Oxygen radicals photo-induced by ferric nitrilotriacetate complex

This study examined the photo-induced generation of reactive oxygen species (ROS) by the carcinogenic iron(III)-NTA complex. Iron(III)-NTA complex (1:1) has three conformations (type (a) in acidic conditions of pH 1−6, type (n) in neutral conditions of pH 3−9, and type (b) in basic conditions of pH 7−10) with two pKa values (pKa1 ≈ 4, pKa2 ≈ 8). The iron(III)-NTA complex was reduced to iron(II) under cool-white fluorescent light without the presence of any reducing agent, and the reduction rates of the three conformations of iron(III)-NTA were in the order type (a) > type (n) > type (b) as reported previously (Akai K. et al., Free Radic. Res. 38, 951-962, 2004). ROS generation was investigated by electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique. Apparent EPR signals attributed to PBN/·13CH3 and PBN/·OCH3 spin adducts were observed after incubation of the iron(III)-NTA complex was mixed with α-phenyl-tert-butylnitrone (PBN) and 13C-DMSO in an aerobic condition. The addition of catalase effectively attenuated the PBN adducts, but superoxide dismutase enhanced them. Taken together, these results indicate that the iron(III)-NTA complex is spontaneously reduced to the iron(II)-NTA complex by light under acidic to neutral pH, and in turn transfers an electron to molecular oxygen to form ROS.