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.

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