Production of Hydroxyl Free Radical by Exposure of N-Methyl-N′-Nitro-N-Nitrosoguanidine to Visible Light in the Absence of Hydrogen Peroxide

The production of hydroxyl free radical (|${}^{\bullet}{\rm OH}$|) by N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) in the absence of H₂ O₂ was investigated by electron spin resonance with the use of the spin-trapping agent 5,5-dimethyl-1-pyrroline-1-oxide (DMPO).|${}^{\bullet}{\rm OH}$| was detected as well as a nitrogen-centered free radical (|${}^{\bullet}{\rm NR}$|) on exposure of 1.25 mM MNNG solution (pH 6.0) containing 25 mM DMPO to visible light from a tungsten-halogen lamp at an intensity of|$6.68\ {\rm mW}/{\rm cm}^{2}$|. Production of|${}^{\bullet}{\rm OH}$| depended on the concentration of MNNG and on the intensity of light exposure, indicating that|${}^{\bullet}{\rm OH}$| production resulted from the photolysis of MNNG solution. Manganese-superoxide dismutase and catalase did not inhibit the production of|${}^{\bullet}{\rm OH}$|, excluding the possibility that|${}^{\bullet}{\rm OH}$| might be produced from|${\rm O}_{2}^{-\bullet}$| or H₂ O₂. When the concentration of DMPO was increased from 25 mM to 125 mM, the yield of|${}^{\bullet}{\rm OH}$| decreased significantly while that of|${}^{\bullet}{\rm NR}$| increased over twice that at 25 mM. When the concentration of DMPO was further increased to over 200 mM, only|${}^{\bullet}{\rm NR}$| was detected and its yield was the same as that at 125 mM, which indicates that DMPO inhibited|${}^{\bullet}{\rm OH}$| production but not through the trapping of|${}^{\bullet}{\rm NR}$|. From these results we suggest that nitric oxide (NO) derived with|${}^{\bullet}{\rm NR}$| from the photolysis of MNNG resulted in|${}^{\bullet}{\rm OH}$| production which was inhibited by DMPO in aqueous solution.