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

Quantitative detection of free radical reactions induced by low linear-energy-transfer (LET) irradiation in an aqueous solution was attempted using nitroxyl radicals. The stability and reactivity of reaction mixtures containing a nitroxyl radical and a hydrogen donor, i.e., glutathione (GSH), nicotinamide adenine dinucleotide (NADH), or nicotinamide adenine dinucleotide phosphate (NADPH), were tested. X-band electron paramagnetic resonance (EPR) measurements of several reaction mixtures were performed to find a suitable preparation to quantitatively detect free radical reactions produced by low LET irradiation. The EPR signal intensity of nitroxyl radicals was decreased by low LET irradiation when a hydrogen donor coexisted in the reaction mixture. The combination of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (4-hydroxy-TEMPO, known as TEMPOL) and GSH was most preferable among other preparations tested in this paper, because of the sensitivity and irreversible reaction. The extent of the free radical reaction, i.e., formation of reactive oxygen species, in the reaction mixture depended on the radiation energy when an identical dose was given.

Visualization of free radical reactions in an aqueous sample irradiated by 290 MeV carbon beam.

The detection of free radical reactions in a gelatin sample irradiated by a heavy-ion beam was tested using electron paramagnetic resonance (EPR) spectroscopic and MRI methods. Geometry and the amount of free radical generation in a sample are described. A reaction mixture containing glutathione and a nitroxyl radical, TEMPOL, was caked with gelatin, and then irradiated with a 290 MeV carbon beam. The amount of free radical generation in a solid sample was almost flat from the surface to the beam end, except for a small peak, the peak radioactivation profile, and then steeply decreased approaching the beam end. Total free radical reactions obtained with carbon-beam irradiation were expected to be less than one-third of X-ray irradiation, when the same dose for a deeper target was considered. Both EPR and MRI are useful tools to visualize free radical generation in samples irradiated by a heavy-ion beam. The EPR-based method is more sensitive and quantitative than the MRI-based method; however, the MRI method can achieve high spatial resolution. This study gives the rationale for a redox regulation trial using antioxidant drugs to reduce the side effects on normal tissues in carbon-beam therapy.