Photochemical Generation of Superoxide Radical and the Cytotoxicity of Phthalocyanines

The effect of the central metal atom on the photodynamic activity of phthalocyanine dyes has been estimated by cytotoxicity to cultured Chinese hamster cells. Chloroaluminium phthalocyanine,, followed by the Zn- derivate, were found to be the only active dyes. In parallel it was found that visible light (615 +/- 10 nm) excitation of phthalocyanines dissolved in dimethylsulphoxide in the presence of oxygen generates superoxide radical anion. O2- radicals were spin--trapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance. The quantum yields for O2- generation range from 10(-5) (Zn-phthalocyanine) to 4.2 X 10(-4) (Ga-phthalocyanine). The efficiency of generating O2- was apparently uncorrelated with the phototoxicity of the same dyes. Furthermore, the biological photodamage could not be inhibited by the addition of superoxide dismutase. It is concluded that O2- is involved very little, if at all, in the phthalocyanine-induced photo-killing of mammalian cells.