Pristine (C60) and Hydroxylated [C60(OH)24] Fullerene Phototoxicity towards HaCaT Keratinocytes: Type I vs Type II Mechanisms

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Abstract
The increasing use of fullerene nanomaterials has prompted widespread concern over their biological effects. Herein, we have studied the phototoxicity of γ-cyclodextrin bicapped pristine C60 [(γ-CyD)2/C60] and its water-soluble derivative C60(OH)24 toward human keratinocytes. Our results demonstrated that irradiation of (γ-CyD)2/C60 or C60(OH)24 in D2O generated singlet oxygen with quantum yields of 0.76 and 0.08, respectively. Irradiation (>400 nm) of C60(OH)24 generated superoxide as detected by the EPR spin trapping technique; superoxide generation was enhanced by addition of the electron donor nicotinamide adenine dinucleotide (reduced) (NADH). During the irradiation of (γ-CyD)2/C60, superoxide was generated only in the presence of NADH. Cell viability measurements demonstrated that (γ-CyD)2/C60 was about 60 times more phototoxic to human keratinocytes than C60(OH)24. UVA irradiation of human keratinocytes in the presence of (γ-CyD)2/C60 resulted in a significant rise in intracellular protein-derived peroxides, suggesting a type II mechanism for phototoxicity. UVA irradiation of human keratinocytes in the presence of C60(OH)24 produced diffuse intracellular fluorescence when the hydrogen peroxide probe Peroxyfluor-1 was present, suggesting a type I mechanism. Our results clearly show that the phototoxicity induced by (γ-CyD)2/C60 is mainly mediated by singlet oxygen with a minor contribution from superoxide, while C60(OH)24 phototoxicity is mainly due to superoxide.

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