The Influence of Depleted Glutathione Levels on the Photodynamic Action of Zinc Phthalocyanine in CHO K1 Cells

Objective: The current study focuses on any influence that depletion of endogenous glutathione in CHO K1 cells may have on the photodynamic action of zinc phthalocyanine (ZnPc). Materials and Methods: Two lasers - a HeNe laser, 632.5 nm, maximum power output 3.5 mW, and a Toshiba semiconducting laser, 670 nm, maximum power of 7 mW - were used. Chinese Hamster Ovary cells (CHO K1) were exposed to light, 2-10 J. Cellular reduced glutathione levels [GSH] were depressed prior to exposure to ZnPc and laser light, using buthionine sulphoximine, a potent inhibitor of γ-glutamylcysteine synthetase. The influence of hypoxic intracellular conditions was studied by reduction of oxygen content of cells by 80% following purging of cell cultures with nitrogen. Results: In well-aerated cells, doubling times are reduced by the photodynamic action of ZnPc by 29 ± 6%, fig 2 (p = 0.01). Cells with lowered [GSH] do not show this effect (p = 0.1). When hypoxic cells are studied at normal [GSH], no photodynamic effect is observed (p = 0.1). When cell viability is studied, using the 670-nm laser, a photodynamic effect is observed, (80% fall from controls, p < 0.001), irrespective of the cellular [GSH] level for a single dose of 6 J. This effect is observed in cells with normal [GSH], for varied doses of 2 J and higher (63% fall at 2 J, p < 0.001). Conclusions: Lowered [GSH] was observed to depress the photodynamic effect of ZnPc when cell-doubling times were the endpoint. The photostimulating effect of ZnPc was similarly suppressed by hypoxic conditions. When cell viability was the endpoint, then a photodynamic effect of ZnPc was observed irrespective of the endogenous [GSH] values.