) tautomers. We find that SOS may be trapped in situ by derivatization with nucleophilic and electrophilic trapping agents and then characterized by high resolution LC MS. In this report, we compare SOS formation from H2S oxidation by a variety of biologically relevant oxidants. These SOS appear relatively long lived in aqueous solution, and thus may be involved in the observed physiological effects of H2S.

Mechanistic Investigations of Photoinduced Oxygenation of Ru(II) Bis-bipyridyl Flavonolate Complexes

We previously reported that a Ru-bound flavonolate model of flavonol dioxygenases, [Ru^II(bpy)₂(3-hydroxyfla)][PF₆], photochemically reacts with dioxygen in two different manners. Broad-band excitation generates mixtures of products characteristic of 1,3-addition of dioxygen across the central pyrone ring, as is observed in enzymatic reactions. However, low temperature excitation at wavelengths longer than 400 nm generates a unique Ru-bound 2-benzoatophenylglyoxylate product resulting from a 1,2-dioxetane intermediate. Herein, we investigate this reactivity in a series of Ru(II)bis-bipyridyl flavonolate complexes [Ru^II(bpy)₂(3-hydroxyfla^R)][PF₆] (bpy = 2,2'-bipyridine; fla = flavonolate; R = p-OMe (1), p-Me (2), p-H (3), p-Cl (4)), and [Ru^II(bpy)₂(5-hydroxyfla)][PF₆] (5). The complexes' structures, photophysical and electrochemical properties, and photochemical reactivity with oxygen were investigated in detail. Two different reaction product mixtures, from 1,2- and 1,3-additions of dioxygen, are observed by illumination into distinct excitation/emission manifolds. By analogy to previous reports of excited state intramolecular proton transfer, the two manifolds are attributed to tautomeric diradicals that predict the observed reactivity patterns.

Synthesis of first row transition metal selenomaltol complexes

Complexation of selenomaltol to transition metal ions promotes a zwitterionic resonance form.

Trapping Reactions of the Sulfenyl and Sulfinyl Tautomers of Sulfenic Acids

Sulfenic acids react as both nucleophiles and electrophiles, which may be attributable to interconversion between sulfenyl and sulfinyl tautomers. We demonstrate one-pot trapping of both tautomeric forms of glutathione sulfenic acid by LCMS. The sulfinyl tautomers are characterized by reaction with nucleophilic reagents such as dimedone and cyanide, giving unique products that are analogous to corresponding adducts of aldehydes. Likewise, we show that aldehyde reactive reagents such as silyl enol ethers also react with glutathione sulfenic acid to give products characteristic of both sulfenyl and sulfinyl tautomers.

Amyloids, melanins and oxidative stress in melanomagenesis

Melanoma has traditionally been viewed as an ultraviolet (UV) radiation-induced malignancy. While UV is a common inducing factor, other endogenous stresses such as metal ion accumulation or the melanin pigment itself may provide alternative pathways to melanoma progression. Eumelanosomes within melanoma often exhibit disrupted membranes and fragmented pigment which may be due to alterations in their amyloid-based striated matrix. The melanosomal amyloid can itself be toxic, especially in combination with reactive oxygen species (ROS) and reactive nitrogen species (RNS) generated by endogenous NADPH oxidase (NOX) and nitric oxide synthase (NOS) enzymes, a toxic mix that may initiate melanomagenesis. Further understanding of the loss of the melanosomal organization, the behaviour of the exposed melanin and the induction of ROS/RNS in melanomas may provide critical insights into this deadly disease.

Ligand-based photooxidations of dithiomaltolato complexes of Ru(II) and Zn(II): photolytic CH activation and evidence of singlet oxygen generation and quenching

The complex [Ru(bpy)2(ttma)](+) (bpy = 2,2'-bipyridine; ttma = 3-hydroxy-2-methyl-thiopyran-4-thionate, 1, has previously been shown to undergo an unusual C-H activation of the dithiomaltolato ligand upon outer-sphere oxidation. The reaction generated alcohol and aldehyde products 2 and 3 from C-H oxidation of the pendant methyl group. In this report, we demonstrate that the same products are formed upon photolysis of 1 in presence of mild oxidants such as methyl viologen, [Ru(NH3)6](3+) and [Co(NH3)5Cl](2+), which do not oxidize 1 in the dark. This reactivity is engendered only upon excitation into an absorption band attributed to the ttma ligand. Analogous experiments with the homoleptic Zn(ttma)2, 4, also result in reduction of electron acceptors upon excitation of the ttma absorption band. Complexes 1 and 4 exhibit short-lived visible fluorescence and long-lived near-infrared phosphorescence bands. Singlet oxygen is both generated and quenched during aerobic excitation of 1 or 4, but is not involved in the C-H activation process.

A singular value decomposition approach for kinetic analysis of reactions of HNO with myoglobin

The reactions of several horse heart myoglobin species with nitrosyl hydride, HNO, derived from Angeli's salt (AS) and Piloty's acid (PA) have been followed by UV-visible, (1)H NMR and EPR spectroscopies. Spectral analysis of myoglobin-derived speciation during the reactions was obtained by using singular value decomposition methods combined with a global analysis to obtain the rate constants of complex sequential reactions. The analysis also provided spectra for the derived absorbers, which allowed self-consistent calibration to the spectra of known myoglobin species. Using this method, the determined rate for trapping of HNO by metmyoglobin, which produces NO-myoglobin, is found to be 2.7 × 10(5)M(-1)s(-1) at pH7.0 and 1.1 × 10(5)M(-1)s(-1) at pH9.4. The reaction of deoxymyoglobin with HNO generates the adduct HNO-myoglobin directly, but is followed by a secondary reaction of that product with HNO yielding NO-myoglobin; the determined bimolecular rate constants for these reactions are 3.7 × 10(5)M(-1)s(-1) and 1.67 × 10(4)M(-1)s(-1) respectively, and are independent of pH. The derived spectrum for HNO-myoglobin is characterized by a Soret absorbance maximum at 423 nm with an extinction coefficient of 1.66 × 10(5)M(-1)cm(-1). The rate constant for unimolecular loss of HNO from HNO-myoglobin was determined by competitive trapping with CO at 8.9 × 10(-5)s(-1), which gives the thermodynamic binding affinity of HNO to deoxymyoglobin as 4.2 × 10(9)M(-1). These results suggest that the formation of HNO-ferrous heme protein adducts represents an important consideration in the biological action of HNO-releasing drugs.

Synthesis and characterization of lithium oxonitrate (LiNO)

The oxonitrate(1-) anion (NO(-)), the one-electron reduction product of nitric oxide and conjugate base of HNO, has not been synthesized and isolated due to the inherent reactivity of this anion. The large scale synthesis and characterization of a stable NO(-) salt is described here. The lithium salt of oxonitrate (LiNO) was formed by the deprotonation of N-hydroxybenzenesulfonamide with phenyllithium in aprotic, deoxygenated conditions. LiNO exhibited antiferromagnetic paramagnetism as determined by SQUID magnetometry, consistent with a triplet ground state of NO(-). LiNO reacted with HCl to yield nitrous oxide consistent with HNO formation and dimerization. LiNO consumed O(2) in a pH-dependent manner to initially produce peroxynitrite and eventually nitrite. Consistent with the reduction potential of NO, LiNO exhibited an oxidation potential of approximately +0.80 V as determined by reactions with a series of viologen electron acceptors. LiNO also reacted with ferric tetraphenylporphyrin chloride (Fe(TPP)Cl), potassium tetracyanonickelate (K(2)Ni(CN)(4)) and nitrosobenzene in a manner that is identical to other HNO/NO(-) donors. We conclude that the physical and chemical characteristics of LiNO are indistinguishable from the experimentally and theoretically derived data on oxonitrate (1-) anion. The bulk synthesis and isolation of a stable (3)NO(-) salt described here allow the chemical and physical properties of this elusive nitrogen oxide to be thoroughly studied as this once elusive nitrogen oxide is now attainable.

Direct oxygen imaging in titania nanocrystals

Recently, rutile nanotwins were synthesized using high temperature organic solvent methods, yielding two kinds of common high-quality rutile twinned nanocrystals, (101) and (301) twins, accompanied by minor rutile nanorods (Lu et al 2012 CrystEngComm 14 3120-4). In this report, the atomic structures of the rutile and anatase nanocrystals are directly resolved with no need for calculation or image simulation using atomic resolution STEM techniques. The locations of the oxygen rows in the rutile twins' boundaries are directly determined from both HAADF images and ABF images. To the best of our knowledge, this is the first time oxygen columns have been distinguished in rutile twin boundaries using HAADF and BF imaging.

Reactions of HNO with heme proteins: new routes to HNO-heme complexes and insight into physiological effects

The formation and interconversion of nitrogen oxides has been of interest in numerous contexts for decades. Early studies focused on gas-phase reactions, particularly with regard to industrial and atmospheric environments, and on nitrogen fixation. Additionally, investigation of the coordination chemistry of nitric oxide (NO) with hemoglobin dates back nearly a century. With the discovery in the early 1980s that NO is biosynthesized as a molecular signaling agent, the literature has been focused on the biological effects of nitrogen oxides, but the original concerns remain relevant. For instance, hemoglobin has long been known to react with nitrite, but this reductase activity has recently been considered to be important to produce NO under hypoxic conditions. The association of nitrosyl hydride (HNO; also commonly referred to as nitroxyl) with heme proteins can also produce NO by reductive nitrosylation. Furthermore, HNO is considered to be an intermediate in bacterial denitrification, but conclusive identification has been elusive. The authors of this article have approached the bioinorganic chemistry of HNO from different perspectives, which have converged because heme proteins are important biological targets of HNO.

Substance P and vasoactive intestinal peptide are reduced in right transverse colon in pediatric slow-transit constipation

BACKGROUND

Slow-transit constipation (STC) is recognized in children but the etiology is unknown. Abnormalities in substance P (SP), vasoactive intestinal peptide (VIP) and nitric oxide (NO) have been implicated. The density of nerve fibers in circular muscle containing these transmitters was examined in colon from children with STC and compared to other pediatric and adult samples.

METHODS

Fluorescence immunohistochemistry using antibodies to NO synthase (NOS), VIP and SP was performed on colonic biopsies (transverse and sigmoid colon) from 33 adults with colorectal cancer, 11 children with normal colonic transit and anorectal retention (NAR) and 51 with chronic constipation and slow motility in the proximal colon (STC). The percentage area of nerve fibers in circular muscle containing each transmitter was quantified in confocal images.

KEY RESULTS

In colon circular muscle, the percentage area of nerve fibers containing NOS > VIP > SP (6 : 2 : 1). Pediatric groups had a higher density of nerve fibers than adults. In pediatric samples, there were no regional differences in NOS and VIP, while SP nerve fiber density was higher in sigmoid than proximal colon. STC children had lower SP and VIP nerve fiber density in the proximal colon than NAR children. Twenty-three percent of STC children had low SP nerve fiber density.

CONCLUSIONS & INFERENCES

There are age-related reductions in nerve fiber density in human colon circular muscle. NOS and VIP do not show regional variations, while SP nerve fiber density is higher in distal colon. 1/3 of pediatric STC patients have low SP or VIP nerve fiber density in proximal colon.

Decrease in nerve fibre density in human sigmoid colon circular muscle occurs with growth but not aging

BACKGROUND

Studies in animals suggest that enteric neurons decrease in density or number with increasing age. Neurons containing nitric oxide (NO), vasoactive intestinal peptide (VIP) and Substance P (SP) have been implicated. In human large intestine, NO-utilizing neurons decrease during childhood or early adulthood but it is not known if the innervation of the muscle changes. This study examined the density of nerve fibres containing these transmitters in sigmoid colon circular muscle from children and adults.

METHODS

Fluorescence immunohistochemistry using antibodies to neuronal NO synthase (nNOS), VIP and SP was performed on sigmoid colon from 18 adults with colorectal cancer, two children with familial adenomatous polyposis, and normal colon from nine children with Hirschsprung's disease. The percentage area of immunoreactive (IR) nerve fibres containing each transmitter in circular muscle was quantified in confocal images.

KEY RESULTS

In the adult sigmoid colon circular muscle, the percentage area of nerve fibres containing nNOS>VIP>SP (6 : 2 : 1). Paediatric groups had significantly higher percentage area of nerve fibres containing nNOS, VIP or SP-IR than adults, with the decrease in nerve fibre density occurring from birth to 30 years. Circular muscle thickness increased between 12 and 30 years. Total nerve fibre area remained constant, while the muscle increased in thickness.

CONCLUSIONS & INFERENCES

In human sigmoid colon circular muscle, there are reductions in nNOS-, VIP- and SP-IR nerve fibre density with growth from newborn to late adolescence but little further change with aging. The reduction in nerve density is due to an increase in circular muscle thickness rather than a loss of nerve fibres.

Nitrosyl hydride (HNO) as an O2 analogue: long-lived HNO adducts of ferrous globins

Nitrosyl hydride, HNO or nitroxyl, is the one-electron reduced and protonated form of nitric oxide. HNO is isoelectronic to singlet O(2), and we have previously reported that deoxymyoglobin traps free HNO to form a stable adduct. In this report, we demonstrate that oxygen-binding hemoglobins from human, soy, and clam also trap HNO to form adducts which are stable over a period of weeks. The same species can be formed in higher yields by careful reduction of the ferrous nitrosyl adducts of the proteins. Like the analogous O(2)-Fe(II) adducts, the HNO adducts are diamagnetic, but with a characteristic HNO resonance in (1)H NMR at ca. 15 ppm that splits into doublets for H(15)NO adducts. The (1)H and (15)N NMR resonances, obtained by HSQC experiments, are shown to differentiate subunits and isoforms of proteins within mixtures. An apparent difference in the reduction rates of the NO adducts of the two subunits of human hemoglobin allows assignment of two distinct nitrosyl hydride peaks by a combination of UV-vis, NMR, and EPR analysis. The two peaks of the HNO-hHb adduct have a persistent 3:1 ratio during trapping reactions, demonstrating a kinetic difference between HNO binding at the two subunits. These results show NMR characterization of ferrous HNO adducts as a unique tool sensitive to structural changes within the oxygen-binding cavity, which may be of use in defining modes of oxygen binding in other heme proteins and enzymes.

Pattern of expression and substrate specificity of chloroplast ferredoxins from Chlamydomonas reinhardtii

Ferredoxin (Fd) is the major iron-containing protein in photosynthetic organisms and is central to reductive metabolism in the chloroplast. The Chlamydomonas reinhardtii genome encodes six plant type [Fe2S2] ferredoxins, products of PETF, FDX2-FDX6. We performed the functional analysis of these ferredoxins by localizing Fd, Fdx2, Fdx3, and Fdx6 to the chloroplast by using isoform-specific antibodies and monitoring the pattern of gene expression by iron and copper nutrition, nitrogen source, and hydrogen peroxide stress. In addition, we also measured the midpoint redox potentials of Fd and Fdx2 and determined the kinetic parameters of their reactions with several ferredoxin-interacting proteins, namely nitrite reductase, Fd:NADP+ oxidoreductase, and Fd:thioredoxin reductase. We found that each of the FDX genes is differently regulated in response to changes in nutrient supply. Moreover, we show that Fdx2 (Em = -321 mV), whose expression is regulated by nitrate, is a more efficient electron donor to nitrite reductase relative to Fd. Overall, the results suggest that each ferredoxin isoform has substrate specificity and that the presence of multiple ferredoxin isoforms allows for the allocation of reducing power to specific metabolic pathways in the chloroplast under various growth conditions.

The effects of nitroxyl (HNO) on soluble guanylate cyclase activity: interactions at ferrous heme and cysteine thiols

It has been previously proposed that nitric oxide (NO) is the only biologically relevant nitrogen oxide capable of activating the enzyme soluble guanylate cyclase (sGC). However, recent reports implicate HNO as another possible activator of sGC. Herein, we examine the affect of HNO donors on the activity of purified bovine lung sGC and find that, indeed, HNO is capable of activating this enzyme. Like NO, HNO activation appears to occur via interaction with the regulatory ferrous heme on sGC. Somewhat unexpectedly, HNO does not activate the ferric form of the enzyme. Finally, HNO-mediated cysteine thiol modification appears to also affect enzyme activity leading to inhibition. Thus, sGC activity can be regulated by HNO via interactions at both the regulatory heme and cysteine thiols.

Melanosomal damage in normal human melanocytes induced by UVB and metal uptake--a basis for the pro-oxidant state of melanoma

Melanins are ubiquitous catecholic pigments, formed in organelles called melanosomes within melanocytes, the function of which is to protect skin against harmful effects of UV radiation. Melanosomes within melanoma cells are characteristically abnormal, with fragmented melanin and disrupted membranes. We hypothesize that the disruption of melanosomal melanin might be an early event in the etiology and progression of melanoma, leading to increased oxidative stress and mutation. In this report, we examine the effect of a combination of UV treatment and metal ion exposure on melanosomes within melanocytes, as well as their ability to act as pro-oxidants in ex situ experiments, and assay the effects of this treatment on viability and cell cycle progression. UVB exposure causes morphologic changes of the cells and bleaching of melanosomes in normal melanocytes, both significantly enhanced in Cu(II) and Cd(II)-treated cells, as observed by microscopy. The promoted bleaching by Cu(II) is due to its ability to redox cycle under oxidative conditions, generating reactive oxygen species; verified by the observed enhancement of hydroxyl radical generation when isolated melanosomes were treated with both Cu(II) ions and UVB, as assayed by DNA clipping. Single-dose UVB/Cu treatment does not greatly affect cell viability or cell cycle progression in heavily pigmented cells, but did so in an amelanotic early stage melanoma cell line.

Unexpected C-H activation of Ru(II)-dithiomaltol complexes upon oxidation

Thione-substituted derivatives of maltol are of interest in several applications of metal-based drugs. In order to investigate the effect of the oxygenation on such thione chelates, Ru complexes of 3-hydroxy-2-methyl-4-thiopyrone (thiomaltol or Htma) and 3-hydroxy-2-methyl-4H-thiopyran-4-thione (dithiomaltol or Httma), [Ru(bpy)2(tma)](+), 1, and [Ru(bpy) 2(ttma)] (+), 2, were synthesized as diamagnetic PF6(-) salts. Peroxidation of 2 unexpectedly generated products of C-H activation at its pendant methyl group; an air-stable aldehyde [Ru(bpy)2(ttma-aldehyde)](+), 4, was the major product. In addition, an intermediate oxidation product [Ru(bpy) 2(ttma-alcohol)](PF6), 3, was characterized. Both 3 and 4 are also formed by reaction of 2 with outersphere oxidants (e.g., Na2IrCl6) and by bulk electrolysis under anaerobic conditions. Similar oxidations of the analogous [Ru(bpy)2(ettma)](+), 2' , complex (3-hydroxy-2-ethyl-4H-thiopyran-4-thione; ethyl dithiomaltol or Hettma) formed the corresponding ketone, [Ru(bpy)2(ettma-ketone)](PF6), 4', by oxidation at the same position adjacent to the conjugated ring. The structures of the aldehyde 4 and starting materials 1 and 2 have been confirmed by X-ray crystallography, and all complexes have been characterized by UV-vis, (1)H NMR, and IR spectroscopies. Initial mechanistic investigations are discussed.

New perspectives on melanoma pathogenesis and chemoprevention

Epidemiologic studies implicate ultraviolet radiation (sunlight) as an etiologic agent for the pathogenesis of melanoma. However, the experimental evidence is less convincing. We present information from recent experimental findings that elevation of reactive oxygen species follows from melanin serving as a redox generator, and that this may play an important role in the etiology and pathogenesis of cutaneous melanoma. These observations offer a new paradigm for the development of preventive (and therapeutic) approaches to this disease.

Oxygenation of Zinc Dialkyldithiocarbamate Complexes: Isolation, Characterization, and Reactivity of the Stoichiometric Oxygenates

S-oxygenation of dithiocarbamate (DTC) complexes has been implicated in their function as industrial anti-oxidants, as well as in their use as pesticides and most recently in their cumulative toxicity, but little is known of the species generated. Several S-oxygenated derivatives of N,N-disubstituted DTCs have been synthesized, characterized by a variety of methods, and their structure and reactivity examined. Low-temperature reaction of bis(N,N-diethyldithiocarbamato)zinc(II), Zn(deDTC)2 1, with oxygenating reagents (hydrogen peroxide, m-chloroperbenzoic acid, urea hydrogen peroxide) yields mono-oxygenated DTC complexes (N,N-peroxydiethyldithiocarbamato)(N,N-diethyldithiocarbamato)zin(II), Zn(O-deDTC)(deDTC), 2 and bis(N,N-peroxydiethyldithiocarbamato)zinc(II), Zn(O-deDTC)2, 3. The tetraoxygenated derivative bis(N,N-diethylthiocarbamoylsulfinato)zinc(II), Zn(O(2)-deDTC)2, 4, was cleanly obtained by initial reaction of the DTC salts with stoichiometric oxidant prior to complexation with Zn(II). X-ray crystallographic analysis of 2, 3, and 4 show that the peroxydithiocarbamate ligands are S,O-bound. Similar derivatives were obtained from the homoleptic dimethyl and pyrollidine DTC Zn complexes. These oxygenated species display unique 1H and 13C NMR variable-temperature spectra, as the symmetry of DTC ligand is broken upon oxygenation; total line shape analysis (TLSA) was used to compare the energetic parameters for rotation about the C-N bond in several derivatives. Compounds 2, 3, and 4 were deoxygenated by alkyl phosphine, regenerating the parent dithiocarbamate 1. The peroxydithiocarbamate complexes were susceptible to base-catalyzed hydrolytic decomposition, giving ligand-based products indicative of S-oxidation and S-extrusion.

The Interaction of Nitric Oxide with Distinct Hemoglobins Differentially Amplifies Endothelial Heme Uptake and Heme Oxygenase-1 Expression

Heme is a strong inducer and substrate of the stress protein heme oxygenase-1 (HO-1), which produces carbon monoxide, iron, and bilirubin. We have reported recently that nitric oxide (NO) augments the incorporation of free hemin in endothelial cells, resulting in amplified HO-1 expression and production of bilirubin. Here, we extend our studies by showing that both NO+ and NO- donors interacted with reduced (HbA0) or oxidized (metHb) hemoglobin, as well as hemoglobin from sickle cell disease (HbS), to strongly magnify HO-1, with a pattern of induction dependent on the oxidation state of the hemoglobin used. A corresponding enhancement of endothelial heme uptake was observed following exposure of HbA0 or HbS to the NO donors, which also increased the uptake of free hemin. We postulated that this effect may be caused by formation of heme-nitrosyl (H-NO) complexes, and indeed endothelial cells exposed to preformed H-NO showed greater heme incorporation than free hemin. Furthermore, NO donors directly affected the permeability of membranes to free hemin. In conclusion, our data indicate a novel role for NO in the modulation of heme transport and HO-1 induction in endothelial cells, which may be relevant for hematological disorders characterized by disruption of the heme-NO equilibrium.

A novel heterocyclic atom exchange reaction with Lawesson's reagent: a one-pot synthesis of dithiomaltol

A one-pot reaction of maltol with Lawesson's reagent generates dithiomaltol, a thiopyran-4-thione, via an unusual heterocyclic atom exchange (HCAE) reaction; only pyrones with proton or aliphatic substituents undergo the HCAE substitution.

Coordination chemistry of the HNO ligand with hemes and synthetic coordination complexes

The coordination chemistry of the one-electron reduced form of nitric oxide, termed a nitroxyl or nitrosyl hydride (NO- or HNO), is described with special focus on its interaction with hemes and heme model complexes. Nitroxyl intermediates have been proposed in the catalytic cycles of several heme-based nitrite and nitric oxide reductases; in fungal cytochrome P450nor, a short-lived nitroxyl-adduct has been observed during catalytic turnover. Ferrous-nitroxyl adducts were first identified in electrochemical reductions of nitrosyl porphyrins and heme proteins, but only recently have these species been characterized in solution. Small molecule HNO complexes of transition metals are rare, and the several reported species are presented with descriptions of their synthesis, and a comparison of available spectroscopic data. Special emphasis is given to the long-lived HNO adduct of myoglobin, including its synthesis by various routes and characterization by 1H NMR, resonance Raman and X-ray absorption spectroscopy. HNO is isoelectronic with 1O2, and as with oxymyoglobin, there are several possible descriptions for its bonding with a ferrous heme; an analogy to the pi-bonding interactions of a Fischer carbene is presented. A survey of the reactivity associated with the characterizable HNO complexes is made, including redox and protonation equilibrium, reactivity with small molecules, and dissociation or displacement reactions.

The putidaredoxin reductase-putidaredoxin electron transfer complex: theoretical and experimental studies

Interaction and electron transfer between putidaredoxin reductase (Pdr) and putidaredoxin (Pdx) from Pseudomonas putida was studied by molecular modeling, mutagenesis, and stopped flow techniques. Based on the crystal structures of Pdr and Pdx, a complex between the proteins was generated using computer graphics methods. In the model, Pdx is docked above the isoalloxazine ring of FAD of Pdr with the distance between the flavin and [2Fe-2S] of 14.6 A. This mode of interaction allows Pdx to easily adjust and optimize orientation of its cofactor relative to Pdr. The key residues of Pdx located at the center, Asp(38) and Trp(106), and at the edge of the protein-protein interface, Tyr(33) and Arg(66), were mutated to test the Pdr-Pdx computer model. The Y33F, Y33A, D38N, D38A, R66A, R66E, W106F, W106A, and Delta106 mutations did not affect assembly of the [2Fe-2S] cluster and resulted in a marginal change in the redox potential of Pdx. The electron-accepting ability of Delta106 Pdx was similar to that of the wild-type protein, whereas electron transfer rates from Pdr to other mutants were diminished to various degrees with the smallest and largest effects on the kinetic parameters of the Pdr-to-Pdx electron transfer reaction caused by the Trp(106) and Tyr(33)/Arg(66) substitutions, respectively. Compared with wild-type Pdx, the binding affinity of all studied mutants to Pdr was significantly higher. Experimental results were in agreement with theoretical predictions and suggest that: (i) Pdr-Pdx complex formation is mainly driven by steric complementarity, (ii) bulky side chains of Tyr(33), Arg(66), and Trp(106) prevent tight binding of oxidized Pdx and facilitate dissociation of the reduced iron-sulfur protein from Pdr, and (iii) transfer of an electron from FAD to [2Fe-2S] can occur with various orientations between the cofactors through multiple electron transfer pathways that do not involve Trp(106) but are likely to include Asp(38) and Cys(39).

Effect of Stacking and Redox State on Optical Absorption Spectra of Melanins−Comparison of Theoretical and Experimental Results

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.

Bonding in HNO-Myoglobin as Characterized by X-ray Absorption and Resonance Raman Spectroscopies

The EXAFS and resonance Raman spectra on the HNO-myoglobin adduct, 1, are consistent with the presence of HNO bound to a heme center. The three-dimensional structure about the heme center of 1 obtained from multiple-scattering (MS) analysis of the EXAFS of the heme protein yielded an Fe-N-O bond angle of 131 degrees and an Fe-N bond length of 1.82 A, which compare well with published values for model complexes containing RNO ligands. Resonance Raman spectra identified the nu(N=O) stretch at 1385 cm-1 (confirmed by 15N labeling), which corresponds well with those reported for small molecule HNO complexes. The wavelength of the nu(Fe-N) at 636 cm-1 of 1 is significantly higher than those of MbIINO and MbIIINO (554 and 595 cm-1, respectively). The XAFS, XANES, and resonance Raman data are all consistent with the structure deduced from the NMR experiments, providing more detail on the bonding between HNO and the metal center.

Disulfiram Facilitates Intracellular Cu Uptake and Induces Apoptosis in Human Melanoma Cells

The alcohol-abuse deterrent disulfiram (DSF) is shown to have a highly selective toxicity against melanoma in culture, inducing a largely apoptotic response, with much lower toxicity against several other cell lines. Melanoma cell lines derived from different stages (radial, vertical, and metastatic phase) were all sensitive to DSF treatment in vitro; melanocytes were only slightly affected. A required role of extracellular Cu is demonstrated for DSF toxicity. Low concentrations of DSF alone decreased the number of viable cells, and the addition of CuCl(2) significantly enhanced the DSF-induced cell death to less than 10% of control. Significantly, the intracellular Cu concentration of melanoma cells increased rapidly upon DSF treatment. Both the intracellular Cu uptake and the toxicity induced by DSF were blocked by co-incubation with bathocuproine disulfonic acid (BCPD, 100 muM), a non-membrane-permeable Cu chelator. Chemical studies demonstrated a complicated, extracellular redox reaction between Cu(II) and DSF, which forms the complex Cu(deDTC)(2) in high yield, accompanied by oxidative decomposition of small amounts of disulfiram. The Cu complex has somewhat higher activity against melanoma and is suggested to be the active agent in DSF-induced toxicity. The redox conversion of DSF was unique to Cu(II) and not engendered by the other common biological metal ions Fe(II or III), Mn(III), and Zn(II). The implications of this work are significant both in the possible treatment of melanoma as well as in limiting the known side-effects of DSF, which we propose may be diminished by cotreatment to decrease adventitious Cu.

Multiple Pathways for the Oxygenation of a Ruthenium(II) Dithiocarbamate Complex: S-Oxygenation and S-Extrusion

The reactions of Ru(bpy)(2)(N,N-dimethyldithiocarbamate)(+), 1, with O-atom-transfer reagents such as hydrogen peroxide, m-chloroperoxybenzoic acid, and oxone have been studied and several resulting derivatives isolated and structurally characterized. Both S-oxygenation and S-extrusion may occur depending upon reagent and conditions. Excess peroxygenation leads to a stable dioxygenate, Ru(bpy)(2)(N,N-dimethylthiocarbamatesulfinate-S,S)(+), 3. Stoichiometric oxygenation leads to mixtures of products from which two forms of monooxygenated species Ru(bpy)(2)(N,N-dimethylperoxydithiocarbamate-S,S), 2a, and Ru(bpy)(2)(N,N-dimethylperoxydithiocarbamate-O,S), 2b, and an S-extruded product, Ru(bpy)(2)(N,N-dimethylmonothiocarbamate)(+), 4, have been isolated as PF(6)(-) salts. The S,S-bound monooxygenate is unstable over time toward either O-atom-transfer reactions via disproportionation or reaction with phosphines or S-extrusion yielding complex 4 in which the thiocarbamate is bonded solely through the remaining S atom. All the complexes have been characterized by (1)H NMR, UV-vis, and mass spectroscopies, and all but the highly reactive 2a structurally determined by X-ray crystallography.

High-Temperature Electrocatalysis Using Thermophilic P450 CYP119: Dehalogenation of CCl4 to CH4

The use of a thermophilic cytochrome P450, CYP119, in electrocatalytic dehalogenations of C1 halocarbon solvents is studied. Temperature stable enzyme-modified electrodes were constructed using sol-gel and polymeric surfactant approaches. CYP119 deposited in a dimethyldidodecylammonium poly(p-styrene sulfonate) (DDAPSS) film has good retention of electrochemical activity up to 80 degrees C. At potentials approaching the FeII/I couple, the CYP119/DDAPSS films demonstrate high catalytic dehalogenations of the C1 chloromethanes CCl4, CHCl3, and CH2Cl2. Product analysis identified mixtures of sequentially dechlorinated products up to methane; no evidence for radical-coupled products was observed. The yield of methane from the CYP119-catalyzed reduction of CCl4 is increased 35-fold from 25 degrees C to 55 degrees C. In combination with the lack of C2 products, the facility of an overall eight-electron reductive dehalogenation suggests that the substrate is constrained within the protein during electrocatalytic turnover.

Efficient trapping of HNO by deoxymyoglobin

Nitrosyl hydride, HNO, also commonly termed nitroxyl, is a transient species that has been implicated in the biological activity of nitric oxide, NO. Herein, we report the first generation of a stable HNO-metal complex by direct trapping of free HNO. Deoxymyoglobin (Mb-Fe(II)) rapidly reacts with HNO produced from the decomposition of methylsulfonylhydroxylamine (MSHA) or Angeli's salt (AS) in aqueous solutions from pH 7 to pH 10, forming an adduct, Mb-HNO. The unique 1H NMR signal of the Fe-bound HNO at 14.8 ppm allows definitive proof of its formation. The generation of Mb-HNO and quantification of various myoglobin byproducts were accomplished by correlation of 1H NMR, UV-vis, and EPR spectroscopies. Typically, the maximum Mb-HNO yield obtained is 60-80%; competitive side reactions with byproducts as well as the further reactivity of the Mb-HNO decrease the overall yield. At pH 10, the observed rate of Mb-HNO generation by trapping HNO from MSHA is close to that for MSHA decomposition; kinetic simulations give a lower limit to the bimolecular rate of trapping as 1.4 x 10(4) M(-1) s(-1). The binding of HNO to deoxymyoglobin is rapid and essentially irreversible, which suggests that the biological activity of nitroxyl may be mediated by its reactivity with ferrous heme proteins such as myoglobin and hemoglobin.