The products of the reduction of doxyl stearates in cells are hydroxylamines as shown by oxidation by 15N-perdeuterated Tempone

Molecular Probes for Evaluation of Oxidative Stress by In Vivo EPR Spectroscopy and Imaging: State-of-the-Art and Limitations

Oxidative stress, defined as a misbalance between the production of reactive oxygen species and the antioxidant defenses of the cell, appears as a critical factor either in the onset or in the etiology of many pathological conditions. Several methods of detection exist. However, they usually rely on ex vivo evaluation or reports on the status of living tissues only up to a few millimeters in depth, while a whole-body, real-time, non-invasive monitoring technique is required for early diagnosis or as an aid to therapy (to monitor the action of a drug). Methods based on electron paramagnetic resonance (EPR), in association with molecular probes based on aminoxyl radicals (nitroxides) or hydroxylamines especially, have emerged as very promising to meet these standards. The principles involve monitoring the rate of decrease or increase of the EPR signal in vivo after injection of the nitroxide or the hydroxylamine probe, respectively, in a pathological versus a control situation. There have been many successful applications in various rodent models. However, current limitations lie in both the field of the technical development of the spectrometers and the molecular probes. The scope of this review will mainly focus on the latter.

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

In vivo evaluation of different alterations of redox status by studying pharmacokinetics of nitroxides using magnetic resonance techniques

Free radicals, particularly reactive oxygen species (ROS), are involved in various pathologies, injuries related to radiation, ischemia-reperfusion or ageing. Unfortunately, it is virtually impossible to directly detect free radicals in vivo, but the redox status of the whole organism or particular organ can be studied in vivo by using magnetic resonance techniques (EPR and MRI) and paramagnetic stable free radicals - nitroxides. Here we review results obtained in vivo following the pharmacokinetics of nitroxides on experimental animals (and a few in humans) under various conditions. The focus was on conditions where the redox status has been altered by induced diseases or harmful agents, clearly demonstrating that various EPR/MRI/nitroxide combinations can reliably detect metabolically induced changes in the redox status of organs. These findings can improve our understanding of oxidative stress and provide a basis for studying the effectiveness of interventions aimed to modulate oxidative stress. Also, we anticipate that the in vivo EPR/MRI approach in studying the redox status can play a vital role in the clinical management of various pathologies in the years to come providing the development of adequate equipment and probes.

Electron spin resonance. Part one: a diagnostic method in the biomedical sciences

A review is presented of some of the ways in which electron spin resonance (ESR) spectroscopy may be useful to investigate systems of relevance to the biomedical sciences. Specifically considered are: spin-trapping in biological media; the determination of antioxidant efficiencies; lipid-peroxidation; the use of nitroxides as probes of metabolic activity in cells and as structumral probes of cell-membranes; ESR coupled with materials for radiation-dosimetry; food- and drug-irradiation; studies of enzyme systems and ofcyclodextrins; diagnosis of cancer and rheumatoid arthritis; measurement of oxidative stress in synovial tissue in preparation for joint replacement; determination of oxidative species during kidney dialysis; measurement of biological oxygen concentrations (oximetry); trapping in living cells of the endothelium-derived relaxing factor nitric oxide (NO); measurement of hydrogen peroxide; determination of drugs of abuse (opiates); ESR measurements of whole blood and as a means to determine the age of bloodstains for forensic analysis are surveyed, and also a determination of the aqueous volume of human sperm cells is described, among other topics.

Membrane chemical stability and seed longevity

Here, we investigate the relationships between the chemical stability of the membrane surface and seed longevity. Dry embryos of long-lived tomato and short-lived onion seeds were labeled with 5-doxyl-stearic acid (5-DS). Temperature-induced loss of the electron spin resonance signal caused by chemical conversion of 5-DS to nonparamagnetic species was used to characterize the membrane surface chemical stability. No difference was found between temperature plots of 5-DS signal intensity in dry onion and tomato below 345 K. Above this temperature, the 5-DS signal remained unchanged in tomato embryos and irreversibly disappeared in onion seeds. The role of the physical state and chemical status of the membrane environment in the chemical stability of membrane surfaces was estimated for model systems containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) dried alone or in the presence of trehalose or glucose. Fourier transform infrared spectroscopy was used to follow temperature-induced structural changes in dry POPC. Spin-label technique was used to relate the chemical stability of 5-DS with the dynamic properties of the bilayer and 5-DS motion behavior. In all the models, the decrease in 5-DS signal intensity was always observed above T_m for the membrane surface. The 5-DS signal was irreversibly lost at high temperature when dry POPC was embedded in a glucose matrix. The loss of 5-DS signal was moderate when POPC was dried alone or in the presence of trehalose. Comparison of model and in vivo data shows that the differences in longevity between onion and tomato seeds are caused by differences in the chemical status of the membrane surface rather than the degree of its immobilization.

Neonatal blood is more resistant to oxidative stress induced by stable nitroxide radicals than adult blood

Neonate erythrocytes are more susceptible to oxidizing drugs than adults; however, there are controversial reports in the literature regarding the total antioxidant capacity of neonate blood. Stable nitroxide radicals (NRs) are reduced by blood and some other biological materials to the corresponding hydroxylamines. The kinetics of the nitroxide's disappearance using electron paramagnetic resonance (EPR) spectroscopy, provides useful biochemical and biophysical information about the antioxidant properties of biological systems. In order to investigate the antioxidant properties in the newborn's blood, we applied this novel method on 38 umbilical vein blood samples and 40 healthy adults. The NR, 5-Dimethylaminonaphthalene-1-sulfonyl-4-amino-2,2,6,6,-tetramethyl-piperidine-oxyl (R), was used for this purpose. Ascorbate is the only known natural antioxidant that reduces R. We found that the reduction rates of R in neonate's whole blood are significantly higher (P < 0.001) than the reduction rates of R in adult's whole blood. However, there were no significant differences in the antioxidant capacity between the two groups. Newborn's blood has significantly higher ability to deal with oxidative stress, caused by R, in comparison with adult blood. We suggest that the system that responds to the recycling of ascorbate is more efficient in neonate blood than in adult's blood.

Factors influencing nitroxide reduction and cytotoxicity in vitro

Nitroxides have been shown to be effective antioxidants, radiation protectors, and redox-active probes for functional electron paramagnetic resonance (EPR) imaging. More recently, the nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-N-oxyl (Tempol) has been shown to exert differential cytotoxicity to tumor compared with normal cell counterparts. Nitroxides are readily reduced in tissues to their respective hydroxylamines, which exhibit less cytotoxicity in vitro and do not provide radiation protection or an EPR-detectable signal for imaging. In order to better understand factors that influence nitroxide reduction, the rate of reduction of Tempol in mouse and human cell lines and in primary cultures of tumor cells was measured using EPR spectroscopy. Additionally, the cytotoxicity of high concentrations of Tempol and the hydroxylamine of Tempol (Tempol-H) was evaluated in wild-type and glucose-6-phosphate dehydrogenase (G6PD)-deficient Chinese hamster ovary cells. The results show that in general Tempol was reduced at a faster rate when cells were under hypoxic compared with aerobic conditions. Neither depletion of intracellular glutathione nor treatment of cells with sodium cyanide influenced Tempol reduction rates. G6PD-deficient cells were found to reduce Tempol at a significantly slower rate than wild-type cells. Likewise, Tempol-induced cytotoxicity was markedly less for G6PD-deficient cells compared with wild-type cells. Tempol-H exhibited no cytotoxicity to either cell type. Tempol-mediated cytotoxicity was enhanced by glutathione depletion and inhibition of 6-phosphogluconate dehydrogenase in wild-type cells, but was unaltered in G6PD-deficient cells. Collectively, the results indicate that while the bioreduction of Tempol can be influenced by a number of factors, the hexose monophosphate shunt appears to be involved in both nitroxide reduction as well as cytotoxicity induced by high levels of exposure to Tempol.

The Use of Spin-Labeled Ligands as Biophysical Probes to Report Real-Time Endocytosis of G Protein-Coupled Receptors in Living Cells

Recycling and degradation of plasma membrane receptors and transporters are fundamental mechanisms for regulating cell signaling and metabolic processes. For many membrane proteins, endocytosis reduces the number of molecules available for transport or signal transduction, providing an attenuation response. Fluorescent reporters attached to either the receptor or ligand have been used to monitor the trafficking of internalization; however, these approaches provide poor resolution for the early endocytic response. Here, we describe the use of a spin-labeled ligand for a heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor for measuring the kinetics of endocytosis in real time. Included are protocols for designing a nitroxide-labeled ligand and measuring receptor endocytosis in live cells using electron paramagnetic resonance (EPR) spectroscopy. Methods for the evaluation of the receptor binding and activation properties of modified ligands and the generation of a cell line stably expressing high receptor levels are also provided.

Probing the binding pocket and endocytosis of a G protein-coupled receptor in live cells reported by a spin-labeled substance P agonist

To probe the molecular nature of the binding pocket of a G protein-coupled receptor and the events immediately following the binding and activation, we have modified the substance P peptide, a potent agonist for the neurokinin-1 receptor, with a nitroxide spin probe specifically attached at Lys-3. The agonist properties and binding affinity of the spin-labeled substance P are similar to the native peptide. Using electron paramagnetic resonance (EPR) spectroscopy, the substance P analogue is capable of reporting the microenvironment found in the binding pocket of the receptor. The EPR spectrum of bound peptide indicates that the Lys-3 portion of the agonist is highly flexible. In addition, we detect a slight increase in the mobility of the bound peptide in the presence of a non-hydrolyzable analogue of GTP, indicative of the alternate conformational states described for this class of receptor. The down-regulation of neurokinin-tachykinin receptors is accomplished by a rapid internalization of the activated protein. Thus, it was also of interest to establish whether spin-labeled substance P could serve as a real time reporter for endocytosis. Our findings show the receptor agonist is efficiently endocytosed and the loss of EPR signal upon internalization provides a real time monitor of endocytosis. The rapid loss of signal suggests that endosomal trafficking vesicles maintain a reductive environment. Whereas the reductive capacity of the lysosome has been established, our findings indicate this capacity in early endosomes as well.

Characterization of redox activity in resting and activated mast cells by reduction and reoxidation of lipophilic nitroxides

1. We measured redox systems in resting and activated rat peritoneal mast cells under anoxia by using the redox metabolism of free doxyl stearic acid (5DS) and phosphatidylcholine with two 5DS molecules esterified to the glycerol (di5DSPC). 2. In the absence of oxygen, 5DS and di5DSPC were reduced to the corresponding hydroxylamines by resting mast cells, with apparent first-order kinetics of 0.085 and 0.078/min, respectively. 3. The activation of mast cells induced by compound 48/80 and bradykinin did not affect the rates of reduction of the nitroxides, and therefore the activation appeared not to be closely coupled to the redox system of these cells; this finding implies that ischemia is unlikely to affect histamine release from mast cells. 4. The oxidation of the nitroxides by the mast cells was very fast and may be nonenzymatic. 5. We concluded that nitroxides can be useful probes of redox metabolism in the mast cells but, because the characteristics of the cellular reduction-reoxidation systems differed from that of other cells, the use of this approach in other cells will require careful characterization of the redox metabolism of nitroxides in those cells.

Aging and caloric restriction affect mitochondrial respiration and lipid membrane status: an electron paramagnetic resonance investigation

Previous studies have indicated that reactive oxygen species (ROS) are likely involved in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). ROS, generated by succinate-stimulated mitochondria, have been reported to be spin trapped and detected by electron paramagnetic resonance (EPR). Our aim in the current study was to study the impact of aging on the effect of increased metabolic stimuli on mitochondrial respiration in terms of oxy-radical generation and possible lipid peroxidative changes in brain neocortical membranes. A mixed population of brain synaptosomes and mitochondria from brown norway male rats of differing ages being fed either ad lib (AL) or a caloric-restricted diet (DR) was prepared and labeled with 5-nitroxyl stearate (5-NS), a membrane lipid-specific spin label. The changes in anisotropic motion of the intercalated 5-NS spin probe also allows one to evaluate the status of the membrane fluidity in the lipid microenvironment via the order parameter. Upon succinate stimulation of mitochondria, the ROS generated resulted in a decrease in the EPR signal amplitude of the 5-NS reporter molecule indicative of the flux of oxy-radicals produced and possible peroxidation-induced changes in the synaptosomal lipid membrane. The line width remained constant, indicating that the overall intensity was reduced. The results showed a significant overall age effect in the ability to generate oxygen-derived radicals following metabolic stimulation (p < .0001). Stimulation of state 4 mitochondrial respiration with 20 mM succinate resulted in greater oxy-radical production in 25-month-old animals as compared to younger animals, suggesting increased mitochondrial leakage with age. Free radical stress induced by metabolic stimulation also causes a concomitant increase in membrane fluidity (p < .0001). There was also a significant age effect (p < .0007) on the order parameter of the mixed population of membranes. Although caloric restriction attenuated the membrane rigidization caused by aging, it was found to play a role in limiting the oxy-radical production following metabolic stimulation of mitochondria. The overall effect of age on membrane spin-label intensities EPR signal upon succinate stimulation suggests that progressive mitochondrial dysfunction may be a key factor in the aging process and in development of age-associated diseases.

Kinetics of nitroxide spin label removal in biological systems: an in vitro and in vivo ESR study

A systematic study on the disappearance of the electron spin resonance (ESR) signal of nitroxides based on six-or five-membered ring and bearing various charges was carried out in vitro and in vivo. The second-order kinetic rate constants of the reaction of spin probes with ascorbate were measured in vitro at various temperatures in phosphate buffered saline, and the relative activation energies were calculated. Clearance rates of the nitroxide radicals in rat brain homogenates and in blood indicate that the ascorbate contribution to nitroxide removal is about 50-70% in brain and 50-90% in blood. These rates can be easily calculated on the basis of the ascorbate concentration and of the second-order kinetic rate constants measured in phosphate buffered saline. ESR spectra acquired in vivo in rat head and tail, by an L-band resonator, indicated that the nitroxide decay rate is a first-order kinetic process in both domains and that the positively charged nitroxides are not retained in the brain, whereas the anionic and uncharged nitroxides are. Once nitroxides with piperidine ring enter the brain, their decay appears controlled mainly by ascorbate, while the ascorbate has a negligible influence on disappearance in brain of five-membered ring proxyl nitroxides.

Reduction of lipid-soluble nitroxides in CHO cells and macrophage tumor cells

There is a need to understand the metabolism of nitroxides because of their usefulness in measurements in living cells of complex phenomena, such as biophysical properties, redox metabolism, and the concentration of oxygen at specific sites. As part of a systematic study of the metabolism of nitroxides in cells, the authors studied Chinese hamster ovary (CHO) cells and mouse macrophage tumor (M5076) cells, using a set of lipophilic nitroxides based on 5 doxyl stearate: the free acid, the methyl ester of the acid, and a phosphatidylcholine with two doxyl stearates esterified to the glycerol. The rates of metabolism of these nitroxides under anoxia differed significantly as a function of both the type of cell and the type of nitroxide. The rates of reduction of the three lipophilic nitroxides depended on their localization. The rates of reduction were first order for all three lipophilic nitroxides, and the only products detected were the respective hydroxylamines. Effects of freeze-thawing and incubation temperature differed in the two cell lines. The authors conclude that the metabolism of nitroxides in different cell lines can be quite different. This may be especially important in understanding studies using nitroxides in living cells, functional organs, and in vivo.

Membrane lipid fluidity affects the nitroxide radical decay of 5-doxyl stearic acids in isolated rat hepatocytes

We investigated the effect of membrane fluidity on the nitroxide radical decay rate of 5-doxyl stearic acid in spin-labeled rat hepatocytes. The half-time (t1/2) for the EPR signal decay of 5-doxyl stearic acids incorporated into the membranes of isolated rat hepatocytes was 12 min (mean value). When spin-labeled hepatocytes were separated into membrane and cytosol fractions, the t1/2 of the membrane fraction was prolonged by more than 2 hrs. However, when the cytosolic fraction was added to the membrane fraction, the radical decay reaction recovered (t1/2 was 27 min). Incubation of hepatocytes with a stream of 95% O2 at 37 degrees C for 2 hrs prolonged t1/2 by 106% and was associated with a 18% decrease in water-soluble antioxidant content. When the measurement temperature was changed from 24 degrees C to 37 degrees C, t1/2 was shortened with a decrease in the order parameter (S). The t1/2 and S in hepatocytes treated with phosphatidylcholine (PC) were reduced by 14% and 0.008, respectively. Conversely, after treatment with phosphatidylethanolamine (PE), PC+cholesterol and PE+cholesterol, t1/2 and S increased by 14% and 0.014, 20% and 0.018 and 29% and 0.040, respectively. These findings suggest that the nitroxide radical decay of 5-doxyl stearic acids incorporated into hepatocyte membranes is mediated by the antioxidants in the cytosol fraction, and that the nitroxide radical decay rate is affected not only by water-soluble antioxidant content but also by the membrane lipid fluidity of the hepatocytes.

Formation of superoxide in the reaction of photolytically altered nifedipine--a nitroso compound--with unsaturated membrane lipids

Nifedipine, which is unstable at light, is photolytically converted to the corresponding 4-[2'-nitrosophenyl]-pyridine (NTP). We reported earlier that NTP react with unsaturated lipids in a pseudo Diels-Alder reaction, thus forming stable nitroxide radicals. In this paper we report that superoxide is being generated in the latter reaction. Superoxide formation was evidenced by SOD-inhibitable cytochrome c reduction in the reaction of NTP with egg phosphatidylcholine at molar ratio 1:1, and 1:3. In this reaction an ESR-observable nitroxide radical was formed. Maximum nitroxide formation was observed after 90 min; the addition of SOD (93 units/ml) increased the concentration of nitroxide. This effect of SOD was reversed by catalase, indicating involvement of hydrogen peroxide in this effect. The nitroxide radical formation appears to be metal-independent, since neither iron salts, nor an iron chelator, desferal, influenced the nitroxide formation. Although production of superoxide in our system was only observed at high concentrations of NTP and of unsaturated lipids, this reaction may be of potential cytotoxic significance due to redox cycling of the nitroxide/hydroxylamine couple in cellular systems.

Selective suppression of lipid resonances by lipid-soluble nitroxides in NMR spectroscopy

The ability of lipid-soluble nitroxides to suppress selectively the peaks of lipid resonances in 31P, 1H, and 13C NMR spectra was investigated in serum as part of studies aimed at using these contrast agents for magnetic resonance imaging and magnetic resonance spectroscopy in vivo. Nitroxides are especially interesting potential contrast agents because they can reversibly be converted in cells to diamagnetic hydroxylamines, with conversion rates that are dependent on the redox potential and the intracellular concentration of oxygen; the characterization of nitroxide-dependent changes in NMR spectra may therefore be a useful means to measure oxygen-dependent redox metabolism in vivo. The fatty acid analogs, doxyl stearates, suppressed the methyl resonance of choline and the methyl and methylene peaks of lipids in the 1H NMR spectra of serum samples. As a consequence, lactate peaks, which were not readily detected became clearly resolved and could be evaluated quantitatively. The 31P resonance of phosphatidylcholine in the 31P NMR spectrum was suppressed by 5-doxyl stearate and 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxy l,iodid e (Cat16). In the 13C NMR spectrum, the resonances of the methyl groups of choline and the lipids also were broadened significantly by addition of 5-doxyl stearate. Differential suppression of lipid resonances can be employed to facilitate quantitation of lactate.

Cellular metabolism of proxyl nitroxides and hydroxylamines

Previous data from model systems indicated that the proxyl nitroxides should be especially resistant to bioreduction and therefore could be an effective solution to this often problematic characteristic of nitroxides. Therefore, we investigated the rate of reduction by cells and by the usual model system, ascorbate, of four proxyl nitroxides and three reference nitroxides. We found that, while the rate of reduction by ascorbate of the proxyl nitroxides was slower than the rate of a prototypic pyrrolidine nitroxide (PCA), the reverse was true for reduction by cells. We also studied the rate of oxidation of the corresponding hydroxylamines. The rate of oxidation by cells of the proxyl hydroxylamines was relatively fast, especially for the most lipophilic derivative. These results indicate that: (i) proxyl nitroxides may not be unusually resistant to bioreduction by functional biological systems; (ii) accurate knowledge of relative rates of metabolism of nitroxides and hydroxylamines in cells and tissues will require direct studies in these systems because the rates may not closely parallel those observed in model (chemical) systems; and (iii) proxyl nitroxides show potential value as agents to measure oxygen concentrations by the rates of oxidation of their corresponding hydroxylamines.

Chemical and electrochemical reduction rates of cyclic nitroxides (nitroxyls)

The reduction rates of five-membered pyrrolidine and pyrroline, and six-membered piperidine nitroxides (alternatively termed nitroxyls) containing various substituents were determined under homogeneous conditions using ascorbate, and electrochemically under heterogeneous conditions. The results were compared with data from the literature. It was shown that the increased rates of reduction of six-membered nitroxides, compared with those of the five-membered nitroxides, cannot be explained on the basis of differences in electrochemical potentials but, rather, can be ascribed to differences in the accessibility of the nitroxide group. A double bond in the five-membered nitroxyls increases the reduction rate. Within any ring system, the reduction rates of nitroxides using ascorbate can be correlated with the inductive substituent constants. The half-way potentials for electrochemical reduction within a series of nitroxides based on the same ring correlate with logarithms of the rates using ascorbate and with the inductive constants. The potentials for one-electron oxidation of the nitroxides were related to the inductive constants.

Principles of the metabolism of nitroxides and their implications for spin trapping

A review of the principal interactions of nitroxides with cells suggests that if these same phenomena occur with spin adducts the result could be considerable experimental confusion and error. In particular, these could lead to differential rates of loss of spin adducts, thereby potentially invalidating conclusions on the amounts or even the types of free radicals that are trapped. In addition, shuttling of electrons between nitroxides and hydroxylamines also very significantly could alter the amounts and types of spin adducts that are observed.

A method to evaluate the antioxidant system for radicals in erythrocyte membranes

The erythrocyte defense system against cellular oxidants is complex and efficient. Free radicals generated in cell membranes, however, are relatively sequestered from the cell's antioxidant mechanisms. When an oxidant challenge exceeds the capacity of the erythrocyte's antioxidant system, membrane damage may occur, causing red cell destruction and hemolytic anemia. In this study, we present a method for monitoring radical reduction in erythrocyte membranes, using fatty acid spin labels with nitroxide radicals on the hydrocarbon chains. About 50 microL of packed (about 5-6 x 10(8)), carbon monoxide (CO)-gassed red blood cells are used. The electron paramagnetic resonance signals of the 5-doxylstearic acid spin labels in the intact cells are obtained as a function of time, at 37 degrees C over a period of 2 h. The pseudo first-order rate constant for reduction of the spin label in normal adult intact cells under our experimental conditions is 4.3 +/- 1.8 x 10(-3)/min. The reproducibility and variability of the measurements are discussed. Since the measurements we describe reflect the extent of radical reductions occurring in cell membranes, we suggest that this method can be used to measure the ability to defend oxidants in membranes of erythrocytes with defective antioxidant systems. This method is particularly useful for measuring the modification of the antioxidant system toward radicals in membranes by drugs, chemicals, or environmental toxins.