Effects of oxygen on the metabolism of nitroxide spin labels in cells

Structural Features Governing the Metabolic Stability of Tetraethyl-Substituted Nitroxides in Rat Liver Microsomes

Nitroxides are potent tools for studying biological systems by electron paramagnetic resonance (EPR). Whatever the application, a certain stability is necessary for successful detection. Since conventional tetramethyl-substituted cyclic nitroxides have insufficient in vivo stability, efforts have recently been made to synthesize more stable, tetraethyl-substituted nitroxides. In our previous study on piperidine nitroxides, the introduction of steric hindrance around the nitroxide moiety successfully increased the resistance to reduction into hydroxylamine. However, it also rendered the carbon backbone susceptible to modifications by xenobiotic metabolism due to increased lipophilicity. Here, we focus on a new series of three nitroxide candidates with tetraethyl substitution, namely with pyrrolidine, pyrroline, and isoindoline cores, to identify which structural features afford increased stability for future probe design and application in in vivo EPR imaging. In the presence of rat liver microsomes, pyrrolidine and pyrroline tetraethyl nitroxides exhibited a higher stability than isoindoline nitroxide, which was studied in detail by HPLC-HRMS. Multiple metabolites suggest that the aerobic transformation of tetraethyl isoindoline nitroxide is initiated by hydrogen abstraction by P450-Fe^V = O from one of the ethyl groups, followed by rearrangement and further modifications by cytochrome P450, as supported by DFT calculations. Under anaerobic conditions, only reduction by rat liver microsomes was observed with involvement of P450-Fe^II.

Unexpected rapid aerobic transformation of 2,2,6,6-tetraethyl-4-oxo(piperidin-1-yloxyl) radical by cytochrome P450 in the presence of NADPH: Evidence against a simple reduction of the nitroxide moiety to the hydroxylamine

Aminoxyl radicals (nitroxides) are a class of compounds with important biomedical applications, serving as antioxidants, spin labels for proteins, spin probes of oximetry, pH, or redox status in electron paramagnetic resonance (EPR), or as contrast agents in magnetic resonance imaging (MRI). However, the fast reduction of the radical moiety in common tetramethyl-substituted cyclic nitroxides within cells, yielding diamagnetic hydroxylamines, limits their use in spectroscopic and imaging studies. In vivo half-lives of commonly used tetramethyl-substituted nitroxides span no more than a few minutes. Therefore, synthetic efforts have focused on enhancing the nitroxide stability towards reduction by varying the electronic and steric environment of the radical. Tetraethyl-substitution at alpha position to the aminoxyl function proved efficient in vitro against reduction by ascorbate or cytosolic extracts. Moreover, 2,2,6,6-tetraethyl-4-oxo(piperidin-1-yloxyl) radical (TEEPONE) was used successfully for tridimensional EPR and MRI in vivo imaging of mouse head, with a reported half-life of over 80 min. We decided to investigate the stability of tetraethyl-substituted piperidine nitroxides in the presence of hepatic microsomal fractions, since no detailed study of their "metabolic stability" at the molecular level had been reported despite examples of the use of these nitroxides in vivo. In this context, the rapid aerobic transformation of TEEPONE observed in the presence of rat liver microsomal fractions and NADPH was unexpected. Combining EPR, HPLC-HRMS, and DFT studies on a series of piperidine nitroxides - TEEPONE, 4-oxo-2,2,6,6-tetramethyl(piperidin-1-yloxyl) (TEMPONE), and 2,2,6,6-tetraethyl-4-hydroxy(piperidin-1-yloxyl) (TEEPOL), we propose that the rapid loss in paramagnetic character of TEEPONE is not due to reduction to hydroxylamine but is a consequence of carbon backbone modification initiated by hydrogen radical abstraction in alpha position to the carbonyl by the P450-Fe^(V)=O species. Besides, hydrogen radical abstraction by P450 on ethyl substituents, leading to dehydrogenation or hydroxylation products, leaves the aminoxyl function intact but could alter the linewidth of the EPR signal and thus interfere with methods relying on measurement of this parameter (EPR oximetry).

Clinical imaging of hypoxia: Current status and future directions

Tissue hypoxia is a key feature of many important causes of morbidity and mortality. In pathologies such as stroke, peripheral vascular disease and ischaemic heart disease, hypoxia is largely a consequence of low blood flow induced ischaemia, hence perfusion imaging is often used as a surrogate for hypoxia to guide clinical diagnosis and treatment. Importantly, ischaemia and hypoxia are not synonymous conditions as it is not universally true that well perfused tissues are normoxic or that poorly perfused tissues are hypoxic. In pathologies such as cancer, for instance, perfusion imaging and oxygen concentration are less well correlated, and oxygen concentration is independently correlated to radiotherapy response and overall treatment outcomes. In addition, the progression of many diseases is intricately related to maladaptive responses to the hypoxia itself. Thus there is potentially great clinical and scientific utility in direct measurements of tissue oxygenation. Despite this, imaging assessment of hypoxia in patients is rarely performed in clinical settings. This review summarises some of the current methods used to clinically evaluate hypoxia, the barriers to the routine use of these methods and the newer agents and techniques being explored for the assessment of hypoxia in pathological processes.

Electron Paramagnetic Resonance Measurements of Reactive Oxygen Species by Cyclic Hydroxylamine Spin Probes

SIGNIFICANCE

Oxidative stress contributes to numerous pathophysiological conditions such as development of cancer, neurodegenerative, and cardiovascular diseases. A variety of measurements of oxidative stress markers in biological systems have been developed; however, many of these methods are not specific, can produce artifacts, and do not directly detect the free radicals and reactive oxygen species (ROS) that cause oxidative stress. Electron paramagnetic resonance (EPR) is a unique tool that allows direct measurements of free radical species. Cyclic hydroxylamines are useful and convenient molecular probes that readily react with ROS to produce stable nitroxide radicals, which can be quantitatively measured by EPR. In this work, we critically review recent applications of various cyclic hydroxylamine spin probes in biology to study oxidative stress, their advantages, and the shortcomings. Recent Advances: In the past decade, a number of new cyclic hydroxylamine spin probes have been developed and their successful application for ROS measurement using EPR has been published. These new state-of-the-art methods provide improved selectivity and sensitivity for in vitro and in vivo studies.

CRITICAL ISSUES

Although cyclic hydroxylamine spin probes EPR application has been previously described, there has been lack of translation of these new methods into biomedical research, limiting their widespread use. This work summarizes "best practice" in applications of cyclic hydroxylamine spin probes to assist with EPR studies of oxidative stress.

FUTURE DIRECTIONS

Additional studies to advance hydroxylamine spin probes from the "basic science" to biomedical applications are needed and could lead to better understanding of pathological conditions associated with oxidative stress. Antioxid. Redox Signal. 28, 1433-1443.

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 paramagnetic resonance: a powerful tool to support magnetic resonance imaging research

The purpose of this paper is to describe some of the areas where electron paramagnetic resonance (EPR) has provided unique information to MRI developments. The field of application mainly encompasses the EPR characterization of MRI paramagnetic contrast agents (gadolinium and manganese chelates, nitroxides) and superparamagnetic agents (iron oxide particles). The combined use of MRI and EPR has also been used to qualify or disqualify sources of contrast in MRI. Illustrative examples are presented with attempts to qualify oxygen sensitive contrast (i.e. T1 - and T2 *-based methods), redox status or melanin content in tissues. Other areas are likely to benefit from the combined EPR/MRI approach, namely cell tracking studies. Finally, the combination of EPR and MRI studies on the same models provides invaluable data regarding tissue oxygenation, hemodynamics and energetics. Our description will be illustrative rather than exhaustive to give to the readers a flavour of 'what EPR can do for MRI'.

Two-photon fluorescence microscopy imaging of cellular oxidative stress using profluorescent nitroxides

A range of varying chromophore nitroxide free radicals and their nonradical methoxyamine analogues were synthesized and their linear photophysical properties examined. The presence of the proximate free radical masks the chromophore's usual fluorescence emission, and these species are described as profluorescent. Two nitroxides incorporating anthracene and fluorescein chromophores (compounds 7 and 19, respectively) exhibited two-photon absorption (2PA) cross sections of approximately 400 G.M. when excited at wavelengths greater than 800 nm. Both of these profluorescent nitroxides demonstrated low cytotoxicity toward Chinese hamster ovary (CHO) cells. Imaging colocalization experiments with the commercially available CellROX Deep Red oxidative stress monitor demonstrated good cellular uptake of the nitroxide probes. Sensitivity of the nitroxide probes to H(2)O(2)-induced damage was also demonstrated by both one- and two-photon fluorescence microscopy. These profluorescent nitroxide probes are potentially powerful tools for imaging oxidative stress in biological systems, and they essentially "light up" in the presence of certain species generated from oxidative stress. The high ratio of the fluorescence quantum yield between the profluorescent nitroxide species and their nonradical adducts provides the sensitivity required for measuring a range of cellular redox environments. Furthermore, their reasonable 2PA cross sections provide for the option of using two-photon fluorescence microscopy, which circumvents commonly encountered disadvantages associated with one-photon imaging such as photobleaching and poor tissue penetration.

Oxidative stress imaging in live animals with techniques based on electron paramagnetic resonance

Oxidative stress has been the object of considerable biological and biochemical investigation. Quantification has been difficult although the quantitative level of products of biological oxidations in tissues and tissue products has emerged as a widely used technique. The relationship between these products and the amount of oxidative stress is less clear. Imaging oxidative stress with electron paramagnetic resonance related magnetic resonance imaging, while not addressing the specific issue of quantification of initiating events, focuses on the anatomic specific location of the oxidative stress. Moreover, the relative quantification of oxidative stress of one location against another is possible, sharpening our understanding of oxidative stress. This promises to improve our understanding of oxidative stress and its deleterious consequences and enhance our understanding of the effectiveness of interventions to modulate oxidative stress and its consequences.

Nitroxides as cancer imaging agents

Nitroxides are low molecular weight (150-400 Da) superoxide dismutase mimics that exhibit antioxidant, radical scavenging, and radioprotective activity. Additionally, the paramagnetic nature of nitroxides makes them viable as both spin probes for electron paramagnetic resonance imaging as well as contrast agents for magnetic resonance imaging. These imaging techniques enable in vivo monitoring of nitroxide metabolism. In biological systems, nitroxide metabolism occurs predominantly via reduction of the nitroxide to a hydroxylamine. The rate of nitroxide reduction can increase or decrease due to either oxidative stress, suggesting that nitroxides can provide an imaging-based assay of tissue redox status. The current review briefly summarizes the potential clinical applications of nitroxides, and focuses on the biochemical and tumor microenvironmental factors that affect the rate of nitroxide reduction. The potential therapeutic applications and bio-reduction mechanisms are discussed in the context of their relevance to oncology.

Magnetic resonance imaging of organic contrast agents in mice: capturing the whole-body redox landscape

Nitroxides are a class of stable free radicals that have several biomedical applications including radioprotection and noninvasive assessment of tissue redox status. For both of these applications, it is necessary to understand the in vivo biodistribution and reduction of nitroxides. In this study, magnetic resonance imaging was used to compare tissue accumulation (concentration) and reduction of two commonly studied nitroxides: the piperidine nitroxide Tempol and the pyrrolidine nitroxide 3-CP. It was found that 3-CP was reduced 3 to 11 times slower (depending on the tissue) than Tempol in vivo and that maximum tissue concentration varies substantially between tissues (0.6-7.2mM). For a given tissue, the maximum concentration usually did not vary between the two nitroxides. Furthermore, using electron paramagnetic resonance spectroscopy, we showed that the nitroxide reduction rate depends only weakly on cellular pO(2) in the oxygen range expected in vivo. These observations, taken with the marked variation in nitroxide reduction rates observed between tissues, suggest that tissue pO(2) is not a major determinant of the nitroxide reduction rate in vivo. For the purpose of redox imaging, 3-CP was shown to be an optimal choice based on the achievable concentrations and bioreduction observed in vivo.

Heterogeneity of regional redox status and relation of the redox status to oxygenation in a tumor model, evaluated using electron paramagnetic resonance imaging

It is widely accepted that redox status, along with the partial pressure of oxygen (pO(2)), determines the efficacy of some therapeutic methods applied to treat tumors, including radiation. Redox status, evaluated by the reduction of a nitroxyl probe, was reportedly heterogeneous in a mouse tumor model. However, neither variation of heterogeneity of the redox status among mice nor the relation of the redox status to pO(2) in tumors has been characterized sufficiently. In this study, the regional reduction status in a mouse radiation-induced fibrosarcoma tumor model was evaluated using sequential three-dimensional electron paramagnetic resonance (EPR) imaging after i.v. injection of a tissue-permeable nitroxyl probe, HM-PROXYL. The regional decay of HM-PROXYL signal obeyed first-order kinetics, and the amplitude of the reduction rate and extent of its heterogeneity in a tumor varied among six mice. The tissue pO(2) was measured using EPR oximetry with lithium phthalocyanine (LiPc) microcrystals implanted within the tumor. The location of LiPc was determined with EPR imaging. A sequential image was obtained following the injection of HM-PROXYL, even after LiPc implantation, by choosing an HM-PROXYL signal peak which does not overlap with the signal of LiPc. The relationship between pO(2) and the reduction rate at the region of pO(2) measurement was found to be low (r = 0.357) in 13 tumor-bearing mice, indicating that the extent of oxygenation does not necessarily affect the redox status under air-breathing conditions. The results strongly indicate the necessity of measurements of both redox status and oxygenation in every tumor to characterize tumor physiology.

In vivo imaging of free radicals and oxygen

Free radicals are highly reactive compounds that play an essential role in many biological processes, both beneficial and deleterious. Detection and quantification of these species is critical to develop a better understanding of normal and pathophysiological functions at the cellular and tissue levels. Electron paramagnetic resonance (EPR) spectroscopy is the technique most commonly used for this purpose through the detection of exogenous probes or spin traps that interact with the free radical species of interest. Over the past several years, the spatial and temporal distribution of free radicals within cells and tissues has been of particular interest. This chapter briefly explains the principles and challenges in the use of EPR for biological samples and introduces the concept of EPR for free radical imaging purposes. In addition, specific examples are given for the use of EPR imaging in four principal areas: free radical probes, nitric oxide (NO), redox state, and oxygen (O(2)) concentration.

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.

Application of in vivo EPR for tissue pO2 and redox measurements

The technique of electron paramagnetic resonance (EPR) spectroscopy is more than 50 years old, but only recently it has been used for in vivo studies. Its limited application in the past was due to the problem of high nonresonant dielectric loss of the exciting frequency because of high water content in biological samples. However, with the development of spectrometers working at lower frequencies (1,200 MHz and below) during the last 15 years, it is now possible to conduct in vivo measurements on a variety of animals and isolated organs. This is further facilitated by the development of new resonators with high sensitivity and appropriate stability for in vivo applications. It now has become feasible to obtain new insights into the complex aspects of physiology and pathophysiology using in vivo EPR. Among several important applications of this technique, the in vivo tissue pO(2) (partial pressure of oxygen) and redox measurements seem to be the most extensive use of this technique. In this chapter, we describe the procedure for in vivo pO(2) and redox measurements in animal models.

Differential metabolic responses to pluronic in MDR and non-MDR cells: a novel pathway for chemosensitization of drug resistant cancers

A synthetic amphiphilic block copolymer, Pluronic, is a potent chemosensitizer of multidrug resistant (MDR) cancers that has shown promise in clinical trials. It has unique activities in MDR cells, which include a decrease in ATP pools and inhibition of P-glycoprotein (Pgp) resulting in increased drug accumulation in cells. This work demonstrates that Pluronic rapidly (15min) translocates into MDR cells and co-localizes with the mitochondria. It inhibits complex I and complex IV of the mitochondria respiratory chain, decreases oxygen consumption and causes ATP depletion in MDR cells. These effects are selective and pronounced for MDR cells compared to non-MDR counterparts and demonstrated for both drug-selected and Pgp-transfected cell models. Furthermore, inhibition of Pgp functional activity also abolishes the effects of Pluronic on intracellular ATP levels in MDR cells suggesting that Pgp contributes to increased responsiveness of molecular "targets" of Pluronic in the mitochondria of MDR cells. The Pluronic-caused impairment of respiration in mitochondria of MDR cells is accompanied with a decrease in mitochondria membrane potential, production of ROS, and release of cytochrome c. Altogether these effects eventually enhance drug-induced apoptosis and contribute to potent chemosensitization of MDR tumors by Pluronic.

Comparison of methods for measuring oxygen consumption in tumor cells in vitro

The oxygen consumption rate of tumor cells affects tumor oxygenation and response to therapies. Highly sensitive methods for determining cellular oxygen consumption are, therefore, needed to identify treatments that can modulate this parameter. We compared the performances of three different methods for measuring cellular oxygen consumption: electron paramagnetic resonance (EPR) oximetry, the Clark electrode, and the MitoXpress fluorescent assay. To compare the assays, we used K562 cells in the presence of rotenone and hydrocortisone, compounds that are known to inhibit the mitochondrial electron transport chain to different extents. The EPR method was the only one that could identify both rotenone and hydrocortisone as inhibitors of tumor cell oxygen consumption. The Clark electrode and the fluorescence assay demonstrated a significant decrease in cellular oxygen consumption after administration of the most potent inhibitor (rotenone) but failed to show any significant effect of hydrocortisone. EPR oximetry is, therefore, the most sensitive method for identifying inhibitors of oxygen consumption on cell assays, whereas the Clark electrode offers the unique opportunity to add external compounds during experiments and still shows great sensitivity in studying enzyme and chemical reactions that consume oxygen (non-cell assays). Finally, the MitoXpress fluorescent assay has the advantage of a high-sample throughput and low bulk requirements but at the cost of a lower sensitivity.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Assessment of tissue redox status using metabolic responsive contrast agents and magnetic resonance imaging

Regulation of tissue redox status is important to maintain normal physiological conditions in the living body. Disruption of redox homoeostasis may lead to oxidative stress and can induce many pathological conditions such as cancer, neurological disorders and ageing. Therefore, imaging of tissue redox status could have clinical applications. Redox imaging employing magnetic resonance imaging (MRI) with nitroxides as cell-permeable redox-sensitive contrast agents has been used for non-invasive monitoring of tissue redox status in animal models. The redox imaging applications of nitroxide electron paramagnetic resonance imaging (EPRI) and MRI are reviewed here, with a focus on application of tumour redox status monitoring. While particular emphasis has been placed on differences in the redox status in tumours compared to selected normal tissues, the technique possesses the potential to have broad applications to the study of other disease states, inflammatory processes and other circumstances where oxidative stress is implicated.

Therapeutic and clinical applications of nitroxide compounds

Nitroxide compounds have been used for many years as biophysical tools, but only during the past 15-20 years have the many interesting biochemical interactions been discovered and harnessed for therapeutic applications. By modifying oxidative stress and altering the redox status of tissues, nitroxides have the ability to interact with and alter many metabolic processes. This interaction can be exploited for therapeutic and research use, including protection against ionizing radiation, as probes in functional magnetic resonance imaging, cancer prevention and treatment, control of hypertension and weight, and protection from damage resulting from ischemia/reperfusion injury. Although much remains to be done, many applications have been well studied, and some are presently being tested in clinical trials. The therapeutic and research uses of nitroxides are reviewed here, with a focus on the progress from initial development to modern, state-of-the art trials.

High-resolution mapping of tumor redox status by magnetic resonance imaging using nitroxides as redox-sensitive contrast agents

PURPOSE

There is considerable research directed toward the identification and development of functional contrast agents for medical imaging that superimpose tissue biochemical/molecular information with anatomical structures. Nitroxide radicals were identified as in vivo radioprotectors. Being paramagnetic, they can provide image contrast in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). The present study sought to determine the efficacy of nitroxide radioprotectors as functional image contrast agents.

EXPERIMENTAL DESIGN

Nitroxide radioprotectors, which act as contrast agents, were tested by EPRI and MRI to provide tissue redox status information noninvasively.

RESULTS

Phantom studies showed that the nitroxide, 3-carbamoyl-PROXYL (3CP), undergoes time-dependent reduction to the corresponding diamagnetic hydroxylamine only in the presence of reducing agents. The reduction rates of 3CP obtained by EPRI and MRI were in agreement suggesting the feasibility of using MRI to monitor nitroxide levels in tissues. The levels of 3CP were examined by EPRI and MRI for differences in reduction between muscle and tumor (squamous cell carcinoma) implanted in the hind leg of C3H mice simultaneously. In vivo experiments showed a T1-dependent image intensity enhancement afforded by 3CP which decreased in a time-dependent manner. Reduction of 3CP was found to be the dominant mechanism of contrast loss. The tumor regions exhibited a faster decay rate of the nitroxide compared to muscle (0.097 min(-1) versus 0.067 min(-1), respectively).

CONCLUSIONS

This study shows that MRI can be successfully used to co-register tissue redox status along with anatomic images, thus providing potentially valuable biochemical information from the region of interest.

Probing the Intracellular Redox Status of Tumors with Magnetic Resonance Imaging and Redox-Sensitive Contrast Agents

Nitroxide radicals are paramagnetic contrast agents, used in magnetic resonance imaging (MRI), that also exert antioxidant effects. Participating in cellular redox reactions, they lose their ability to provide contrast as a function of time after administration. In this study, the rate of contrast loss was correlated to the reducing power of the tissue or the "redox status." The preferential reduction of nitroxides in tumors compared with normal tissue was observed by MRI. The influence of the structure of the nitroxide on the reduction rate was investigated by MRI using two cell-permeable nitroxides, 4-hydroxy-2,2,6,6,-tetramethyl-1-piperidynyloxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3CP), and one cell-impermeable nitroxide, 3-carboxy-2,2,5,5,5-tetramethylpyrrolidine-1-oxyl (3CxP). Pharmacokinetic images of these nitroxides in normal tissue, tumor, kidney, and artery regions in mice were simultaneously obtained using MRI. The decay of Tempol and 3CP in tumor tissue was significantly faster than in normal tissue. No significant change in the total nitroxide (oxidized + reduced forms) was noted from tissue extracts, suggesting that the loss in contrast as a function of time is a result of intracellular bioreduction. However, in the case of 3CxP (membrane impermeable), there was no difference in the reduction rates between normal and tumor tissue. The time course of T(1) enhancement by 3CxP and the total amount of 3CxP (oxidized + reduced) in the femoral region showed similar pharmacokinetics. These results show that the differential bioreduction of cell-permeable nitroxides in tumor and normal tissue is supported by intracellular processes and the reduction rates are a means by which the intracellular redox status can be assessed noninvasively.

In vivo proton electron double resonance imaging of the distribution and clearance of nitroxide radicals in mice

Proton electron double resonance imaging (PEDRI) is an emerging technique that utilizes the Overhauser effect to enable in vivo and in vitro imaging of free radicals in biological systems. Nitroxide spin probes enable measurement of tissue redox state based on their reduction to diamagnetic hydroxylamines. PEDRI instrumentation at 0.02 T was applied to assess the ability to image the in vivo distribution, clearance, and metabolism of nitroxide radicals in living mice. Using phantoms of 2,2,5,5-tetramethyl-3-carboxylpyrrolidine-N-oxyl (PCA) in normal saline the dependence of the enhancement on RF power and spin probe concentration was determined. Enhancements of up to -23 were obtained in phantoms with 2 mM levels. Maximum enhancement of -7 was observed in vivo. Coronal images of nitroxide-infused mice enabled visualization of the kinetics of spin probe uptake and clearance in different organs including the great vessels, heart, lungs, kidneys, and bladder with an in-plane spatial resolution of 0.6 mm. PEDRI of living mice was also performed using 3-carbamoyl-proxyl and 2,2,6,6-tetramethyl-4-oxopiperidine-N-oxyl to compare the different rate of clearance and metabolism among different nitroxide probes. PCA, due to its intravascular compartmentalization, provided the sharpest contrast for the vascular system and highest enhancement values in the PEDRI images among the three nitroxides.

Evaluation of adriamycin nephropathy by an in vivo electron paramagnetic resonance

A rat model for human minimal change nephropathy was obtained by the intravenous injection of adriamycin (ADR) at 5 mg/kg. By using an in vivo electron paramagnetic resonance (EPR) spectrometer operating at 700 MHz, the temporal changes in signal intensities of a nitroxide radical, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), in the kidneys of rats with ADR nephropathy were investigated. The decay rate of the EPR signal intensity obtained in the kidney is indicative of the renal reducing ability. It was found that the reducing ability in the kidney declined on the 7th day after ADR administration and recovered after the 14th day. Impairment of the reducing ability occurred before the appearance of continuous urinary protein. The in vitro EPR study showed that this impairment of in vivo renal reducing ability is related to impairment of the reducing ability in the mitochondria.

In vivo detection of gastric cancer in rats by electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) enables noninvasive spatial mapping of free radical metabolism and has recently been shown to provide in vivo physiologic information regarding alterations in the redox state of tumors and neoplastic tissues. With the use of nitroxide spin probes, it has been shown that certain tumors possess a highly reduced state. To determine whether EPRI can be used for early detection and visualization of gastric carcinoma based on its altered redox metabolism, studies were performed in a rat gastric cancer model induced by 1-methyl-3-nitro-1-nitrosoguanidine. Using a specialized 750 MHz resonator and EPRI instrument, a technique was developed for imaging nitroxide radicals in the whole stomach. In vivo three-dimensional EPRI of the stomach of rats with continuous intravenous administration of nitroxide 3-carboxamido-2,2,5,5-tetramethylpyrrolidine-N-oxyl (3-carbamoyl-proxyl) [3-CP] was performed. Whereas electron paramagnetic resonance images from untreated controls provide a uniform visualization of the stomach mucosa and wall, in the treated rats with gastric cancer, holes were present in the image at the locations of tumors. With localized spectroscopy, it was confirmed that the tumor regions were devoid of signal, and this was largely due to the presence of a more reduced state with rapid reduction of nitroxide. Pharmacokinetic studies indicated that 3-CP in tumors was rapidly reduced to an undetectable level, whereas the 3-CP levels in normal stomach tissue persisted. Near-infrared reflectance measurements of indocyanine green dye uptake indicated that there were no significant differences in tumor versus normal mucosal perfusion. From these results, we concluded that gastric cancer tumors could be distinguished from normal tissue based primarily on the marked difference in their rate of radical metabolism. Because alterations in cellular redox state and radical metabolism are of critical importance in tumor biology and treatment, this methodology should provide an important new tool for the study and visualization of gastric carcinoma and may also be of use in other cancer models.

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.

A low molecular weight antioxidant decreases weight and lowers tumor incidence

Stable free radical nitroxides are potent antioxidants possessing superoxide dismutase- and catalase-mimetic activity that protect cells and animals against a variety of oxidative insults. Tempol, as a representative nitroxide, was evaluated for its influence on weight maintenance and spontaneous tumor incidence in C3H mice. Tempol administered in either the drinking water or food did not show any untoward effects and prevented animals from becoming obese. Tempol-treated animals' leptin levels were reduced. Long-term treatment with Tempol significantly decreased tumorigenesis when compared to controls (10 vs. 40%, respectively). Selected tissues from Tempol-treated animals exhibited elevated levels of mitochrondrial uncoupling protein-2 (UCP-2) and HSP70. The present data suggest that nitroxides upregulate UCP-2, obviate weight gain, and decrease age-related spontaneous tumor incidence. As a class, nitroxides may provide overall health benefits by contributing to decreased obesity and tumor incidence.

Identification by an EPR technique of decreased mitochondrial reducing activity in puromycin aminonucleoside-induced nephrosis

The temporal changes in the electron paramagnetic resonance (EPR) signal intensities of a nitroxide radical, 4-hydroxy 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), in the kidney in rat puromycin aminonucleoside (PAN) nephrosis were investigated in vivo and in vitro. The rats of the PAN nephrosis group received intraperitoneal injections of PAN at 75 mg/kg body weight while those of control group received saline. The in vivo renal half-lives of TEMPOL were calculated from the decay curve of EPR signal intensities after the intravenous injection of the TEMPOL solution. The mitochondrial half-lives were obtained from the decay curve of the EPR signals after mixing the mitochondrial fraction of the kidney and TEMPOL solution. The in vivo half-lives of TEMPOL of the kidney from 7 to 14 d after PAN administration were significantly longer than those of the controls. The mitochondrial half-lives of TEMPOL on the 9th day after the PAN administration prolonged remarkably compared to the controls (378 +/- 69 vs. 676 +/- 183 s, p <.01). These findings indicate that the in vivo and mitochondrial reducing activity in PAN treated rats decreased markedly, because the half-life of TEMPOL in the kidney reflects the renal reducing activity.

Feasibility and assessment of non-invasive in vivo redox status using electron paramagnetic resonance imaging

PURPOSE

To test the feasibility of electron paramagnetic resonance imaging (EPRI) to provide non-invasive images of tissue redox status using redox-sensitive paramagnetic contrast agents.

MATERIAL AND METHODS

Nitroxide free radicals were used as paramagnetic agents and a custom-built 300 MHz EPR spectrometer/imager was used for all studies. A phantom was constructed consisting of four tubes containing equal concentrations of a nitroxide. Varying concentrations of hypoxanthine/xanthine oxidase were added to each tube and reduction of the nitroxide was monitored by EPR as a function of time. Tumor-bearing mice were intravenously infused with a nitroxide and the corresponding reduction rate was monitored on a pixel-by-pixel basis using 2D EPR of the tumor-bearing leg and normal leg serving as control. For animal studies, nitroxides were injected intravenously (1.25 mmol/kg) and EPR projections were collected every 3 min after injection using a magnetic field gradient of 2.5 G/cm. The reduction rates of signal intensity on a pixel-by-pixel basis were calculated and plotted as a redox map. Redox maps were also collected from the mice treated with diethylmaleate (DEM), which depletes tissue thiols and alters the global redox status.

RESULTS

Redox maps obtained from the phantoms were in agreement with the intensity change in each of the tubes where the signals were decreasing as a function of the enzymatic activity, validating the ability of EPRI to accurately access changes in nitroxide reduction. Redox imaging capability of EPR was next evaluated in vivo. EPR images of the nitroxide distribution and reduction rates in tumor-bearing leg of mice exhibited more heterogeneity than in the normal tissue. Reduction rates were found to be significantly decreased in tumors of mice treated with DEM, consistent with the depletion of thiols and the consequent alteration of the redox status.

CONCLUSION

Using redox-sensitive paramagnetic contrast agents, EPRI can non-invasively discriminate redox status differences between normal tissue and tumors.

Mechanism of anesthetic action: oxygen pathway perturbation hypothesis

Although more than 150 years have passed since the discovery of general anesthetics, precisely how they work remains a mystery. We propose a novel unitary mechanism of general anesthesia verifiable by experiments. In the proposed mechanism, general anesthetics perturb oxygen pathways in both membranes and oxygen-utilizing proteins, such that the availability of oxygen to its sites of utilization is reduced, which in turn triggers cascading cellular responses through oxygen-sensing mechanisms, resulting in general anesthesia. Despite the general assumption that cell membranes are readily permeable to oxygen, existing publications indicate that these membranes are plausible oxygen-transport barriers. The present hypothesis provides a unified framework for explaining phenomena associated with general anesthesia and experimental results on the actions of general anesthetics. If verified by experiments, the proposed mechanism also has other significant medical and biological implications.

Accurate and sensitive measurements of pO(2) in vivo using low frequency EPR spectroscopy: how to confer biocompatibility to the oxygen sensors

Within the last few years, there has been a significant amount of progress using EPR oximetry, which has resulted in the availability of instrumentation and paramagnetic materials capable of measuring pO(2) in tissues with an accuracy and sensitivity comparable to or greater than that available by any other method. While the results obtained with EPR so far indicate that criteria for the measurements of pO(2)-such as accuracy, sensitivity, repeatability, and noninvasiveness-can be met, some of the paramagnetic materials with optimum spectroscopic properties (i.e., strong simple signals which are appropriately responsive to changes in pO(2)) may have some undesirable interactions with tissues, causing reactions with and/or losing responsiveness to oxygen. In this paper, several approaches are discussed, such as encapsulation procedures, which can result in the availability of oxygen-sensitive materials in a suitable configuration for long-term studies (absence of toxicity and preservation of the responsiveness to oxygen).

Effect of a polymeric surfactant on electron transport in HL-60 cells

To assess the effect of a polymeric surfactant, Pluronic P-105 on the activity of electron transport chains in the mitochondria of HL-60 cells, the bioreduction rates of two membrane-localized lipophilic spin probes, 16-doxylstearic acid methyl ester (16-DSME) and 5-doxylstearic acid (5-DS), were studied. In addition, the effect of Pluronic on the bioreduction rate of the DNA-intercalating spin-labeled anthracyclin drug, Ruboxyl (Rb) was evaluated. For 16-DSME, the bioreduction kinetics was zero order with regard to the nitroxide concentration, indicating that the rate was controlled by the concentration of the reducing enzyme(s), which depends on the activity of the electron transport chains. The introduction of Pluronic at concentrations higher than 0.01% resulted in the decrease of the 16-DSME bioreduction rate. The data suggested that short-term cell incubation with Pluronic resulted in reduced activity of the electron transport chains in the mitochondria of HL-60 cells. This was corroborated by the results of an MTT assay. For 5-DS, the bioreduction kinetics was first order in the absence of Pluronic, but did not follow any simple kinetic law after a short-term cell incubation with Pluronic. For Rb, the degree of nitroxide bioreduction dropped progressively with increasing Pluronic concentration. Thus, incubating cells with polymeric surfactants modulates the intracellular energy metabolism, which can affect the rates of energy-dependent intracellular processes.

Analysis of hepatic oxidative stress status by electron spin resonance spectroscopy and imaging

Real-time detection of free radicals generated within the body may contribute to clarify the pathophysiological role of free radicals in disease processes. Of the techniques available for studying the generation of free radicals in biological systems, electron spin resonance (ESR) has emerged as a powerful tool for detection and identification. This article begins with a review of spin trapping detection of oxygen-centered radicals using X-band ESR spectroscopy and then describes the detection of superoxide and hydroxyl radicals by the spin trap 5,5-dimethyl-1-pyrroline-N-oxide and ESR spectroscopy in the perfusate from isolated perfused rat livers subjected to ischemia/reperfusion. This article also reviews the current status of ESR for the in vivo detection of free radicals and in vivo imaging of exogenously administered free radicals. Moreover, we show that in vivo ESR-computed tomography with 3-carbamoyl-2,2,5, 5-tetramethylpyrrolidine-1-oxyl may be useful for noninvasive anatomical imaging and also for imaging of hepatic oxidative stress in vivo.

N-t-butyl hydroxylamine, a hydrolysis product of alpha-phenyl-N-t-butyl nitrone, is more potent in delaying senescence in human lung fibroblasts

Alpha-phenyl-N-t-butyl nitrone (PBN), a spin trap, scavenges hydroxyl radicals, protects tissues from oxidative injury, and delays senescence of both normal human lung fibroblasts (IMR90) and senescence-accelerated mice. N-t-butyl hydroxylamine and benzaldehyde are the breakdown products of PBN. N-t-Butyl hydroxylamine delays senescence of IMR90 cells at concentrations as low as 10 microM compared with 200 microM PBN to produce a similar effect, suggesting that N-t-butyl hydroxylamine is the active form of PBN. N-Benzyl hydroxylamine and N-methyl hydroxylamine compounds unrelated to PBN were also effective in delaying senescence, suggesting the active functional group is the N-hydroxylamine. All the N-hydroxylamines tested significantly decreased the endogenous production of oxidants, as measured by the oxidation of 2', 7'-dichlorodihydrofluorescin and the increase in the GSH/GSSG ratio. The acceleration of senescence induced by hydrogen peroxide is reversed by the N-hydroxylamines. DNA damage, as determined by the level of apurinic/apyrimidinic sites, also decreased significantly following treatment with N-hydroxylamines. The N-hydroxylamines appear to be effective through mitochondria; they delay age-dependent changes in mitochondria as measured by accumulation of rhodamine-123, they prevent reduction of cytochrome C(FeIII) by superoxide radical, and they reverse an age-dependent decay of mitochondrial aconitase, suggesting they react with the superoxide radical.

In vivo L-band ESR and quantitative pharmacokinetic analysis of stable spin probes in rats and mice

Free radical species in animals have been measured by X-band ESR spectrometric method on a block of organs or a portion of homogenized samples. However, a nondestructive in vivo ESR measurement has been realized by using a recently developed L-band ESR spectrometry. With this L-band ESR method, we measured ESR spectra in animals, who received stable nitroxide radicals. L-band ESR spectra were observed at the upper abdomen of mice as well as at the heads of mice and rats at various ages immediately after the intravenous injections of nitroxide radicals such as 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3-carbamoyl-PROXYL), in which ESR measurements of the radicals were performed noninvasively at the real time. On the basis of the observed time-dependent free radical clearance curves, the following important results were obtained: (1) Free radical clearances were able to analyze by the pharmacokinetic method. (2) The radicals at the head of mice, given 4-hydroxy-TEMPO, were determined quantitatively by a new analytical method using L-band ESR for the first time. (3) The elimination of the radical was found to be saturated in mice. (4) The clearance rate constant of 4-hydroxy-TEMPO detected at the head of mice was decreased in dose- and age-dependent manners. While, no age-dependent clearance rate constant of 4-hydroxy-TEMPO was observed at the upper abdomen of mice. (5) Ratios of the amount of the detected radicals to that of the administered radicals were decreased age-dependently, but they were independent of the dose of the radicals, suggesting the age-dependent decrease of distribution capacity ratio of the radical at the head of animals. (6) Clearance rate constants of 4-hydroxy-TEMPO and 3-carbamoyl-PROXYL, that were estimated by X- and L-band ESR for the collected blood of mice and rats, were found to be remarkably smaller than those in whole living animals observed by in vivo L-band ESR method. The results suggest that the clearance of the nitroxide radical is relevant to the alteration of the radical in animals following the change of organ distribution and metabolism. (7) Both the radical and its corresponding hydroxylamine, which is the reduced form of the radical, were detectable by X-band ESR method in the collected urine of mice and rats without and with an oxidizing agent, respectively. On the basis of the results on L-band ESR spectrometry, the first quantitative pharmacokinetic analysis of stable spin probes in animals is proposed.

Mechanisms related to reduction of radical in mouse lung using an L-band ESR spectrometer

Reduction of radicals in mouse lung was characterized in whole animals using an L-band ESR technique and nitroxide radicals as probes. An aqueous solution of nitroxide radical was immediately instilled intratracheally to mouse after euthanasia. Nitroxide radicals without charged groups were reduced significantly in the lung, while radicals with charged groups were only slightly reduced. Permeation rates across lung plasma membrane were not rate limiting of the stage of reduction of the noncharged nitroxides. Michaelis parameters, apparent Km and apparent Vmax, were obtained from the Lineweaver-Burk plots of the reduction. Among noncharged nitroxides with constant apparent Vmax, radicals with a larger n-octanol/water partition coefficient showed a lower apparent Km, thereby suggesting that the concentration of these nitroxides in the membrane contributes to apparent Km. The reduction rate of noncharged nitroxide, hydroxy-TEMPO, was influenced by noncharged SH reagents instilled together with the nitroxide; dithiothreitol stimulated the reduction, while the oxidized reagent inhibited it. The Lineweaver-Burk plots of the nitroxide reduction in the presence of various concentrations of dithiothreitol suggest the possibility that the reduction system for hydroxy-TEMPO is based on a kind of ping pong bi-reactant mechanism, and that the reduction system utilizes SH as an electron donor. Endogenous glutathione contributed partially to the reduction.

Bioreduction of Tempone and spin-labeled gentamicin by gram-negative bacteria: kinetics and effect of ultrasound

The primary objective of this study is the investigation of bioreduction kinetics of hydrophilic spin probes, 2,2,6,6, -tetramethyl-4-oxo-piperidinyl-1-oxyl (Tempone), and spin-labeled antibiotic gentamicin by gram-negative bacteria maintained at various oxygen tensions, with emphasis on the effect of probe penetration rate. This information was used to evaluate the effect of ultrasound on the penetration of hydrophilic compounds, including antibiotics, into Pseudomonas aeruginosa and Escherichia coli cells. Penetration of spin-labeled compounds into the cells was assessed by the reduction rate of the nitroxyl moiety measured by EPR. In cell suspensions, both Tempone and spin-labeled gentamicin were localized predominantly in the aqueous phase surrounding the cells. However, a gradual reduction of the probes in contact with the cells indicated that the probes penetrated through the outer membrane and periplasmic space into the cytoplasmic membrane, where the electron transport chains and other metabolic activities of gram-negative bacteria are localized. The kinetics of probe reduction depended on oxygen tension and presence of electron transport chain blockers. It was found that probe penetration rate through the outer cell membrane affected the rate of probe reduction; damaging the permeability barrier by cell incubation with EDTA or by powerful insonation above the cavitation threshold increased the rate of probe reduction. In contrast, insonation below the cavitation threshold did not affect the rate of probe reduction. These findings imply that the recently observed synergistic effect between hydrophilic antibiotics and low frequency ultrasound in killing gram-negative bacteria did not result from the enhanced antibiotic penetration through bacterial cell walls.

Stability of water-soluble and lipid-soluble paramagnetic probes in Bacillus subtilis

Batch cultures of the Gram-positive bacterium Bacillus subtilis PB19 have been investigated for their metabolic action to electron spin resonance (ESR) probes. Five- and six-membered water-soluble and lipid-soluble nitroxides have been used, which were reduced most probably to the corresponding hydroxylamine derivatives. The reduction was followed by the ESR signal intensity and found to be dependent on chemical structure and stability, lipophilic/hydrophilic character, charge, concentration, and temperature. Water-soluble nitroxides did not show apparent toxicity towards B. subtilis, in contrast with n-DXSA (n=5, 12, 16) which were found to be strongly cytotoxic. The cytotoxicity depended on the position of the doxyl unit along the hydrocarbon chain. The hydrophilic nitroxides were reduced at a much slower rate relative to the lipophilic ones. Membrane diffusion was suggested to be a slower process relative to chemical reduction for water-soluble nitroxides. The lipophilic nitroxides were solubilized into the membrane where they were rapidly reduced with a reduction maximum at 303-310 K, which is the optimal growth temperature of B. subtilis, while an inactivation at higher temperatures was observed. Both toxicity and reduction rates of nitroxides strongly indicated that the reduction was an enzyme-mediated process taking place near the outer surface of the periplasmic membrane.

Inhibition of in vitro lipid peroxidation by stable steroidic nitroxyl radicals

4',4'-dimethylspiro (5 alpha-cholestane-3,2'-oxazolidin)-3'-yloxy (IK-1) and 7 alpha,12 alpha-dihydroxy-4',-4'-dimethylspiro (5 beta-cholan-24-oic-3,2'-oxazolidin)-3'-yloxy acid (IK-2), two stable steroidic nitroxyl radicals, were newly synthesized and tested as possible inhibitors of lipid peroxidation, induced by Fenton's reagent in both rat liver microsomes and egg phosphatidylcholine liposomes. The inhibitory activity, evaluated through the formation of thiobarbituric acid reactive substances (TBARS) and the conjugated diene, was compared with that of alpha-tocopherol and 2,2,6,6-tetramethylpiperidine-1-yloxy (TEMPO). In each model system IK-1 and IK-2 exhibited an IC50 of 8 microM and reduced the formation of TBARS and conjugated diene, showing IK-1 a potency comparable to alpha-tocopherol and higher than TEMPO. Moreover IK-1 and, to a lesser extent IK-2, reduced the lipid peroxidation induced in the microsomes by the water-soluble azo-initiator 2,2'-Azobis (2-methylpropionamidine) dihydrochloride (AMPH), indicating the IK-1 and IK-2 ability as chain-breaking antioxidants. The hydroxylamine 4',4'-dimethylspiro (5 alpha-cholestane-3,2'-oxazolidin)-3'-hydroxide (IK-3), obtained by chemical reduction of IK-1, was completely inactive as an inhibitor of lipid peroxidation in heat pre-treated microsomes and in liposomes. However in microsomes it was active since it was oxidized to the corresponding nitroxyl radical IK-1.

Factors affecting the permeability of Pseudomonas aeruginosa cell walls toward lipophilic compounds: effects of ultrasound and cell age

The objective of this research was to elucidate the factors effecting the permeability of cell membranes of gram-negative bacteria toward hydrophobic compounds. Ultrasound treatment, cell age, and the phase state of phospholipid membranes were considered. Spin-labeling EPR method was used to quantify the penetration and distribution of a lipophilic spin probe, 16-doxylstearic acid (16-DS), in Pseudomonas aeruginosa cell membranes. This bacterium was chosen because of its reported resistance to the action of hydrophobic antibiotics caused by the low permeability of the outer cell membrane for hydrophobic compounds. EPR spectra were collected from cell pellets and cell lysates. The overall spin probe uptake was measured in 10% SDS-cell lysates. Lysis with 0.6% SDS revealed the fraction of the probe located in membrane sites readily accessible to the surfactant. The results indicated a structural heterogeneity of P. aeruginosa membranes, with the presence of structurally "stronger" and "weaker" sites characterized by different susceptibility to the SDS treatment. The intracellular concentration of 16-DS was higher in insonated cells and increased linearly with the sonication power. EPR spectra indicated that ultrasound enhanced the penetration of the probe into the structurally stronger sites of the inner and outer cell membranes. The effect of ultrasound on the cell membranes was transient in that the initial membrane permeability was restored upon termination of the ultrasound treatment. These results suggest that the resistance of gram-negative bacteria to the action of hydrophobic antibiotics was caused by a low permeability of the outer cell membranes. This resistance may be reduced by the simultaneous application of antibiotic and ultrasound. This hypothesis was confirmed in our experiments with P. aeruginosa exposed to erythromycin.

MRI contrast enhanced study of cartilage proteoglycan degradation in the rabbit knee

Early degeneration of cartilage is accompanied by a loss of proteoglycans and consequent changes in the content of water. Conventional magnetic resonance imaging (MRI) cannot reliably detect this change, since the relaxation properties of the cartilage are dominated by its collagen content. The applicability of a positively charged nitroxide as an MRI contrast agent in detection of the content of the negatively charged proteoglycans within the cartilage was investigated. The results from both MRI and electron paramagnetic resonance (EPR) spectroscopy indicate that the accumulation of the contrast agent reflects the amount of proteoglycans within the cartilage, presumably due to the electrostatic interactions between the negatively charged proteoglycans and the positively charged nitroxide. Such a contrast agent could be useful in the detection and study of early stages of the degeneration of joints.

Direct evidence for in vivo nitroxide free radical production from a new antiarrhythmic drug by EPR spectroscopy

The new Class I anti-arrhythmic agent 2,2,5,5-tetramethyl-3-pyrroline-1-carboxamide derivative, is currently being evaluated in clinical trials in patients with a high risk for cardiac arrhythmias. In this study we show that this antiarrhythmic drug can be chemically converted to the nitroxide free radical analog. Further, using an in vivo Electron Paramagnetic Resonance (EPR) spectroscopy model by detecting free radicals in the distal portion of the tail of an anesthetized mouse, we demonstrate that the drug is oxidized to the corresponding nitroxide. In vitro studies using Chinese hamster V79 cells suggest that the oxidation products of the drug, namely, the hydroxylamine and the nitroxide protect against oxidative damage induced by hydrogen peroxide (H2O2). Taken together, our results suggest that, in addition to the antiarrhythmic effects of the parent drug, sufficient levels of nitroxides may accumulate from the parent drug in vivo to provide antioxidant defense to cardiac tissue that may be subject to ischemia and oxidation-driven injury.

In vivo electron paramagnetic resonance studies on oxidative stress caused by X-irradiation in whole mice

The effect of x-irradiation on the reduction rates of nitroxyl radicals was examined in whole mice using in vivo EPR. One hour after irradiation, the reduction rates of nitroxyl increased up to 15 Gy irradiation, but decreased over this dose. The enhancement of the reduction rate of nitroxyl was suppressed by preadministration of a radioprotector, cysteamine, suggesting that the enhancement of nitroxyl reduction is related to the radiation damage. Thiobarbituric acid-reactive substances (TBARS) in liver homogenate were increased by x-irradiation, indicating that x-irradiation induced oxidative stress in mice. Endogenous antioxidant, alpha-tocopherol, and the activities of antioxidative enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase were not induced by x-irradiation under these experimental conditions. Eventually the nitroxyl reduction in whole mice should be enhanced by the oxidative stress due to x-irradiation. An in vivo EPR system probing the nitroxyl reduction should be applicable to the noninvasive study on the oxidative stress caused by radiation.