Detection of free radical reactions in an aqueous sample induced by low linear-energy-transfer irradiation

Towards liquid EPR dosimetry using nitroxides in aqueous solution

Objective. Water-equivalent dosimeters are desirable for dosimetry in radiotherapy. The present work investigates basic characteristics of novel aqueous detector materials and presents a signal loss approach for electron paramagnetic resonance (EPR) dosimetry.Approach. The proposed principle is based on the radiation dose dependent annihilation of EPR active nitroxides (NO·) in aqueous solutions. Stable nitroxide radicals (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP), 3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in aqueous solutions containing dimethyl sulfoxide (DMSO) as an additive were filled in glass capillaries for irradiation and EPR readout. Radiation doses ranging from 1 to 64 Gy were applied with a clinical 6 MV flattening filter free photon beam. EPR readout was then performed with a X-band benchtop spectrometer. The dose response, temporal stability and reproducibility of the samples' EPR signal amplitudes as well as the influence of the nitroxide concentration between 10 and 160μM on the absolute signal loss were investigated using MmP. CmP was used to examine the dependence of the dose response on DMSO concentration between 0 and 10 vol%. An indirect effect model was fitted to the experimental data assuming irradiation induced radical reactions as the underlying mechanism.Main results. For an initial MmP concentration of 20μM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(-NO·) of approximately 0.4μmol J-1. Within five weeks upon sample irradiation to doses between 0 and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0-5 vol%. An indirect effect model is applicable to describe the reaction mechanism resulting in the observed dose response curve.Significance. Liquids consisting of nitroxides in aqueous solution and small amounts of DMSO (2 vol%) show promising basic characteristics for application as water-equivalent EPR dosimeter materials in radiotherapy. The EPR signal loss is based on an indirect effect mediated by diffusing radicals originating from the radiolysis of the water/DMSO mixture.

Enhanced virus inactivation by copper and silver ions in the presence of natural organic matter in water

Natural organic matter (NOM) is present in water matrix that serves as a drinking water source. This study examined the effect of low and high NOM concentrations on inactivation kinetics of a model RNA virus (MS2) and a model DNA virus (PhiX 174) by copper (Cu²⁺) and/or silver (Ag⁺) ions. Cu and Ag are increasingly applied in household water treatment (HHWT) systems. However, the impact of NOM on their inactivation kinetics remains elusive despite its importance for their application. The presence of NOM in water led to faster virus inactivation by Cu²⁺ but slower by Ag⁺. The fastest inactivation kinetics of MS2 (K_obs = 4.8 h^-1) were observed by Cu in water containing high NOM (20 mg C/L). Meanwhile, for PhiX 174, the fastest inactivation kinetics (av. K_obs = 3.5 h^-1) were observed by Cu and Ag synergism in water containing high NOM. Altogether, it can be concluded that the combination of Cu and Ag is promising as a virus disinfectant in treatment options allowing for multiple hours of residence time such as safe water storage tanks.

Protein photodegradation in the visible range? Insights into protein photooxidation with respect to protein concentration

Visible light (400-800 nm) can lead to photooxidation of protein formulations, which might impair protein integrity. However, the relevant mechanism of photooxidation upon visible light exposure is still unclear for therapeutic proteins, since proteinogenic structures do not absorb light in the visible range. Here, we show that exposure of monoclonal antibody formulations to visible light, lead to the formation of reactive oxygen species (ROS), which subsequently induce specific protein degradations. The formation of ROS and singlet oxygen upon visible light exposure is investigated using electron paramagnetic resonance (EPR) spectroscopy. We describe the initial formation of ROS, most likely after direct reaction of molecular oxygen with a triplet state photosensitizer, generated from intersystem crossing of the excited singlet state. Since these radicals affect the oxygen content in the headspace of the vial, we monitored photooxidation of these mAb formulations. With increasing protein concentrations, we found (i) a decreasing headspace oxygen content in the sample, (ii) a higher relative number of radicals in solution and (iii) a higher protein degradation. Thus, the protein concentration dependence indicates the presence of higher concentration of a currently unknown photosensitizer.

Reduction of molecular oxygen by redox active thiols: comparison of glutathione, N-acetylcysteine, cysteine, and homocysteine

The reaction properties of the thiol compounds, cysteine (Cys), N-acetyl-l-cysteine (NAC), the reduced form glutathione (GSH), and homocysteine (HCS) were compared. The main purpose of this study was to find a thiol-based anti-oxidant suitable for biological experiments and to provide clear reasoning for its selection. The availability of thiol compounds to generate superoxide by reducing molecular oxygen (O₂) at a hyperthermal temperature was discussed. An oxidative atmosphere, i.e., superoxide generation by the hypoxanthine-xanthine oxidase reaction, hydroxyl radical generation by X-ray irradiation, or direct one-electron oxidation by ferricyanide, was prepared in a reaction mixture containing 0.1 mM TEMPOL and 1 mM test compound, and the EPR signal decay of TEMPOL was observed. A reaction mixture containing 0.1 mM TEMPOL and 1 mM thiol compound was incubated at 44°C, and the EPR signal decay of TEMPOL was observed. Thiols could function as H-donors to the oxoammonium cation and produce the hydroxylamine form of TEMPOL in an oxidative atmosphere. Thiols could also irreversibly react with the oxoammonium cation. GSH and Cys could reduce O₂ to form superoxide/hydroperoxyl radical at hyperthermal temperatures, but HCS and NAC could not reduce O₂. GSH and Cys may cause reductive stress, whereas NAC is a simple tractable antioxidant.

Reactivity of redox sensitive paramagnetic nitroxyl contrast agents with reactive oxygen species

The reactivity of nitroxyl free radicals, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), with reactive oxygen species (ROS) were compared as typical 6-membered and 5-membered ring nitroxyl compounds, respectively. The reactivity of the hydroxylamine forms of both these nitroxyl radicals (TEMPOL-H and CmP-H) was also assessed. Two free radical species of ROS, hydroxyl radical (^•OH) and superoxide (O₂^•−), were subjected to a competing reaction. ^•OH was generated by UV irradiation from an aqueous H₂O₂ solution (H₂O₂-UV system), and O₂^•− was generated by a reaction between hypoxanthine and xanthine oxidase (HX-XO system). ^•OH and O₂^•− generated by the H₂O₂-UV and HX-XO systems, respectively, were measured by electron paramagnetic resonance (EPR) spin-trapping, and the amount of spin adducts generated by each system was adjusted to be equal. The time courses of the one-electron oxidation of TEMPOL, CmP, TEMPOL-H, and CmP-H in each ROS generation system were compared. A greater amount of TEMPOL was oxidized in the HX-XO system compared with the H₂O₂-UV system, whereas the reverse was observed for CmP. Although the hydroxylamine forms of the tested nitroxyl radicals were oxidized evenly in the H₂O₂-UV and HX-XO systems, the amount of oxidized CmP-H was approximately 3 times greater compared with TEMPOL-H.

Detection of Reactive Oxygen and Nitrogen Species by Electron Paramagnetic Resonance (EPR) Technique

During the last decade there has been growing interest in physical-chemical oxidation processes and the behavior of free radicals in living systems. Radicals are known as intermediate species in a variety of biochemical reactions. Numerous techniques, assays and biomarkers have been used to measure reactive oxygen and nitrogen species (ROS and RNS), and to examine oxidative stress. However, many of these assays are not entirely satisfactory or are used inappropriately. The purpose of this chapter is to review current EPR (Electron Paramagnetic Resonance) spectroscopy methods for measuring ROS, RNS, and their secondary products, and to discuss the strengths and limitations of specific methodological approaches.

Roles of Sulfur Metabolism and Rhodanese in Detoxification and Anti-Oxidative Stress Functions in the Liver: Responses to Radiation Exposure

Organisms must confront various environmental stresses. The liver is central to protecting against such stresses in mammals, and it has many detoxification and anti-oxidative stress functions. Radiation is a source of oxidative stress and is known to affect the liver and induce anti-oxidative responses. The detoxification enzyme rhodanese, which is also called thiosulfate sulfurtransferase (TST), has been demonstrated to be induced in the liver in response to radiation. Cyanide detoxification is a function of the liver, and rhodanese is a key enzyme involved in sulfur metabolism in that detoxification. Though the anti-oxidative stress system in which sulfur molecules such as thiol compounds are involved has attracted attention as a defense against radiation, detoxification enzymes may have other roles in this defense. Understanding how these functions are affected by alterations of sulfur metabolism (including thiol compounds) after irradiation would help uncover their roles in defense against cancer and other deleterious health effects, as well as environmental stress responses. This article reviews the roles of sulfur-related metabolism in oxidative stress regulation and detoxification for recovery from liver damage after radiation exposure, with particular attention to recent findings of sulfur-related enzymes such as rhodanese, which is unique in sulfur metabolism.

Scavenging of reactive oxygen species induced by hyperthermia in biological fluid

The scavenging activity of rat plasma against hyperthermia-induced reactive oxygen species was tested. The glutathione-dependent reduction of a nitroxyl radical, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, which was restricted by adding superoxide dismutase or by deoxygenating the reaction mixture, was applied to an index of superoxide (O₂^•−) generation. A reaction mixture containing 0.1 mM 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl and 1 mM glutathione was prepared using 100 mM phosphate buffer containing 0.05 mM diethylenetriaminepentaacetic acid. The reaction mixture was kept in a screw-top vial and incubated in a water bath at 37 or 44°C. The time course of the electron paramagnetic resonance signal of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl in the reaction mixture was measured by an X-band EPR spectrometer (JEOL, Tokyo, Japan). When the same experiment was performed using rat plasma instead of 100 mM PB, the glutathione-dependent reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, i.e., generation of O₂^•−, was not obtained. Only the first-order decay reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, which indicates direct reduction of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl, was obtained in rat plasma. Adding 0.5% albumin to the phosphate buffer reaction mixture could almost completely inhibit O₂^•− generation at 37°C. However, addition of 0.5% albumin could not inhibit O₂^•− generation at 44°C, i.e., hyperthermic temperature. Ascorbic acid also showed inhibition of O₂^•− generation by 0.01 mM at 37°C, but 0.02 mM or more could inhibit O₂^•− generation at 44°C. A higher concentration of ascorbic acid showed first-order reduction, i.e., direct one-electron reduction, of 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl. Hyperthermia-induced O₂^•− generation in rat plasma can be mostly inhibited by albumin and ascorbic acid in the plasma.

Method for assessing X-ray-induced hydroxyl radical-scavenging activity of biological compounds/materials

A method for correctly assessing hydroxyl radical scavenging activity of antioxidative chemicals and/or biological compounds/materials was proposed. This method can simultaneously assess two factors, i.e. hydroxyl radical-scavenging and 5,5-dimethyl-2-hydroxy-1-pyrrolidine-N-oxide (hydroxyl radical adduct of 5,5-dimethyl-1-pyrroline-N-oxide)-reducing ability, as antioxidative properties. In this paper, some biologically common hydrophilic molecules, cell culture media, and rat plasma were tested. X-ray-induced hydroxyl radical can be detected using the electron paramagnetic resonance spin trapping technique. Using X-ray irradiation of the reaction mixture as the hydroxyl radical source, the true hydroxyl radical-scavenging ability of the subjected antioxidant can be assessed. In addition, the method simultaneously measures the reduction of 5,5-dimethyl-2-hydroxy-1-pyrrolidine-N-oxide, to estimate the reducing ability of the antioxidant. Biological materials, such as sugars and proteins, could abolish hydroxyl radical at the biological concentration. Ascorbic acid showed reducing ability at the biological concentration. The simultaneous assessment of hydroxyl radical-scavenging and reducing ability of antioxidants can be an informative index for antioxidants.

Temperature-dependent free radical reaction in water

Temperature-dependent free radical reactions were investigated using nitroxyl radicals as redox probes. Reactions of two types of nitroxyl radicals, TEMPOL (4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl) and carbamoyl-PROXYL (3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl), were tested in this paper. Heating a solution containing a nitroxyl radical and a reduced form of glutathione (GSH) caused temperature-dependent decay of electron paramagnetic resonance (EPR) signal of the nitroxyl radical. Heating a solution of the corresponding hydroxylamine form of the nitroxyl radical showed EPR signal recovery. The GSH-dependent reduction of nitroxyl radicals at 70°C was suppressed by antioxidants, spin trapping agents, and/or bubbling N₂ gas, although heating carbamoyl-PROXYL with GSH showed temporarily enhanced signal decay by bubbling N₂ gas. Since SOD could restrict the GSH-dependent EPR signal decay of TEMPOL, O₂^•− is related with this reaction. O₂^•− was probably generated from dissolved oxygen in the reaction mixture. Oxidation of the hydroxylamines at 70°C was also suppressed by bubbling N₂ gas. Heating a solution of spin trapping agent, DMPO (5,5-dimethyl-1-pyrroline-N-oxide) showed a temperature-dependent increase of the EPR signal of the hydroxyl radical adduct of DMPO. Synthesis of hydroxyl radical adduct of DMPO at 70°C was suppressed by antioxidants and/or bubbling N₂ gas. The results suggested that heating an aqueous solution containing oxygen can generate O₂^•−.

Brain redox imaging

Nitroxyl contrast agents (nitroxyl radicals, also known as nitroxide) are paramagnetic species, which can react with reactive oxygen species (ROS) to lose paramagnetism to be diamagnetic species. The paramagnetic nitroxyl radical forms can be detected by using electron paramagnetic resonance imaging (EPRI), Overhauser MRI (OMRI), or MRI. The time course of in vivo image intensity induced by paramagnetic redox-sensitive contrast agent can give tissue redox information, which is the so-called redox imaging technique. The redox imaging technique employing a blood-brain barrier permeable nitroxyl contrast agent can be applied to analyze the pathophysiological functions in the brain. A brief theory of redox imaging techniques is described, and applications of redox imaging techniques to brain are introduced.