[Generation, Detection and Bio-protection of Reactive Oxygen Species/Free Radicals]

In the present paper, generation, detection and protection of reactive oxygen species (ROS)/free radicals in relation to the author's research over about 20 years are reviewed. ROS/free radicals are generally generated physically, chemically and biologically, and they are harmful to living organisms by inducing various disorders and diseases. To prevent the harmful effects of ROS/free radicals, antioxidants are believed to be useful. Among many methods to detect ROS/free radicals, ESR technique is a direct method and is described in detail in this review. Several topics such as the production of ROS/free radicals by low temperature atmospheric pressure plasma, the evaluation of antioxidant activity using hemolysis of erythrocytes and the protective effects of antioxidants against X-ray induced damage to mice, are presented.

Effects of LET on oxygen-dependent and-independent generation of hydrogen peroxide in water irradiated by carbon-ion beams

Linear energy transfer (LET) dependence of yields of O₂-dependent and O₂-independent hydrogen peroxide (H₂O₂) in water irradiated by ionizing radiation was investigated. The radiation-induced hydroxyl radical (•OH) generation in an aqueous solution was reported to occur in two different localization densities, the milli-molar (relatively sparse) and/or molar (markedly-dense) levels. In the milli-molar-level •OH generation atmosphere, •OH generated at a molecular distance of ∼7 nm are likely unable to interact. However, in the molar-level •OH generation atmosphere, several •OH were generated with a molecular distance of 1 nm or less, and two •OH can react to directly make H₂O₂. An aliquot of ultra-pure water was irradiated by 290-MeV/nucleon carbon-ion beams at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, NIRS/QST, Chiba, Japan). Irradiation experiments were performed under aerobic or hypoxic (<0.5% oxygen) conditions, and several LET conditions (13, 20, 40, 60, 80, or >100 keV/μm). H₂O₂ generation in irradiated samples was estimated by three methods. The amount of H₂O₂ generated per dose was estimated and compared. O₂-independent H₂O₂ generation, i.e. H₂O₂ generation under hypoxic conditions, increased with increasing LET. On the other hand, the O₂-dependent H₂O₂ generation, i.e. subtraction of H₂O₂ generation under hypoxic conditions from H₂O₂ generation under aerobic conditions, decreased with increasing LET. This suggests that the markedly-dense •OH generation is positively correlated with LET. High-LET beams generate H₂O₂ in an oxygen-independent manner.

Heavy-ion beam-induced reactive oxygen species and redox reactions

Quantification and local density estimation of radiation-induced reactive oxygen species (ROS) were described focusing on our recent and related studies. Charged particle radiation, i.e. heavy-ion beams, are currently utilized for medical treatment. Differences in ROS generation properties between photon and charged particle radiation may lead to differences in the quality of radiation. Radiation-induced generation of ROS in water was quantified using several different approaches to electron paramagnetic resonance (EPR) techniques. Two different densities of localized hydroxyl radical (•OH) generation, i.e. milli-molar and molar levels, were described. Yields of sparse •OH decreased with increasing linear energy transfer (LET), the yield total •OH was not affected by LET. In the high-density, molar level, •OH environment, •OH can react and directly make hydrogen peroxide (H₂O₂), and then possible to form a high-density H₂O₂ cluster. The amount of total oxidation reactions caused by oxidative ROS, such as •OH and hydroperoxyl radial (HO₂^•), was decreased with increasing LET. Possibilities of the sequential reactions were discussed based on the initial localized density at the generated site. Water-induced ROS have been well investigated. However, little is known about radiation-induced free radical generation in lipidic conditions. Radio-chemistry to understand the sequential radio-biological effects is still under development.

A quantitative analysis of carbon-ion beam-induced reactive oxygen species and redox reactions

The amounts of reactive oxygen species generated in aqueous samples by irradiation with X-ray or clinical carbon-ion beams were quantified. Hydroxyl radical (^•OH), hydrogen peroxide (H₂O₂), and the total amount of oxidation reactions, which occurred mainly because of ^•OH and/or hydroperoxy radicals (HO₂ ^•), were measured by electron paramagnetic resonance-based methods. ^•OH generation was expected to be localized on the track/range of the carbon-ion beam/X-ray, and mM and M levels of ^•OH generation were observed. Total ^•OH generation levels were identical at the same dose irrespective of whether X-ray or carbon-ion beam irradiation was used, and were around 0.28-0.35 µmol/L/Gy. However, sparse ^•OH generation levels decreased with increasing linear energy transfer, and were 0.17, 0.15, and 0.09 µmol/L/Gy for X-ray, 20 keV/µm carbon-ion beam, and >100 keV/µm carbon-ion beam sources, respectively. H₂O₂ generation was estimated as 0.26, 0.20, and 0.17 µmol/L/Gy, for X-ray, 20 keV/µm carbon-ion beam, and >100 keV/µm carbon-ion beam sources, respectively, whereas the ratios of H₂O₂ generation per oxygen consumption were 0.63, 0.51, and 3.40, respectively. The amounts of total oxidation reactions were 2.74, 1.17, and 0.66 µmol/L/Gy, respectively. The generation of reactive oxygen species was not uniform at the molecular level.

A New Approach for Quantifying Radio-Biological Effects Using the Time Course of Mouse Leg Contracture

A digitization approach to the time course of radiation-induced mouse leg contracture was proposed for quantifying the radiation effect on an individual living mouse. The shortening of the mouse leg length can be easily measured with a caliper/ruler to offer a very simple digitalized index of the radiation effect. Left hind legs of mice were irradiated with single dose of 32 Gy of 290 MeV carbon-ion beam using 0, 50, or 117 mm binary filter (BF). The right legs were used as a control. The lengths of both hind legs of the mice were measured using a digital caliper before irradiation and every week after irradiation. The degree of leg contracture, ΔS_t, at the time point t was estimated by subtraction of the left irradiated leg length from the right control leg length. Equation was fitted on the daily time course of ΔS_t, and two parameters, ΔS_max and T_s, were estimated. ΔS_t=ΔS_max×(1-exp(t/T_s)), where ΔS_max is the maximum degree of leg contracture, and T_s is time of leg contracture. The effect of carbon-ion irradiation on a living mouse was quantified by ΔS_max and T_s of the leg contracture, and then compared to that of X-rays. By 32 Gy irradiation, ΔS_max was largest for the BF117 experiment, followed by X-ray~BF50>BF0. T_s was shortest for the BF50 experiment, while other irradiation conditions give similar T_s. A logarithmic function was successfully repurposed for the evaluation of radio-biological response.