The effects of radiation on living animals are caused by two different mechanisms, i.e., direct and indirect effects. The direct effect is mainly DNA strands breaks, and the indirect effect is oxidative damage mediated by reactive oxygen species (ROS). Various ROS, such as hydroxyl radical ( · OH), superoxide ( · O 2 Ϫ ), and hydrogen peroxide (H 2 O 2 ), were produced by ionizing radiation in living animals. It has been assumed that the generation of · OH, hydrated electrons, and hydrogen radicals could be attributed to the ionization of water, and that · O 2 Ϫ and H 2 O 2 are formed by reacting with dissolved oxygen. Around 70% of the radiation effects on living organisms are based on indirect effects. It is well known that the lethal effect of X-rays on mammalian cells under aerobic conditions is higher than under anaerobic conditions, the so-called oxygen effect of radiation; therefore, the detection and quantification of oxygen-mediated free radical reactions is quite important to estimate the biological efficiency of radiation.Nitroxyl radicals have been used as in vivo redox probes for experimental animals by means of in vivo electron paramagnetic resonance (EPR) spectroscopy and/or imaging. 1,2) Nitroxyl radicals underwent one electron reduction by reactions with oxidoreductases in mitochondria and microsomes, 3,4) antioxidants, 5,6) and other free radical species 7,8) in a living body. The nitroxyl radicals lost their paramagnetism by one electron reduction and were mainly converted to the corresponding diamagnetic hydroxylamines. 9,10) When a living body was exposed to oxidative stress of · O 2 Ϫ and/or · OH, the in vivo EPR signal decay rate of the nitroxyl radicals increased.11,12) A common oxidative stress producing such ROS is ionizing radiation. The b-ray irradiation can significantly decrease the EPR signal of a nitroxyl radical in a solution containing glutathione (GSH).13) The in vivo decay rate of a nitroxyl radical in the rat bile flow significantly increased by b-ray irradiation to the liver.
13)The chemical reduction process of nitroxyl radicals by ROS was moved by two steps. First, the nitroxyl radicals are oxidized to oxoammonium cation by · O 2 Ϫ and/or · OH. Next, oxoammonium cations are reduced to hydroxylamine by receiving a hydrogen atom from hydrogen donors (H-donors), such as reduced GSH, reduced b-nicotinamide adenine dinucleotide (NADH) and reduced b-nicotinamide adenine dinucleotide phosphate (NADPH). Overall, the nitroxyl radicals undergo one-electron reduction by a reaction with · O 2 Ϫ and/or · OH in the presence of H-donors; therefore, the total ROS, i.e., · O 2 Ϫ and · OH, generation can be estimated indirectly by the amount of reduction of nitroxyl radicals.In this study, the stability and reactivity of nitroxyl radicals in the reaction mixture containing an H-donor, i.e., GSH, NADH, or NADPH, were tested. Using a suitable reaction mixture, the amounts of free radical reactions caused by low linear-energy-transfer (LET) irradiation, i.e., b-, g-, and Xray, at a dose lower ...