The conversion of
singlet oxygen ((1)O2) to
hydroxyl radical (*
OH) during
photosensitization of uroporphyrin (UP) in the presence of
NADPH was examined by a spin-trapping technique with
5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Significant electron spin resonance (ESR) signals of
DMPO-OH adduct were observed during irradiation of the UP-
NADPH system with visible light. Scavengers of *
OH reduced the signal intensity to 3-30% of control, indicating that more than 70% of
DMPO-OH results from freely diffusing *
OH. The ESR signal was almost completely lost when quenchers of (1)O2 were added, and was enhanced when the amount of deutrated
solvent was increased. The appearance of (1)O2, as determined by the oxidation of
2,2,6,6-tetramethyl-4-piperidone (TEMPD), was delayed with an increase in the concentration of
NADPH, whereas the production of *
OH was upregulated. These observations indicate that conversion of (1)O2 to *
OH occurs quickly in the presence of
NADPH.
Hydrogen peroxide (H2O2) was produced (1)O2-dependently during irradiation of UP in the presence of
NADPH. However, neither
catalase nor
desferrioxamine decreased the
DMPO-OH signal, and addition of H2O2 did not increase the signal. SOD increased the signal only slightly. These results suggest that the production of *
OH from (1)O2 involves neither
superoxide anion radical nor H2O2.